Corrosion Behavior Of New Aluminum Alloy And The Effect Of Surface Modification

Several new aluminum alloys are developed to meet the requirements of aviation industry. These include 2E12 aluminum alloy with excellent fatigue property, 7475 aluminum alloy with high strength and high toughness, and 7A85 aluminum alloy with good comprehensive performance and low quenching sensitivity. These aluminum alloys would inevitably suffer the damage caused by localized corrosion or corrosion under stress in their service, so that the safety, reliability and service life of aircrafts are weakened. The corrosion behavior and corrosion mechanism of these alloys are not clear, and effective protection methods need to be developed. Consequently, it is necessary to investigate on the corrosion mechanisms of these newly developed aluminum alloys and corresponding surface treatment technologies. Shot peening（SP） is an important method to improve the fatigue resistance of aluminum alloys, but there are few published studies on the influence of SP on the localized corrosion behavior of aluminum alloys. Micro arc oxidation（MAO） treatment is considered to be superior to the traditional anodic oxidation technology. However, studies on the effect of MAO treatment on the localized corrosion, especially those on the corrosion fatigue behavior of aluminum alloys are inadequate, and the inner mechanism is still unclear. As is mentioned above, the intergranular corrosion（IGC）, exfoliation corrosion（EXCO）, stress corrosion cracking（SCC） and corrosion fatigue（CF） behavior, as well as the effect of SP, MAO and the combined treatment on the corrosion behavior of these newly developed aluminum alloys were studied in this paper aimed at revealing the corrosion mechanism and improving their service performance. The main results are as follows:（1） It was found that 2E12-T3 and 7A85-T7452 Al alloy were both very sensitive to IGC, and the IGC sensitivity of 2E12-T3 Al alloy was higher; 2E12-T3 Al alloy also has a higher EXCO sensitivity, while that of 7A85-T7452 Al alloy was relatively low. The continually distributed cathodic θ phase（CuAl₂） along the grainboundary accelerated the dissolution of nearby copper depleted zone, which accounted for the IGC mechanism of 2E12-T3 Al alloy. The IGC mechanism for 7A85-T7452 Al alloy was that the preferential dissolution of continually distributed anodic η phase（MgZn₂） led to the selective corrosion along the grainboundary. The 2E12-T3 Al alloy rolling sheet had highly textured microstructure, serious IGC sensitivity, as well as low strength, so it was very sensitive to EXCO. The forged 7A85-T7452 Al alloy sheet had slightly textured microstructure, relatively low IGC sensitivity, and high strength, so it had a slight EXCO sensitivity. 2E12-T3 Al alloy was sensitive to SCC for its high IGC sensitivity, and its SCC mechanism was dominated by anodic dissolution.（2） Pre-corrosion in 3% NaCl+0.5% H₂O₂ solution caused deterioration of mechanical properties of 7475-T761 Al alloy, the elongation reduced more seriously than strength. Corrosion and corrosion induced hydrogen embrittlement both played a part in the mechanical properties deterioration, and the development of pitting corrosion might be a major cause. Tensile stresses promoted the deterioration by accelerating both the corrosion and the hydrogen embrittlement process.（3） 7A85-T7452 Al alloy was not sensitive to CF when the maximum cyclic stress was 60% higher than its yield strength. This was because that the detrimental effect of aggressive solution was not noticeable when exposure time was short; when the maximum cyclic stress reduces to 38% of its yield strength, the CF sensitivity became significant for its high IGC sensitivity and the combined cyclic stress. The CF mechanism for the 7A85-T7452 Al alloy could be described as: the synergetic effect of cyclic stress and IGC promotes the fatigue crack initiation, then the synergetic effect of cyclic stress and hydrogen embrittlement accelerates the fatigue crack early propagation.（4） SP treatment at a reasonable intensity and coverage significantly reduced the IGC, EXCO and SCC sensitivity of 2E12-T3 Al alloy. This was because that the favorable factors such as compressive residual stresses and structure refinement played the leading role. However, SP treatment at an excessive high intensity or coverage will damage the surface integrity of 2E12-T3 Al alloy, and thus the unfavorable factors such as surface roughness and surface damage played the leading role and the IGC, EXCO as well as SCC were accelerated.（5） SP treatment significantly improved the IGC resistance and the CF performance of 7A85-T7452 Al alloy, while the effect on CF resistance was not monotonic with the SP intensity. This was because that the surface compressive residual stress induced by SP was the main favorable factor to improve corrosion fatigue behavior, while surface roughness and surface damage were factors of second importance.（6） The corrosion resistance of 7A85-T7452 Al alloy in neutral salt fog and 3.5% NaCl aqueous solution was significantly improved by MAO treatment. However, MAO coating failed to protect 7A85-T7452 Al alloy from corrosion in acidic NaCl solution. The corrosion resistance of MAO coating in acidic NaCl solution was improved by Ce（NO₃）₃ sealing, K₂C_r2O₇ sealing and SiO₂ sol-gel sealing treatment. The improvements from high to low were: SiO₂ sol-gel sealing > K₂Cr₂O₇ sealing > Ce（NO₃）₃ sealing. Only the SiO₂ sol-gel sealed MAO coating protected the base material from corrosion. This was attributed to the good barrier function of the SiO₂ gel layer.（7） MAO treatment resulted in the decrease of CF resistance of 7A85-T7452 Al alloy in 3.5% NaCl aqueous solution, This could be explained as follows: the fatigue resistance of MAO treated samples was reduced for the low toughness, high porosity, as well as poor coordinated deformation capacity of the MAO coating; cyclic stress easily introduced cracks to the MAO coating, then occluded corrosion cell and the “big cathode（MAO coating）-small anode（exposed aluminum alloy）” galvanic corrosion was formed at some locations, by coupling with the cyclic stress, the initiation and early growth of the CF crack was promoted and the CF life was reduced. The combined treatment involved SP and subsequence MAO（SP+MAO） can prolong the CF life of 7A85-T7452 Al alloy to more than that of the base material. This was because that SP induced a layer of compressive residual stress, as well as the structure refinement that can inhibit the initiation and early growth of the CF crack. The combined treatment including SP, polishing and MAO in sequence could further improve the CF resistance. The reason was that the polish treatment can relieve the equivalent notch effect induced by excessive ingrowth of MAO coating coupled with peening profile.（8） The CF resistance of MAO treated 7A85-T7452 Al alloy samples was significantly improved by acrylic anodic electrophoretic paint（ACC）. The CF life of the ACC sealed MAO samples was raised to 254% of that of the MAO treated samples, while exceeded 2 times of that of the base materials. This could be explained as: acrylic anodic electrophoretic paint had the ability to deposit on the defects of the MAO coating, and the deposited ACC layer with high toughness improved the resistance MAO coating to fatigue cracking. However, The CF resistance of MAO treated samples was dramatically reduced by SiO₂ sol-gel sealing treatment. It was attributed to the poor toughness of the SiO₂ gellayer.