抄録
Formation of corrosion tubercles on carbon steels were carried out in the aerated neutral solutions composed of boric-borate buffer (0.01M) and water with some suspensions of CaCO3 and CaSiO3 powders at pH=7.5-8.8. The solutions containing NO2-, CrO42-, or HS- of less than 50ppm in concentration were also examined. The solution temperature was at 60-65°C.
The tubercles thus formed were classfied into three types: the active passive, the general corrosion, and the sulfide initiative, depending on the potential. The active passive-type tubercles formed at the potential range where the second peak of current appeard on the anodic polarization curve. On the other hand, the general corrosion-type tubercles grew at the potentials where general corrosion of steels tested appeard. In the latter case, calcium scale deposited on the steel surface, but the corrosion spots formed partially at which the scale deposition was insufficient.
The mechanism for the sulfide initiative-type tubercles is unclear whereas it forms certainly in aerated and neutral solutions containing HS- at the potential range -0.50 to -0.55V vs. Ag/AgCl.
