Improvement in Pitting Resistance of Stainless Steel Surfaces by Prior Anodic Treatment in Metasilicate Solution

Mild anodizing treatment of 304 stainless steel microelectrodes in sodium metasilicate solution reduces the frequency of pit nucleation events markedly when the metal is subsequently exposed to acidic chloride solutions. The same treatment of larger specimens increases the pitting potential substantially. The results confirm that the frequency and/or amplitude of the current spikes that nucleate pitting corrosion on an ultramicroscopic scale have a strong bearing on the pitting characteristics of the metal. Identical surface treatment, but in hydroxide solution of pH similar to the metasilicate solution, yields a much smaller improvement in pitting characteristics, as demonstrated by both analysis of the pit nucleation events on microelectrodes as well as the pitting potential. The effect of the metasilicate treatment is due to a specific action of the metasilicate anion with the pit nucleation sites. The effect on the overall passive state is marginal. It is proposed that the metasilicate treatment effectively caps the pit nucleation sites by forming a silicate- or silica-rich coating over those sites, a process we term "electrochemical pit site capping." © 2004 The Electrochemical Society. All rights reserved.