Corrosion Theory

Corrosion Theory

Corrosion is the primary means by which metals deteriorate. Most metals corrode on contact with water (and moisture in the air), acids, bases, salts, oils, aggressive metal polishes, and other solid and liquid chemicals. Metals will also corrode when exposed to gaseous materials like acid vapors, formaldehyde gas, ammonia gas, and sulfur containing gases. Corrosion specifically refers to any process involving the deterioration or degradation of metal components.

When metal atoms are exposed to an environment containing water molecules they can give up electrons, becoming themselves positively charged ions, provided an electrical circuit can be completed. This effect can be concentrated locally to form a pit or, sometimes a crack, or it can extend across a wide area to produce general wastage. Localized corrosion that leads to pitting may provide sites for fatigue initiation and, additionally, corrosive agents like seawater may lead to greatly enhanced growth of the fatigue crack. Pitting corrosionalso occurs much faster in areas where microstructural changes have occurred due to welding operations.

The corrosion process (anodic reaction) of the metal dissolving as ions generates some electrons, as shown in the simple model on the left, that are consumed by a secondary process (cathodic reaction). These two processes have to balance their charges. The sites hosting these two processes can be located close to each other on the metal's surface, or far apart depending on the circumstances. This simple observation has a major impact in many aspects of corrosion prevention and control, for designing new corrosion monitoring techniques to avoiding the most insidious or localized forms of corrosion.

 Figure 1: Corrosion reaction

Corrosion sample

Shown below are typical examples of common corrosion failures

Fencing

This fence is located on a western-facing hillside a few miles from the coast.  As a result, it came in direct contact with the moist, salt-laden, onshore breezes.  The coating system applied to the fence was not robust enough for this environment.  The corroded areas were cut out and replaced.  The fence was stripped of its original coating and re-coated with a much thicker coating consisting of an epoxy primer followed by a urethane topcoat.

Figure 2: Fencing corrosion

Buried pipe

Two examples of water main failures are shown.

The first water main developed a leak after 8 years of service.  Although primed and tape-wrapped for corrosion protection, the pipe surface showed extensive pitting and corrosion damage.  In contrast, the inside of the pipe was undamaged.  The resistivity of the soil was measured.  Based upon the readings, the soil was found to be corrosive.  It was recommended that the water main be cathodically protected to prevent future

The second water main was in place since the 1920s.  It was bare cast iron, buried beneath the foundation of a home.  When the pipe began to leak it caused the surrounding soil to settle and the foundation of the home above it to crack.  In this instance, the hole in the water main started on the inside surface.

Figure 3: Buried pipe corrosion

Copper Water Lines

If the soil is corrosive enough it can even attack copper water lines as these examples show.  Typically, the surface of the water line at the bottom of the trench experiences the most corrosion.  The difference in oxygen concentration between the soil which was dug up to create the trench and then replaced (thus high oxygen content) and the undisturbed soil at the bottom of the trench (low oxygen content) creates a corrosion cell.  This cell, in addition to the soil’s general corrosiveness, causes the copper tube at the bottom of the trench to corrode.

Figure 4: Copper water line pipe corrosion

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