Effects of N on the passivity and $Cr_2N$ formation of high nitrogen stainless steels and their effects on localized corrosion

Abstract

High nitrogen stainless steels (HNSs) have been developed to replace the expensive conventional austenite stainless steels (γ-SS) containing high Ni content of 8~10 wt. %. Alloyed N improves significantly the resistance to corrosion of HNS, therefore, effects of N on the composition, structure and electronic properties of passive film formed on HNS were investigated in order to clarify a mechanistic role of N in an improvement of the resistance to general and localized corrosion of HNS. Since breakdown of the passive film due to Cl&#61485; adsorption can be controlled by solution pH, as a prerequisite study for the investigation of passive film, the effects of pH on the physico-electronic properties of the passive film and adsorption of Cl&#61485; on the film on Fe were examined to determine critical solution pH at which the Cl&#61485; adsorption was inhibited and hence the most protective passive film was formed. Further, because HNS has a problem of forming of Cr-nitride when aged at 500~1,050 ℃, the influences of N content on the precipitation behavior of Cr2N with its relevant influences on the resistance to localized corrosion of HNS were examined. Passive metal forms an interfacial diffuse layer on the surface of passive film by its reaction with H+ or OH&#61485; ions in solution and so the dissolution reaction of the passive film is determined depending on solution pH. There is a critical pH, called pH point of iso-selectivity (pHpis) at which the ion-selective nature of the film/solution interfacial diffuse layer is changed from the anion-permeable at pH<pHpis to the cation-permeable at pH>pHpis. Therefore, the pHpis for a passivated Fe was determined by examining the effects of pH on the thickness of passive film and on the dissolution reaction occurring on the passive film at steady-state in borate-phosphate buffer solutions at various pH of 7~11. The steady-state thickness of passive film formed on Fe showed the maximum at pH 8.5~9, and further the ...