Corrosion protection of anodized aluminum alloys and aluminum metal matrix composites and stainless steels

Boric-sulfuric acid anodized (BSAA) Al alloys Al 2024, Al 6061 and Al 7075 were sealed in various cerium and yttrium salt solutions in a search for alternative sealing methods to replace toxic chromate sealing. Electrochemical impedance spectroscopy (EIS) was used to evaluate the effectiveness of the different sealing methods. Several cerium salt and yttrium salt sealing processes produced BSAA Al alloys with similar corrosion resistance as that produced by chromate sealing.; Considerable corrosion mainly in the form of pitting occurred for a Al 6092/SiC{dollar}sb{lcub}rm p{rcub}{dollar} metal matrix composite (MMC) exposed to 0.5N NaCl. Various corrosion protection methods such as the Ce-Mo process, sulfuric acid anodizing with hot water sealing, cerium nitrate sealing and dichromate sealing were applied to the MMC in an attempt to improve the corrosion resistance. The formation of continuous barrier layer and porous oxide layer produced by anodizing is hindered by the presence of large insulating SiC particulates. The reinforcement introduces additional interfaces, discontinuities and flaws in the MMC making the common corrosion protection methods for Al alloys less effective.; Different cerium treatments were applied to stainless steels type SS 316 and SS 304 to improve the localized corrosion resistance. The different cerium treatments affected the anodic behavior to different extents. The passive current density was reduced possibly due to an increase of Cr/Fe ratio and/or increase in the passive film thickness. The pitting potential increased due to the removal of MnS inclusions which act as pit nucleation sites. However, no inhibition of the cathodic oxygen reduction as published in the literature was found.; Scanning Kelvin probe analysis (SKPA) was performed to study localized corrosion phenomena by measuring the potential distribution in the classic Evans drop experiment and in similar drop experiments. Drops of sodium chloride solution with different concentrations with and without inhibitor were applied on galvanized steel, Al 2024 and SS 316 to evaluate the influences of Cr and inhibitor on localized corrosion. The potential distribution measured by SKPA showed local anode and cathode due to differential aeration cell and demonstrated the pit initiation effect of Cl{dollar}sp-{dollar} and the inhibition effect of Cr{dollar}sp{lcub}6+{rcub}{dollar}.