Seawater corrosion rates

In aerated seawater, corrosion rates of more than 11.5 mm/yr (450 mpy) or (18 Ib/sq-ft/yr) have been measured with down-hole coupons. In diilhng fluids the control of corrosion rates below 1.27 mm/yr (50 mpy) or (2 Ib/sq ft/yr) with uniform corrosion is considered a practical objective. 

The titanium oxide film consists of mtile or anatase (31) and is typically 250-A thick. It is insoluble, repairable, and nonporous in many chemical media and provides excellent corrosion resistance. The oxide is fully stable in aqueous environments over a range of pH, from highly oxidizing to mildly reducing. However, when this oxide film is broken, the corrosion rate is very rapid. Usually the presence of a small amount of water is sufficient to repair the damaged oxide film. In a seawater solution, this film is maintained in the passive region from ca 0.2 to 10 V versus the saturated calomel electrode (32,33). 

The rate of self-corrosion of zinc anodes is relatively low. In fresh cold water, it amounts to about 0.02 g m h , corresponding to a corrosion rate of 25 /rm a. In cold seawater, the value is about 50% higher [10]. These figures refer to stagnant water. In flowing water the corrosion rates are significantly greater. Zinc is not practically suited for use in warm waters because of its tendency to passivate. 

The thermodynamic driving force behind the corrosion process can be related to the corrosion potential adopted by the metal while it is corroding. The corrosion potential is measured against a standard reference electrode. For seawater, the corrosion potentials of a number of constructional materials are shown in Table 53.1. The listing ranks metals in their thermodynamic ability to corrode. Corrosion rates are governed by additional factors as described above. 

For many cooling waters, including seawater and also drinking water, where corrosion rates are 70 to 100% of the limiting diffusion current, the use of dimensionless group analysis can then be applied. 

The corrosion rates of wrought iron and mild steel when immersed in seawater or buried in soil are not significantly different when the copper contents are similar. 

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. ... 

The general corrosion rate of zinc and zinc alloys in practice often have been shown to be much less than in simulated conditions this is because many naturally occurring substances act as inhibitors. Figure 4.42 is a good example of this. The diagram is valuable for the qualitative relationship between acid, neutral and alkaline conditions but, in practice, the corrosion rates are usually very much lower than indicated by the pH because of the effect of other dissolved constituents and the barrier effect of corrosion products. Seawater around the British Isles is much less corrosive to zinc than tropical seawater. 

Most of the published evidence suggests that marine fouling cover— particularly where it is continuous and well established — reduces corrosion rates of steels . Indeed, 35%o seawater is by no means the most corrosive of saline environments towards steel. Brackish water, as found in estuarine or certain other coastal areas, is considerably more aggressive towards steel, and careful design measures should be taken to ensure that effective corrosion control is achieved in such circumstances. 

Comparative tests between HSI and HSCI in seawater at 93° C and 10-8Am showed consumption rates of 8-4kg A y and 0-43 kg A y , respectively . These figures show that the consumption rate of HSI when used in seawater without the addition of chromium may approach that of steel, but because of the very deep pitting and its fragility, it is in most cases inferior to steel. However, in fresh waters HSI has a far lower corrosion rate than steel. The consumption rate of HSCI freely suspended in seawater in the current density range 10-8 to 53-8 Am increases from 0-33 kg A y at 10-8Am to 0-48 kg A" y at 53-8Am Direct burial in seawater silt or mud will also increase the consumption rate, with values of 0-7kg A y at 8-5 Am increasing to 0-94 kg A " y at 23-4 Am . 

If all the oxygen produced were to combine with the carbon the maximum theoretical wastage rate would be of the order of 1 kg A" y However, in practice the rate is usually of the order of 0-2 kg A y and in coke breeze may be as low as 0-05 kg A" y ". In seawater, where chlorine is the predominant gas produced, to which carbon is immune, any oxygen formed will be quickly removed and the corrosion rate may be very low. 

Figure 4.11 Simulated reaction pathway between 25 cm3 ceramic and 1 kg seawater, at STP, showing the predicted alteration of the minerals in the ceramic. Total corrosion rate = 104.62 mg/year. [From Wilson, 2004 199, generated using The Geochemist s Workbench (Bethke, 1996), with permission of the author.]...

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)...

Assuming corr in seawater is —650 mV, then the corrosion rate at —850 mV is only... 

The four important areas of application of carbon steels are (i) atmospheric corrosion (ii) corrosion in fresh water (iii) corrosion in seawater and (iv) corrosion in soils. The atmospheric corrosion of steel is caused by major environmental factors such as (i) time of wetness as defined by ISO 9223-1992 (ii) sulfur dioxide in the atmosphere due to the combustion of fossil fuels and (iii) chloride carried by the wind from sea. The equations for corrosion rates of carbon steel by multiple regression analysis have been obtained.1... 

The environmental factors that influence the corrosion rate vary with the depth in seawater as detailed8 in Table 4.3. The variation of the factors in seawater at different global locations has been documented in the literature.9... 

The corrosion rate of carbon steel increases with increase in velocity until a critical velocity is reached. This behavior is different from that of the carbon steel in fresh water where the corrosion rate decreases beyond a critical velocity due to the formation of a passive him. In seawater passive films are not formed because of the presence of high concentrations of chloride. The erosion corrosion occurs after critical velocity 20 m/s is reached. The maximum corrosion rate of 1,0/mm/yr is reached at velocities up to 4 m/s. 

The source of chloride is seawater as well as the deicing salts used on roads. The hydrogen chloride is also present in seawater aerosols. The corrosion of copper and its alloys in marine atmospheres has been studied and a corrosion rate of 600-700 pg/cm2 yr averaged over a period of 8 yr has been reported.50... 

The corrosion rate of a totally immersed copper sample in seawater is about 0.02-0.07 mm/yr and at half-tide the rate is 0.02-0.1 mm/yr. In this respect the corrosion resistance of copper is 2-5 times greater than mild steel under total immersion conditions and even greater under half-tide conditions. The copper loses its corrosion resistance in seawater of velocities greater than 1 m/s and the rate of dissolution is such that toxic copper species produced are beneficial in that they are used in marine antifouling agents. 

Some typical applications of the alloys are in propeller shafts, propellers, pump impeller blades, casings, condenser tubes and heat exchanger tubes. The corrosion rate in flowing seawater is <0.025 mm/yr, but can pit under stagnant water. Alloy 400 is immune to chloride SCC. 

The corrosion rates of lead and its alloys are low in seawater, as seen from the data in Table 4.58. 

Corrosion rates in seawater range between 20 and 70 pm/yr, depending upon factors such as location, duration of exposure and type of zinc sample with the rates decreasing over time. 

Depending on the velocity of fluid flow, the thickness varies from 10 to 100 pm, and it may cover from less than 20% to more than 90% of the metal surface. Biofilms or macrofouling in seawater can cause redox reactions that initiate or accelerate corrosion. Biofilms accumulate ions, manganese and iron, in concentrations far above those in the surrounding bulk water. They can also act as a diffusion barrier. Finally, some bacteria are capable of being directly involved in the oxidation or reduction of metal ions, particularly iron and manganese. Such bacteria can shift the chemical equilibrium between Fe, Fe2+, and Fe3+, which often influences the corrosion rate. (Dexter)5... 

Galvanic corrosion rates (mils) of some couples after 16 yr exposure to seawater and fresh water are given in Table 7.23. In the cae of carbon steel/aluminum the data show that in fresh water carbon steel corrodes to a greater extent than aluminum which provides further evidence for polarity reversal of the steel/Al couple in fresh water. 

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. 

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