Corrosion Protection - Offshore Structures: Design, Construction and Maintenance - page 388

E c

E cor 3
E cor 2
E cor 1

E a

Increa sing flow or oxygen conce ntration

I cor 1 I cor 2 I cor 3 Log I

FIGURE 6.3 Polarization diagram with increasing oxygen concentration.

negative from E c . This has the effect of depleting the oxygen immediately adja-
cent to the metal surface, thus rendering the reaction more difficult. Ultimately,
a point is reached where the surface concentration of oxygen has fallen to zero
and oxygen can then be reduced only as and when it reaches the surface. Further
lowering of the potential cannot increase the cathodic reaction rate, because the
kinetics are now governed by potential-independent diffusion processes, and a
plateau, or limiting, current is observed. Figure 6.3 shows that the corrosion rate
is then equal to this limiting current. The limiting current can be increased by
increasing the oxygen flux, either by raising the bulk oxygen concentration (the
concentration gradient gets steeper) or increasing the flow rate (the oxygen-
depleted layer gets thinner). Both serve to increase the corrosion rate, as
shown in Figure 6.3 .
To a first approximation, it may be stated that the rate of corrosion of clean
steel in aerated seawater under turbulent flow conditions is directly proportional
to the bulk oxygen concentration and the linear velocity. Fick s first law of dif-

fusion and the Chilton-Colbourn analogy can be used to calculate the precise
effect of oxygen concentration and Reynolds number (flow rate) on corrosion.
Accordingly, Ashworth (1994) estimated the maximum corrosion rates of clean
steel in North Sea water at 7

'

C, as shown in Table 6.1 .
In practice, corrosion products and marine fouling build up on steel as it cor-
rodes in seawater, and they generally produce lower corrosion rates.
Rowlands (1994) suggests that the corrosion rate of fully immersed steel is
fairly rapid in the first few months of exposure but falls progressively with time.

°