Four primary techniques are used for limiting corrosion rates to practical levels:
- Material Selection
- Cathodic Protection
- Chemical Corrosion Inhibitors
Material selection entails picking an engineering material - either metal alloy or non-metal - that is inherently resistant to the particular corrosive environment and also meets other criteria. Variables that will affect corrosion are established along with materials that may provide suitable resistance for those conditions. Obviously other requirements such as cost and mechanical properties of the potential materials must be considered.
Data needed to thoroughly define the corrosive environment include many of its chemical and physical characteristics plus application variables such as its velocity (or is it ever stagnant?) and possible extremes caused by upset conditions. Non-corrosion considerations include mechanical strength, type of expected loading and possibly the compatibility of the different candidates with the required fabrication method. After these criteria and other more unique ones are considered the list of materials that can generally satisfy all requirements usually becomes short. Final selection is then made but trade-off's from optimal meeting of each criterion often are necessary.
Coatings are the most widely used method for controlling corrosion. The possibilities cover a wide range and include such things as paints of many types, electroplating, weld overlays and bonding a thin, corrosion-resistant metal or non-metal onto a stronger substrate metal that is susceptible to corrosion. In many cases the coating simply acts as a barrier between the corrosive environment and the substrate material. In some cases such as in galvanized steel the coating (zinc in this case) provides a barrier but it also acts as a sacrificial anodic material to protect the steel below by preferentially corroding instead of the steel. This effect is cathodic protection.
The coating selection, surface preparation, application and proper quality control throughout the process to attain an optimal coating for the given application requires special experience. This is because - like selecting inherently resistant materials by the first method - there are many variables to be considered. When using organic and inorganic paint coatings there are engineering standards that aid the coating specialist. For those coatings the specific surface preparation required on the substrate is often critical to final success depending on the type of coating being used.
Cathodic protection is a corrosion control technology with a long history. It functions due to a fundamental characteristic of corrosion, i.e., when the electrochemical process of corrosion occurs there is a flow of DC electric current from the surface being attacked. Cathodic protection (often known as CP) provides a flow of DC current onto the protected surface to counteract corrosion current flow. The resulting rate of corrosion is greatly reduced to allow practical, long-term use of the protected metal, e.g., for 10 to 20 years or more, although corrosion is not stopped. The amount of current necessary to be supplied to the surface to control the rate to practical levels depends on the area exposed. Thus CP is most often used in conjunction with some type of coating. This greatly lowers the current needed for protection. No coating is 100% free of small areas where the substrate is exposed. Using CP with a coating means current is only needed at these bare spots.
There are two types of CP. One is the sacrificial anode (also called galvanic) type in which a metal more susceptible to corrosion in the given electrolyte is electrically connected to a less susceptible metal to be protected. The former metal becomes the anode and is consumed over time while the latter metal becomes the cathode in a galvanic corrosion cell. Thus the zinc on galvanized steel is the anode while the steel substrate - as the cathode - is protected. The second type of CP is impressed current CP. Here an electric power rectifier is used to lower the voltage of AC line voltage feed to it while changing the AC to DC current. The rectifier is connected to non-consumable anodes that supply DC current to the metal surface to be protected. Each type of CP has its separate advantages and disadvantages.
Chemical Corrosion Inhibitors are solid, liquid or gaseous compounds that are added in small quantities to the given corrosive environment to change its interaction with the metal to be protected. Corrosion is an electrochemical process that consists of an oxidation reaction on the anodic site (or sites) of the metal plus one or more reduction reactions on the cathodic site (or sites). These two types of reactions must always occur at the same rate. An effective inhibitor functions by chemically changing one or both of the two reactions so as to slow their rates. Thus the rate of the overall corrosion reaction is reduced to practical levels when a proper inhibitor is applied.
Persons that specify inhibitors must have in-depth knowledge. They need to thoroughly understand what corrosion reactions occur without the use on an inhibitor in a given application, which inhibitors will have the desired control affect in that application and whether or not a given inhibitor will have extraneous, undesirable effects in the application. Using and maintaining the correct concentration of inhibitor is essential. Using too much or too little can each have negative consequences. Inhibitors are most often used in circulating cooling water systems or to treat steam boiler feedwater before usage in a boiler.
Gerald O. Davis, PE, President and co-owner of DM&ME, has over 40 years experience in Materials Engineering and Business. Mr. Davis is a Forensic Expert in Materials Usage, Corrosion, Metallurgy, Mechanical Failure, & Root-Cause Failure Analysis. His recent background includes work as a corrosion researcher, senior engineer, and program manager for Battelle Memorial Institute, DNV, Inc., Henkels & McCoy, Inc., respectively and, since 2004, as president of DM&ME.
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