Steel in seawater

Fig. 2-21 Effect of potential on the current density and on the reduction in area for X65 steel in seawater (slow strain test, e = 2 X 10" s ), (a) Air purging, (h) nitrogen purging, (c) COj purging.

For example, Oldfield and Todd have confirmed that for mild steel in seawater then the rate of corrosion can be predicted from ... 

The corrosion rates for both maraging steel and the low alloy steels in seawater are similar initially, but from about 1 year onwards the maraging steels tend to corrode more slowly as indicated in Fig. 3.32. The corrosion rates for both low alloy and maraging steel increase with water velocity . During sea-water exposure the initial attack was confined to local anodic areas, whereas other areas (cathodic) remained almost free from attack the latter were covered with a calcareous deposit typical of cathodic areas in sea-water exposure. In time, the anodic rust areas covered the entire surface. ... 

Fig. 8.65 Corrosion fatigue crack growth data for structural steel in seawater at 0.1 Hz, / = -I to 0.85 and -1.10 V (Ag/AgCI) (after Scott...

Turnbull, A., Review of the electrochemical conditions in cracks with particular reference to corrosion fatigue of structural steel in seawater . Reviews in Coatings and Corrosion, 5, Nos. 1-4, 43-160 (1982)... 

Guide for crevice corrosion testing of iron base and nickel base stainless steels in seawater and other chloride-containing aqueous environments... 

R. Gundersen, B. Johansen, P. O. Gartland, L. Fiksdal, I. Vintermyr, R. Tunold, and G. Hagen. The effect of sodium hypochlorite on the electrochemical properties of stainless steels in seawater with and without bacterial films. Corrosion, 47(10) 800-807, October 1991. 

Stainless steel 316L material used for piping and equipment shows considerable corrosion resistance because of the beneficial effect of molybdenum on the surface properties. It is also observed that the surface treatment (pre-reduced, polished, passivated and chemically treated surfaces) of stainless steel equipment and piping reduces the corrosion process in seawater applications. The corrosion resistance of stainless steel in seawater applications can also be enhanced by bulk alloying the stainless steel with nitrogen, chromium, molybdenum and nickel by converting the stainless steel into super austenitic stainless steel. From leaching studies it is also observed that the release of iron, chromium and nickel from the super austenitic stainless steel to seawater is considerably... 

A practical illustration of the application of more negative (cathodic) potential to carbon steel in seawater to reduce the corrosion rate is provided in Figure 1.69. The figure shows that the more cathodic the applied potential, the lower is the corrosion rate. In the figure rp is the maximum acceptable (or allowed) corrosion rate with a corrosion current density of ip and protection potential of Ep. For this particular case the protection potential range is —800 to —900 mV. The corrosion rate iv may be written as 75... 

Figure 1.69 Schematic diagram showing the variation of cathodic potential with current density for steel in seawater, and the correlation of corrosion rate measured by weight loss. (Reproduced from Corrosion for Science and Engineering, Tretheway and Chamberlain, Copyright Pearson Education Ltd)...

Uniform corrosion usually occurs in fairly aggressive environments that attack the whole surface. Examples include carbon steel in seawater or acids, or aluminum alloys in strong alkali. The rate of metal loss is usually rather high, but, because it is distributed over the whole surface, the performance can usually be predicted, and managed with corrosion allowances, in most situations. Thus, sheet steel piling is often used in seawater without any corrosion protection, the corrosion rate of around 0.1 mm/yr, coupled with the relatively thick steel sections, giving an acceptable life. 

Table 2.1 shows the eonversion factors between the units of corrosion rates that are most frequently used in the literature. Note that, for most of the listed materials, a corrosion current density of 1 pA/cm corresponds to a thickness reduction of roughly 0.01 mm/year. As an example of practical corrosion rates it can be mentioned that structural steels in seawater normally corrode by 0.1-0.15 mm/year = 10-15 pA/cm on average. The corrosion rate can be a few times higher locally. 

An example of measured corrosion rates on steel in seawater is shown in Figure 6.8. If this diagram is redrawn with v as the variable along the horizontal axis, we will get an S-shaped curved in this case also, as we have in Figure 6.6. 

Figure 6.8 Corrosion rate as a function of flow velocity. Steel in seawater at 23 C...

Figure 6.16 shows how different concentrations of chlorine can affect cathodic overvoltage curves for stainless steel in seawater. The corrosion risk for stainless steel at higher CI2 concentrations arises because the increased cathodic reaction rate lifts the potential so that critical potentials for local corrosion arc exceeded (see also Section 8.3). Chlorine may cause corrosion on several other materials as well. 

Bardal E, Drugli JM, Gartland PO. The behaviour of corrosion-resistant steels in seawater A review. Corrosion Science, 35,1-4,1993 257-267. 

Example 2 For copper alloys in contaet with stainless steel in seawater, wifli an area ratio 1 1 between the materials, the galvanie coupling may give an inerease in the corrosion rate on eopper by a factor of 4-8 [7.8]. (In this case, stainless steel is the most effieient cafliode.)... 

Figure 7.23 Potential as a function of time for various stainless steels in seawater at a flow velocity of 1.2 m/s and temperature 9 C. The potential drop after about 20 days for one of the materials is due to initiation of crevice corrosion.

Figure 7.66 Crack surface after fatigue of steel in seawater under cathodic protection. (From the hotel platform Alexander KieUand . Photo B. Lian, Statoil.)...

Wallen B, Anderson T. Galvanic corrosion of copper alloys in contact with a highly alloyed stainless steel in seawater. Stockholm 10th Scandinavian Corrosion Congress, 1986. 

Figure 8.7 Corrosion rate on steel in seawater as a function of depth. (Reproduced from Marine and Offshore Corrosion by K.A. Chandler. Reprinted by permission of Elsevier Limited.)...

In most cases it is appropriate to use a potential criterion for CP. The main criterion is Ec < Ep, where Ep is a protection potential defined mainly upon an empirical basis. For steel in seawater, the following protection potential has usually been recommended, referred to the various reference electrodes ... 

Reference [10.25] speeifies a current density of 10 mA/m for thermally sprayed aluminium on steel in seawater (must possibly be adjusted for higher temperatures) and 20 mAJvci for bare steel buried in the seabed. For eonerete structures they have specified 1-3 mA per m surface of reinforeement steel depending on the climatic zone and the water depth. Contrary to the praetiee for steel structures, the highest design values of CD on eonerete are specified for the warm climatic zones. 

For CP of steel in seawater, Zn and A1 anodes give sufficient driving voltage. For CP in environments with low conductivity (soil, fresh water), however, either impressed current or Mg saerifieial anodes are usually necessary. 

Cr5Ni3Mo ferritic-austenitic stainless steel in seawater with some sand... 

FIGURE 19.1 Overvoltage diagram (E-logI) for steel in seawater [5]. 

FIGURE 19.3 Overvoltage diagram for steel in seawater with protection current 7p included [5]. 

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