Investigation into causes of corrosion in 316-type stainless steel

Corrosion was detected on a number of 3mm diameter wires made from 316-type stainless steel, and evidence was found to indicate that this had happened even before the wires were exposed to service. Analytical tests of chemical composition showed that the wires (irrespective of their corrosion behaviour) met the specifications of ASTM A314 for 316-type stainless steel. Randomly selected samples that contained corrosion stains and some that did not were subjected to microscopic and corrosion examination. Samples of wires were immersed in 10%FeCl ₃ solution for 72 hours as per ASTM G48 and the weight loss was measured. It was found that the wires that had previous corrosion stains showed excessive weight loss. The SEM images in general showed surface folds which are thought to have been produced during manufacture. Some wires with corrosion stains also showed spiral surface defects. After the immersion in 10%FeCl ₃ solution at 20°C it was found that pitting was generally associated with the surface folds on the wires. In addition to this there was excessive pitting that had generally occurred along the spiral defects of samples with previous corrosion stains. The defects in the surface conditions of wires are considered to be the result of poor quality control during manufacture.