Hybrid metal matrix composite: a comprehensive review on its fabrication, corrosion behaviour and inhibition

Abstract

Corrosion remains a major global challenge with substantial economic consequences. Aluminium alloys and their metal matrix composites find extensive use in various industrial applications due to their exceptional strength-to-weight ratio and natural resistance to corrosion. These materials are commonly used in structures, storage tanks, pipelines, and containers, where they can be exposed to corrosive conditions and are vulnerable to degradation in such aggressive environments. Use of inhibitors is an effective corrosion control strategy which enhances their long-term durability and reliability. This review provides an in-depth coverage of how reinforcements impact the corrosion behavior of aluminium hybrid composites, their fabrication techniques, methods for assessing corrosion rates, surface characterization techniques, and corrosion prevention strategies, with a strong emphasis on inhibitors. Additionally, recent developments in the application of hybrid heterocyclic derivatives as corrosion inhibitors for different metals are also covered.

Article Highlights

The significance of corrosion study, the different types of electrolytes employed for it, various preventive methods with a focus on inhibitors, their stability, and the general factors influencing the corrosion rate are discussed.

Importance of Hybrid metal matrix composites, its fabrication methods, effects of various reinforcing particulates and its role in enhancing the mechanical properties.

Corrosion behaviour of Aluminium hybrid metal matrix composites in various corrosive medium, different techniques employed to measure the corrosion rate.

The application of inhibitors for protecting the hybrid composites from undergoing corrosion. Importance of hybrid heterocycles as corrosion inhibitors for various metals and alloys. The corrosion inhibition mechanism of the hybrid composite and various surface characterization techniques.

Introduction

Metals gradually corrode over time due to chemical or electrochemical interaction with their surroundings [1, 2]. This challenge significantly impacts various industries, including construction, automotive, oil and gas, aerospace, and marine sectors [3]. Preventing corrosion is vital to preserve the integrity and longevity of metal structures and equipment. Industrial operations such as pickling and oil-well acidization often involve aggressive acids, which can accelerate metal degradation [4, 5–6]. Consequently, corrosion can result in serious issues, including equipment failure, plant shutdowns, reduced operational efficiency, increased maintenance expenses, product losses, and even structural failures [7, 8–9]. These setbacks place a substantial financial burden on industries. To counter these risks, adopting effective corrosion control is essential, as they help extend material’s lifespan.

Studies indicate that the losses incurred by industries and government sectors due to corrosion have exceeded several billion dollars annually, resulting in significant economic consequences. The World Corrosion Organization estimates that the global annual cost of corrosion is approximately $2.5 trillion, accounting for about 3% of the global GDP [10, 11–12]. In many developed nations, the economic impact can range from 3.5 to 4.5% of their GNP [13]. These figures highlight the potentially devastating effects of corrosion. To address this issue, regular corrosion detection, including monitoring and inspection, is conducted to evaluate key variables such as the type and rate of corrosion. Therefore, implementing effective corrosion prevention and control strategies is essential to minimize these impacts while ensuring safety and operational efficiency [14].

The selection of corrosion control methods depends on the type of metal and the surrounding corrosive environment. Common approaches include choosing suitable materials, improving design, applying metal coatings, utilizing cathodic and anodic protection, and employing corrosion inhibitors (CIs) [15]. Figure 1 presents various methods to control corrosion. Among these, CIs are often considered the most effective and convenient option when process conditions permit [16, 17–18]. These inhibitors provide a cost-effective and adaptable approach to mitigate metal degradation in a wide range of industries. Their ease of application either directly onto surfaces or through fluid systems such as pipelines and boilers. Moreover, their compatibility with other protective measures and minimal environmental impact make them an efficient and sustainable solution for comprehensive corrosion management.

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Fig. 1

Various corrosion control techniques

These inhibitors work by adsorbing onto the metal surface, forming a protective barrier that decreases further corrosion by hindering either the anodic and/or cathodic reactions [19, 20–21]. The stability factor is one of the fundamental aspects in the evaluation of inhibitors performance. The importance of thorough testing to select inhibitors that provide reliable corrosion protection while meeting environmental and economic criteria emphasizes on three main steps: laboratory evaluation, compatibility assessment, and field evaluation. Laboratory methodologies simulate operational conditions to test inhibitor efficiency, using techniques like wheel tests, bubble tests, and rotating disk electrodes. Compatibility evaluation considers cost, environmental impact, and quality control. Field evaluation ensures inhibitors effectively reduce the corrosion rates through methods such as weight loss coupons, linear polarization resistance probes, and pipeline integrity gauges [22, 23]

Different electrolytes are employed in corrosion measurements to examine material interactions and replicate particular environmental conditions. Because of their potent corrosive qualities and capacity to mimic industrial environments, acidic electrolytes such as hydrochloric, sulfuric, and nitric acids are selected. An understanding of corrosion in alkaline conditions is aided by basic electrolytes like potassium and sodium hydroxide. To simulate road salt conditions and marine salt conditions, respectively, neutral electrolytes such as calcium chloride and sodium chloride are utilized. NaCl saturated with CO₂ and CO₂ + H₂S are the examples of mixed electrolytes that mimic circumstances found in the oil and gas sector. These electrolytes were chosen because they can simulate real-world conditions, speed up corrosion processes, and reveal information about the mechanisms and corrosion resistance of different materials [24].

From the literature it can be seen that acid pickling is a pre-treatment process used to clean metal and alloy surfaces by utilizing acids such as sulfuric acid, hydrochloric acid, or a combination of both [25, 26]. This process effectively removes oxide layers from the metal surface. However, while these acids efficiently eliminate surface contaminants, they also contribute to significant metal dissolution, thereby increasing the corrosion rate [27]. Additionally, the presence of chlorides and other foreign anions in the solution further accelerates metal degradation. To mitigate these issues, inhibitors are essential additives in pickling baths, as they help maintain the efficiency of the pickling process while protecting metal components from the corrosive effects of acids [28].

The corrosion behaviour and its inhibition are largely affected by the several factors such as temperature, pH, conductance of the medium, humidity, corrosive concentration, velocity, presence of oxygen and oxidizers, and metallurgical aspects [29]. Generally, higher temperatures and lower pH levels increase corrosion rates. Solutions with higher conductivity and higher humidity also promote corrosion. The concentration of corrosive species can either increase or stabilize the corrosion rate, while fluid velocity can either reduce or enhance corrosion depending on the environment. Oxygen and oxidizers can accelerate or inhibit corrosion, and the presence of defects in metals or alloying elements can significantly impact corrosion behavior. Smooth surfaces tend to be less susceptible to corrosion compared to rough surfaces [30]. Thus, it is evident that the rate of corrosion of material in a given environment is influenced to a varying degree by the above listed factors.

The present article aims at highlighting the applications of hybrid metal matrix composites (HMMCs), which exhibit enhanced mechanical properties through the combination of various reinforcing particulates and fabrication techniques. It further explores the corrosion behavior of aluminium-based hybrid metal matrix composites (Al-HMMCs), with a detailed discussion on how different reinforcing elements influence corrosion rates. The role of hybrid heterocyclic molecules in corrosion mitigation across various metals and alloys is summarized. Additionally the different surface characterization methods employed to analyse morphological changes are described in detail.

Aluminium and it’s alloys

Aluminium and it’s alloys remain extensively utilized across diverse industries, including aerospace, marine, defence and automotive [31, 32]. Renowned for it’s exceptional properties, aluminium is regarded as the third most abundant element on the earth’s crust, after oxygen and silicon [33]. It is lightweight, with a density just one-third that of steel, and is characterized by its silvery appearance, non-magnetic nature, ductility, and softness [34]. Aluminium alloy consists primarily of aluminium combined with other elements such as copper, magnesium, iron, chromium, zinc, and manganese, which enhance properties like strength, hardness, and corrosion resistance [35]. The International Alloy Designation System (IADS) classifies aluminium alloys into eight series based on alloying elements as shown in the Fig. 2.

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Fig. 2

Classification of aluminium alloys

The selection of the appropriate aluminium alloy series depends on the specific requirements of the intended application [36]. Aluminium alloys have good strength as well as excellent corrosion resistance due to a protective oxide layer, but under harsh conditions such as seawater exposure or acidic environments these alloys are vulnerable to degradation [37, 38–39]. Moreover, certain aluminium alloys possess relatively low mechanical strength, which limits their use in more demanding engineering applications. To address these challenges composites have been developed.

Aluminium HMMCs (Al-HMMCs)

In response to the increasing demands of modern engineering, high mechanical strength, and superior strength-to-weight ratios are essential. To meet these requirements, advanced metal matrix composites reinforced with particulates have been developed, gradually replacing conventional alloys [40]. When multiple types of reinforcements are used, improved mechanical properties are observed and the resulting composites are referred to as HMMCs [41].

To further optimize their performance, researchers have explored the incorporation of diverse ceramic-based reinforcements such as silicon carbide (SiC), aluminium oxide (Al₂O₃), boron carbide (B₄C), tungsten carbide (WC), graphite (Gr), single or multi-walled carbon nanotubes (CNTs), silica (SiO₂), titanium diboride (TiB₂), and silicon nitride (Si₃N₄) [42, 43]. Due to the limited availability and high cost of ceramic reinforcements, industrial by-products and agro waste derivatives are also used as reinforcements [44]. Research studies indicate that various combinations of these reinforcements have been utilized in developing HMMCs, such as blends of ceramic particles with agro-waste derivatives, ceramic particles with industrial by-products, and combinations of different ceramic particulates.

Reinforcement particles in Al-HMMCs

The incorporation of reinforcements improves the mechanical properties of aluminium-based composites, including hardness, tensile strength, compressive strength, impact strength, greater stiffness, better thermal and corrosion resistance [45]. The extent of these enhancements in HMMCs depends on factors such as the type of reinforcement particles, stirring parameters, and the uniform distribution of reinforcements while minimizing agglomeration during fabrication. Figure 3 shows various kinds of reinforcements that can be utilized during the fabrication of Al-HMMCs and the brief overview of different reinforcing particulates are given in Table 1.

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Fig. 3

Reinforcements in Al-HMMCs

Table 1. Different reinforcing particulates incorporated into Al-HMMCs

Type	Reinforcements	Role of reinforcement particulates	References
Ceramic particulates	SiC	High thermal conductivity, low thermal expansion coefficient, extreme hardness and resistant to wear	[46]
	B4C	High thermal and chemical stability, superior bonding property and higher hardness	[47]
	Al2O3	Increase in compression strength, flexural strength, and tensile strength in composites	[48]
	SiO2	Enhanced strength and hardness	[49]
	TiB2	Better corrosion resistance, increase in tensile strength and toughness	[50]
	Si3N4	Increase in hardness in composites	[51]
	WC	Good thermal and electrical conductivity, high strength to weight ratio	[52]
	Gr	Improvement in wear behaviour in composites	[53]
	CNTs	Light weight, excellent physical and mechanical properties	[54]
Agro-waste derivatives	Bamboo Leaf Ash (BLA)	Increase in hardness and tensile strength in composites	[55]
	Rice Husk Ash (RHA)	Light weight filler, contributes to improved tensile strength and hardness	[56]
	Palm Kernel Shell Ash (PKSA)	Improved strength and hardness	[57]
	Bagasse Ash	Addition of these enhanced compressive strength and hardness in HMMCs	[58]
Industrial by-products	Fly ash	Less weight, improves mechanical strength, reduces the thermal expansion coefficient and improve wear resistance in composites	[59]
	Red mud	Improvement in wear behavior in composites	[60]

Research on the development of Al-HMMCs continues to grow, reflecting their suitability for numerous high-performance applications across diverse industrial sectors [61]. For instance, alloy matrix reinforced with ceramic particles are used in the fabrication of storage tanks, reactors, heat exchangers, and pipelines where they are exposed to extreme acidic conditions [62]. The alloy matrix reinforced with industrial by products such as fly ash, red mud, agro-waste derivatives such as rice husk ash, coconut shell ash, and bamboo fiber, are gaining attention for sustainable engineering applications. They are used in automotive parts, lightweight panels, and household appliances, offering reduced cost, lower environmental impact, and good strength-to-weight performance [44]. Similarly, together, these applications take Al-HMMCs to new levels of performance that are critical in these demanding industries.

Fabrication of hybrid metal matrix composites (HMMCs)

The properties of composites are significantly influenced by the fabrication process. Processing methods are primarily classified into two categories: solid-state and liquid-state [45]. This classification is based on the processing temperature, which is either above the melting point of the matrix material in liquid-state processing or below it in solid-state processing. Solid-state metallurgy encompasses processes such as powder metallurgy, diffusion bonding, and friction stir processing. On the other hand, liquid-state metallurgy includes techniques like stir casting, squeeze casting, and gas pressure infiltration. Figure 4 illustrates the different manufacturing methods.

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Fig. 4

Various methods of fabrication of HMMCs

Liquid state processing

Stir casting: Stir casting is one of the most widely used methods for fabricating HMMCs. Figure 5 depicts fabrication of HMMC using stir casting method. In this process, preheated reinforcement particles are introduced into the molten metal matrix and thoroughly mixed using a stirrer, followed by reheating. Continuous stirring ensures uniform dispersion of the reinforcement particulates and prevents agglomeration. Achieving high-quality casting requires careful optimization of key process parameters, including reinforcement particle size, stirring speed, stirring duration, preheating of reinforcements, and preheating of moulds [63, 64–65].

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Fig. 5

Fabrication of HMMC in a stir casting furnace

Squeeze casting: This is a widely employed technique for the commercial manufacturing of composites. The squeeze casting procedure begins with preheating the reinforcing particles and the furnace, followed by the addition of alloy ingots. To the melted alloy, preheated reinforcing particles are added, and to assure wettability, a pinch of wetting agents are added and mixed, followed by heating. Melted metal is poured into the moulds under pressure. This reduces porosity in the castings. Several factors, including melting temperature, die temperature, pressure, and pressure hold duration, must be properly managed for a successful fabrication. There are two types of squeeze casting techniques: direct and indirect [66]. Direct squeeze casting is used to produce composites with very basic shapes, and also the casting dies for direct method are simple and reasonably priced. The use of indirect squeeze casting allows for the fabrication of more complicated composite components, but it requires more expensive casting dies [67, 68]. The corrosion of squeeze cast composite is relatively less compared to friction stir processed composite of the same composition. This may be due to uniform distribution of reinforcement in the metal matrix. This has been noticed especially in Al-HMMCs. Figure 6 shows squeeze casting technique for the production of HMMCs.

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Fig. 6

Squeeze casting technique

Gas pressure infiltration: Gas pressure infiltration process is shown in Fig. 7. It involves injecting an inert gas from outside to a porous preform, allowing molten metal to penetrate within. Melting and metal infiltration are carried out using an appropriate pressure vessel. Nitrogen and argon are commonly used as inert gases. Compared to mechanical infiltration, they result in minimal damage to the preform [69, 70].

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Fig. 7

Schematic diagram of gas pressure infiltration

Solid state processing

Powder metallurgy: Powder metallurgy is regarded as one of the most effective metal matrix composite manufacturing processes [71]. Figure 8 shows various stages involved in the powder metallurgy process. Metal powder and reinforcement are initially blended and then introduced into a mould of the desired shape. Blending can be performed either in dry form or within a liquid suspension. Subsequently, pressure is applied to further compact the powder (cold pressing). The compact is then heated to a temperature below the melting point but sufficiently high to facilitate significant solid-state diffusion (sintering). Secondary operations such as machining and finishing are optional and can be carried out as needed [72].

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Fig. 8

Stages involved in powder metallurgy

Diffusion bonding: Diffusion bonding involves the solid-state diffusion of atoms facilitated by the application of pressure and heat. The schematic representation of diffusion bonding process is shown in Fig. 9. The temperature used in this process typically ranges from 0.5 to 0.8 times the melting point of the chosen material. This technique effectively prevents the formation of cracks and the distortion of grain boundaries. The holding time is carefully controlled to ensure a strong chemical bond across the interface, while avoiding the formation of an excessively thick inter-diffusion layer. Despite its advantages, diffusion bonding is a complex, costly, and challenging method to implement [73, 74].

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Fig. 9

Schematic illustration of diffusion bonding process

Friction stir processing (FSP): FSP is an effective technique for achieving a uniform distribution of reinforcement particles in a metal matrix. It enhances the material’s properties through localized and intense plastic deformation [75]. In this process, a cylindrical tool with a profiled probe is rotated at high speed and gradually plunged into the metal sheet or plate. The interaction between the wear-resistant tool and the workpiece generates frictional heat, which raises the temperature of the material. A sufficient dwell time is maintained to allow adequate heat generation, softening the material without reaching its melting point. This softened material enables the tool to move along a predefined path, modifying the microstructure. As the material experiences intense plastic deformation during FSP, it undergoes dynamic recrystallization, leading to the formation of fine, equiaxed grains. This refined microstructure enhances mechanical properties, making FSP a valuable technique for improving the performance of metals and composites [76]. However, these composites may have porosity. These composites are more vulnerable to galvanic corrosion because the interior (bulk) is chemically different from the surface and potential difference exist between them. Corrosion will confine to the interface and proceeds with acceleration once the corrosion is initiated. Figure 10 depicts schematic diagram of friction stir process.

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Fig. 10

Schematic diagram of friction stir process

Thus, the method used to fabricate hybrid composites greatly influences their internal microstructure, which is a critical factor in determining their corrosion resistance. Manufacturing techniques such as stir casting, powder metallurgy, and squeeze casting each impart distinct microstructural traits, including the distribution of reinforcements, matrix-reinforcement bonding quality, porosity levels, and residual stresses. These structural variations play a direct role in how the material behaves in corrosive environments, particularly in terms of electrochemical stability. For instance, achieving a uniform dispersion of reinforcements and minimizing porosity are possible through precise processing can significantly reduce localized galvanic reactions, thereby enhancing corrosion resistance. Therefore, understanding the interplay between processing methods and corrosion performance is essential for designing. The summary of all the discussed fabrication techniques with process temperature, complexity, cost, advantages, disadvantages and industrial applicability are given in Table 2.

Table 2. Comparison between fabrication methods of HMMCs

Fabrication Method	Process temp	Process complexity	Cost	Industrial applicability	Advantages	Disadvantages
Stir casting	700–800 °C	Less complex	Less expensive	Water pump, housings, manifolds	Simple, ideal for mass production	Slow process, poor wettability
Squeeze casting	600–750 °C	Moderate	Medium	Used for manufacturing components in automotive and aerospace industries	Improved wettability, lower porosity, fewer casting defects	High pressure requirements, tooling expenses
Gas pressure infiltration	600–900 °C	High	Medium	Hydraulic parts, brake calipers	Versatile for any matrix-reinforcement combination, handles high melt temperature	High cost of pressurized inert gas
Powder metallurgy	520–600 °C (sintering)	High	Medium	Large-scale production of automotive components	Energy-efficient, suitable for complex shapes, Lower processing temperature	Non-uniform reinforcement distribution may degrade mechanical properties, elevated sintering temperatures can cause grain coarsening
Diffusion bonding	450–550 °C	High	Very high	Used for blades, sheets, and structural components	Prevents formation of cracks and the distortion of grain boundaries	Challenging method, expensive
Friction stir process	Temperature is below melting point	Moderate	Medium	Aerospace and automotive industries	Uniform distribution of reinforcement particles, ease of operation	Porosity issues, exit holes are formed during tool withdrawal

Factors influencing the rate of corrosion of Al-HMMCs

Corrosion of Al-HMMCs occurs due to the presence of reinforcements and their interaction with the matrix. In Al-HMMCs, the aluminium alloy matrix acts as the anode, while the reinforcing particulates serve as the cathode [77]. The difference in electrode potentials between these components can lead to galvanic corrosion. Additionally, exposure to chloride ions in the environment can damage the protective oxide layer, making Al-HMMCs susceptible to pitting corrosion. They may also experience intergranular corrosion due to the segregation of impurities or reinforcements at the grain boundaries. Below are the some of the factors that influence the corrosion rate of Al-HMMCs.

Metal matrix effects

The corrosion behaviour in Al-HMMCs is largely influenced by the interaction between the reinforcement and the matrix material. Since aluminium alloy matrix is less noble than the reinforcement particulates, it tends to corrode faster when paired with conductive or noble reinforcements, which act as cathodic sites [77, 78]. This galvanic effect accelerates the degradation of the matrix [79].

Environmental effects

Electrolyte can greatly influence the corrosion behavior of Al-HMMCs. For instance, exposure to corrosive media containing chloride ions can lead to pitting corrosion, as these ions can damage the protective oxide layer formed on the aluminium matrix, resulting in localized damage [80].

Reinforcement effects

The impact of reinforcements on the corrosion behavior depends on factors such as their purity, structure, electrical resistivity, and area fraction. The type of reinforcement plays a crucial role in determining the rate of galvanic corrosion. Additionally, the composition of the reinforcement influences the kinetics of hydrogen evolution and oxygen reduction, which can significantly affect the overall corrosion process [81].

Reinforcement resistivity

For reinforcements with high resistivity, significant ohmic losses may occur, effectively limiting galvanic corrosion. However, if the reinforcements in the Al-HMMCs are not of high purity, their resistivity can decrease considerably, potentially allowing galvanic corrosion to occur more readily.

Reinforcement area fraction

The rate of galvanic corrosion tends to increase with a higher area fraction of the reinforcement in the composite material [82].

Interphase effects

During the fabrication of Al-HMMCs, interactions between the reinforcement and matrix, or the precipitation of compounds, can result in the formation of an interphase or intermetallic layer at the reinforcement-matrix interface [81, 83]. If this interphase or intermetallic acts as a more effective cathode than the reinforcement itself, the galvanic corrosion could be more intense than that predicted based on the virgin constituents.

Methods to investigate the corrosion behaviour

Potentiodynamic polarization (PDP) technique

Electrochemical measurements are carried out using an electrochemical work station and a conventional three electrode Pyrex glass cell with platinum as counter electrode, saturated calomel electrode (SCE) as reference electrode and test coupons as working electrode [84]. In PDP method the test specimens are immersed in a corrosive medium and allowed to attain steady-state open circuit potential (OCP). The specimen is polarized from − 250 mV cathodically to + 250 mV anodically with respect to the OCP, to obtain the Tafel plot of current versus potential [85]. The extrapolation of linear portion of the curve representing the Tafel region in either cathodic or anodic polarization curve to the corrosion potential will give the corrosion current density (i_corr) [86]. The other parameters cathodic (β_c) and anodic (β_a) slopes are also obtained from the linear portion of the curve are used to calculate polarization resistance (R_p) using Eq. (1). R_p is further used to calculate i_corr using Stern-Geary Eq. (2) [1].

1

$$R_p=\frac{\delta E}{\delta i_{\mathit{app}}}$$

2

$$i_{\mathit{corr}}=\frac{{\beta }_a+{\beta }_c}{\left({\beta }_a+{\beta }_c\right)R_p}$$

The obtained i_corr will be used to calculate the corrosion rate (CR) according to ASTM-G102-89 using the following equation [87].

3

$$CR\left(m,m,y^{-1}\right)=\frac{3270\times EW\times i_{\mathit{corr}}}{d}$$where, 3270 is proportionality constant in mmy⁻¹, EW is the equivalent weight, d is the density of the metal in g/cm³.

Further, inhibition efficiency (% IE) can be calculated using the following Eq. (4)

4

$$IE\left(\%\right)=\frac{i_{\mathit{corr}}-{i_{\mathit{corr}}}_{\left(i,n,h\right)}}{i_{\mathit{corr}}}\times 100$$where, i_corr and i_corr(inh) = corrosion current densities in the absence and presence of inhibitor.

Linear polarization method (LPR)

The linear polarization (LPR) technique is highly useful to measure corrosion rate in highly conductive solution. In this method a small potential of -10 to -30 mV is applied to the corroding specimen with respect to the OCP and the resulting current response will be measured. The plot of potential (E) versus current density gives a linear polarization curve. The slope (δE/δi_app) of the curve is related to the polarization resistance (R_p) of the system by the relation (1). The further calculations of i_corr, CR and % IE are evaluated as discussed under PDP method [88].

Electrochemical impedance spectroscopy (EIS) technique

Electrochemical impedance is usually measured by applying an AC potential to an electrochemical cell and then measuring the current through the cell, a small amplitude signal, usually a voltage between 5 to 50 mV, is applied to a specimen over a range of frequencies of 0.001 Hz to 100,000 Hz. The Nyquist and Bode plots are two ways to represent the EIS data. If the real part (Zʹ) is plotted on the x-axis and the imaginary part (−Z″) on the y-axis of a chart, the plot obtained is called Nyquist plot. In this plot the y-axis is negative and each point on the Nyquist is the impedance at one frequency [89].

Further, the Nyquist plots are fitted into an appropriate equivalent circuit depending upon their shape. These circuits are used to interpret impedance data, providing insights into corrosion mechanisms, reaction kinetics, and interface characteristics. For example, the Randles equivalent circuit as shown in Fig. 11 is a fundamental model in EIS for representing electrochemical systems. It consists of solution resistance (R_s), double-layer capacitance (C_dl), and charge transfer resistance (R_ct), with the optional addition of inductance (L) and Warburg element (Z_w) to account for diffusion effects.

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Fig. 11

Schematic representation of Randle’s equivalent circuit

The R_ct/R_p values are used to calculate the % IE using Eq. (5) [90].

5

$$IE\left(\%\right)=\frac{{R_p}_{\left(i,n,h\right)}-R_p}{{R_p}_{\left(i,n,h\right)}}\times 100$$where, R_{p (inh)} and R_p are the polarization resistances in the presence and absence of inhibitor respectively. Generally higher the R_p values indicate the higher resistance towards corrosion. The increase in R_p values in the presence of inhibitor suggests the decrease in the corrosion rate [91].

An electrical double layer forms at the interface between an electrode and the surrounding electrolyte. It arises as ions from the solution migrate toward the electrode surface, creating a separation between the charged electrode and the oppositely charged ions. This separation is extremely small, typically on the scale of angstroms. The capacitance of the double layer is influenced by several factors, including electrode potential, temperature, ion concentration and type, presence of oxide layers, surface roughness, and adsorbed impurities [92].

To better represent the behavior of the electrical double layer, a constant phase element (CPE) is used in the circuit instead of an ideal capacitor. The impedance of the CPE is given by Eq. 6:

6

$$Z=Q^{-1}{\left(i\omega \right)}^{-n}$$

Here, Q is the proportionality constant, ω is the angular frequency, i is the imaginary unit, and n is an exponent that reflects the degree of surface inhomogeneity. The value of n ranges from 0 to 1 (0 ≤ n ≤ 1), indicating deviations from ideal capacitive behavior. The true double-layer capacitance (C_dl) is then corrected using the following expression (7) [93].

7

$$C_{\mathit{dl}}=Q{\left({\omega }_{max}\right)}^{n-1}$$

Generally, the C_dl values are typically higher due to increased exposure of the metal surface to the electrolyte in the absence of inhibitor. This indicates more active sites for charge exchange thereby promoting corrosion. Whereas, the adsorbed inhibitor molecules significantly reduce the charge accumulation at the metal–solution interface. This reflects the formation of a protective film, blocking active sites and hindering corrosion.

Another important method is Bode plot, in which impedance is plotted with log frequency on the x-axis and both the absolute value of the impedance (|Z|= Z₀) and phase shift on the y-axis. The Bode plots help to reveal the key electrochemical processes like charge transfer and diffusion. The high-frequency (HF) range typically represents the solution resistance (R_s), while the low-frequency (LF) range provides insight into corrosion behavior. A constant or flat impedance magnitude at low frequencies generally indicates good corrosion resistance. The difference between the HF limit and LF limit in the Bode plot is equal to polarization resistance (R_p) [94].

Weight loss (WL) method

This is a direct method to measure the CR. It involves the use of test specimens, which can be fabricated in various sizes and shapes. A pre-weighed metal or alloy sample is exposed to the corrosive environment for a specified duration. Weight changes are monitored over specified time intervals. The inhibition efficiency can be calculated using Eq. (8) [95].

8

$$IE\left(\%\right)=\frac{v^{\circ }-v^{\prime }}{v^{\circ }}\times 100$$where, $v^{\circ }$ and $v^{\prime }$ are the corrosion rates of metal samples in the absence and presence of inhibitor respectively.

Hydrogen evolution method

Hydrogen evolution method also known as gasometric method ensures a more sensitive monitoring in situ of any perturbation by the inhibitor through gas evolution on the metal corrodent interphase. The gasometric assembly is essentially an apparatus that measures the volume of gas evolved from a corrosion reaction system.

In this monitoring technique, known volume of the corrodent was introduced into the two-necked flask and the initial volume of air in the burette was noted. Thereafter, a test coupon weighed approximately was dropped into the corrodent and the flask was quickly closed. The volume of hydrogen gas evolved from the corrosion reaction was monitored by volume changes in the level of paraffin oil in the graduated burette at fixed time intervals. From the measured volume the corrosion rate can be calculated using Eq. 9 [2].

9

$$CR=\frac{V_t-V_i}{t_t-t_i}$$where V_t and V_i are the volumes of hydrogen evolved at time t_t and t_i, respectively.

Salt spray method

The salt spray method is a corrosion monitoring technique used to evaluate the corrosion resistance of materials by exposing samples to a controlled environment of a neutral salt solution. In the salt spray test, pre-weighed specimens are placed in a closed chamber where a fine mist of NaCl solution is continuously sprayed at a controlled temperature, following ASTM B117 standards for a specified duration. After exposure, samples are reweighed. The difference in weight before and after exposure is used to calculate the corrosion rate as per Eq. 10 [96].

10

$$CR=\frac{k\times w{t}_{\mathit{loss}}}{A\times t\times \rho }$$k is a proportionality constant, A = exposed area of the test coupons, t = exposure time and ρ = density of the test coupon in g/cm³.

Corrosion behaviour of Al-HMMCs

The corrosion behavior of Al-HMMCs is also influenced by the type, composition and the properties of the reinforcement particulates [97]. Analysing the interaction between reinforcements and the aluminium matrix will help in understanding the corrosion behaviour of these composites in various environments.

Researchers have used various combinations of reinforcements such as SiC, Al₂O₃, B₄C, WC, Gr, CNTs, SiO₂, TiB₂ and Si₃N₄ along with agro- waste derivatives and industrial by-products during the fabrication of HMMCs to improve the properties of the composites and also studied their corrosion behaviour in various corrosive media [98]. Moreover, from the literature it could be observed that the addition of reinforcement particulates in some cases have increased the CR, while in others decreased the CR.

Preethi Kumari et al. [99] explored the corrosion behaviour of a hybrid composite of Al7075 reinforced with Ni coated duralumin powder using stir casting method in 0.1 M HCl and 3.5 wt% NaCl by adopting PDP and EIS techniques. It was observed that the reinforcement of nickel coated duralumin powder into the base alloy caused the discontinuities throughout the matrix phase. These discontinuities acted as reactive sites and resulted in higher CR of hybrid composite compared to base alloy. Also, the specimen's CR increased with increase in temperature from 31.77 to 47.61 mmy⁻¹ in 0.1 M HCl, and from 0.26 to 1.28 mmy⁻¹ in 3.5 wt% NaCl respectively in the range of 30 to 50 °C. In comparison to 3.5% NaCl medium, the hybrid composite exhibited a faster rate of corrosion in the 0.1 M HCl medium due to the increased conduction of H⁺ ions, which accelerated the hydrogen evolution reaction and had a direct impact on the metal dissolution process.

The deterioration of AA6063 composites dispersed with a mixture of SiC (4 and 6 wt%) and WS₂ (4 wt%) in 3.5 wt% NaCl was studied using PDP technique by Vijayasarathi Prabakaran et al. [100]. It was observed that the reinforced composite material showed improved corrosion resistance when compared to the base alloy, further CR decreased from 0.0456 mmy⁻¹ to 0.0112 mmy⁻¹ with increase in the weight percentages of SiC by preventing material loss. The HMMC showed CR of 0.0112 mmy⁻¹ at the 6 wt% SiC and 4 wt% WS₂, whereas AA6063 had a CR of 0.0456 mmy⁻¹. Superior corrosion resistance was attained by improving the interface interaction and bonding between the SiC/WS₂ particles and the aluminium matrix. Summary of recent works on the corrosion behaviour of Al-HMMCs is given in Table 3.

Table 3. Summary of recent works on the corrosion behaviour of Al-HMMCs

Aluminium hybrid composite	wt% of Reinforce-ment particles	Medium and method used	PDP results	EIS results	Reason	References
AA7075/SiC/Al2O3	SiC + Al2O3 (2.5, 5, 7.5 and 10 wt% each)	3.5 wt% NaCl-PDP and EIS (pH = 7)	CR ↓ from 1.699 to 0.630 mmy−1 in comparison with the base alloy	Polarization resistance (Rp) ↑ from 283.8 Ω cm2 to 543.5 Ω cm2 with ↑ in reinforcements	Reinforcement addition enhanced interfacial/ intermetallic bonding, forming a barrier over composite surface	[101]
AA‒Si/Al2O3/ graphene nanoplatelets (GNP)	Al2O3 (6 wt%) + GNP (1, 2, and 3 wt%)	3.5 wt% NaCl-PDP (pH = 7)	icorr ↓ from 7.33 × 10−4 Acm−2 to 3.78 × 10−6 Acm−2 as the wt% of GNP ↑ in composites	–	Protective Al- oxide layers and GNP’s stability reduces Cl− attack and improved corrosion resistance	[102]
AA6061/SiC/Al2O3/ CeO2	Al2O3 + SiC (2.5, 5 and 7.5 wt% each) + CeO2 (0.5, 1.5 and 2.5 wt%)	2.5 and 3.5 wt. % NaCl- PDP (pH = 7)	CR of friction stir welded (FSW) samples ↓ from 0.0248 mmy−1 to 0.00406 mmy−1 and from 9.347 × 10–3 mmy−1 to 8.636 × 10–4 mmy−1 with ↑ in wt% of CeO2 in 3.5 and 2.5 wt% NaCl	–	Hydrophobic surface formation and CeO2/Al2O3 incorporation resulted in hydroxide layer formation, reducing corrosion	[103]
AA5083/B4C/carbon nanotubes (CNTs)	B4C + CNTs	1% HCl (pH = 0.5), 1% HNO3 (pH = 0.8) and 3.5 wt% NaCl (pH = 7) – PDP	FSW HMMCs showed lower CR (0.013, 0.0118, 0.0034 mmy−1) compared to base alloy	–	Higher CR in base alloy was due to the presence of Mg content in Al5083 which led to β precipitation causing intergranular corrosion	[62]
AA6063/SiC/palm kernel shell ash (PKSA)	SiC + PKSA (0, 2 and 8 wt% each)	1.0 M H2SO4 (pH≈ 0)—PDP	icorr ↑ with ↑ in the immersion time. At 24 h, icorr varied from 423.81 to 860.23 µAcm−2, while at 72 h, they ranged from 1075.65 to 3057.16 µAcm−2	–	Sulphate ions formed soluble complexes with Al-oxide accelerating the corrosion over time with prolonged exposure	[104]
AA2024/ B4C/ SiC	B4C + SiC (0.125, 0.25, 0.5,1 and 2 wt% each)	3 wt% NaCl (pH = 7)—PDP	Base alloy exhibited highest CR (0.809 mmy−1) compared to HMMCs (0.055 to 0.152 mmy−1)	–	Barrier formation on the addition of B4C/SiC, ↑ corrosion resistance in HMMCs	[105]
Al-Zn-Cu/SiC/ TiB2	SiC + TiB2 (0, 2.5, 5 and 7.5 wt% each)	3.5 wt% NaCl (pH = 7)—PDP	CR ↓ with ↑ in wt% of SiC and TiB2 from 367 to 261 mmy−1, improving HMMC corrosion resistance	–	Barrier effect and improved passivation	[106]
Al/ZrO2/Gr	ZrO2 (12 wt%) + Gr (2 wt%)	3.5 wt% NaCl (pH = 7) and 0.1 M H2SO4- (pH = 1) PDP and EIS	CR in NaCl and H2SO4 were 0.0262 mmy−1 and 2.598 mmy−1	Larger high-frequency capacitive loop indicates oxide layer formation in NaCl	Lower CR in NaCl due to passive film formation on surface	[107]
Al/BN/SiC/Rice husk ash (RHA)	BN (5 wt%) + SiC (5 wt%) + RHA (1.5, 3 and 5 wt%)	5 wt% NaCl – (pH = 7) PDP and EIS	Corrosion resistance improved (1 to 0.7 mAcm−2) with increased RHA reinforcement	Addition of RHA ↑ the charge transfer resistance (Rct) in Al–5BN–5SiC–5RHA (Rct = 22.50 Ω cm2) than Al–5BN–5Si (Rct = 14 Ω cm2)	Barrier effect of RHA, which reduced charge transfer across the surface in HMMCs	[108]
AA7005/industrial waste-based fly ash (FA)/glass fiber (GF)	FA (6 wt%) + GF (5 wt%)	1.0 M HCl (pH = 0) – PDP and EIS	HMMC showed better corrosion resistance (0.055 mmy−1) than the base alloy (0.344 mmy−1)	Larger diameter curves in Al7005 + 6% FA indicates ↑ in Rct due to insulating surface film formed by FA, enhancing corrosion resistance	FA addition forms an insulating surface film, protecting the metal-solution interface	[109]
AA7075/ MoS2/ CeO2	MoS2 + CeO2 (1:1)	3.5 wt% NaCl (pH = 7)—PDP	HMMC exhibited lowest icorr of 0.225 × 10−6 Acm−2 when compared to the base alloy (icorr = 2.131 × 10−6 Acm−2)	–	CeO2 formed passive layer, limiting oxygen supply and reducing corrosion in HMMC	[110]
AA7075/ SiC/ Al2O3/B4C	SiC (20 wt%) + Al2O3 (2, 4, 6 and 8 wt%) + B4C (2, 4, 6 and 8 wt%)	3.5 wt% NaCl (pH = 7)—PDP	HMMCs comprising larger quantity B4C and less Al2O3 showed highest corrosion resistance	–	Smaller quantity of B4C resulted in the formation of brittle Al4C3 phase, which reacted with media, increasing CR	[111]
AA6063/ Si3N4/ Cu(NO3)2	Si3N4 (12 wt%) + Cu(NO3)2 (0, 2, 4 and 6 wt%)	5 wt% NaCl- (pH = 7) PDP and EIS	icorr ↓ from 2.190 to 1.950 mAcm−2 with ↑ in the wt% of Cu(NO3)2. HMMC with 12%Si3N4 and 6% Cu(NO3)2 had the lowest icorr of 1.950 mAcm−2	Rp values ↑ from 86.86 Ω cm2 to 95.45 Ω cm2 with ↑ in wt. of Cu(NO3)2	Cu(NO3)2 and Si3N4 promoted passive layer formation, enhancing corrosion resistance	[112]
AA6061/ BN/ Al2O3/ graphite (Gr)	BN (20, 30 and 45 wt%) + Al2O3 (10, 20 and 45 wt%) + Gr (5 wt%)	5 wt. % NaCl (pH = 7)—PDP	icorr ↓ from 0.525 to -0.983 µAcm−2 as wt% of BN and Al2O3 particulates ↑	–	BN and Al2O3 increased the surface stability and reduced active corrosion sites	[113]
AA6063/ TiC/ Al2O3	TiC (5 wt%) + Al2O3 (1.5 wt%)	3.5 wt% NaCl (pH = 7)—PDP and EIS	HMMCs exhibited the lowest icorr of 13.0 μA than the base alloy (icorr = 42.4 μA)	Addition of reinforcements ↑ the Rct in HMMCs (Rct = 737 Ω cm2) than the base alloy (Rct = 104.6 Ω cm2)	Protective oxide layer formation in HMMCs	[114]
Al-TiB2/ TiC	TiB2/TiC (5 wt. %)	3.5 wt% NaCl (pH = 7)—PDP and EIS	As rolling reduction ↑ from 20 to 90%, the composites' icorr values also ↑ from 4.6 × 10−6 Acm−2 to 231.6 × 10−6 Acm−2	Larger diameter of EIS curve indicated better corrosion resistance after rolling. Corrosion resistance decreased with increasing rolling reduction	Higher rolling reduction led to more surface defects, decreasing the corrosion resistance	[115]
Al alloy/ RHA/ SiC	RHA:SiC (1:0, 3:1, 1:1, 1:3, and 0:1)	3.5 wt% NaCl (pH = 7) and 0.3 M H2SO4 (pH = 1)—PDP	HMMCs with higher RHA showed better corrosion resistance. HMMC Al/1:1 RHA-SiC and Al/3:1 RHA-SiC had lowest icorr values of 5.67 × 10–5 Acm−2 and 4.46 × 10–5 Acm−2 in H2SO4 and 2.34 × 10–6 Acm−2 and 2.67 × 10–6 Acm−2 in NaCl	–	Greater RHA content improved the matrix's resistance to corrosion by enhancing barrier properties and reducing reactivity	[116]
AA7075/ Al2O3/ Gr	Al2O3 (3, 6 and 9 wt%) + Gr particles (3 wt%)	3.5 wt% NaCl (pH = 7)—PDP and EIS	CR ↓ from 8.255 to 0.635 mmy−1 with ↑ in wt% of Al2O3. HMMC with 9% Al2O3 and 3% Gr had the lowest CR of 0.63 mmy−1, than the base alloy having CR of 8.255 mmy−1	Nyquist plot indicates impedance of the circuit ↑ with ↑ in wt% of reinforcements	Al2O3 acts as a barrier to pit formation and refines the matrix structure, thereby reducing CR and potential	[117]
AA7075/Al2O3/SiC	Al2O3 + SiC (1, 2, 3 and 4 wt% each)	3.5 wt% NaCl (pH = 7)—PDP	CR ↓ from 56.22 mmy−1 to 44.642 mmy−1 as the wt% of reinforcing component SiC ↑. HMMC with 4% SiC exhibited the lowest CR of 44.642 mmy−1	–	The inert nature of SiC particulates blocked the active corrosion sites enhancing surface stability and overall corrosion resistance than the base alloy	[118]
AA7075/ SiC/ TiC	SiC + TiC (0, 2.5, 5 and 7.5 wt% each)	3.5 wt% NaCl (pH = 7)—PDP	CR ↓ from 0.0996 mmy−1 to 0.0101 mmy−1 as the wt% of reinforcement ↑. AST15 (85 wt% Al7075 + 7.5 wt% SiC + 7.5 wt% TiC) had the lowest CR of 0.0101 mmy−1	–	Reinforcements acted as barriers which improved corrosion resistance in HMMCs	[119]
AA5083/ CeO2/ SiC	CeO2 (25, 50 and 75 wt%) + SiC (25, 50 and 75 wt%)	3.5 wt% NaCl (pH = 7)—PDP	Corrosion resistance improved as the wt% of CeO2 ↑	–	CeO2 acted as an effective cathodic inhibitor, forming a passive oxide layer that enhances surface passivity and blocks oxygen and electron transfer, preventing corrosion	[120]
Al–Mg-Si alloy/ bamboo leaf ash (BLA)/ Al2O3	BLA (0, 2, 3 and 4 wt%) + Al2O3 (6, 7, 8 and 10 wt%)	3.5 wt% NaCl (pH = 7)—PDP	icorr values ↑ from 1.936 to 5.583 µAcm−2 with ↑ in the addition of BLA	–	Increased BLA content led to more surface defects, increasing icorr by enhancing reactivity	[121]
Al–Mg-Si alloy/ Rice husk ash (RHA)/ Al2O3	RHA (0, 2, 3 and 4 wt%) + Al2O3(6, 7, 8 and 10 wt%)	3.5 wt% NaCl (pH = 7)—PDP	The icorr values ↑ from 2.379 to 7.614 µAcm−2 as the wt% of RHA ↑	–	Higher RHA content led to pit formation at the Al2O3/Al matrix, increasing icorr	[122]
Al–Mg-Si/ RHA/ SiC	RHA:SiC (0:1, 1:3, 1:1, 3:1, and 1:0)	3.5% NaCl (pH = 7)—PDP	HMMC showed a higher icorr (7.837 µAcm−2) than the base alloy (0.819 µAcm−2)	–	Increased RHA content disrupted the protective layer, making HMMC more susceptible to corrosion	[123]

Corrosion inhibition studies on Al-HMMCs using inhibitors

According to the literature, extensive research has been conducted on the mechanical properties of Al-HMMCs. However, there is still considerable scope for further exploration of their corrosion behavior. Additionally, research on corrosion inhibition of Al-HMMCs is limited, and the use of hybrid heterocyclic derivatives as CIs are scarcely reported. Investigating effective CIs are crucial to enhance the durability and performance of these materials, especially in the aggressive closed environments.

The impact of glucosamine sulfate (GAS) on the prevention of corrosion in AA6061 reinforced with 2 wt% SiC and 2 wt% B₄C was investigated by Lavanya M. et al. [124] in 0.1 M HCl. The PDP and EIS approaches were employed to investigate the inhibition performance. The maximum IE of 81.22% was reported at 323 K for 1.25 × 10^–3 M concentration of GAS. By obeying Freundlich's adsorption isotherm (FAI) the inhibitor underwent chemisorption and functioned as a mixed type. The protective film created through electron pair interactions served as a barrier between the composite material and the corrosive medium, enhancing the efficiency of inhibition.

The corrosion inhibition effect of disodium salt of ethylenediaminetetraacetic acid (EDTA) on AA6061 reinforced with 3 wt% SiC and 3 wt% B₄C was investigated by Juan David et al. [125] in 0.5 M HCl utilizing WL, PDP and EIS techniques. The % IE decreased with increase in temperature (303 K to 323 K). EDTA acted as mixed inhibitor and followed Langmuir’s adsorption isotherm (LAI) by showing maximum IE of 82% at 1.074 mM concentration of the inhibitor. Addition of EDTA to the acid solution has blocked the active sites on the hybrid composite surface, slowing the corrosion process and increasing the corrosion inhibition effectiveness. Also, the interaction of π electrons in EDTA with the empty p-orbital of Al atoms on the composite surface resulted in the formation of coordinate bonds, which improved the % IE.

Hybrid heterocyclic derivatives as CIs

Literature studies reveal that various heterocyclic compounds serve as effective CIs for different metals. Their efficiency is attributed to the presence of heteroatoms such as oxygen (O), nitrogen (N), and sulfur (S), along with electron-donating groups and aromatic rings containing π-electrons, which enhance their adsorption onto metal surface [126, 127, 128–129]. Research focused on exploring hybrid heterocyclic derivatives as CIs for different metals has gained the attention in recent years.

Hybrid heterocyclic derivatives are the molecules that consist of two or more heterocyclic rings within a single structure. Various heterocycles, including imidazole, pyridine, quinoline, pyrazole, indole, oxazole, and pyrimidine derivatives, are commonly used to synthesize CIs [126, 130]. The incorporation of multiple rings in a single molecule enhances its properties, making it more effective in different corrosive environments. This structural feature improves adsorption onto metal surfaces, increases it’s IE, and provides greater thermal and chemical stability. A summary of the effectiveness of hybrid heterocyclic derivatives as CIs for various metals is presented in the Table 4.

Table 4. Summary of hybrid heterocyclic derivatives as CIs on different metals

Metal	Inhibitor	Method used and medium	Structure	Inhibitor conc	% IE	Inhibitor type and Isothermmodel	References
Mild steel (MS)	POTC, PCPD (pyrimidine derivatives)	PDP, EIS (15% HCl)		200 ppm	PDP: POTC- 96.2% PCPD- 93.4% EIS: POTC- 96.8% PCPD- 93.3%	Mixed, Langmuir isotherm	[97]
MS	P2PZ, O2PZ (pyrazole derivatives)	PDP, EIS (1 M HCl)		1 × 10–3 M	PDP: P2PZ- 95.83% O2PZ- 84.29% EIS: P2PZ- 91.29% O2PZ- 77.94%	Mixed, Langmuir isotherm	[131]
MS	N-BPPC, N-PMPPC, 4-MPPM (carboxamide derivatives)	PDP, EIS (1 M HCl)		3.4 × 10–4 M	PDP: 4 -MPPM- 94.6% N -BPPC- 92.8% N- PMPPC- 80.6% EIS: 4 -MPPM- 94.2% N -BPPC- 92.9% N- PMPPC- 79.2%	Mixed, Langmuir isotherm	[132]
MS	M2PyAz, B2PyAz (pyrazole derivatives)	PDP, EIS (1 M HCl)		10–3 M	PDP: M2PyAz- 98.1% B2PyAz—85% EIS: M2PyAz- 98.5% B2PyAz- 83.5%	Mixed, Langmuir isotherm	[133]
MS	DPP, PP (pyrimidine derivatives)	PDP, EIS (1 M HCl)		10–3 M	PDP: DPP- 81% PP- 92% EIS: DPP- 77% PP – 88%	Mixed, El- Awady adsorption	[134]
MS	BTT, BTI (triazine-thiourea derivatives)	PDP, EIS (15% HCl)		200 ppm	PDP: BTT- 97.4% BTI- 92.68% EIS: BTT- 98% BTI- 94.6% [134, 135]	Mixed, Langmuir isotherm	[135]
MS	PMTTA, PATT, PMTA, PTA (thiadiazole derivatives)	PDP, EIS (1 M HCl)		125 ppm	PDP: PMTTA- 90.4% PATT- 88.2% PMTA- 82.5% PTA – 77.7% EIS: PMTTA- 94.9% PATT- 94.8% PMTA – 87.3% PTA – 80.3%	Mixed, Langmuir isotherm	[136]
MS	IICH, ICCH, IBCH, and IMCH (indole derivatives)	PDP, EIS (1 M HCl)		200 mgL−1	PDP: IMCH- 99.3% IICH- 97.9% ICCH- 95.3% IBCH- 92.2% EIS: IMCH—96% IICH—91.1% ICCH—90.9% IBCH—90.2%	Mixed, Langmuir isotherm	[137]
MS	Hqcq and Hqpzc (quinoline derivatives)	EIS (1 M HCl)		100 ppm	EIS: Hqcq – 94.34% Hqpzc – 92.18%	Mixed, Langmuir isotherm	[138]
MS	ME-1 and ME-2 (triazine derivatives)	PDP, EIS (1 M HCl)		34 × 10–5 mML−1	PDP: ME- 1: 91.5% ME- 2: 93.98% EIS: ME- 1: 91.59% ME- 2: 93.81%	Cathodic, Langmuir isotherm	[139]
MS	9-Hydroxyris-peridone (HRD)	PDP, EIS (1 M HCl)		5 × 10–4 M	PDP: HRD—88% EIS: HRD—90%	Mixed, Langmuir isotherm	[140]
MS	P2 (pyrazole derivatives)	PDP, EIS (1 M HCl)		10–3 M	PDP: P2—95.38% EIS: P2—95.19%	Mixed, Langmuir isotherm	[141]
Carbon steel (CS)	2-(Benzoxazol-2-ylthio)-N-(4-phenyl-5-phenylazo- thiazol-2-yl)-acetamide (BNP)	PDP, EIS (1 M HCl)		18 × 10–6 M	PDP: BNP—91.8% EIS: BNP—90.8%	Mixed, Temkin model	[142]
CS	Hybrid heterocyclic pyrazole derivative (PPA), and a non-hybrid heterocyclic pyrazole derivative (PMB)	PDP, EIS (1 M HCl)		10–3 M	PDP: PPA—94% PMB—92.1% EIS: PPA—93% PMB – 91.1%	PPA- anodic PMB- mixed, Langmuir isotherm	[143]
CS	L1, L2 and L3 (quinoline derivatives)	PDP, EIS (1 M HCl)		1 × 10−3 M	PDP: L1—92.06% L2—83.38% L3—73.22% EIS: L1—88.88% L2—80.91% L3—79.03%	Mixed, Langmuir isotherm	[144]
CS	2-[(5-methylpyrazol-3-yl)methyl]benzimidazole (MPMB)	PDP, EIS (1 M HCl)		5 mM	PDP: MPMB—91.7% EIS: MPMB—92.19%	Mixed, Langmuir isotherm	[145]
CS	bis-pyrazoline derivative (Bis-Pyr)	PDP, EIS (1 M HCl)		400 ppm	PDP: Bis-Pyr: 92.73% EIS: Bis-Pyr: 90.41%	Mixed, Langmuir isotherm	[146]
Stain-less steel (SS)	Pyrazole-Pyrimidine (PYR-PER) hybrids PYR-PER1, PYR-PER2 and PYR-PER3	PDP (1 M HCl)		600 ppm	PDP: PYR-PER3- 97.45% PYR-PER1- 93.57% PYR-PER2- 90.76%	Mixed, Langmuir isotherm	[147]
Aluminium	Thiophene derivatives (A, B, C and D)	PDP, EIS (1 M HCl)		21 × 106 M	PDP: A—92.5% B—90.9% C—89.9% D—87.1% EIS: A—90% B—89.4% C—88.8% D—85.5%	Mixed, Langmuir isotherm	[148]
Copper	pyrimidine-bichalcophene derivatives, MA-1230, MA-1231, MA-1232	PDP, EIS (1 M HNO3)		21 µM	PDP:MA-1230: 90.3% MA-1231: 91.3% MA-1232: 92.1% EIS:MA-1230: 91.54% MA-1231: 92.07% MA-1232: 93.27%	Mixed, Langmuir isotherm	[149]

From the Table 4 it is seen that there is scope for exploring these hybrid derivatives as corrosion inhibitors for mitigating the corrosion of Al-HMMCs.

Mechanism of Al-HMMC corrosion

In Al-HMMCs, matrix aluminium, with a lower standard electrode potential of -1.66 V, functions as the anode and undergoes rapid oxidation. The reinforced phase acts as a cathodic site, facilitating the hydrogen evolution reaction. During the anodic process, aluminium transforms into metal ions which diffuse into the acidic medium while releasing electrons. Meanwhile, hydrogen ions are discharged during the cathodic process, leading to hydrogen gas evolution [99].

The following anodic and cathodic reactions occur when Al is in contact with HCl

11

$$\left(A,l,\to ,{\mathit{Al}}^{3+},+,3,e^{-}\right)\times 2$$

12

$$\left(2,H^{+},+,2,e^{-},\to ,H_2,\uparrow \right)\times 3$$

The overall reaction that takes place during the corrosion of the hybrid composite is

13

$$2Al+6H^{+}\to 2{\mathit{Al}}^{3+}+3H_2\uparrow$$

In contrast, in a NaCl medium, the cathodic reactions are as follows:

14

$$2H^{+}+2e^{-}\to H_2\uparrow$$

15

$$O_2+2H_{2}O+4e^{-}\to 4O{H}^{-}$$

Once $i_{\mathit{corr}}$ and $E_{\mathit{corr}}$ are reached, the anodic current rises as a result of the aggressive nature of chloride ions.

16

$$Al\to {\mathit{Al}}^{3+}+3e^{-}$$

17

$${\mathit{Al}}^{3+}+3{\mathit{Cl}}^{-}\to Al{\mathit{Cl}}_3+3e^{-}$$

The adsorption of inhibitor molecules is mainly influenced by the nature and surface charge of the metal, chemical structure of the inhibitor, and type of aggressive electrolyte. The two principle types of interactions involved during the adsorption of inhibitors on the metal surface are physical (or electrostatic) adsorption (physisorption) and chemical adsorption (chemisorption). Physical adsorption is caused by electrostatic forces which exist between the inhibitor and the metal surface. The second type of adsorption is chemisorption, which form a coordinate bond either by charge transfer or charge sharing with the metal surface and impede ongoing electrochemical dissolution reactions. A schematic representation for the metal dissolution and adsorption of inhibitor molecule onto the metal surface is shown in Fig. 12 [124].

[See PDF for image]

Fig. 12

Schematic representation for the a metal dissolution and b adsorption of inhibitor molecules onto the metal surface

Effect of pitting potential (E_pit) on the corrosion process of Al-alloy and inhibition mechanism

Pitting potential can be determined fairly in agreement (within ± 10 mV accuracy) by both potentiodynamic and potentiostatic (electrochemical measurements) methods [150]. In the potentiodynamic method, the current increases with increase in potential at a constant scan rate. At one stage current remains constant (in the passive region) and suddenly starts increasing at a potential much below the trans passive region. This potential is called pitting potential (E_pit) and it is more noble to primary passive potential (E_pp). In the potentiostatic method, current is measured as a function of time at a set most noble potential. In case of Al alloys, it was found that the values obtained for E_pit by potentiostatic method were only 10–20 mV more active than those for potentiodynamic method. It is also observed that E_pit is somewhat dependent on the nature of alloying element and on it’s structure, defects, inclusions, whether intermetallic or precipitates formed, solution chemistry of medium [151].

Depending upon the alloy content, intermetallics (IM) may be formed if alloying content is high or form precipitates (soluble or insoluble) causing separate phases which may be more anodic or cathodic to the Al- matrix. This leads to galvanic corrosion at the interphase and may also influence to some extent E_pit, E_corr and mechanism. The works of [151] showed that the nature and amount of secondary phases of many alloying elements e.g. Cr, Mn do not have influence upon the E_pit of Al- alloy. Secondly, the values of E_pit do not necessarily indicate the susceptibility to pitting corrosion. High E_pit values do not represent high pitting up resistance. Al- 4% Cu has E_pit value about 100 mV more than Epit value of many sea- water resistant alloys but still more vulnerable compared to those alloys. Pitting corrosion susceptibility is rather more dependent on the difference between E_OCP, E_pit values and it’s variation with time [151].

Metastable pitting in Al alloys occurs as a precursor to stable pit formation and is strongly influenced by the properties of the passive film and the presence of aggressive anions, especially chloride ions. In chloride-containing environments such as NaCl solutions, pitting initiates at weak sites typically regions with structural defects, dislocations, or inhomogeneities ultimately leading to breakdown of passive film leading to metastable pit formation [152]. These metastable pits are very small in size and grow and repassivate in less than a few seconds or grow in to stable pits [153]. The stabilization of these pits into stable pits depends on factors such as high Cl⁻ ion concentration, low pH etc. Hence, in NaCl environment, Al alloys exhibit frequent metastable pitting activity, which may or may not transition into stable pitting depending on the electrochemical and microstructural conditions.

Mechanism of pit formation in Al alloys

Pitting is a highly localized form of corrosion that commonly occurs in aluminium and its alloys when exposed to aggressive environments containing Cl⁻ ions, such as NaCl solutions [154]. Pits typically initiate at weak points, where Cl⁻ ions penetrate and locally break down the passive film. Pits propagate according to the following anodic reactions

18

$$Al\to {\mathit{Al}}^{3+}+3e^{-}$$

19

$${\mathit{Al}}^{3+}+3H_{2}O\to Al{(OH)}_3+3H^{+}$$

Simultaneously, cathodic reactions occur as explained by the Eqs. (14) and (15).

As indicated by reaction (19), which results in decrease in pH as pitting progresses, thereby altering the chemical environment within the pit. To maintain electrochemical charge neutrality, Cl⁻ ions migrate into the pit and results in the formation (regeneration) of HCl inside the pit and causes accelerated (auto catalytic) pit propagation. It is proposed that at the critical pitting potential; the passive film undergoes localized breakdown through a mechanism involving field-assisted adsorption of Cl⁻ ions onto the hydrated oxide surface. This process facilitates the formation of soluble basic chloride salt such as AlCl₃ or Al(OH)₂Cl, which readily dissolve into the electrolyte and sustain the corrosive conditions within the pit. If the aggressive conditions inside the pit are maintained, metastable pits transition into stable pits, resulting in continuous and localized material degradation [155]. To mitigate localized corrosion, the use of CIs, particularly those that stabilize the passive film and counteract chloride ion attack from the surrounding environment has been proven effective.

CIs play a vital role in preventing the initiation and growth of pits by interacting with the passive film or the electrolyte–metal interface. In particular, chromate-based inhibitors have been shown to significantly hinder the transition from metastable to stable pitting in Al alloys. Chromates may act through multiple mechanisms: (i) they can adsorb onto defective regions on the passive film, thereby increasing the overall passivity of the surface; (ii) chromates can get incorporated into the defective regions of the oxide film and be reduced to form chromium oxides, and protect the film from chloride attack; and (iii) they can alter the electronic properties of the passive film. These combined actions strengthen the film’s integrity, raise the pitting potential, and suppress the initiation and stabilization of pits [153].

Surface characterization techniques

Surface characterization is essential for gaining insights into the surface properties of materials. Figure 13 depicts various surface characterization techniques.

[See PDF for image]

Fig.13

Various surface characterization techniques

Scanning electron microscopy (SEM)

SEM generates images of a sample by scanning its surface with a focused electron beam. During scanning, the beam interacts with surface atoms generating signals like secondary and back scattered electrons. These signals are captured by detectors and analysed to reveal the surface topography and morphology of the material [156]. Being non-destructive, it allows detailed analysis without damaging the sample. Figure 14 [157] shows the SEM images of the base alloy AA7075 and AA7075 HMMC. As depicted in Fig. 14, the SEM image of the polished surface of the base alloy exhibits a smooth, uncorroded surface. In contrast, the HMMC demonstrates a uniform distribution of TiB₂ and Si₃N₄ reinforcement particulates within the matrix, along with noticeable agglomeration of these particles. The uniform distribution of reinforcements within the matrix material indicates the successful fabrication of HMMCs.

Fig.14 [Images not available. See PDF.]

SEM images of a AA7075 b AA7075 HMMC.

Adapted from ref. [157] copyright 2023, Springer Nature

The SEM image of freshly polished AA7075 HMMC revealed a fine surface with visible boundaries due to the incorporation of duralumin reinforcement as depicted in Fig. 15 [99]. When the sample was immersed in 0.1 M HCl, pits were observed on the surface, indicating the possibility of pitting corrosion. However, in the presence of 3.5 wt% NaCl, the surface appeared relatively smoother compared to that in 0.1 M HCl, attributed to the passivation of the oxide film.

Fig.15 [Images not available. See PDF.]

SEM images of Al- HMMC a Freshly polished AA7075 HMMC b AA7075 HMMC immersed in 0.1 M HCl c AA7075 HMMC immersed in 3.5 wt% NaCl.

Adapted from ref. [99] under the CC BY license

Figure 16 [124] depicts the SEM micrographs of AA6061 composite reinforced with 2 wt% SiC and 2 wt% B₄C immersed in 0.1 M HCl in the absence and presence of inhibitor (glucosamine sulfate). Figure 16 (a) displayed high surface roughness, cavities, and small pits due to the incorporation of SiC and B₄C particles creates microstructural heterogenicity and interfaces that disrupts the uniformity of the matrix there by enhancing the CR. The added inhibitor molecules block these active sites affectively by getting adsorbed onto the metal surface and hence minimising the CR.

Fig. 16 [Images not available. See PDF.]

SEM images of 6061 AA-HMMC a Freshly polished HMMC immersed in 0.1 M HCl b HMMC immersed in 0.1 M HCl + GAS.

Adapted from ref. [123] under Creative Commons Attribution 4.0 licence. (https://creativecommons.org/licenses/by-nc/4.0/)

Energy dispersive X- rays (EDX)

EDX is a technique used for elemental analysis. In this method, a high-energy beam of charged particles, such as electrons, protons, or even X-rays, is directed onto the sample. This high-energy interaction excites an electron from the inner shell, ejecting it and creating a vacancy. An electron from the outer shell then fills this vacancy, releasing X-rays corresponding to the energy difference between the two shells. Since these emitted X-rays are characteristic to specific elements, they enable the elemental analysis of the sample [158].

As depicted in Fig. 17 [157], the EDX spectrum of the base alloy confirms the absence of Si₃N₄ and TiB₂ reinforcements, as indicated by the lack of nitrogen, silicon, titanium and boron peaks. In contrast, the EDX spectrum of the HMMC containing TiB₂ and Si₃N₄ exhibits distinct peaks for titanium, boron, silicon and nitrogen, confirming the presence and distribution of these reinforcement particles within the composite.

Fig. 17 [Images not available. See PDF.]

EDX image of a AA7075 b AA7075 HMMC.

Adapted from ref. [157] copyright 2023, Springer Nature

Similarly, Fig. 18 [159] shows the EDX spectrum of AA6061 in the absence and presence of imidazolium-based ionic liquid. In the corroded sample, a higher chloride content was observed, predominantly concentrated at the corrosion sites. In contrast, the inhibited sample showed an increase in carbon content and a higher weight percentage of aluminium, indicating the formation of a protective film onto the metal surface, which effectively reduced the CR.

Fig. 18 [Images not available. See PDF.]

EDX spectrum of a AA6061 + 1 M HCl b AA6061 + 1 M HCl + inhibitor (imidazolium-based ionic liquid).

Adapted from ref. [152] under the CC BY license. (https://creativecommons.org/licenses/by/4.0/)

X- ray diffraction (XRD)

XRD is a widely used technique for determining the crystal structure, phase composition, and unit cell lattice parameters of a sample [160]. The principle of XRD is based on the Bragg’s law. When X- rays are incident on a crystal, the atoms present in crystal lattice scatter them. When these X-rays constructively interfere with a crystalline solid, it produces a diffraction pattern that reveals material’s crystal structure. As shown in Fig. 19 [111], the XRD analysis of the AA7075 HMMC revealed distinct peaks corresponding to SiC, B₄C, and Al₂O₃, alongside those of aluminium. This confirms the incorporation of reinforcements within the matrix material.

Fig.19 [Images not available. See PDF.]

XRD analysis of HMMC AA7075/SiC/B₄C/Al₂O_3.

Adapted from ref. [111] copyright 2021, Elsevier

Figure 20 displays the XRD spectrum of 6061Al-10 vol% SiC in 0.05 M HCl in the absence and presence of inhibitor [161]. An ionic liquid namely1-methyl-1-propyl-piperidinium bromide (MPPB) was used as inhibitor and the corresponding peak intensities obtained in the XRD analysis were compared with the blank. The higher peak intensities were observed for the specimen dipped in 0.05 M HCl, while a decrease in peak intensities for the inhibited sample indicated adsorption of MPPB onto the Al-composite surface.

Fig. 20 [Images not available. See PDF.]

XRD spectra of a corroded sample in 0.05 M HCl b Sample with 0.05 M HCl and inhibitor (MPPB).

Adapted from ref. [154] under the CC BY license. (https://creativecommons.org/licenses/by/4.0/)

Optical microscope analysis (OM)

Optical microscopy, also known as light microscopy, typically uses light as the illuminating source, and the magnified image is formed based on the varying light absorption across different regions of the specimen. As shown in Fig. 21 [162], the optical microscope image of the base alloy (AA6063) clearly displays grain boundaries with a uniform distribution of precipitation, while no reinforcement particulates are observed. In the case of AA6063 HMMCs, an increase in the wt% of SiC and WS₂ particles results in a decrease in grain quantity. Figure 21 (b) and (c) depicts a uniform and well-integrated composite structure, as indicated by the distinct and bright appearance of particles at the centre of the matrix. These images confirms the presence of reinforcements of SiC and WS₂ within the matrix material.

Fig. 21 [Images not available. See PDF.]

a AA6063 b AA6063 + 4 wt% SiC + 4 wt%WS₂ c AA6063 + 6 wt% SiC + 4 wt% WS_2.

Adapted from ref. [155] copyright 2024, Elsevier

Figure 22 presents optical microscope images of AA2024-T3 in NaCl medium in the absence and presence of sodium molybdate (Na₂MoO₄) [163]. Figure 22 (a) indicates that in the absence of the inhibitor, significant corrosion attack occurred, affecting both the intermetallic particles and the matrix. However, in the presence of Na₂MoO₄, the extent of corrosion was noticeably reduced. This improvement is attributed to the formation of a thick protective oxide layer on the metal surface, resulting from the adsorption of Na₂MoO_4.

Fig. 22 [Images not available. See PDF.]

Optical micrographs of a AA2024-T3 + NaCl b AA2024-T3 + NaCl + inhibitor (Na₂MoO₄).

Adapted from ref. [156] under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/)

Atomic force microscopy (AFM)

AFM is a powerful tool that provides high-resolution 3D images of a sample surface without requiring vacuum [156]. In this method, a fine sharp tip of the order of nanometer attached to a cantilever is brought close to the sample surface. Van der Waals forces between the tip and the surface causes the cantilever to deflect. This deflection is measured by a laser beam reflected from the cantilever tip onto a photodetector, which converts the laser light into an electric signal, enabling the imaging of the sample surface. Figure 23 (b) revealed increased surface roughness for the specimen immersed in the acid solution without an inhibitor. However, in the presence of the inhibitor (Fig. 23 c), the specimen's surface appeared smooth, suggesting the adsorption of inhibitor and the subsequent formation of a protective film on the Al-HMMC surface [125].

Fig. 23 [Images not available. See PDF.]

AFM images of AA6061 HMMC specimen a freshly abraded; immersed in b 0.5 M HCl, and c 0.5 M HCl + 1.074 mM EDTA.

Adapted from ref. [125] under the CC BY-NC 4.0 license (https://creativecommons.org/licenses/by-nc/4.0/)

Similarly, as shown in Fig. 24, corrosion inhibition studies of Al-6061 reinforced with SiC were conducted using Pullulan, a green inhibitor derived from a fungal polysaccharide containing maltotriose units [164]. The results clearly indicate that the average surface roughness (R_a = 39.2 nm) and root mean square roughness (R_q = 52.0 nm) of the inhibited samples were significantly lower compared to the uninhibited specimen. This suggests that Pullulan adsorbed onto the composite surface, thereby reducing the CR.

Fig. 24 [Images not available. See PDF.]

AFM images of a Freshly polished Al6061-SiC b AA6061-SiC + 0.025 M HCl c AA6061-SiC + 0.025 M HCl + pullulan.

Adapted from ref. [164] copyright 2018, Elsevier

Transmission electron microscopy (TEM)

TEM is a powerful tool that offers highly magnified images of a specimen by utilizing the interaction between a high-energy electron beam and the sample. This interaction generates transmitted electrons, which strike a fluorescent screen to produce a detailed and magnified image of the specimen [165]. It offers exceptional resolution and is useful for analysing interfaces, identifying different phases, and detecting cracks, voids, or grain boundary defects in composite materials. David et al. [166] investigated the microstructure and mechanical characterization of AA6061/(TiB₂ + Al₂O₃) HMMC, and their findings are illustrated in Fig. 25, which depicts TEM micrographs of AA6061 HMMC reinforced with 15 wt% (TiB₂ + Al₂O₃). Figure 25 a highlight the presence of nano-sized TiB₂ reinforcement particles within the composite material, while Fig. 25b shows the presence of ultra-fine Al₂O₃ particulates. These observations confirm that no needle-shaped structures were formed, indicating the stability of the particulates under the applied casting conditions.

Fig. 25 [Images not available. See PDF.]

TEM micrographs of AA6061 + TiB₂ + Al₂O₃ showing a TiB₂b Al₂O₃ particulates.

Adapted from ref. [166] copyright 2018, Elsevier

Figure 26 [167] displays TEM images of the AA2009/SiC composite, indicating a uniform distribution of SiC reinforcement within the metal matrix. Figure 26a reveals a high density of coarse precipitates in the bottom layer of the composite, identified as S (Al₂CuMg) phases. In contrast, Fig. 26b shows that the top layer contains fewer coarse precipitates, and the S-phase precipitates present are significantly smaller compared to those in the bottom layer. The formation of such coarse precipitates can negatively affect the composite's properties by reducing its overall strength and hardness.

Fig. 26 [Images not available. See PDF.]

TEM images of AA2009/SiC composite a Bottom layer b Top layer.

Adapted from ref. [167] copyright 2024, Elsevier

Summary and future scope

This review presents an in-depth examination of the corrosion behavior of Aluminium-based Hybrid Metal Matrix Composites (Al-HMMCs), with particular attention to the role of different reinforcements, fabrication processes, and corrosion evaluation techniques. Although extensive research has been conducted on the mechanical properties of Al-HMMCs, their corrosion behavior and inhibition strategies remain relatively underexplored. The review underscores how the type of reinforcement significantly influences corrosion performance across various environmental conditions.

Additionally, it highlights the critical role of corrosion rate assessment and surface characterization methods in revealing corrosion mechanisms and interactions between the matrix and reinforcements. For future research, the review suggests exploring the combined effects of different reinforcements to enhance corrosion resistance under diverse conditions. It also points to advanced fabrication techniques, including additive manufacturing and spark plasma sintering, as promising approaches for improving corrosion performance.

Moreover, the development of smart, eco-friendly corrosion inhibitors, particularly those derived from natural sources or hybrid heterocyclic compounds, aligns with sustainable engineering practices. The review strongly emphasizes the need for long-term corrosion testing in real-world marine, aerospace, and automotive environments. Bridging these research gaps will significantly advance the durability, sustainability, and industrial adoption of Al-HMMCs.

Author contributions

Divya Nayak: Writing—review, Writing—original draft, Data curation, Conceptualization. Suma A Rao and Santosh L Gaonkar: Writing—review & editing, Visualization, Supervision, Methodology, Investigation, Formal analysis. Preethi Kumari P: Writing—review & editing, Visualization, Validation, Supervision.

Funding

Open access funding provided by Manipal Academy of Higher Education, Manipal.

Data availability

Not applicable.

Declarations

Research involving human participants and/or animals

Not applicable.

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing Interests

The authors declare no competing interests.

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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