The considerably corrosive nature of seawater (because of its Cℓ–, SO42–… ions) for steel will vary with its oxygen content and its temperature. Very deep, cold water will thus be less corrosive than hot water. The Ryznar index is totally unsuitable for characterising this corrosivity.
In an area that is constantly submerged and in calm water, overall steel corrosion, governed by the rate at which the oxygen transfers through the oxide film, will be equal to approximately 100 to 200 μm per year. In pipes and tanks where water is circulating, pitting forms at the rate of 400 to 700 μm per year, prohibiting the use of bare steel.
Seawater is systematically used in coastal refrigeration systems; however, these systems require materials (stainless steel, titanium …) and a design that guarantee efficient protection.
protection against corrosion
- The use of concrete or FRP or coated steel pipes.
- Construction of heat exchanger, water box and manifolds with plastic coated (Rilsan or Secaphene).
- Construction of shell and tube exchangers in admiralty brass or in 76.22 brass which is less sensitive to dezincification or, alternately, cupro-nickel or titanium.
- Use of high PREN stainless steels (duplex ….).
Cathodic protection (seecathodic protection)
Cathodic protection (see corrosion inhibitors)
Eliminating dissolved oxygen
- Through vacuum degasification or gas stripping (see degasification, odour control, evaporation). This is the method that is systematically used for seawater that is injected into oil wells.
- Through chemical reduction using catalysed sodium bisulphite.
protection against soiling
Soiling is the major cause of corrosion that occurs under deposits and of degraded heat exchange capacity.
- Organic mucous produced by algae and bacteria.
- Soft organisms such as sea squirts and sea anemones that attach themselves to the metal.
- Colonies of hard organisms (crustaceans, mussels, oysters, barnacles) that become incrusted and that are difficult to destroy; these colonies cause perforating corrosion to occur under deposits through differential aeration.
In slow circulation, copper alloy circuits, copper dissolution will be enough to prevent this type of soiling.
Chlorination or the use of other oxidising biocides as a shock or ongoing treatment has been found to be effective against micro-organisms. Only crustaceans are capable of withstanding high amounts of chlorine for prolonged periods. Non-oxidising biocides may be effective against crustaceans but care must be taken when using these products which must be eliminated from the water or sufficiently downgraded to avoid harming other sea creatures.
The use of chlorine can contribute to corrosivity. This will apply in concentrations greater than 0.5 mg · L–1.