ELECTRO CHEMICAL BEHAVIOR OF PASSIVE FILMS ON MOLYBDENUM-CONTAINING STAINLESS STEELS IN AQUEOUS SOLUTIONS

Abstract

Austenitic stainless steels containing both chromium and nickel have good corrosion resistance. Experience has shown that small addition of molybdenum improves the pitting resistance of the material. The present study aims to examine the nature, properties, characteristics and composition of the surface films, as well as, to evaluate the corrosion resistance properties of high molybdenum stainless steels in aqueous solutions and under different experimental conditions.

The Thesis includes three main chapters: The first chapter includes the introduction involving the literature review. The second chapter involves the experimental techniques used, chemicals, cell and instruments. The third chapter involves three sections.

Section A describes the open-circuit transients behavior of Me-containing stainless steels in aqueous solutions. It was found that in naturally aerated and N2-deaerated buffer solutions as well as, chloride and sulphate solutions, the open-circuit transients measured for the different steels are similar where the potential changing towards more noble values after immersion indicating film formation and thickening. Also the steady state potential Ess rises with

increasing of Mo % indicating that Mo enhances the thickening of the passive films of the steels.

Section B involves the polarization behavior of Me-containing stainless steels in aqueous solutions. The corrosion current Icorr values were strongly dependent on the Mo content at the same pH and decreases with pH for the same electrode. In general Icorr values are lower in N2-deaerated buffer solutions than those in naturally aerated buffer solutions. The corrosion current also decreases with increase in Mo content at the same concentration in both NaCl and NazS04