Corrosion by Water at Low Flow Velocity

It is postulated that water qaulity is a primary factor in corrosion by fresh waters, especially at low velocities, and that the influence of dissolved oxygen and pH on corrosion rates is secondary. Citing classic articles on pH adjustment, the authors say the conclusions reached by cited experimenters are true only for the water tested and cannot properly be applied generally to all waters. By means of an electrolysis cell the authors tested the effect on anodes and cathodes of passing through water moving at various low velocities an increasing volume of current and noted changes in the character of the water. After the proportion of sodium chloride or sulfate reached a given value the corrosion rate was not influenced by further additions of the chemicals but was in proportion to the dissolved oxygen present. For specific chloride content corrosion rates might be considerably greater for solutions of low mineral content, leading to the conclusion the rate is controlled more by specific mineral quality than total mineral content. The inhibitive effect of bicarbonate was noted in both 3-day and longer tests. Relatively lower corrosion rates were experienced with nitrates. Authors doubt the corrosion product adjacent to iron is ferrous hydroxide when carbonate or bicarbohate is present. Instead it is contended the initial corrosion product in fresh water is soluble ferrous chloride, hydrolized insoluble ferrous hydroxide being secondary.