Effects of Rolling Direction and Tensile Stress on the Corrosion of a Naval Carbon Steel in Natural Marine Environments: II. Harbour anaerobic sediments

Abstract

The influence of rolling direction and tensile stress on the corrosion behaviour of a naval carbon steel has been studied by immersing specimens under natural anaerobic conditions in the marine sediments of Genoa Harbour. The tests were performed on specimens cut both parallel and perpendicular to the rolling direction of ships’ plate. Some of the specimens were subjected to tensile stress. Tests lasted 150 weeks, but some specimens were removed after shorter periods of immersion and examined visually and metallographically, and the corrosion products examined by X-ray diffraction.

Corrosion was found to be due both to the action of the bacterial H₂S of the sediments and of the oxygen present in the outer layers of the sediments. The corrosion rate remained constant until the 90th week of exposure and then showed a marked increase owing to the breakdown of the protective films. The rolling direction affects the transverse surfaces of the specimens and not the surfaces parallel to the rolling plane.The transverse surfaces which are cut parallel to the rolling direction corroded more rapidly than did those perpendicular to the rolling direction. This is thought to be related to the more closely packed fibres on these surfaces and to the higher density of interdendritic segregates. The effect of tensile stress was found to be considerable only for the highest permanent deformations and only on those surfaces which, owing to the rolling direction, were in any case the least resistant to corrosion.