Fundamentals of Metallic Corrosion
in Fresh Water

By J.R. Rossum

In preparation for this paper, I've examined some of the available literature on water well corrosion. I find that much of the material is either wrong, terribly confusing, or else completely misses the point. For example:

"When water contains less iron than the maximum that it is capable of carrying in solution, it corrodes iron or steel rapidly - unless a protective film or crust of some material covers the metal surface. The unsaturated water tends to dissolve metal from the surface of well screens, well casing or piping systems until it becomes saturated with respect to iron. If the mineral content of the water is such that a protective film is not formed by deposition of insoluble materials, severe corrosion results." [1]

This concept is true only if we are talking about metallic iron rather than ionized iron. However, the solubility of metallic iron in water is not reported in the literature, and is probably too small to measure. Serious corrosion of steel casing from the solution of metallic iron would take centuries or millenia.

Corrosion experiments have not produced entirely satisfactory results. When a commercially useful metal is immersed in water, we know it's going to corrode. The question is, how fast? Since the useful life of most water facilities is often several decades, some corrosion experiments take too long to be practical. If the rate of corrosion is accelerated, the very thing we want to know has been distorted. While corrosion rates can be studied experimentally, in general these rates change with time.

Corrosion in fresh water very often results in pitting so that, because of statistical variation in pit geometry, experiments under identical conditions will not yield identical results.

The result of changes in experimental conditions may appear to be contradictory. For example, normally an increase in temperature will increase the corrosion rate, but it is possible for an increase in temperature to increase the Lagelier Index to a point where the corrosion rate is greatly reduced.

Because of these inherent difficulties, the results of experiments have failed to yield enough information to enable corrosion engineers to calculate the useful life of pipe or other metallic facilities exposed to water. Nor is chemical thermodynamics particularly useful, since this subject deals largely with equilibrium conditions. The information presented herein is based upon observation of corroding structures and the application of elementary chemical theory.

In conversations, I have found that well drillers often use the term "corrosion" to mean any process that causes incrustation. I will use the term to express a chemical attack on the metal. Since corrosion products sometimes adhere to the metallic surface, and are more voluminous than the corroded metal, corrosion often causes incrustation. However, there are many instances where incrustation occurs without corrosion.

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