AGAINST SULFURIC ACID ATTACK OF PASSIVE FILM FORMED ON SUS 304 STAINLESS STEEL IN FLUORIDE SOLUTIONS

摘要

Electrochemical conditions for improving the resistance to active corrosion of SUS 304 stainless steel using potentiostatic pretreatment in a total fluoride concentration of .kmollm*. m~-3 solution with various pH at 303K were studied. Active corrosion susceptibility after treatment was evaluated by measuring the self-activation time,τa, in deaerated 1.0 kmol lm~-3 sulfuric acid solution at 303K. In an acidic fluoride solution, maximum stability is obtained at -0.4V(SCE) of pretreatment potential in a solution pH of 1.8.XPS analysis of treated steels showed the Cr content extensively increased in the oxide layer of passive film improving resistance to active corrosion, and F ions located in the hydroxide layer, assuming that passive film consists of three layers, hydrocarbon contaminant covering a hydroxide layer of Cr and an underlying Cr and Fe oxide layer. In a neutral solution, maximum stability is obtained at 0.6V(SCE) of pretreatment potential in a solution pH of 8.6.Cr enrichment was not seen in the passive film by SPS. A nodic polarization measurements suggest that structural changes in the passive film take place at 0.6V(SCE).In an　alkaline solution, however , treated steel can not acquire a high resistance to active corrosion over a wide range of potentials for pretreatment. XPS showed that the hydroxide layer of passive film was very thin compared to that formed in an acidic solution and F ions were entirely absent in the passive film.