| Essays on Aggregate Fluctuations with Micro Heterogeneity 5XXX series Al-Mg alloys have been used for marine applications due to their low density, good formability, weldability, and, most notably, resistance to corrosion. However, they can be susceptible to intergranular corrosion (IGC) attack and intergranular stress corrosion cracking (IGSCC). This is due to a highly anodic beta phase (Mg2Al3) relative to the matrix that forms along the grain boundary and decreases corrosion resistance of 5XXX aluminum alloys. The purpose of this study is to develop the proper method to test corrosion inhibition on Al-Mg alloys and investigate the efficiency of inhibitors on sensitized alloys to prevent IGC and IGSCC.;During the study of the electrochemical behavior of sensitized AA5083, another issue arose. It became clear that an altered surface layer (ASL) with a recrystallized microstructure having no beta phase at the grain boundaries forms on sensitized AA5083 during polishing, which prevents the exposure of subsurface beta phase on an as-polished surface. Selective dissolution of beta phase occurred when it was exposed on the surface as the result of either etching in alkaline solution or cathodic polarization, which caused an increase in the local pH near intermetallic particles. The different corrosion behavior of solutionized and sensitized AA5083 was examined by testing etched samples. Therefore, removal of the ASL is necessary to understand the influence of inhibitors on sensitized AA5083.;The effectiveness of corrosion inhibitors was studied on sensitized and solutionized AA5083 with and without the ASL in 3.5 wt% NaCl solution containing potassium chromate (K2CrO4), sodium silicate (Na 2SiO3), or sodium vanadate (NaVO3). Examination of the materials by polarization experiments demonstrated that the ASL was active on solutionized samples and protective on sensitized samples. All inhibitors exhibited a suppression of the cathodic reaction, with chromate also exhibiting anodic inhibition on sensitized material without the ASL as the result of an increase in the pitting potential. X-ray Photoelectron Spectroscopy (XPS) was used to determine the concentration and oxidation state of species on the surface of AA5083 samples with and without an ASL after 7 days immersion in 3.5 wt% NaCl with and without inhibitors.;The stress corrosion cracking (SCC) behavior of sensitized AA5083 also was studied by slow strain rate testing in 3.5 wt% NaCl solutions with and without K2CrO4, Na2SiO3, and NaVO 3 additions. Sensitized AA5083 was susceptible to IGSCC at the open circuit potential (OCP) in solution with no inhibitor. However, it became immune to IGSCC when the added inhibitors shifted the OCP to a potential below the breakdown potential (Eb) of the beta phase. Chromate and vanadate inhibited IGSCC in this fashion whereas silicate did not. Sensitized AA5083 samples exhibited severe IGSCC when polarized above the Eb of the beta phase during testing, even in the presence of chromate. When the samples were polarized below the Eb of the beta phase, ductility was fully recovered with no evidence of IGSCC. A high rate of beta phase dissolution, which is essential for IGSCC, can be prevented by shifting the OCP below the Eb of the beta phase. |
