Grey cast iron and unalloyed spheroidal graphite iron behave in a similar manner to steel and the corrosion rates are comparable. However, over time, cast iron is also prone to graphitisation, a form of corrosion in which the iron is corroded, leaving behind a deposit of graphite, silica and iron corrosion products. Although its implications are discussed later, the metal surface may appear uncorroded but a sharp tool can reveal that serious loss of section has occurred.


Corrosion of steels and irons are largely understood and corrosion can be controlled by including an extra thickness as a corrosion allowance, applying coatings and/or cathodic protection (CP). Another alternative is to select more resistant alloy systems.


Stainless Steels


A stainless steel, by definition, is an iron alloy with more than 10.5% chromium. There are many grades of stainless steel with a range of other alloying additions to influence the alloy structure, mechanical properties and corrosion resistance. These alloys owe their corrosion resistance to the formation of a passive chromium oxide film on their surface. This forms in air and there is enough oxygen under most seawater conditions to maintain the film and protect the underlying metal from corrosion.


The general corrosion rate for the types of stainless steels used in seawater is very low at about 0.002 mm/yr. By improving oxygen availability, the flow of seawater


across a stainless steel surface helps maintain the passivity and prevent corrosion, thus above 1 m/s and even at velocities as high as 40 m/s—or until cavitation occurs— the corrosion rate remains low.


Movement of seawater across the steel surface over about 1 m/s also tends to prevent deposits and biofouling and discourages local pitting. However, under quiet conditions and at crevices or under macrofouling or other deposits, where the oxygen becomes locally depleted, the oxide film may break down. Aided by seawater chlorides, a dynamic electrochemical cell forms such that the exposed metal rapidly corrodes (Figure 2). Crevice corrosion occurs more readily and at lower temperatures than pitting in seawater in the same bulk environment.


1.2 1 0.8


The corrosion behaviour of metals is largely influenced by oxygen in the seawater as well as flow velocity, temperature, pollution and marine organisms.


0.6 0.4


0.2


0 0 1 2 Velocity (m/s) Figure 1 - Effect of velocity on the corrosion rate of carbon steel(1) 3 4 5


The Report • June 2018 • Issue 84 | 35


Corrosion Rate (mm/y)


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