The corrosion behaviour of a low carbon steel in natural and synthetic seawaters

摘要

The corrosion behaviour of a low carbon steel was investigated in natural seawater aad various synthetic seawaters. It was found that the steel corroded nearly four times faster In a 3.5 percent NaCl solution than in natural seawater for an exposure time of 21 days. The corrosion rate after immersion in synthetic seawaters (ASTM D! 141 and Marine Biological Laboratory seawater) is similar to the corrosion rate after immersion in natural seawater. Calcium carbonate (aragonite) deposits were found on the surface of the steel after immersion fa natural seawater and the synthetic seawaters. Some magnesium-containing deposits were also found after immersion in the natural seawater. These deposits act as a barrier against oxygen diffusion and thereby lower the corrosion rate. The morphology of the calcium carbonate deposits that formed daring immersion in the natural seawater is different from those formed during immersion in the synthetic seawaters. This may explain the slightly lower corrosion rates obtained in the natural seawater. X-ray diffraction also showed that the oxy-hydroxides formed in the 3.5 percent NaCl solution differed from those formed in the other solutions.