Atmospheric corrosion in marine environments along the Gulf of Mexico

Atmospheric corrosion in Mexico is creating significant structural problems in the tropical marine regions of the Gulf of Mexico. This is especially true along the southern Gulf coast where increasing petrochemical pollution is present with the extremely high chloride concentrations resulting from strong onshore winds, and the ever-present high time-of-wetness. The climatic and environmental parameters have been measured, between 1993 and 1998, at 12 coupon exposure sites at marine locations around the Gulf of Mexico. Corrosion classifications for iron, aluminum, copper and zinc have been calculated for each site using mean values of the time-of-wetness, chloride and sulfur dioxide concentrations. The theoretically predicted corrosion rates have been determined from the site classification data, and compared to the 12-month corrosion rates measured by coupon mass loss for the four metals exposed at the same sites. The data show that all sites have at least a C4 classification with most having C5 even though many are located over 1 km from the shoreline. The measured corrosion rates of different metals are often not in good agreement with those predicted using the environmental parameters, providing evidence that other climatic factors such as frequency of precipitation can modify the corrosion rates and distort the correlation with theoretical corrosion predictions and models. Spectroscopic analysis of corrosion products from exposed carbon steel coupons shows that large fractions of akaganeite form in high chloride environments, along with Lepidocrocite and goethite. The oxides form in distinct layers on the steel surface. The amount of akaganeite formed is related to some extent by the precipitation frequency which controls if pollutants are regularly flushed from the steel surfaces.