Seawater

Seawater systems are used by many industries, such as shipping, offshore oil and gas production, power plants, and coastal industrial plants. The main use of seawater is for cooling purposes, but it is also used for firefighting, oilfield water injection, and desalination plants. The corrosion problems in these systems have been well studied over many years, but despite published information on materials behaviour in seawater, failures still occur. Most of the elements that can be found on earth are present in seawater, at least in trace amounts. However, 11 of the constituents account for 99.95 percent of the total solutes.

For many engineering materials, seawater is known to be the most corrosive of all natural elements. The corrosivity depends on various factors. Of primary interest is the content of oxygen, since the dominating cathodic reaction in the corrosion process is the reduction of oxygen. This reduction reaction may be strongly affected by the presence of microorganisms, which therefore constitute one of the most important factors. The effects of the microorganisms are particularly strong on some materials which are usually considered as corrosion-resistant, such as conventional stainless steels, e.g. the AISI 304 and 316 types, and Ni-Cu-alloys. In some cases the effects may be rather dramatic, and lead to unpleasant and expensive surprises for users of these materials.

Water treatment

Water treatment is applied:

To prevent bio-fouling in seawater systems

Chlorination by chlorine gas or hypo chlorite, typically up to 0.5 ppm residual chlorine (Cl₂ + HOCl + OCl^-)
Combined Cu and chlorine, typically 5 ppb Cu and 0.1 ppm residual chlorine

To prevent corrosion in pipelines

Oxygen scavenger
Biocide
Corrosion inhibitors

Materials for seawater systems

For piping:

CS, cement lined CS, galvanised CS, Cu alloys, high alloyed SS (PREN>40), titanium, GRP

For vessels:

Titanium, GRP, CS with rubber lining

For components:

Titanium, GRP, CS clad with nickel alloys, solid nickel alloys, SS, Cu-alloys, NiAl bronz, GLASS REINFORCED PLASTIC (GRP), Carbon steel (CS), Stainless Steel (SS)