Ship Hulls
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:
- Hull roughening
- Loss of speed
- Increased fuel consumption
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.
Ship Cargo Tanks
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.
Fixed Structures
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.
Return to the Principles of Cathodic Protection. |
