Water corrosivity

K. N. Reichek, K. J. Clark, and J. E. HiUis, Controlling the Salt Water Corrosion Peformance of Magnesium AZ91 Alloy, paper 850417, Society of Automotive Engineers, Detroit, Mich., 1985. 

The marine environment is highly aggressive. Materials in marine service are constantly exposed to water, corrosive salts, strong sunlight, extremes in temperature, mechanical abuse, and chemical pollution in ports. This climate is very severe on ships, buoys, and navigational aids, offshore stmctures such as drilling platforms, and faciUties near the shore such as piers, locks, and bridges. 

Sulfate reducers. The best-known form of microbiologically influenced corrosion involves sulfate-reducing bacteria.- Without question, sulfate reducers cause most localized industrial cooling water corrosion associated with bacteria. Desulfovibrio, Desulfomonas, and Desulfotomacu-lum are three genera of sulfate-reducing bacteria. 

White, friable corrosion products composed of Bayerite AI2O3 3H2O, caustic, and NaA102 cover corroded areas (Fig. 8.3). The white corrosion product and deposit usually test as distinctly alkaline when mixed with distilled water. Corrosion products usually cling tenaciously to the underl3dng metal and do not form voluminous lumps. Instead, corrosion products line and coat generally wasted surfaces below. 

Alter the chemistry of the common fluid to render it less conductive and/or less corrosive. Generally, water corrosivity increases with an increase in temperature and oxygen content and a decrease in pH. Inhibitors may he effective. Note that in mixed-metal systems, higher dosages of inhibitors may be required than would be necessary in single-metal systems in the same environment. 

Greenblatt, J. H., Initial Corrosion of A1 Alloys in High Temperature Water , Corrosion, 19, 295t(1963)... 

Murray, G. A. W., Artificial Pits for Quantitative Studies of Corrosion of Aluminium Alloys in Natural Waters , Corrosion, 20, 329 (1964)... 

Wilde, B. E., A Critical Appraisal of Some Popular Laboratory Electrochemical Tests for Predicting the Localised Corrosion Resistance of Stainless Alloys in Sea-water , Corrosion, 28, 283 (1972)... 

Fraker, A. C., Effect of Solution pH on the Saline Water Corrosion of Titanium Alloys , Proc. 

Serious pitting may occur in the area of welds, particularly in sea-water. Corrosion rates of up to lOmm/y have been reported in weld joints of ice-breakers. The severe corrosion has been attributed to galvanic effects between the weld metal and the steel plate. The use of more noble electrodes for welding are reported to overcome this problem . 

Table 3.49 Sea-water corrosion/erosion test carried out at 8 m/s at 28° C for 60 d...

Symposium on Corrosion by High Purity Water, Corrosion, 13, 151t (1957)... 

Addition of about 0 04% arsenic will inhibit dezincification of a brasses in most circumstances and arsenical a brasses can be considered immune to dezincification for most practical purposes . There are conditions of exposure in which dezincification of these materials has been observed, e.g. when exposed outdoors well away from the sea , or when immersed in pure water at high temperature and pressure, but trouble of this type rarely arises in practice. In other conditions, e.g. in polluted sea-water, corrosion can occur with copper redeposition away from the site of initial attack, but this is not truly dezincification, which, by definition, requires the metallic copper to be produced in situ. The work of Lucey goes far in explaining the mechanism by which arsenic prevents dezincification in a brasses, but not in a-/3 brasses (see also Section 1.6). An interesting observation is that the presence of a small impurity content of magnesium will prevent arsenic in a brass from having its usual inhibiting effect . 

LaQue, F. L., Marine Corrosion, Causes and Prevention, John Wiley Sons, New York (1975) Schumacher, M., Sea Water Corrosion Handbook, Noyes Data Corp., Park Ridge, N.J. (1979) Gilbert, P. T., Mater. Performance, 21 (2), 47 (1982)... 

Reichek, Clark, Hillis, Controlling the Soli Water Corrosion Performance of Magnesium AZ91 Alloy, SAE Paper 850417... 

Motojima, S., andKosaki, H., Resistivities Against Sea-Water Corrosion and Sea-Sand Abrasion of TiB2 Coated Copper Plates, J. Mater. Sci. Lett, 4(11) 1350-1352 (Nov., 1985)... 

The corrosion of iron occurs particularly rapidly when an aqueous solution is present. This is because water that contains ions provides an oxidation pathway with an activation energy that is much lower than the activation energy for the direct reaction of iron with oxygen gas. As illustrated schematically in Figure 19-21. oxidation and reduction occur at different locations on the metal surface. In the absence of dissolved ions to act as charge carriers, a complete electrical circuit is missing, so the redox reaction is slow, hi contrast, when dissolved ions are present, such as in salt water and acidic water, corrosion can be quite rapid. 

Formulations (clear liquids or coloured formulations) based on an ethanol solution of various silica or aluminosilica precursors are marketed for application on glass, metallic or plastic surfaces to achieve hard 1 10 pm coatings that form strong chemical bonding with the surfaces and protect against staining or water corrosion. 

The quality of the ethanol has a strong influence on its corrosive effects. Three categories of problems have been identified general corrosion (caused by ionic impurities, mainly chloride ions and acetic acid), dry corrosion due to ethanol polarity, and wet corrosion caused by azeotropic water. Corrosion inhibitors should thus be incorporated in ethanol-diesel blends. 

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