Anomalous dissolution of metals and chemical corrosion

An overview is given of the anomalous behavior of some metals, in particular Fe and Cr, in acidic aqueous solutions during anodic dissolution. The anomaly is recog- nizable by the fact that during anodic dissolution more material dissolves than would be expected from the Faraday law with the use of the expected valence of the formed ions. Mechanical disintegration, gas bubble blocking, hydrogen embrittlement, passive layer cracking and other possible reasons for such behavior have been discussed. It was shown, as suggested by Kolotyrkin and coworkers, that the reason can be, also, the chemical reaction in which H2O molecules with the metal form metal ions and gaseous H2 in a potential independent process. It occurs simultaneously with the electrochemical corrosion process, but the electrochemical process controls the corrosion potential. On the example of Cr in acid solution it was shown that the reason for the anomalous behav- ior is dominantly chemical dissolution, which is considerably faster than the electro- chemical corrosion, and that the increasing temperature favors chemical reaction, while the other possible reasons for the anomalous behavior are of negligible effect. This ef- fect is much smaller in the case of Fe, but exists. The possible role of the chemical disso- lution reacton and hydrogen evolution during pitting of steels and Al and stress corro- sion cracking or corrosion fatigue are discussed.