Corrosion in water systems is a costly problem. In the United States alone, the costs associated with corrosion in water and wastewater systems are estimated to exceed $50 billion annually. The metal loss resulting from corrosion reduces system life, increases maintenance costs, and ultimately results in premature equipment failure. Corrosion can also produce deposits that impede water flow, foul heat exchange surfaces, and reduce overall water system efficiency. A basic understanding of the different types of corrosion can help you diagnose and solve corrosion related problems.
Corrosion is typically categorized as either general or localized. Generalized Corrosion proceeds uniformly over an exposed metal surface. With generalized corrosion, fouling is usually a more serious problem than equipment failure.
In contrast, Localized Corrosion is characterized by depressions or pits on metal surfaces. It usually is a more serious problem than generalized corrosion. Forms of localized corrosion include pitting, galvanic corrosion, under-deposit corrosion, and microbiologically influenced corrosion. Localized corrosion can lead to rapid metal perforation and system failure. Erosion-corrosion is another form of corrosion which cannot be neatly categorized as either generalized or localized.
Pitting Corrosion
Pitting is one of the most destructive forms of corrosion and one of the most difficult to predict. Pitting corrosion occurs when discrete areas of a metal undergo rapid localized attack while the vast majority of the surface remains virtually unaffected. Pitting is generally promoted by low velocity or stagnant conditions and by the presence of corrosive ions (i.e., oxygen, chlorides, sulfates, etc.). Once a pit is formed, the solution inside it is isolated from the bulk environment. Over time, it becomes increasingly corrosive, and the process becomes self-sustaining.
Under-Deposit Corrosion
Under-deposit corrosion, also called crevice corrosion, is localized corrosion that occurs within a crevice or any shielded area. Solutions underneath deposits become highly concentrated and corrosive just as within a pit. Alloys that depend on oxide films for protection such as stainless steel, copper, and aluminum are highly susceptible to under-deposit corrosion attack. It is more prevalent at the bottom of horizontal lines, on lower floors, at the end of lines, and where flow rates are slowest. The best way to prevent under-deposit corrosion is to keep metal surfaces clean.
Galvanic Corrosion
Galvanic corrosion occurs when two dissimilar metals are in contact while immersed in a conductive solution. The difference in galvanic potential between the two metals essentially results in a battery, where the anode deteriorates and the cathode is protected from corrosion by the corroding anode. The greater the difference in galvanic potential between the two metals, the faster the reaction will progress. A galvanic potential chart can be used to predict whether galvanic corrosion is likely, which metal will act as the anode to corrode, and how rapidly the corrosion will proceed. A dielectric union or similar device may be used to prevent the two metals from coming in contact, therefore breaking the circuit and preventing galvanic attack.
Erosion-Corrosion
Erosion-corrosion arises from a combination of corrosion and the physical abrasion resulting from fluid flow. Virtually all metals are susceptible to some type of erosion-corrosion. Soft metals or metals that rely on a passive layer for corrosion protection are especially sensitive to erosion-corrosion. Once the passive layer has been removed, the bare metal surface is exposed to the corrosive material. Fluids that contain suspended solids can directly cause or aggravate erosion-corrosion. The best way to limit erosion-corrosion is to design systems that will maintain flow velocities within the limits of the metals being used and minimize the number of changes in direction. Impingement and cavitation are specialized forms of erosion corrosion.
Tuberculation
Tubercles are accumulations of corrosion by-products and other deposits that cap localized regions of metal loss. The structure depends on water chemistry, dissolved oxygen concentration, temperature, flow, and corrosion rates. Tubercles grow internally as metal is removed and externally as corrosion by-products build up. In the absence of significant microbiologically influenced corrosion (MIC), the corroded areas beneath tubercles are usually broad, irregular depressions. Not all red-brown iron oxide deposits in cooling water systems are tubercles. If there is no metal loss beneath the deposit, the iron in the deposit may be material transferred from elsewhere in the system.
Microbiologically Influenced Corrosion (MIC)
The term "microbiologically influenced corrosion" (MIC) describes corrosion processes in which bacteria play a significant role. MIC is often a contributing factor in tuberculation and other forms of corrosion. However, the bacteria do not directly attack metal. In most cases of MIC, the metabolic by-products associated with bacterial growth attack or react with the underlying metal.
Chem-Aqua has the knowledge and expertise to help safeguard your important water systems. We offer a wide range of treatment strategies to protect your systems from corrosion. Contact us today to learn how we can help you manage your water treatment challenges.
Written by: Matt Schnepf, PE, CWT