Seawater corrosion factors

Pseudomonas spp. are IRB species reported to have corrosive effects. " - However, there is an increasing body of evidence that IRB could actually slow down corrosion. Hernandez et al. reported that in the presence of bacteria like aerobic Pseudomonas sp. and facultative anaerobic Serratia marcescens in synthetic seawater, corrosion of mild steel is inhibited. The effect seemed to disappear with time in natural seawater. Although no particular mechanism was proposed to address this phenomenon, it seems that it is related to the biofilm produced, thus reducing the contribution of factors such as diffusion gradients that normally enhance corrosion. 

Seawater is a unique environment. A recent worldwide test program was recently completed by ASTM Task Group Gl.09.02.03 to evaluate the corrosivity of seawater at a number of sites [/5]. Though these results indicated the uniqueness of natural seawater, corrosivity was site-specific and influenced by numerous factors. Seawater can vary widely in terms of chemical composition, dissolved oxygen content, temperature, salinity, pH, carbonate levels, flow, degree of fouling, biological activity, and pollution [16. ... 

For the seawater corrosion resistant steels sold commercially under various different designations, the reported improvements in corrosion behaviour by a factor of 2 or 3 also apply only to the immersion or non-immersion zone. In the tidal zone, the corrosion rates practically fall into the general scatter band of the low-alloyed steels [47, 51]. Therefore, these steels also require corrosion protection in the tidal zone. 

The amount of chloride, sulfate, thiosulfate, or other aggressive anions dissolved in water necessary to produce noticeable attack depends on many interrelated factors. Extraordinarily, if the water is quite aggressive, general corrosion may occur so rapidly outside the crevice that concentration differences cannot easily develop between the crevice interior and exterior. However, it is usually safe to assume that as the concentration of aggressive anions increases in solution, crevice attack is stimulated. Seawater chloride concentrations produce severe attack in most stainless crevices in a few weeks. 

Metallurgy is another cost versus operability factor for heat exchangers. For example, for seawater service some companies specify 90/10 Cu-Ni tubes as minimum and do not allow the cheaper, but more prone to corrosion, aluminum-brass. One can even consider going on up to 70/30 Cu-Ni, or to one of the modern, high-performance... 

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. 

Chlorides are often found as the salt aerosols of the atmosphere, and consequently may strongly influence the corrosion performance of structures and plant, particularly in marine or coastal situations. This influence on corrosivity reduces proportionately with distance from the seawater surface, though local environmental factors such as prevailing wind direction, level... 

As with the calcareous tests, BSi dissolution rates depend on (1) the susceptibility of a particular shell type to dissolution and (2) the degree to which a water mass is undersaturated with respect to opaline silica. Susceptibility to dissolution is related to chemical and physical factors. For example, various trace metals lower the solubility of BSi. (See Table 11.6 for the trace metal composition of siliceous shells.) From the physical perspective, denser shells sink fester. They also tend to have thicker walls and lower surface-area-to-volume ratios, all of which contribute to slower dissolution rates. As with calcivun carbonate, the degree of saturation of seawater with respect to BSi decreases with depth. The greater the thermodynamic driving force for dissolution, the fester the dissolution rate. As shown in Table 16.1, vertical and horizontal segregation of DSi does not significantly coimter the effect of pressure in increasing the saturation concentration DSi. Thus, unlike calcite, there is no deep water that is more thermodynamically favorable for BSi preservation they are all corrosive to BSi. 

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... 

Exposure to seawater results in decrease in critical stress intensity factor and the susceptibility to SCC68 0.2% Fe improves the resistance to SCC presence of >5 wt percent of A1 increases the velocity of cracking Sn in the alloy decreases SCC resistance chloride bromide and iodide induce or accelerate SCC69 Occurs by trangranular cleavage of a-phase in which a-phase controls the crack propagation rate Intergranular corrosion due to formation of titanium methoxide... 

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. 

S)D.J. Astley, J.C. Rowlands, Modelling of Bimetallic Corrosion in Seawater Systems," Brit. Corms. J. 20(1985) p. 90. (b Factors in parentheses have been derived on the basis of a limited amount of evidence.18... 

Measurements have been made by a Joint Norwegian-Russian Expert Group of the release of activity at the disposal sites for reactor compartments at Novaya Zemlya. Surface sediment samples at the sea bottom taken close to the dumped reactor compartments and the sunken submarine showed in a few cases activities of up to about 10 kBq/kg, but in most cases the activity of the samples was 100 Bq/kg or lower. At some distance from the dumped objects the level of radionuclides in sediments was close to the range obtained in the open Kara Sea [2]. It may also be mentioned that the activity of fish from the Baltic, measured in Bq/kg is more than a factor of 10 higher than the activity of fish from the Barents Sea. Thus the contamination caused by the dumped reactor compartments has so far been quite limited. Corrosion of the seawater will of course gradually increase the release of fuel and activated materials to the environment, but at the same time the activity will decrease due to radioactive decay... 

Corrosion damage is a major factor in ship maintenance and availability. Coatings combined with the impressed current cathodic protection (ICCP) system are the most common means for shipboard corrosion control. They interact with each other to protect the shipboard. Coatings provide primary corrosion protection by isolating the hull metal from seawater, while ICCP systems protect the hull by applying an external source of current to the ship where the paint is damaged or degraded. 

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