Electrochemical Analyses of Etchants Used to Detect Sensitization in Marine-Grade 5XXX Aluminum-Magnesium Alloys

The 5XXX series aluminum-magnesium alloys (AA5XXX) are susceptible to intergranular corrosion (IGC) in marine environments due to the preferential precipitation and dissolution of the β-phase (Al3Mg2) at grain boundaries. Detecting sensitization, or the presence and extent of grain boundary precipitation, contributes to the determination of IGC susceptibility and the prediction of intergranular stress corrosion cracking (IGSCC) propagation in IGC-susceptible materials. Etchants such as concentrated nitric acid, 40 vol% phosphoric acid, and acidified ammonium persulfate are used to detect sensitization in AA5XXX via selective attack of the β-phase. The objective of this work was to characterize the selectivity of these etchants by assessing the corrosion behaviors of the bulk alloy (AA5083) and pure β-phase during etchant exposure. Methods used to analyze corrosion behavior included open circuit potential (OCP) etching, 24-hour mass loss tests according to ASTM G67, and anodic polarization scans. Ammonium persulfate was found to exhibit the highest selectivity amongst the etchants over the range of sensitization conditions, surface orientations (LT, LS), and exposure temperatures considered in this study.