Corrosion of Aluminized Steel in Aggressive Natural Water

Aluminized steel Type 2 (AST2) has a similar to 50 mu m thick coating with outer and inner layers of mostly Al and Fe-Al intermetallic respectively. AST2 is used for culvert and drainage pipes that are required to have long service life. The use of calcium carbonate-rich backfill (limestone) found to be a key contributing factor to the observed early deterioration. Limestone contributed to form a high pH environment sufficient to cause passivity breakdown of the passive film on aluminum. AST2 specimens were exposed in limestone-water cells with water being continuously replenished at a slow rate. A high steady state pH developed that was aggressive to aluminum passive film. Electrochemical measurements and direct inspection indicated onset of severe early corrosion with consequent fast wastage of the outer film. Corrosion rate decreased later as a thick corrosion product layer covered the surface and formed a transport limitation barrier.