Corrosion – OVERVIEW
There are many types of corrosion that can occur in water treatment systems. Most corrosion is caused by anodic (electron producing) and cathodic (electron consuming) sites that exist, and/or are created within a water system. These sites can occur for a variety of reasons. Some of the more common causes include, impurities in metal components, differences in metal composition, mechanical stresses, and environmental differences in the system, such as oxygen or salt concentration. Corrosion will be more or less consistent when the anodic and cathodic sites shift along metal surfaces. A more serious problem is â€œlocalized corrosionâ€ that occurs when the these sites remain stationary. Examples of localized corrosion are, pitting and stress corrosion cracking.
When large differences in surface conditions create stationary anodic and cathodic sites it will likely cause pitting. Once pitting begins it creates an environment for further damage rate will increase. Because of this and because pitting is difficult to predict, pitting is one of the more problematic forms. In many cases it is possible to control pitting with inhibitors, however it is critical that those inhibitors are applied correctly for the pitting to be effectively controlled.
Galvanic corrosion is the result of having two dissimilar metals in contact with each other and with a liquid, such as water. For this type, the contact must conduct electricity in order to create an electric potential difference. The level of corrosions will increase for the anodic metal, while this level will actually decrease for the cathodic metal. A common and very serious example of Galvanic corrosion occurs when dissolved copper attaches to the steel surface in a cooling system. Corrosion inhibitors are recommended to prevent this from occurring. Another way to control galvanic corrosion is through the use of sacrificial anodes. Proper placement of the anodes is extremely important and should only be done by a well trained professional.
Dezincification is a common type of selective leaching. Generically, selective leaching is when one element of an alloy experiences corrosion. Dezincification is more likely to occur when pH is low (<6.0) and/or there are high levels of free chlorine (>1.0ppm). An effective water treatment program with monitoring of pH and chlorine levels is the best defense against selective leaching.
Biofilms that are created by microorganisms are likely to lead to corrosion in several ways. The exact mechanism will vary, depending on the species of microbe(s) involved, and the specific conditions that exist. Most microorganisms will excrete corrosive by-products, hydrogen sulfide for example, which will increase the rate of corrosion on the affected surfaces. Anti-microbial chemicals and/or non-chemical disinfection, such as UV or ozone, are an important part of a water treatment program to eliminate the risk of microbiological growth.
Erosion corrosion is essentially abrasion that causes an increased rate of metal deterioration in a system. Erosion is evidenced by grooves or rounded holes in the metal surface. As flow rates and suspended solids increase in water system, the erosion will also increase.
Other types include
Intergranular Corrosion, Crevice Corrosion, and Stress Corrosion Cracking.
How to Prevent
The best defense against corrosion of all types is a well planned and well executed water treatment program that is administered by a qualified water treatment specialist.