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Electron beam welding is used for high precision welds mainly in aerospace industries. It is a type of fusion welding process in which a high beam of electrons hits the metals, produce heat and melts the base metals and then solidifies to form a weldment which is generally stronger than the individual base metals. This paper presents the corrosion testing of Electron beam welded pure copper and stainless steel 304 weldment. Tafel polarization technique is used to find the electrochemical corrosion behaviour of electron beam welded copper and SS304 dissimilar metal joints in NaCl saturated solution. The experiment was carried out using a CHI electrochemical workstation with a three-electrode setup where the welded specimen act as the working electrode. Tafel plots were generated to analyze the corrosion characteristics. An active-passive transition is found that indicates the formation of a protective oxide layer. The corrosion potential (Ecorr) and corrosion current density (Icorr) of the weldments proves that the weldment has superior corrosion resistance when compared with the base metals. A significant increase in oxygen content and decrease in Fe, Cr, Ni and Cu concentrations are noted in the energy dispersive X-ray analysis (EDX) which indicates the metal degradation due to electrochemical reactions. Small corrosion pits are observed during the microstructural analysis proves the presence of pitting corrosion. However, the low Icorr values of the weldment is very low when compared with pure copper and SS304 which shows the enhanced corrosion resistance in the weldment due to microstructural refinement and alloying effects in the fusion zone. These findings prove that Cu-SS304 dissimilar weld using EBW is suitable for aerospace applications.
INTRODUCTION
Dissimilar metal joints are a challenging process in various industries particularly in aerospace industry as it demands high precision defect free welds. The combination of different metals with different properties enhances corrosion resistance and mechanical properties but the electrochemical interactions between the joined metals can introduce challenges such as pitting corrosion to optimize the performance of the welded joints [1].
Electron beam welding is a suitable method for joining dissimilar metals due to its high precision, minimum heat affected zone and deep penetration capability. Studies have proved that welding parameters highly influence microstructural stability and corrosion resistance particularly in stainless steel [2]. Selection of welding parameters plays an important role in high precision welds with corrosion resistance in the weldment [3].
The corrosion behaviour of stainless steel and copper-based weldments have been studied in detail highlighting the formation of passive oxide layer and localized corrosion behaviour [4].
Stainless steels exhibit complex corrosion characteristics due to their dual phase microstructure. The corrosion resistance of these materials can be optimized by regulating the welding parameters [5]. The response of stainless steel 304 to the electrochemical environments such as NaCl solutions play a crucial role in ensuring long term durability in marine and industrial applications [6].
Microstructural modifications in the weldment can affect the electrochemical corrosion resistance and this can be enhanced by the application of protective coatings by inhibiting pitting corrosion in adverse environments [7]. The presence of residual stress and microstructural heterogeneities in dissimilar metal joints also influence their corrosion behaviour.
Advanced welding techniques such as ultrasonic vibration assisted welding and laser welding are also investigated towards corrosion resistance [9]. Ultrasonic welding has been reported for localized corrosion resistance by refining the weld structure and reducing residual stresses, whereas laser welding techniques have shown good results in modifying surface topography and enhancing corrosion resistive behaviour of stainless-steel weldment [10, 11].
The role of post weld heat treatment and its effect towards corrosion resistance has also been studied. It is evident that the laser surface alloying and diode laser surface modification can enhance the wear and corrosion resistance of the weldment [12]. Additionally, computational analysis of the ultrasonic effects on welding processes has given few insights in optimizing welding parameters for enhanced corrosion resistance [13].
In this paper, the electrochemical corrosion behaviour of electron beam welded Cu-SS304 dissimilar metal joints in NaCl-saturated solutions is studied. The study focuses to evaluate the influence of microstructural modifications on the corrosion resistance of the weldment providing understanding of mechanisms governing localized corrosion, passive film stability and potential for galvanic interactions. The findings will contribute to an optimized process parameter for dissimilar metal joints used in corrosive environments ensuring durability and structural reliability [14, 15].
EXPERIMENTAL
Specimen Preparation
Copper and stainless steel 304 plates are machined to size 100 × 60 × 6 mm and weld coupons are made using EBW of size 100 × 120 × 6 mm as shown in Fig. 1. Input parameters such as beam current 30 mA, voltage 60 KV, weld speed 8 mm/s and work distance 260 mm are finalized for this weld after various trials. Since electron beam welding is done inside a vacuum chamber the possibility of oxide formation during welding is very limited. Great care has been taken to pre clean the materials to ensure that both metals are free from dust and oxides. Initially both copper and SS 304 are cleaned with trichloroethylene. Then the materials are soaked for about 10 s in a tank containing a solution prepared with nitric acid and hydro fluoric acid diluted with distilled water. Finally, the materials are rinsed with distilled water and dried in hot air and subjected to electron beam welding with zero fit up gap and precise beam alignment. Two operations such as seal and cosmetic are carried out and finally a high-quality weld is produced. Post weld cleaning is executed by means of electrolytic etching by means of 10% H2SO4 at 6 V and 17°C for about 5 min. The cleaned specimen is inspected visually and found to be a perfect weld.
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Fig. 1.
Copper-SS304 welded joint using EBW.
Electrochemical Test
Tafel polarization curves was recorded using CHI electrochemical workstation Model CHI 604D as shown in Fig. 2. A cell containing three compartments are used for electrode. The working polished welded specimen with exposing the weldment was immersed in NaCl solution. Platinum wire is used as the counter electrode and Ag/AgCl electrode were used as the reference electrode respectively. All electrochemical measurements were studied at room temperature at 2.5% NaCl Solution in stationary condition.
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Fig. 2.
CHI electrochemical workstation.
The experiment is conducted with an initial potential of –5 V and then linearly increased to a final potential of 2.5 V at a scan rate of 0.01 V/s that confirms a controlled potential change. This process contains a single segment with no holding time at the final potential and a holding time of 2 s before the initiation that makes the system stable. This technique is used to generate the Tafel slopes for Cu-SS304 weldment which provides reaction mechanisms, charge transfer coefficients and corrosion rate of the weldment.
EDX Analysis
Energy dispersive X-ray spectroscopy (EDX) is used to analyze the composition of elements in the Electron Beam welded Cu-SS304 dissimilar metal joints before and after electrochemical corrosion testing. This test is used to investigate the distribution of alloying elements across the weldment and to detect the degradation of metals induced by the corrosion process after electrochemical corrosion testing. This analysis helps in understanding the degradation of metals, passive layer formation and its stability contributing to the optimization of welding parameters for improved corrosion resistance specifically for aerospace applications.
Microstructural Characterization
Microstructural characterization of the weldments is completed using scanning electron microscope (SEM). The specimen before and after corrosion test are subjected to electrolytic etching and then polished using diamond paste and finally cleaned using selvet cloth. Microstructures are taken to analyse the pores in the weldment before and after the corrosion test.
RESULTS AND DISCUSSION
Electrochemical Corrosion Behaviour
The corrosion characteristics of the weldment of Copper and SS304 can be analysed using the Tafel plot as shown in Fig. 3. This graph shows an active-passive transition which indicates the presence of a protective oxide layer that temporarily reduces the corrosion rate. The corrosion potential (Ecorr) is located around a negative value that signifies the weakness to corrosion. The corrosion current density (Icorr) suggests the rate at which the material degrades in a high corrosive environment. A sharp dip of current at the lower potential region indicates the active corrosion in that region whereas the relatively stable passive region at higher potentials suggests temporary resistance before the possible breakdown.
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Fig. 3.
Tafel plot of corrosion test.
The observed corrosion behaviour suggests that the Cu-SS304 weldment exhibits high corrosion resistance. The presence of a separate breakdown region where the current increases sharply, indicates the potential for localized corrosion such as pitting corrosion. The electrochemical differences between copper and SS304 could also lead to galvanic effects in the weld interface between Cu and SS304 reduces the degradation of the weldment.
From the Tafel plot it is evident that the corrosion resistance of the electron beam weldment of Cu-SS304 is very high when compared with that of the individual base metals. The corrosion current density (Icorr) exhibited by the weldment is very low compared to pure copper and SS304 which proves a low corrosion rate. Also, the presence of the passive region is an indicative of the presence of s protective oxide layer which enhances the resistance to corrosion. Pure copper is more likely subjected to uniform corrosion because of the high Icorr value whereas SS 304 resists corrosion to some extent unless it is exposed to aggressive environments. But the weldment shows an improved resistive performance that can be attributed to the formation of a better microstructure and with better alloying effects at the fusion zone. This proves that the weldment possesses benefits from the combined electrochemical properties of both metals resulting in greater corrosion resistance suitable for aerospace applications.
EDX Analysis
The weldment before corrosion test and after corrosion test are subjected to EDX analysis and the results of the tests with alloying elements present are recorded in Tables 1 and 2 and the corresponding peaks are shown in Figs. 4 and 5.
Table 1. . Alloying elements in the weldment before corrosion test
Elements | Appl. conc. | Corr. intensity | Wt % | Wt % | At % |
|---|---|---|---|---|---|
O | 9.46 | 1.2892 | 2.50 | 0.70 | 8.51 |
Si | 0.80 | 0.5761 | 0.48 | 0.11 | 0.92 |
Cr | 36.15 | 1.0977 | 11.23 | 0.21 | 11.75 |
Mn | 1.91 | 1.0130 | 0.64 | 0.15 | 0.64 |
Fe | 126.30 | 1.0277 | 41.90 | 0.47 | 40.82 |
Ni | 12.77 | 0.9441 | 4.61 | 0.23 | 4.27 |
Cu | 103.56 | 0.9138 | 38.64 | 0.48 | 33.09 |
Table 2. . Alloying elements in the weldment after corrosion test
Elements | Appl. conc. | Corr. intensity | Wt % | Wt % | At % |
|---|---|---|---|---|---|
O | 22.00 | 1.2832 | 18.00 | 0.85 | 38.00 |
Al | 1.16 | 0.4593 | 2.01 | 0.21 | 3.24 |
Si | 0.89 | 0.5768 | 1.22 | 0.17 | 1.90 |
Cr | 6.50 | 1.0590 | 4.50 | 0.20 | 4.00 |
Fe | 25.00 | 1.0233 | 16.00 | 0.45 | 14.00 |
Ni | 2.00 | 0.9408 | 1.50 | 0.25 | 1.30 |
Cu | 40.00 | 0.9063 | 35.00 | 0.65 | 28.00 |
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Fig. 4.
EDX spectrum before corrosion test.
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Fig. 5.
EDX spectrum after corrosion test.
The EDX results provide a comparative study of the element composition of the electron beam welded Cu-SS304 dissimilar metal joints before and after corrosion test. The weldment shows a balanced distribution of alloying elements before the corrosion test. It is noted that the presence of oxygen is minimal indicating limited oxidation at the initial stage. The composition suggests a stable microstructure with contributions from both Cu and SS304 ensuring corrosion resistance.
After the corrosion test few changes are noted in the element composition where the effects of electrochemical interactions in the NaCl saturated environment. A significant increase in oxygen content from 2.50 to 18 wt % is noted which shows oxidation and the formation of corrosive products at the weldment. The reduction in Cr content from 11.23 to 4.50 wt % shows the degradation of the passive layer making the joint subjected to localized corrosion. Similarly, the decrease in Fe from 41.90 to 16 wt % and Ni from 4.61 to 1.50 wt % shows the material loss due to corrosion. A decrease in Cu content from 38.64 to 35 wt % shows the material dissolution likely due to galvanic interactions between Cu and SS304 in the chloride medium.
These results confirm that the electrochemical corrosion in NaCl saturated solution affects the elemental integrity of the weldment. The increase in oxygen content and material loss indicates pitting corrosion occurs in the weldment which affects the elemental integrity of the weldment. However, the combination of Cu and SS304 in a welded joint exhibits a balanced corrosion resistance when compared with the corrosive resistance behaviour of the individual base metals that makes the weld to be more suitable for aerospace applications.
The EDX results provide a comparative study of the element composition of the electron beam welded Cu-SS304 dissimilar metal joints before and after corrosion test. The weldment shows a balanced distribution of alloying elements before the corrosion test. It is noted that the presence of oxygen is minimal indicating limited oxidation at the initial stage. The composition suggests a stable microstructure with contributions from both Cu and SS304 ensuring corrosion resistance.
After the corrosion test few changes are noted in the element composition where the effects of electrochemical interactions in the NaCl saturated environment. A significant increase in oxygen content from 2.50 to 18 wt % is noted which shows oxidation and the formation of corrosive products at the weldment. The reduction in Cr content from 11.23% by weight to 4.50% by weight shows the degradation of the passive layer making the joint subjected to localized corrosion. Similarly, the decrease in Fe from 41.90 to 16 wt % and Ni from 4.4.61 to 1.50 wt % shows the material loss due to corrosion. A decrease in Cu content from 38.64 to 35 wt % shows the material dissolution likely due to galvanic interactions between Cu and SS304 in the chloride medium
These results confirm that the electrochemical corrosion in NaCl saturated solution affects the elemental integrity of the weldment. The increase in oxygen content and material loss indicates pitting corrosion occurs in the weldment which affects the elemental integrity of the weldment. However, the combination of Cu and SS304 in a welded joint exhibits a balanced corrosion resistance when compared with the corrosive resistance behaviour of the individual base metals that makes the weld to be more suitable for aerospace applications.
SEM Analysis
The SEM images of the Cu-SS304 weldments before and after the corrosion tests are shown in Figs. 5 and 6. The SEM image taken before the corrosion test shows a uniform microstructure with a well-defined fusion zone between Cu and SS304. The surface of the Cu-SS304 weldment is smooth and has no visible pores or metal degradation indicating the integrity of the weldment before exposed to the corrosive medium.
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Fig. 6.
(a) SEM image of the weldment before corrosion test; (b) SEM image of the weldment after corrosion test.
The SEM image taken after the corrosion test shows certain significant changes in the fusion zone and the Cu region. The formation of corrosion pits in the Fusion zone proves the localized corrosion due to electrochemical interactions between the dissimilar metals. The increased surface roughness and the pitting in the fusion zone indicates the presence of passive oxide layer due to the presence of NaCl solution. Even though pitting corrosion is found in the weldment after corrosion test, the structural integrity of the weldment remains undamaged with only a minimum material loss. This indicates that the Cu-SS304 weldment provides better corrosion resistance when compared with Cu alone which is one of the base metals and exhibits enhanced durability making it more suitable for aerospace applications.
CONCLUSIONS
The electrochemical corrosion behaviour of the Electron Beam welded Cu-SS304 dissimilar metal joints exposed to NaCl saturated solution is analyzed using Tafel polarization, EDX and SEM analysis. The results shows that the weldment shows an active-passive transition which confirms the formation of a protective oxide layer and enhance the corrosion resistivity. The corrosion potential (Ecorr) and corrosion current density (Icorr) values of the weldments are found to be better than that of the corresponding values of the base metals. The EDX analysis shows significant elemental changes after the corrosion test with increased oxygen content and reduced Cr, Fe, NI and Cu concentrations that shows metal degradation due to electrochemical interactions. The SEM analysis confirms the presence of pitting corrosion in the fusion zone, but still the structural integrity of the weldment is not damaged. This suggests that even localized corrosion occurs, the weldment provides good corrosion resistance compared with the base metals and making this weld to be suitable for long term applications more particularly in aerospace applications.
FUNDING
This work was supported by ongoing institutional funding. No additional grants to carry out or direct this particular research were obtained.
CONFLICT OF INTEREST
The authors of this work declare that they have no conflicts of interest.
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