On a ship's hull in seawater, corrosion forms on what might appear to be a uniform surface. Since corrosion results in metal loss, uncoated plates will experience deterioration and wastage. Where an imperfect coating exists, corrosion will take the form of accelerated pitting at the location of the bare spots. In the absence of protection, corrosion will cause:
Anodic areas on submerged steel must be eliminated in order to obtain optimum ship performance. Protection with the best and most expensive coatings alone is not enough because paint becomes damaged. It is also prone to breakdown unless it is carefully applied under favorable atmospheric conditions on correctly prepared surfaces.
Corrosion can be particularly severe when it occurs at breaks in a coated surface where deep pitting may develop rapidly. Once pitting is initiated, it can be exceedingly difficult to contain. Having commenced at the bare areas, corrosion spreads under adjacent sound paint, accelerating the breakdown of the coating as a whole.
The corrosion pattern is different in the cargo tanks of crude oil carriers where cargos are alternated with seawater ballast. In this environment, corrosion takes the form of pitting on horizontal surfaces such as the inner bottom shell plating, the upper surfaces of stringer platforms and the face plates of longitudinal and traverse members. It also occurs in other places where water may be present beneath oil cargos and during void periods.
Oftentimes, a steel structure (e.g., oil platform or pipeline) submerged in seawater is in contact with mud or deposits which may be conducive to the growth of sulphate-reducing bacteria. A particularly aggressive type of environment results in which paints are prone to breakdown. Steel pilings below the seabed level and buried pipelines are susceptible to this form of attack making cathodic protection essential.
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