Corrosion Resistance of 3105 and 6110 Aluminium Alloy in Neutral Chloride- and Sulphate-Contaminated Artificial Seawater

Abstract

Corrosion resistance of 3105 and 6110 aluminium alloy (Al3105 and Al6110) in neutral chloride and sulphate-chloride was evaluated by potentiodynamic polarization, open-circuit potential measurement (OCP) and optical microscopy. Corrosion rate results of Al3105 and Al6110 in the sulphate-chloride solution were significantly higher than the values from the neutral chloride solution. Al6110 exhibited higher corrosion resistance at 0.5% and 1% NaCl concentration (0.097 mm/y and 0.128 mm/y) after which Al3105 was more corrosion resistant (0.269 mm/y to 0.295 mm/y) till 3.5% NaCl. In the acid chloride solution, Al6110 exhibited higher corrosion resistant (0.344 mm/y to 20.017 mm/y). Polarization plot of Al3105 in the neutral chloride solution showed significant cathodic reduction beyond 0.5% NaCl compared to Al6110 whose polarization plots appeared under activation control. Both alloys displayed substantial passivation on their anodic polarization plots. OCP plots for Al6110 display lower corrosion tendency than the plots obtained for Al3105. Plots for Al3105 and Al6110 at 0.5% NaCl and 3.5% NaCl shifted anodically before relative thermodynamic stability. The plots at 3.5% NaCl exhibited lower electropositive characteristics with significant active–passive behaviour. SO42− ions at 0.00625 M and 0.1 M concentration decreased the potentials of the OCP plots coupled with improved thermodynamic stability and decrease in active–passive shift. Corrosion pits were visible on Al3105 from neutral chloride solution compared to Al6110 which displayed general surface deterioration and a pitted surface. Addition of SO42− ion to the electrolyte enlarged the pits on Al3105, while the surface degradation on Al6110 increased with more corrosion pits.