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How Molybdenum prevents pitting corrosion in SS316?
MARCH 20, 2016   CONIC Press

The effects on pitting corrosion resistance of alloying with for instance molybdenum or nitrogen have been investigated, but the picture is not yet completely clear. In the case of alloying with molybdenum improved metal passivation has been found. When pitting attack occurs the molybdenum assists in repairing the passive layer so that pit nucleation is stopped. According to one theory molybdate ions are formed from dissolved molybdenum. The molybdate ions then remain at the outer surface of the diffusion layer so that it becomes cation selective. The aggressive anions, such as chlorides, are thereby prevented from reaching the surface. At the interface between the oxide and the diffusion layer anion selectivity prevails so that oxide growth can continue. After the initiation of attack increased amounts of molybdate ions have also been detected in pitted areas.

The effects of molybdenum seems to be enhanced by nitrogen which influences the molybdate concentrations at the surface. This has been explained by the production of ammonium ions which increases the pH which, in turn, makes the formation of molybdate ions more likely. Surface analysis has also proved that iron dissolution is increased with increased nitrogen amounts, whereas the dissolution of chromium and molybdenum decreases. In alloys with increased nitrogen amounts the passive films have been found to contain higher ratios of chromium in the outer layer. Below this exists a thin layer enriched in nitrogen, nickel and molybdenum.

Yet another theory states that the effect of molybdenum is to block active sites (steps) at the surface. This blocking reduces the amount of aggressive chloride ions at active sites, and thereby increases the resistance to chloride attack.

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