I. Nature of Initial Corrosion of Stressed Austenitic Stainless Steel by Chloride Ions. II. Platinum Decoration of Active Sites

It is shown that there is a relationship between surface sites on bulk specimens of Type 304 austenitic stainless steel at which chloride ion reaction occurs and defect structure of the face-centered cubic austenite lattice. Utilizing oxide extraction replicas and electron stereo-microscopy techniques the paths of initial corrosion attack are found to be tunnels of sub-micronic cross section. The chloride ion is believed to diffuse preferentially down the “pipes” of extended dislocations or stacking faults which lie in the {111} <110> slip system of the lattice. Prolonged exposure to boiling 42 percent MgCl₂ produces lateral corrosion and merging of the tunnels to form micro-crevices.

The sites at which passivity breaks down in the stainless surface due to chloride ion attack can be “decorated” by platinum deposition. The technique delineates reactive sites numbering as many as 10¹¹ per square centimeter and provides further confirmation of electrochemical activity along slip traces in a cold-worked surface.