Corrosion profiles of mild steel in varying tidal seawaters

Corrosion loss of mild steel in tidal conditions varies with height, as most commonly seen in examining harbour infrastructure such as sheet piling. Conventional understanding of the variation of the corrosion losses for piling or steel vertical strips in the tidal zone is based largely on a very small set of field trials carried out in the 1950’s in US East coast waters. The corrosion profile shows a distinctly greater corrosion loss at about high water level. Since then very few systematic field observations have been made. However, there is increasing concern, particularly in Europe, about excessive corrosion at about the low water level, known as Accelerated Low Water Corrosion (ALWC). This phenomenon is currently not understood. The present paper reports early results from an extensive investigation of corrosion of steel strips in the tidal zone. It involves ten field trials at various locations on the Australian eastern seaboard, ranging from tropical to cooler temperate waters. The results show considerable differences in corrosion profiles both between sites and with exposure time. Typically, there are two distinct zones of increased corrosion, at the high water mark and at the low water mark. The relative magnitudes of the corrosion at these levels vary with location and also with exposure period. In the trials to date, the highest annual loss for high water corrosion was greater than 900 microns, for low water corrosion over 600 microns and the annual mid –tide loss varied from less than 50 microns to over 600 microns. Investigations show that there is no significant loss effect due to water velocity but that there is an apparent effect related to nutrient levels in the water. This is known to cause increased localized microbiologically-influenced corrosion.