Nickel-base alloys can suffer extensive transpassive corrosion when exposed to nearly neutral environments in pulp bleach plants. One consequence is that highly alloyed austenitic stainless steels, commonly welded with nickel-base fillers, will suffer preferential weld attack under these conditions. There are also reports showing the same phenomenon in desalination plants, although the corrosion rate has been extremely low without causing any damage. The transpassive corrosion phenomena on the high alloy stainless steels UNS
S31254 and UNS S32654 have been investigated and compared to the nickel-base alloys UNS N06625 and UNS N10276. The investigation was based on electrochemical measurements in chloride and nitrate solutions in the pH range 1 - 7. The investigation showed that an oxide layer with some protective capacity, i.e. a pseudo-passive layer, could form on the surface of the stainless steels in the transpassive range but not so on the Ni-base alloys. This pseudo-passivity may be effective in neutral and moderately acid solutions. The composition of the pseudo-passive oxide was found to be quite different from the
passive layer formed at lower potentials. A higher content of iron was measured and a certain amount of iron seems to be necessary to give the oxide its protective capacity.