GENERAL CORROSIONDOE-HDBK-1015/1-93CorrosionCH-02Rev. 0Page 20General Corrosion SummaryThe two conditions that contribute to general corrosion are:Metal and water in the same environmentChemical reaction between the metal and water to form an oxideThe corrosion rate is affected by the following:A temperature rise in the range of 20?F to 50?F doubles the corrosion rateuntil the formation of the protective oxide film is complete.An extremely high water velocity, 30 to 40 ft per second, tends to removethe oxide film allowing the corrosion rate to increase. The presence of oxygen in water to which iron is exposed increases thecorrosion rate. The reason for the corrosion rate increase is due to therapid reaction between the oxygen and the polarizing layer of hydrogenabsorbed on the oxide layer.A pH between 4 and 10 results in minimal corrosion rate. If the pH fallsbelow or above this range, the corrosion will normally increase.The condition and composition of the metal surfaces affects the corrosionrate. Deposits, scale, or irregular surfaces create areas on the metal wherelocal corrosion can initiate and proceed at a faster rate than normal.Dissolved solids tend to make it easier for current to flow, which resultsin a higher corrosion rate.The three products formed from general corrosion of iron are FeO, Fe O , and2 3Fe O .3 4SummaryThe major points of this chapter are summarized below.
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