Initiation and Growth of Pits on High-Purity Iron and its Alloys with Chromium and Copper in Neutral Chloride Solutions

The pitting of pure iron, and its alloys with chromium and copper, in neutral chloride solutions, has been studied using microscopy and time-lapse photography, and measurements have been made of local variations of pH value and electrode potential using micro-electrodes. Primary pitting at a few discrete points is followed by secondary pitting at a large number of points. On pure iron the pits are generally polygonal in shape, while with chromium-iron alloys the shape is less regular and attack is confined to grain boundaries. Facetting of the metal surface within the pits is commonly observed.

The initial growth of pits corresponds to a current density of up to 1 A/cm² but this rapidly decreases and, following accumulation of corrosion product, the corrosion rate settles down to a relatively low figure. The long-term penetration rate is reduced significantly by small alloying additions of copper.

pH differences of up to 8 units have been measured between the pitted area (acid) and the surrounding unattacked area (alkaline). The electrode potential in the pitted area may be up to 100 mV lower than that a short distance away.