Initiation and Repassivation of Pits on Austenitic Stainless Steel
Doctoral thesis, 2000
Stainless steels can be damaged by pitting corrosion in presence of aggressive anions, such as halide ions. The aim of this work was to study different aspects on initiation and repassivation of pits on austenitic stainless steel in chloride solutions at ambient temperature. The role of the alloying elements molybdenum and nitrogen in improving the corrosion properties as well as the influence of the solution composition were studied. A review of Mo-Cl salts and co-ordination complexes in hydrochloric acid solutions is given. Moreover, the properties of molybdenum in the apparent passive region were investigated. The techniques applied were mainly electrochemical methods and surface analysis by angular-dependent X-ray photoelectron spectroscopy (XPS).
Three high alloyed steels with a systematic variation in composition (Fe20Cr20Ni, Fe20Cr20Ni6Mo, and Fe20Cr20Ni6Mo0.2N) were polarised in electrolytes with varying acidity and Cl- concentration. Chloride ions are incorporated throughout the 10-25 Å thick passive film and the Cl- content increases with the acidity of the solution. It was found that alloying with Mo decreases the amount of Cl- in the passive film and it is concluded that this is due to formation of soluble Mo-oxochloro complexes. The pitting corrosion resistance of Mo alloyed stainless steels is enhanced by the decreased Cl- content in the film. Insoluble .BETA.-MoCl2 was detected in the passive film and it was suggested that the formation of Mo-chlorides with low solubility contributes to the stability of the passive film. Electrochemical results showed that alloying with N improves the repassivation properties of stainless steels. The OH- content of the passive film increases with the acidity, suggesting that the deprotonation of the initially formed hydroxide film is hindered by a low pH. A 304 stainless steel (Fe18Cr9Ni) was studied in neutral Cl- solutions with and without phosphate, respectively. It was concluded that phosphate enhances the repassivation of initiated pits.
Molybdenum was polarised in 1 M HCl. A three dimensional passive film is not formed at potentials below the transpassive dissolution. In this apparent passive region, anodic current is only observed if the sample has first been polarised at low potentials. It is suggested that hydrogen diffuses into the molybdenum metal at cathodic potentials and is oxidised upon positive polarisation.
austenitic stainless steel