1998 Volume 47 Issue 9 Pages 591-598
The effects of alloyed nitrogen and molybdenum on the pitting behavior of austenitic stainless steels were examined comparing with that of corresponding oxyacid salts (i.e. nitrate and molybdate) in a solution. The comparison was made through measurements of pitting potential as a function of temperature up to 423 K and that of pitting temperature as a function of applied potential. Both alloyed nitrogen and nitrate ion in a solution showed inhibitive effect at lower temperatures rather than at high temperatures. Pitting temperatures measured in a solution containing nitrate showed a unique dependency on potential: the nobler the potential, the higher the pitting temperature. However, it could not be revealed that alloyed nitrogen inhibited pitting through formation of nitrate ion because stainless steels alloyed with nitrogen did not show the similar dependency of pitting temperature on applied potential. Both alloyed molybdenum and molybdate in a solution enhanced pitting potential more effectively at higher temperatures, possibly because the film formation reaction in which molybdenum species involved proceeded more rapidly at higher temperatures. However molybdate ion could miss its distinctive effect at high temperatures when a specimen was polarized before heating. Excellent inhibition of pitting was realized at every temperature when nitrate and molybdate ions were mixed into a chloride solution: nitrate worked at nobler potentials even at lower temperatures and molybdate worked at higher temperatures even at less noble potentials. The stainless steel which contained both nitrogen and molybdenum also showed good performance against chloride solutions, uniting the individual advantages.
