RESISTANCE TO PITTING AND CHEMICAL-COMPOSITION OF PASSIVE FILMS OF A FE-17-PERCENT-CR ALLOY IN CHLORIDE-CONTAINING ACID-SOLUTION

The resistance of pitting of a Fe-17%Cr alloy in Cl- -containing solut ion (0.5M H2SO4 + 0.3M NaCl) and its relationship with the chemical co mposition of the passive film have been studied by electrochemical and x-ray photoelectron spectroscopy analyses. Aging of the passive film in the acid solution without or with Cl- (at a potential below the pit ting potential) improves the resistance to pitting. The passive films formed in 0.5M H2SO4 (at +500 mV/SHE) have a bilayer structure (hydrox ide and oxide) already well developed after 5 min of passivation. The thickness of the passive film (approximately 26 to 28 angstrom) remain s constant during polarization. The average cation fraction of chromiu m in the oxide increases with increasing polarization time (from 35% a fter 5 min of polarization to 80% after 24 h of polarization). The ben eficial effect of aging on the resistance to pitting is attributed to the enhancement of the chromium oxide enrichment at the film/alloy int erface. The passive films exposed to Cl- or formed in Cl- -containing solution (0.3 M NaCl) have similar composition and thickness as the fi lms formed in Cl- free solution, but they contain Cl-. The Cl- ions ar e located mainly in the hydroxide layer (approximately 0.016 mol . cm- 3). A very small amount of Cl- is found in the inner oxide layer.