Surprising Intergranular "Non-Corrosion" of a 304L Stainless Steel

Abstract
A low chloride content solution, representative of an artificial saliva, was used to study the pitting and crevice resistance of a 304L wire used to fix teeth against each other in the mouth. On an industrial 304L alloy, corrosion inside deep pits showed a special character in which grain boundaries were not attacked : a honeycomb-like structure of the corroded surface was observed in which grain boundaries were the walls of the cells. This result was reproduced in a 1. 66 NaCl g/l solution, pH=7, on polished sections of ultra high purity base alloys, covered with a varnish layer in order to create a crevice-like situation. The electrochemical potential was imposed at values near passivity breakdown. The exposure times were 40 to 90 h at room temperature. UHP-alloys, representative of 304L steels, with subsequent additions of C, P and Mo were prepared and tested to determine the possible role of intergranular segregation or precipitation on honeycomb corrosion, in the as-quenched condition and after annealing (600°C, 30h). As a function of exposure time, different corrosion stages under the varnish layer were observed : crystallographic pitting, honeycomb corrosion and general dissolution.