Metals and their


Corrosion Behaviour in Seawater


A brief overview of metals used in seawater and common corrosion issues to be avoided


By CAROL POWELL BSc CEng FIMMM MIMarEST 1 1 Consultant to Nickel Institute and Copper Development


Association 2


Consultant to Copper Development Association


The majority of metals are susceptible to corrosion in one form or another in seawater. Their successful application relies on knowledge of the types of corrosion each alloy might be susceptible to and how to avoid potential problems by good material selection, design, fabrication and operational practices. Seawater, of course, is very complex and its composition varies. It contains many different salts, dissolved gases, trace elements, suspended solids, decomposed organic matter and living organisms. Also, temperature and biological activity alter with local climate and season.


34 | The Report • June 2018 • Issue 84 and MARK TUR MIMMM MCIM MBA 2


The corrosion behaviour of metals is largely influenced by oxygen in the seawater as well as flow velocity, temperature, pollution and marine organisms. As it is a good electrolyte, seawater may also lead to galvanic and localised corrosion in susceptible alloys. The following overview describes the relative corrosion resistance of several alloy families used in seawater systems, the more common types of corrosion which can occur and how these can be mitigated. The main properties covered are resistance to general corrosion, erosion corrosion, pitting and crevice corrosion, selective phase corrosion, stress corrosion cracking, galvanic response and biofouling. Suggestions for further reading are given at the end.


Carbon Steel and Cast Irons(1)


Steel is the basic alloy used for marine structures in seawater and its corrosion resistance is primarily governed by the diffusion of oxygen in the water to the steel surface. General corrosion rates in quiescent seawater are in the order of 0.1-0.2 mm/yr but with pitting at several times this depth. Flow of seawater across the steel surface increases the corrosion rate and at 3 m/s, a velocity often used in pipe systems, the corrosion rate is increased to about 0.75 mm/ yr (Figure 1). At higher velocities, the corrosion rate continues to increase, for example, 5 mm/yr at 40 m/s and velocities of this order may well be reached in areas of local high turbulence.


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