Treatment of Susceptible Areas
Area Below the Waterline
This underwater area is protected from corrosion following the hull construction before it is launched. One of the more popular protection used is two part coal-tar epoxy in conjunction with a vinyl tar coat.
This combination has been found satisfactory and can be repaired at the ships yearly drydock survey.
- Decks and Hulls Above the Waterline.
Ships decks are also liable to corrosion due to being immersed in seawater during adverse weather depositing salt on the plating. When I was at sea the sailors were forever chipping away and repainting the deck or hanging over the side in a bosun’s chair, attacking the hull. This was not complimentary to my after lunch snooze before going on the 4-8 watch.
Anyway, protection of deck and above waterline areas is carried out using a well tested combination of alkyd and chlorinated rubber.
Ballast tanks are very susceptible to corrosion due to constant humidity promoted by their surface wet and dry conditions depending if the ship is ballasted or carrying a full cargo.
Ballast tanks have been accused of being the importer and spreading of non-indigenous seawater borne marine bacteria, organisms and barnacles. Coatings to eradicate these have been developed and are applied after a coating of anticorrosive material.
Ballast tanks used to be coated with coal tar epoxy (CTE) but now epoxy products produced in hydrocarbon refining has been developed. This is applied in two coats and is one of the current methods used against corrosion in ballast tanks.
Freshwater Storage Tanks
These tanks used to be cement washed and then chlorinated to prevent corrosion and protect against E-Coli and Legionnaires disease. I well remember the taste of the newly bunkered fresh water after the mate had liberally dosed it with chlorine. Yuck! Not nice in afternoon tea, to say nothing about the rum!
Nowadays an application of pure epoxy, applied under strict guidelines by an experienced contractor is one method. This can be supplemented with a strictly controlled addition of silver nitrate or chlorine to the tank being used to control any bacteria in the water.
This is a vast subject due to the various cargoes carried by today’s merchant vessels from crude oil to chemicals, so will cover this fully in a future article on Cargo Tank Protective Coatings.
However, in the interim we will have a quick look at t
he protection of oil storage tanks in a crude oil tanker. The corrosion in these areas is caused by the sulphurous and water contained in the crude, combined with other water vapour and the flexing of the ship’s structure. Microbes also compliment corrosion as they ingress into the protective coating. The new high tensile steels used in ships construction and the innovation of double hulled vessels has had a detrimental effect on tanks, being sited to have exacerbated corrosion and pitting instances.
Crude oil tank internals can be protected by applications of Coal tar and pure epoxy coatings.
Anyway I will leave it there as I am coming back to this subject in a later article.
1. calthelco: (ICCP) Impressed Current Cathodic Protection
2. DNV: Corrosion protection of ships.
3. sintef: Corrosion protection in marine environment.