Increasing the Resistance of Steel and Austenitic Stainless Steels Against Pitting Corrosion by a γ-Irradiated Self-Assembled Amphiphilic Molecular Layer

Abstract

This study, based on our previous research, aims to quantitatively determine the enhanced protection of austenitic stainless steels against pitting corrosion in NaCl solution by self-assembled molecular (SAM) layers, in their original form and after γ-irradiation. This study focuses on four stainless steels of varying compositions, covered by self-assembled undecenyl phosphonic acid layers. The metal dissolution in corrosion experiments was measured by a special, highly sensitive analytical technique using the inductively coupled plasma–optical emission spectrometry (ICP-OES). The comparison of the dissolved metal ion concentrations measured in the presence of different metals with and without nanocoatings allowed the evaluation of the anticorrosion effectiveness of nanofilms as well as the importance of the alloying elements. The ICP-OES results demonstrated that the quality of layers have a significant impact on anticorrosion efficacy. The γ-irradiated self-assembled layers were the most effective in controlling the dissolution of stainless steels. The mechanisms of the inhibition in the presence of these nanolayers were elucidated by infrared spectroscopy. First of all, it revealed the differences in the adsorption of the undecenyl phosphonic acid self-assembled layer, both with and without γ-irradiation. The other important observation that confirmed the increased anticorrosion efficiency after γ-irradiation proved the formation of a more compact, polymer-like layer over the metal surface. The increased anticorrosion efficacy, defined as the enhancement in Pitting Resistance Equivalent Numbers (PRENs) in the presence of self-assembled layers (either pre- or post-γ-irradiation), can be documented.

1. Introduction

The importance of inhibitors against corrosive deterioration is well known. Molecules (inorganic, organic or bioorganic ones) can control corrosion processes, decrease material losses and increase the life span of instruments. The inhibitors are designed to isolate the metal surface from aggressive environments. Generally, different polar or ionic groups (e.g., –OH, –COOH, –OCH₃, –PO₃H₂, –NH₂, –CONH₂, etc.) are responsible for the anticorrosion effect of inhibitors [1,2,3,4,5].

Water- or oil-soluble inhibitors used at low concentrations can spontaneously adsorb onto the active sites of metals or form complex films or deposits on metal surfaces. The most active inhibitors are those that contain heteroatoms (N, O, S, P) and non-bonding or π electrons. All these components enable interactions between the inhibitors and metal surfaces via chemisorption (by coordinate-type bonding and interaction of non-bonding or π electrons with the d-orbital of metals), or via physisorption, where electrostatic interactions play an important role [1,6,7,8,9].

Corrosive deterioration could be controlled by coatings of solid surfaces before they get in contact with a corrosive environment; these special surface layers can modify metal surfaces by thin films [10], or by thicker paint layers of complex composition. Nanolayers could be prepared by special techniques like dipping the solid surface into an organic or aquatic solution of special molecules, Langmuir–Blodgett film deposition [11,12] and the self-assembling technique. In the formation of self-assembling molecular layers, amphiphilic molecules play important roles, with head groups (generally containing heteroatoms such as O, N, P or S in the form of carboxylic, sulfonic, or phosphonic acids and amine derivatives) and hydrophobic molecular parts (e.g., alkyl, alkenyl or aralkyl groups). Both parts determine the quality of the nanolayers. As our works focuses on the deposition, characterization and application of a special self-assembled molecule, this technique is summarized in detail. Some of the amphiphiles require oxide-free metal surfaces (e.g., R-SH forms nanolayers on copper or gold), while others can adhere only to metal oxide/hydroxide (carboxylic and phosphonic acids on iron, aluminum and alloys). The layer stability and thickness depend on the strength of bonds between solid surfaces and head groups and on the hydrophobic interactions (hydrogen bond, van der Waals and/or π-π interaction, etc.) among the hydrophobic molecular parts (e.g., alkyl or alkenyl groups). The phosphorous atom plays an important role in corrosion inhibition. There are very effective phosphonic acid molecules without substituents in the hydrophobic part or they could be substituted by amino, hydroxyl or carboxyl groups that make the molecules either water- or organic solvent-soluble. Bigger molecules with higher hydrophobicity can form more compact nanolayers [13,14,15,16,17,18]. The phosphono groups can be bound to a metal oxide layer via mono-, bi- and tridentate bindings [18].

The self-assembled nanolayers (SAMs) should have compact 2D structures [13,14] that depend on the amphiphiles and the metal surface composition. The quality of the surface layer is determined not only by the amphiphiles but also by the metal surface. In our experiments, stainless steel coupons (their compositions are listed in Table 1) were the solids on which the self-assembled molecular layers were deposited. Stainless steels are iron-based alloys that can resist general corrosion and higher temperatures [15]. The alloying metals influence the anticorrosion characteristics of steels significantly; i.e., a chromium oxide layer forms across the metal surface and this very thin film can protect the metal from corrosion. The Cr absorbs the oxygen more readily than the iron, which reacts with oxygen, producing different iron oxides/hydroxides, e.g., rust with a porous structure that allows contact between the metal and oxygen. The Cr₂O₃/Cr(OH)₃ oxide/hydroxide layer forms a hard, protective surface layer that is passive and non-reactive; it prevents the interaction between the iron and the oxygen by blocking oxygen diffusion to the underlying metal surface. A greater Cr concentration results in higher corrosion resistance. When the Cr concentration is higher than 10.5%, a continuous oxide layer is formed on the surface, which can protect the base metal from corrosive deterioration. Generally, the thickness of a passive layer on stainless steels is approximately 1–3 nm. The enrichment of Cr in the surface oxide layer is due to the lower solubility of Cr₂O₃ compared to that of Fe(II)/Fe(III) oxides.

The presence of nickel in stainless steels affects the passive oxide layer by increasing the anticorrosion activity (but less significantly than Cr₂O₃).

Molybdenum is especially useful in acidic media as it increases the passivity of the metal surface by improving the density of the passive film, improving anticorrosion resistance [16,17,18].

It is easier to understand the role of the main alloying components when the ΔG normal free enthalpy values of the formation of the Cr-, Fe-, Ni- and Mo-oxides are compared (Table 1).

Manganese enhances passivation and improves anticorrosion efficacy against pitting corrosion.

When the ΔG value is small, the metal is very reactive in the presence of oxygen, and it is not easy to reduce it (like in the case of Cr) [19].

The presence of Ti in the stainless steel increases its anticorrosion ability [20]. The oxide layers are more compact than those of the metal hydroxides [16]. It is important to mention that stainless steels are generally corrosion-resistant, but under special conditions (moisture, aggressive environment, chloride ions, etc.), they can corrode. The most frequent form of their corrosion is pit formation, which starts with de-passivation of the surface oxide layer initiated by certain chemicals (e.g., chloride ions). Pitting corrosion happens in three steps: (a) breakdown of the passive film, (b) growth of metastable pits and (c) formation of stable larger pits. NaCl solution can destroy the previously formed passive oxide layer.

The stainless steels we chose for research are often applied in the chemical and petrochemical industry, the food industry and in heat exchangers (because of their strong heat resistance), to mention only some of the application possibilities.

To improve the characteristics of stainless steels—without changing their composition—there are several methods: increasing the passive layer, mechanical polishing, electropolishing, mechanical cleaning and applying coatings. All these techniques improve their special characteristics (corrosion resistance, welding properties, etc.). The steels we used in our experiments are applicable across a wide pH range, but they are reactive to pitting corrosion, generally caused by chloride ions. Our work focuses on improving the pitting resistance by a special type of coating, via a self-assembled layer.

In the case of stainless steel surfaces, if they are covered by coatings, their sensitivity towards pitting corrosion changes. The compactness of the nanocoatings is important. Generally, the self-assembled molecular layers formed from proper amphiphiles (active head group, high hydrophobicity) meet this requirement. If not, the compactness of self-assembled molecular films can be improved by special techniques. Previously, several authors studied crosslinking by classical methods [21]; by heat treatment, which can improve the removal of the water formed at the condensation step of phosphonic groups to the metal oxide layer, as well as to expel the rest of the organic solvents [22,23,24]; by UV irradiation [25]; and by γ-irradiation [24,26], when the double bonds in the carbon chain can be polymerized, and the crosslink of double bonds increases the anticorrosion efficiency. γ-Irradiation proved to be the most successful technique for improving the quality of SAM layers [26,27]. The irradiation effect depends on the dosage, on the environment, on the presence of other molecules and the position of the double bonds in the amphiphile, as Marušić and co-workers reported [28]. They investigated alkenyl carboxylic acid amphiphiles on copper surfaces.

Previously, our research focused on the influence of conditions of self-assembled layer (SAM) formation, as well as on the effect of some post-treatments of the undecenyl phosphonic acid SAM. Those results have been summarized in several papers [24,29,30,31].

The experiments discussed in this paper are the continuation of our previous works, in which morphological changes were visualized by AFM imaging and characterized by roughness parameters.

The main aim of this paper is the quantitative determination of metal dissolution from different steels with and without nanocoatings in a chloride solution and the demonstration of inhibition mechanisms caused by the nanolayers.

The present publication discusses four iron alloys covered by SAM layers of undecenyl phosphonic acid, before and after γ-irradiation. Important changes in the SAM structure caused by post-treatment were demonstrated by infrared spectroscopy; this technique also helped in understanding the influence of the alloy compositions on the inhibiting activity. To prove increased anticorrosion efficacy, inductively coupled plasma–optical emission spectrometry (ICP-OES) was used to measure the dissolved metal ion concentrations in the presence of chloride ions. The Pitting Resistance Equivalent Number (PREN) values [32] were correlated with the concentration of dissolved metal ions measured in sodium chloride solution after corrosion experiments. The influence of alloying metal concentrations in stainless steels on the compactness of nanolayers, as well as on anticorrosion efficacy in aggressive environments, was demonstrated. Infrared spectroscopy helped to understand the enhanced anticorrosion activity of the irradiated nanolayers.

2. Experimental Procedures

2.1. Materials

2.1.1. Metals

The steel and stainless steel coupons (their compositions are presented in Table 2; one contains mostly iron (>99%), and in the others, the alloying components are Cr, Ni, Mn and Mo at different concentrations, which improve the quality of the stainless steels) used in our experiments are as follows: 1.0330: ISD Dunaferr, Hungary; 1.4541; 1.4571, 1.4841: APERAM, Genk, Belgium; they were cut into sizes of 12 × 10 × 1 mm (for AFM measurements) and 30 × 10 × 1 mm (for infrared measurements). The surfaces were first mechanically ground by silicon carbide paper (240 → 2000 mesh), and then polished by diamond pastes (grain sizes: 15–12–9–6–3–1 µm) to produce a smooth surface. Between the polishing steps and at the end of the smoothing process, the metals were washed by deionized water and methanol ultrasonically and dried under atmospheric conditions.

2.1.2. Amphiphile

In all experiments, undecenyl phosphonic acid molecules (CH=CH-[CH₂]₉–PO(OH)₂, MW:234, Specific Polymers, Castries, France) were used for self-assembled layer formation. The amphiphile was dissolved in methanol at a concentration of 5 × 10⁻³ M.

[Figure omitted. See PDF]

2.2. Nanolayers

2.2.1. SAM Layer Preparation

The properly polished metal coupons were dipped into the amphiphile solution for 24 h. Then, the coupons were removed from the solution, the excess solution was rinsed off with methanol and the nanocoated metal samples were dried in air.

2.2.2. Post-Treatment of the SAM Layers by γ-Irradiation

The irradiation experiments of undecenyl phosphonic acid SAM layers deposited on stainless steel coupons were conducted at room temperature in air or in an inert atmosphere at room temperature, in the panoramic type 1.8 PBq activity ⁶⁰Co source of the Institute of Isotopes Co., Ltd. (Budapest, Hungary), applying a dose rate of 11.5 kGy/h (equal to 11.5 kJ/kg.h). The dose rate measured using ethanol–chlorobenzene dosimetry was 20 kGy/h [33].

2.2.3. Fourier Transform Infrared Spectroscopy

Infrared spectroscopy was used to identify and quantify the functional groups in the nanolayers before and after post-treatment [27,34].

The Jasco FT-IR-4600 instrument equipped with Jasco ATR multireflection (5 reflections) ZnSe crystal ATR accessory (incident angle 45°), and a DTGS detector used in the 4000–400 cm⁻¹ region. A resolution of 4 cm⁻¹ and co-addition of 128 individual spectra were applied. An ATR correction was performed on raw spectra.

2.3. Nanolayers in Corrosive Environments

2.3.1. Corrosion Experiments

The polished and SAM-coated coupons (with and without post-treatment) were immersed in NaCl solution (3%; pH 6.8; 25 °C) for 1 and 5 days (depending on the type of steel). The rest of the NaCl solution was first removed by filter paper and then by dipping the coupons into MilliQ water, and at last, the coupons were dried in air.

2.3.2. Determination of Metal Ion Concentrations by Inductively Coupled Plasma–Optical Emission Spectrometry

The concentrations of metal ions dissolved in the NaCl electrolyte were measured by inductively coupled plasma–optical emission spectrometry (ICP-OES); this is a beneficial technique for solution analysis [33], which can measure about 30 elements at the same time, at the concentration range between γ g/L and some percent levels. The plasma is very stable due to the special operating frequency and broad bandwidth; the instrument is equipped by a CCD detector (it allows us to take measurements in the wavelength range of 175–775 nm). The instrument we worked with is a Spectro Genesis ICP-OES spectrometer (with axial plasma observation).

2.3.3. Relationship Between the Metal Ion Dissolution and the Pitting Resistance Equivalent Number (PREN)

This number indicates the resistance to localized corrosion of stainless steels determined by their chemical composition. A higher PREN means that the stainless steel is more resistant to pit formation in the presence of chloride ions. For example, considering the influence of sea water (high Cl concentration), a PREN value higher than 32 is acceptable.

For calculation of PREN, the frequently used formula is as follows:

PREN = %Cr + 3.3 × %Mo + 16 × %N(1)

The alloying elements must be calculated in weight (%).

The concentration of the dissolved metal ions was correlated with the PRENs.

The experimental conditions are summarized in Table 3.

The adsorption of the SAM layers onto the metal oxides was followed by infrared spectroscopy. The corrosion inhibition effectiveness of the SAM nanofilms, i.e., the metal dissolution before and after SAM post-treatment, was determined by ICP-OES.

3. Results and Discussion

The changes in the SAM layer characteristics caused by γ-irradiation were demonstrated in our previous publication by AFM imaging and roughness parameters [34]. Data measured on SAM surfaces after γ-irradiation proved that the nanolayer turned out to be smoother and more hydrophobic. This paper shows how these changes observed on post-treated SAM surfaces affect the anticorrosion behavior of nanolayers.

The composition of the stainless steels under investigation is listed in Table 1. We started with a metal without alloying components (1.0330), and for the subsequent metals, the composition changed systematically: not only did the concentrations of the more important components (Cr, Ni) change, but the quantities of other elements that can improve stainless steel quality also changed in parallel.

3.1. Corrosive Metal Dissolution of Metals (1.0330, 1.4541, 1.4571 and 1.4841) with and Without SAM Layers, Before and After γ-Irradiation (Table 4)

First, the metal dissolution caused by the NaCl solution is presented in the case of the 1.0330 metal, as shown in the table below.

The metal ion concentrations unequivocally indicate and prove the important effect of the alloying elements in stainless steel. The 1.0330 alloy does not contain those alloying components that could help in the formation of a passive layer on the metal surface. This was the reason why it was necessary to stop the corrosion experiments after only one day, as the concentration of the precipitated rust indicated severe corrosion. Before the determination of iron ions by the ICP-OES technique in these experiments, the rust particles were dissolved in acid.

In the other set of experiments, the 1.0330 metal surfaces were covered by SAM layers of undecenyl phosphonic acid in order to coat the metal surface and control the metal dissolution in a corrosive environment. Previous experiments showed the importance of dipping time on layer formation and surface hydrophobicity. This value indicated the compactness of the surface layer. Through these corrosion experiments, we aimed to prove the influence and importance of the compactness of SAM layers.

After the corrosion experiments, the analysis of iron concentrations in the sodium chloride solutions proved the positive effect of the nanolayers formed on the metal surface was, as well as the importance of the nanolayer formation time: with a shorter deposition time (4 h), the layer could not fully cover the solid surface, which resulted in a higher iron ion concentration in the corrosive solution. A longer SAM dipping time (24 h) resulted in a more compact surface layer, which was already able to control the metal dissolution more effectively. This was reflected not only in the Fe ion concentrations but also in the Mn ion concentrations: after a longer SAM deposition time, not only Fe but also Mn appear in much lower concentrations in the sodium chloride solution (Table 3). The compactness of the layers was reflected in the higher water contact angles [31].

As for the temperature of the layer formation, it is clear that at an increased temperature (35 °C instead of 23 °C), even for a short duration (1–3 h), the SAM layer seems to be more homogeneous: this is proven by the increased contact angle values and the concentration of the dissolved metal.

3.2. Influence of γ-Irradiation on the Anticorrosion Efficiency of the SAM Layers

The γ-irradiation of the SAM layers resulted in a significant decrease in the metal dissolution in a corrosive environment not only in the case of the 1.0330 metal but also in the other stainless steels under investigation. It is caused by a more compact layer of the γ-irradiated SAM (Table 3).

The SAM nanolayer developed on the 1.4541 stainless steel—without any post-treatment—could decrease the corrosive metal dissolution by one order of magnitude! When the layer was post-treated by γ-irradiation, the decrease in metal ion concentration reached levels three hundred times lower than those measured on the pure metal. The positive effect of the post-treated layers was demonstrated not only by the iron, but also the decreases in the chromium, nickel and manganese ions (Table 5).

In the case of stainless steel 1.4571, the observation was similar to that for 1.4541: in the presence of the SAM layer, the iron dissolution decreased to one-tenth of the level observed without the nanolayer; the concentrations of iron and the other alloying metal ions were further diminished after post-treatment of the SAM layers (Table 6).

As the results in Table 5, Table 6 and Table 7 show, the passive layer of these stainless steels can control pitting corrosion: taking the Fe ion concentration measured in the corrosive solution with the pure steel as 100%, the iron ion concentration in the NaCl solution after 5 days with the SAM layer decreased to 6.25%. When the quality of the SAM layer was improved by γ-irradiation, this ratio decreased further to 2.1%.

Table 8 unequivocally proves that the SAM layers in all cases significantly decrease corrosive deterioration, even in the case of the unalloyed 1.0330 metal. In the latter case, dissolution is much higher after one day than in all other cases of stainless steels. Iron dissolution from the alloyed metals is significantly lower than that of the 1.0330 metal after five days. The effectiveness of the SAMs and γ-irradiated SAMs is evident. The post-treated SAMs increase the isolation of the metal surface from chloride ions. These phenomena are clearly explained by the infrared measurements.

In the corrosion experiments, the calculation was based on the iron ion concentrations measured for pure metals without any coatings, which were taken as 100%. In other experiments, the iron ion concentrations measured in the presence of SAMs, as well as when the metal was covered by γ-irradiated SAMs, were compared to this baseline. The evaluation of the numerical values unequivocally proves the increased insulatory properties of the SAM layers, which are further improved by γ-irradiation (Table 9).

The structure of metals 1.4541, 1.4571 and 1.4841 are austenitic and relatively sensitive to pitting corrosion induced by chloride ions. The evaluation of their corrosion experiments shows the importance of Cr in the alloy. Of course, other alloying components (Ni, Mo, Mn, etc.) also play a role in the formation of the passive layers. The absence of alloying metals in 1.0330 is clearly shown by the ratio of dissolved iron concentrations measured without and with the SAM nanofilm, before and after post-treatment.

When stainless steels come in contact with oxidative environments, passive films are formed on their surfaces, which consist mainly of Cr₂O₃ and other metal oxides, which play a role in controlling corrosion processes. The ratio of Cr₂O₃ to Fe₂O₃ on the surface depends mainly on the presence of Cr₂O₃, but the other alloying components can also influence it.

When stainless steels come into contact with a solution of chloride ions, the presence of Cr₂O₃ in the passive film primarily helps in the inhibition of metal dissolution. In the presence of higher chloride ion concentrations or at higher temperatures, chloride ions attack the passive layer, making it unstable and resulting in metal dissolution. The passive film in contact with chloride ions contains chloride and sodium ions, as well as ions of the alloying metals (Cr³⁺, Fe²⁺ and Fe³⁺), and water molecules. The concentrations of these components in the passive films are determined by the composition of the bulk metal and the corrosive environment.

The iron dissolution data in Table 2, Table 3, Table 4, Table 5 and Table 6 clearly demonstrate the primary role of Cr in resisting pitting corrosion in the presence of chloride ions. The presence of alloying elements in metals plays an unambiguous role in the inhibition of corrosion. The lack of alloying components, like in the case of the 1.0330 metal, leads to enhanced metal dissolution (in our case, to pitting corrosion). The presence of Cr and other metals like Ni, Mo and Mn in the bulk metal can reduce metal dissolution to a great extent. But it should be kept in mind that the greater the amount of Cr, the higher the corrosion resistance.

As shown by the metal dissolution data in corrosive environments in Table 2, Table 3, Table 4, Table 5 and Table 6, the presence of molecular layers on the metal surfaces can reduce corrosive dissolution to less than one-tenth of the control. γ-Irradiation of the SAM nanolayers can further improve the positive effect of the nanofilms. In our case, this post-treatment promotes the formation of a more compact layer, as demonstrated by the change in the interaction between the head group of the undecenyl phosphonic acid and the metal oxide surface, as well as the polymerization of double bonds in the amphiphilic molecules in the SAM layer (These findings are summarized in Section 3.4 by presenting results obtained by infrared spectroscopy). The post-treatment produces a surface layer, which can mask the surface. It isolates the metal from the aggressive environment and inhibits pit formation on the surface.

3.3. Relationship Between PREN (Pitting Resistance Equivalent Number) Values and the Metal Ion Dissolution Rate in Corrosive Environments

The susceptibility of the stainless steels to pitting corrosion could be characterized by the PREN values that measure the relative pitting corrosion resistance of stainless steels in corrosive chloride ion-containing environments. This number is determined by the alloy components (the most important ones are Cr, Mo and N).

To quantify the contribution of the alloying components to pitting corrosion, Equation (1) can be used.

It is important to mention that higher PREN value indicate greater corrosion resistance [35].

The numerical PREN values of our steel and stainless steels are as follows (Table 10):

1.0330: PREN = 0.04 + 3.3 × 0.1 + 16 × 0.004 = 0.434

1.4541: PREN = 18.5

1.4571: PREN = 16.9 + 3.3 × 2.04 +16 × 0.012 = 23.824

1.4841: PREN = 24.1 + 3.3 × 0.37 +16 × 0.022 = 25.673

The individual PREN values and the dissolution of iron in NaCl solution for metals without coatings, with SAM nanolayers and with post-γ-irradiated SAM layers are listed in Table 11 and Figure 1.

The metal with the highest Cr content (1.4841) can control the iron dissolution in the presence of chloride ions; in other words, in this case, the passive layer is the most stable. Another important observation is that the metal without any alloying component (1.0330) cannot resist the chloride ion attack, as iron dissolution in this case is not inhibited due to the absence of the Cr₂O₃ oxide layer.

3.4. Demonstration of SAM layer Composition After Adsorption onto the Metal Oxide Surfaces (Without or with γ-Irradiation) by Infrared Spectroscopy

This characterization technique helped us to understand what happens when the self-assembled layer is formed from this amphiphilic molecule—undecenyl phosphonic acid—on stainless steels, which types of interactions keep the molecule on the surface and what happens when the SAM layer is altered by γ-irradiation under different conditions (in air or in an inert atmosphere).

The first important question is the structural changes in the phosphonium head group when it comes into contact with metal oxide surfaces. Additionally, what happens to the alkenyl chain of the amphiphile built in the SAM layer after γ-irradiation, either in an inert atmosphere or in the presence of air? These questions were answered by infrared spectroscopy based on IR bands. To better understand the situation, calculations were performed on a model molecule. For the proper assignments of the different adsorbed phosphonium groups, DFT calculations were performed for the “free” CH₃CH₂CH₂ P=O(OH)₂, the single Fe-bonded CH₃CH₂CH₂-(HO)₂ P=O–Fe and the double-bonded CH₃CH₂CH₂-(OH) P=O–Fe (O–Fe) species. The selected results belonging to vibrations of the –P=O(OH)₂ group and its single- and double-coordinated species are shown in Figure 1.

The vibrational forms of the “free” phosphonium group are separated into a high-frequency P=O double bond stretching at 1160 cm⁻¹ and two antisymmetric and symmetric P–O stretching modes of the P(OH)₂ group at 900 and 880 cm⁻¹. The calculated bond distances are rather different: 1.48 and 1.61 Å for the Fe=O double bond and the P–OH single bond, respectively. The P=O stretching is localized to a pure single stretching mode, without coupling to the P–OH stretching. The band separation between the P=O stretching and the averaged value of the P–OH stretching is 280 cm⁻¹, due to the large differences in the PO bond lengths (first row in Figure 2).

In the case of one-oxygen coordination to iron, the PO bond distances become much shorter; i.e., P=O is 1.51 Å and the two P–OHs are 1.60 Å. This bond length equalization significantly changes the PO stretching vibrational modes. The P=O stretching is coupled with the P–O stretching in an out-of-phase mode at 1150 cm⁻¹, denoted as νP = O − ν_s PO₂. This combination can be simply called ν_a PO₃, representing one component of a double degenerate mode of an approximate C_3v local symmetry. The band separation between the P=O stretching and the averaged value of the P–O stretching is 250 cm⁻¹, due to the smaller differences in the PO bond lengths (row in Figure 2).

The corresponding atomic displacements are shown in the second row in Figure 2.

The antisymmetric P(OH)₂ stretching mode as the second component of the above “double degenerate” mode is localized on the two P(OH)₂ bond stretching modes at 920 cm⁻¹, as shown in the second picture of the second line in Figure 1. This displacement refers to PO₂ antisymmetric stretching, ν_a PO₂.

The symmetric displacement of the three oxygens can be derived as an in-phase coupling of the P=O and symmetric PO₂ stretchings at 880 cm⁻¹, denoted as νP = O + ν_s PO₂. The general pattern refers to a symmetric PO₃ stretching, ν_s PO₃.

In the case of two-oxygen-atom coordination to iron, the calculated bond lengths become even closer. Namely, in the P=O–Fe fragment, the length is 1.57 Å; in the P–O–Fe fragment, the length is 1.60 Å; and in the P–OH group, the length is 1.56 Å. This geometry is even closer to a “symmetric” PO₃ group with C_3v local symmetry, similar to the single-bonded PO₃ group; therefore, the highest calculated mode at 1040 cm⁻¹ can be called antisymmetric PO₃ stretching, ν_aPO₃. The next mode is localized on the opposite displacement of the two coordinated oxygens as the antisymmetric P(O–Fe)₂ mode at 900 cm⁻¹, ν_a PO₂. The symmetric PO₃ stretching was calculated as a band at 870 cm⁻¹ marked as ν_sPO₃. In this case, the band separation between the ν_aPO₃ stretching and the averaged value of ν_a PO₂ and the ν_s PO₃ stretching is only 150 cm⁻¹, due to the little differences in the PO bond lengths (first row in Figure 2). These modes are presented in the third line of Figure 2.

In the last line of Figure 2, the experimental observations show that the triple-coordinated PO₃ group exhibits true C_3v local symmetry, with antisymmetric (~1000 cm⁻¹) and symmetric (~900 cm⁻¹) fundamental modes. In this case, the three PO bonds have bond lengths of about 1.58 Å (approximated from the above-calculated PO bond distances) and the band separation between the two PO₃ stretching modes is only 100 cm⁻¹.

The above-calculated results were used for the interpretation of infrared spectra of differently produced and modified SAM layers of H₂C=CH-(CH₂)₉-P=O(OH)₂ on two different steel surfaces.

The infrared ATR spectrum of a powdered sample of H₂C=CH-(CH₂)₉-P=O(OH)₂ is shown at the bottom of Figure 3 (red-colored spectrum). The assignments of its IR bands in the spectral region of 1300–700 cm⁻¹ are summarized in Table 10.

When the SAM layer is formed on the 1.4571 stainless steel surface, the spectrum (Figure 3, gray) exhibits drastic changes compared with the spectrum of the solid material (Figure 3, red spectrum). The major P=O stretching band shifted down from 1178 to 1077 cm⁻¹, indicating a strong interaction with the iron surface via the P=O oxygen atom.

The band characteristic of the end vinyl group at 993 cm⁻¹ disappeared possibly due to the disappearance of the C=C double bond. In addition, the doublet corresponding to the out-of-plane POH deformations at 779 and 718 cm⁻¹ (the symmetric γ_s POH and antisymmetric γ_aPOH modes, respectively) observed in the spectrum of the “free” sample are not clearly visible. Instead, a medium-intensity feature was recorded at 785 cm⁻¹ and only a weak one was recorded at 705 cm⁻¹. This spectral change suggests that one of the OH groups is deprotonated during the formation of the SAM layer. The presence of the 1077 cm⁻¹ band and the single out-of-plane POH deformation suggest the formation of a double-coordinated species with one remaining POH group.

After irradiation in air, the SAM layer (second spectrum from the bottom) exhibits a strong and broad PO stretching feature at 1082 cm⁻¹ and a broad shoulder near 980 cm⁻¹. A very weak band at 750 cm⁻¹ may indicate traces of the remaining POH structure.

The top spectrum in Figure 3 refers to the results from the irradiated SAM layer in an inert atmosphere. The strongly down-shifted intensive feature close to 1000 cm⁻¹ and a weak band at 989 cm⁻¹ can be assigned to antisymmetric and symmetric –PO₃ stretchings, respectively. This is clear evidence of the triple-coordinated –PO₃ group, exhibiting C_3v local symmetry. It means that the three P–O bonds are equalized during irradiation in an inert atmosphere; i.e., no more different P=O and P–O bonds exist. The spectral difference is clear between SAM samples irradiated in the atmosphere and inert atmosphere. After treatment in an inert atmosphere, stronger triple-oxygen-coordinated surface species are formed (yellow spectrum in Figure 3) compared to those of double-coordinated phosphonium group developed under atmospheric treatment, e.g., formation of –CH₂-P=O–Fe O–Fe (OH) species on the surface (Figure 3 blue spectrum, Table 12).

Infrared ATR spectra of SAM layers developed on 1.4841 stainless steel are shown in Figure 4.

The “free” P=O stretching of the powdered sample at 1178 cm⁻¹ is shifted down to 1132 cm⁻¹ in the spectrum of the starting SAM layer. The downshift can be interpreted as the single-coordinated P=O–Fe stretching, while the two non-coordinated P(OH)₂ stretching bands at 943 and 915 cm⁻¹, corresponding to the symmetric and antisymmetric PO₂ stretching modes, respectively, remained visible in the spectrum. The two spectral features at 1089 and 1060 cm⁻¹ are the up-shifted in-plane deformation bands of the two P–O–H groups (β_s POH and β_aPOH modes, respectively). These bands in the powdered sample are observed at slightly lower positions of 1064 and 1045 cm⁻¹.

The missing wagging mode of the ethylene =CH₂ group at 993 cm⁻¹ indicates the absence of the olefin end group after the formation of the SAM layer and irradiation in an inert atmosphere. The strong shoulder feature at 1120 cm⁻¹ supports a surface structure with a single coordination to iron via the P=O group. Furthermore, it confirms the presence of the symmetric and antisymmetric PO₂ stretching modes of the P(OH)₂ group at 937 and 930 cm⁻¹, and also the out-of-plane doublet of the P(OH)₂ deformation modes at 787 and 718 cm⁻¹. The strong band at 1070 cm⁻¹ can be attributed to the antisymmetric PO₃ stretching of a double-coordinated phosphonium species. Since it is a very strong band, it cannot be assigned to the in-plane deformation of the P(OH)₂ group (βPOH) (see third line of Table 10).

The above spectral evidence supports the coexistence of mono- and double-coordinated PO₃ groups.

The spectrum of the irradiated SAM layer in air (Figure 3 blue-colored spectrum) confirms the pure formation of double-coordinated species with characteristic spectral features at 1086, 968 and 835 cm⁻¹ assigned to the antisymmetric PO₃ stretching, to the antisymmetric PO₂ stretching and to the symmetric PO₃ stretching, respectively. The DFT calculation resulted in these three bands at 1040, 900 and 870 cm⁻¹ (Table 13).

According to above-mentioned results, it can be stated that on different stainless steel surfaces (which vary not only in Cr and Ni contents but in the ratio of Mn to Mo), the spectra of SAM layers are different, which indicates the influence of the alloying elements on the SAM structure. Another important observation is that the SAM spectra after post-treatment are significantly influenced by the irradiation environment: the spectra of SAM layers after γ-irradiation differ significantly depending on whether the post-treatment is performed in air or in an inert atmosphere.

The infrared results demonstrated how the SAM nanolayers are bound to the metal oxide surfaces and how the nanofilms reduce direct contact between chloride ions and the metal surface as a corrosive agent. The result is significant decrease in metal dissolution and increased corrosion inhibition. Further analyses of the infrared spectra proved that γ-irradiation altered both the binding between the oxide layer and the SAM film, as well as polymerized the double bonds in the alkenyl groups of the amphiphiles, resulting in a compact surface layer that increased the anticorrosion efficiency of the SAM nanolayers.

4. Conclusions

In sodium chloride solution, the metal dissolution measured by ICP-OES in the presence and absence of SAM layers developed on four different steels was quantitatively determined by the ICP-OES method. The anticorrosion activity of SAM layers prepared under different conditions was numerically demonstrated, which proved the importance of increased deposition time, temperature and the composition of the alloys onto which the nanofilms were adsorbed. The results quantitatively highlighted not only the importance of the alloying elements in the stainless steels but also the influence of nanolayers: alloying components, especially chromium, significantly reduce metal dissolution. These experiments also proved that the presence of SAM layers on stainless steels significantly decreases metal dissolution in chloride solutions, and their anticorrosion activity increases to a large extent. The dissolution rate results were correlated with the PREN values. Infrared spectroscopy helped us to understand the reason for the enhanced corrosion-inhibiting activity of SAM layers. One reason is the proper bonding of the head groups onto the metal oxide surface, which results in stable layers, whereas the hydrophobic interaction of the alkenyl part of the amphiphile prevents the aggressive, corrosion-relevant chloride ions from reaching the metal surface. Further experiments proved the positive effect of γ-irradiation of SAM layers: the significant increase in the anticorrosion activity is due to more effective bonding between the head groups and the oxide layer, as well as the formation of a more compact, polymer-like layer over the metal surface.

Author Contributions

Methodology, Z.M., M.P.M. and J.M.; Investigation, É.K.P.; Resources, I.G.G.; Writing—review & editing, J.T. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article.

Acknowledgments

The authors are grateful to László Trif (Research Centre for Natural Sciences, Hungary) for experimental support.

Conflicts of Interest

The author declares no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. The correlation between the PREN values and the dissolution of iron ions from different uncoated metals as well as after their coating by self-assembled layers.

Figure 2. Vibrational modes and frequencies of the phosphonium group in different coordinations. First line: “free” P=O(OH)2 group (uncoordinated phosphate group); second line: single-coordinated PO3—Fe group (phosphate group coordinated to one Fe atom); third line: double-coordinated PO3(—Fe)2 group (phosphate group coordinated to two Fe atoms); fourth line: triple-coordinated P(O—Fe)3 group (phosphate group coordinated to three Fe atoms). * The final data refer to experimental observations.

Figure 3. Infrared spectra of characteristic phosphonium group vibrations of powdered sample and its SAM layers before and after irradiation formed on 1.4571 stainless steel surfaces at different conditions. From bottom to top: powder sample (red); SAM layer irradiated in air (blue-colored spectrum); SAM layer (gray); SAM layer irradiated in inert atmosphere (yellow-colored spectrum).

Figure 4. Infrared spectra of characteristic phosphonium group vibrations of powdered sample and its SAM layers before and after irradiation formed on 1.4841 stainless steel surfaces at different conditions. From bottom to the top: powder sample (red); SAM layer irradiated in air (blue); SAM layer; SAM layer irradiated in inert atmosphere (yellow).

Table 1

The molar free enthalpy of metal oxides formation together with the partial oxygen equilibrium pressure at 550 °C.

Reaction	∆G (kJ/mol)	PO2 (atm)
4/3 Cr + O2 = 3/2 Cr2O3	−610	10−37
3/2 Fe + O2 = 1/2 Fe3O4	−430	10−28
2 Ni + O2 = 2 NiO	−340	10−22
Mo + O2 = MoO2	−591	10−36

Table 2

Composition of stainless steels used in experiments.

Steel	Fe%	C%	Si%	Mn%	P%	S%	Cr%	Ni%	Mo%	Ti%	N%
1.0330	balance	0.42	0.4	0.8	0.045	−	0.04	−	0.1	−	−
1.4541	balance	0.08	1.00	2.00	0.045	0.015	18.5	12.00	−	0.7	−
1.4571	balance	0.029	0.44	0.89	0.034	<0.001	16.9	10.50	2.04	0.31	0.012
1.4841	balance	0.048	1.63	1.45	0.025	0.0010	24.1	19.09	0.37	0.022	0.022

Table 3

Summary of the experimental conditions.

Metal	SAM (5 × 10−3 M)			Corrosive Solution (3% NaCl)
	Time [h]	t [°C]	γ-Irradiation	1 Day	5 Days
1.0330	−	−	−	+	−
	1	35	−	+	−
	3	35	−	+	−
	4	23	−	+	−
	24	23	+	+	−
1.4541	−	−	−	−	+
	24	23	−	−	+
	24	23	+	−	+
1.4571	−	−	−	−	+
	24	23	−	−	+
	24	23	+	−	+
1.4841	−	−	−	−	+
	24	23	−	−	+
	24	23	+	−	+

Table 4

Metal ion dissolution from 1.0330 steel in NaCl solution (3%; pH 6.8; room temperature), with and without self-assembled layer (SAM deposition from 5 × 10⁻³ M undecenyl phosphonic acid in methanol at 25 °C and at 35 °C), before and after γ irradiation.

	Bulk Composition of 1.0330 [%]
	Fe	Cr	Ni	Mn	Mo
1.0330	99.06	0.04	−	0.80	0.10
Metal Ion Content of NaCl Solution After 1 day [mg/L]
	Fe	Cr	Ni	Mn	Mo
1.0330 + NaCl	55.29	<k.h.	<k.h.	0.161	<k.h.
1.0330 + 4 h SAM + NaCl	47.02	<k.h.	<k.h.	0.128	<k.h.
(1.0330 + 24 h SAM) + NaCl	34.48	<k.h.	<k.h.	0.106	<k.h.
(1.0330 +1 h SAM 35 °C) + NaCl	32.96	<k.h.	<k.h.	0.078	<k.h.
(1.0330 +3 h SAM 35 °C) + NaCl	27.76	<k.h.	<k.h.	0.056	<k.h.
γ (1.033 +24 h SAM) + NaCl	21.50	<k.h.	0.013	0.089	<k.h.

k.h.= detection limit.

Table 5

Metal ion dissolution from 1.4541 stainless steel in NaCl solution (3%; pH 6.8; room temperature), with and without self-assembled layer (deposition from 5 × 10⁻³ M undecenyl phosphonic acid in methanol, at 23 °C), before and after γ irradiation.

	Bulk Composition of 1.4541 [%]
	Fe	Cr	Ni	Mn	Mo
1.4541	67.50	18.50	12.00	0.80	0.10
Metal Ion Content of NaCl Solution After 5 days [mg/L]
	Fe	Cr	Ni	Mn	Mo
1.4541 + NaCl	0.899	0.086	0.163	0.024	−
1.4541+ 24 h SAM + NaCl	0.094	0.009	0.023	0.002	−
γ (1.4541+ 24 h SAM) + NaCl	0.003	0.008	0.001	0.001	−

Table 6

Metal ion dissolution from 1.4571 stainless steel in NaCl solution (3%; pH 6.8; room temperature), with and without self-assembled layer (deposition from 5 × 10⁻³ M undecenyl phosphonic acid in methanol, at 25 °C), before and after γ irradiation.

	Bulk Composition of 1.4571 [%]
	Fe	Cr	Ni	Mn	Mo
1.4571	69.67	16.9	10.50	0.89	2.04
Metal Ion Content of NaCl Solution After 5 Days [mg/L]
	Fe	Cr	Ni	Mn	Mo
1.4571 + NaCl	0.113	0.005	0.021	0.003	−
1.4571+ 24 h SAM + NaCl	0.010	<k.h.	0.017	0.001	−
γ (1.4571)+ 24 h AM) + NaCl	0.004	<k.h.	0.005	0.001	−

k.h.= detection limit.

Table 7

Metal ion dissolution from 1.4841 stainless steel in NaCl solution (3%; pH 6.8; room temperature), with and without self-assembled layer (deposition from 5 × 10⁻³ M undecenyl phosphonic acid in methanol, at 25 °C), before and after γ irradiation.

	Bulk Composition of 1.4841 [%]
	Fe	Cr	Ni	Mn	Mo
1.4841	54.99	24.1	19.09	1.45	0.37
Metal Ion Content of NaCl Solution After 5 days [mg/L]
	Fe	Cr	Ni	Mn	Mo
1.4841 + NaCl	0.048	<k.h.	0.011	0.004	<k.h.
1.4841+ 24 h SAM + NaCl	0.003	<k.h.	0.005	0.003	<k.h.
γ (1.4841+ 24 h AM) + NaCl	0.001	<k.h.	0.002	0.002	<k.h.

k.h.=detection limit.

Table 8

Iron dissolution from basic metals before and after coatings in 3% NaCl solution, at room temperature. SAM: self-assembled layer formed from 5 × 10⁻³ M undecenyl phosphonic acid solution at 23 °C; γ-SAM: γ-irradiated SAM layer coatings.

Metal	[Fe2+] (mg/L) in 1 Day	Metal	[Fe2+] (mg/L) in 5 Days	Metal	[Fe2+] (mg/L) in 5 Days	Metal	[Fe2+] (mg/L) in 5 Days
1.0330	55.29	1.4541	0.899	1.4571	0.113	1.4841	0.048
+SAM	34.48	+SAM	0.094	+SAM	0.010	+SAM	0.003
+γ-SAM	21.50	+γ-SAM	0.003	+γ-SAM	0.004	+γ-SAM	0.001

Table 9

Percentage of improvement upon treating the metal samples differently. Influence of SAM layers and γ-irradiated SAM layers as well as of the composition of metals on the iron dissolution.

Metal	[%]	Metal	[%]	Metal	[%]	Metal	[%]
1.0330	100	1.4541	100	1.4571	100	1.4841	100
+SAM	62.40	+SAM	10.50	+SAM	8.80	+SAM	6.25
+(γ-SAM)	38.90	+(γ-SAM)	0.33	+(γ-SAM)	3.54	+(γ-SAM)	2.10

Table 10

Composition of metal concentrations used for the PREN calculation and the resulting PREN values.

Metal	Metal Content in Bulk [%]
	Fe	Cr	Ni	Mn	Mo	PREN
1.0330 (N = 0.004%)	balance	0.04	−	0.8	0.1	0.434
1.4541 (N = 0%)	balance	18.5	12.00	2.00	−	18.500
1.4571 (N = 0.012%)	balance	16.9	10.50	0.89	2.04	23.824
1.4841 (N = 0.022%)	balance	24.1	19.09	1.45	0.37	25.673

Table 11

Relationship between the PREN values and the iron concentration dissolved in NaCl solution in the presence of metal, metal + SAM and metal+ (γ-irradiated SAM).

Metal	PREN Value	Metal; Dissolved Fe [mg/L] in NaCl Solution	Metal + SAM; Dissolved Fe [mg/L] in NaCl Solution	Metal + (γ SAM); Dissolved Fe [mg/L] in NaCl Solution
1.0330 after 1 day	0.434	55.29	34.48	21.50
1.4541 after 5 days	18.500	0.899	0.094	0.003
1.4571 after 5 days	23.824	0.113	0.010	0.004
1.4841 after 5 days	25.673	0.048	0.003	0.001

Table 12

SAM layers developed on 1.4571 stainless steel and their spectra on phosphonic group vibration region (1300–700 cm⁻¹) after irradiation in inert atmosphere and in air.

Powder	Starting Layer	Irradiation in an Inert Atmosphere	Irradiation in Air	Assignments
1178 vs				νP=O (free)
1127 m, sh
		1006 vs c		νaPO3 (fully deprotonated)
1064 w	a			νs POH
	1077 vs b		1082 vs	νP=O –νs PO2 (out-of-phase coupling) d
1045 w				βa POH
993 s				w = CH2
938 s	932 m		983 m	νsPO2
908 s				νaPO2
		899 m c		νs PO3 (fully deprotonated)
	845 vw	821 vw	819 vw	νP=O +νs PO2 (in- phase coupling) e
789 s	785 s	b		γsPOH
			750 vw	γaPOH (single POH deformation)
718 m	705 vw	b

Remarks: ^a Overlap with strong band at 1077 cm⁻¹. ^b Fully deprotonated –P=O(OH)₂ group, triple-coordinated –PO₃ (see last row in Figure 1). ^c Averaged value. ^d This mode is similar to the antisymmetric PO₃ stretching of closely equalized P–O bonds. ^e This mode is similar to the symmetric PO₃ stretching of closely equalized P–O bonds.

Table 13

SAM layers developed on 1.4841 stainless steel and their spectra on the phosphonic group vibration region (1300–700 cm⁻¹) after irradiation in inert atmosphere and air.

Powder	Starting Layer	Irradiation in an Inert Atmosphere	Irradiation in Air	Assignments
1178 vs				νP=O (free)
1127 m, sh
	1132 vs	1120 s, sh		νP=O coordinated with Fe
1064 w	1089 s	a		βs POH
1045 w	1060 m, sh	a		βa POH
		1070 vs	1086 vs	νP=O –νsPO2 (out-of-plane coupling)
			~1007	βOH single P–OH) group
993 s	−	−	−	w = CH2
			968 m	νaPO2 (in double coordination)
938 s	943 vw			νsPO2
908 s	915 m	920 w, sh		νaPO2
	841 w b		835 vw	νsPO3 (νP=O + νsPO2, in-phase coupling)
	789 m		787 m	γPOH (in double coordination)
789 s	789 m	787 vw	787 m	γsPOH
718 m	(~735 vw)	718 w	(~723 vw)	γaPOH

Remarks: ^a Overlap with the strong band at 1070 cm⁻¹. ^b Fully deprotonated –P=O(OH)₂ group.

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