Metal corrosion can be defined as the destructive attack of a metal through interaction with its environment. Most metals used in the construction of facilities are subject to corrosion, due to the high energy content of the elements in metallic form.
The corrosion of metals takes place through the action of electromechanical cells. Although this single mechanism is responsible, the corrosion can take many forms. Below are examples of various forms of corrosion and suggested methods for combating and/or mechanical means for reducing the possibility of each.
Uniform Attack is a form of electrochemical corrosion that occurs with equal intensity on the entire surface of the metal. Iron rusts when exposed to air and water (hence the need for corrosion inhibitors), and silver tarnishes due to exposure to air. Potentially very risky, this type of corrosion is very easy to predict and is usually associated with &qout;common sense" when making material decisions.
Galvanic Corrosion occurs when two metals having different composition are electrically coupled in the presence of an electrolyte. The more reactive metal will experience severe corrosion while the more noble metal will be quite well protected. Perhaps the most infamous examples of this type of corrosion are combinations such as steel and brass or copper and steel. Typically, the steel will corrode the area near the brass or copper, even in a water environment and especially in a seawater environment. Probably the most common way of avoiding galvanic corrosion is to attach a third, anodic metal to the other two. This method is often referred to as "using a sacrificial anode." Sacrificial anodes should be checked regularly and replaced as necessary.
Crevice Corrosion is a consequence of concentration differences of ions or dissolved gases in an electrolytic solution. To prevent crevice corrosion, it is best to use welds rather than rivets or bolted joints whenever possible. Another alternative is the use of nonabsorbent gaskets. Remove accumulated deposits frequently and design containment vessels to avoid stagnant areas as much as possible.
Pitting is the formation of small pits on the surface of a metal or alloy. Pitting is suspected to occur in much the same way crevice corrosion does, but on a flat surface. A small imperfection in the metal is thought to begin the process, and then a "snowball" effect takes place. Pitting can go on undetected for extended periods, until a failure occurs. A textbook example of pitting would result from subjecting stainless steel to a chloride-containing stream such as seawater. Pitting would overrun the stainless steel in a matter of weeks due to its very poor resistance to chlorides, which are notorious for their ability to initiate pitting corrosion. Alloy blends with more than 2% Molybdenum show better resistance to pitting attack. Titanium is usually the material of choice if chlorides are the main corrosion concern.
Occurring along grain boundaries for some alloys, Intergranular Corrosion can be a real danger in the right environment. The heating of some materials causes chromium carbide to form from the chromium and the carbon in the metals. This leaves a chromium deficient boundary just shy of the where the metal was heated for welding. To avoid this problem, the material can be subjected to high temperatures to redissolve the chromium carbide particles. Low carbon materials can also be used to minimize the formation of chromium carbide. Finally, the material can be alloyed with another material such as titanium that forms carbides more readily so that the chromium remains in place.
When one element or constituent of a metal is selectively corroded out of a material, it is referred to as Selective Leaching. The most common example is the dezincification of brass, which can occur in cooling systems exposed to high chlorine levels. After leaching has occurred, the mechanical properties of the metal are obviously impaired and some metal will begin to crack.
Erosion-corrosion arises from a combination of chemical attack and the physical abrasion because of the fluid motion. Virtually all alloy or metals are susceptible to some type of erosion-corrosion as this type of corrosion is very dependent on the fluid. Materials that rely on a passive layer are especially sensitive to erosion-corrosion. Once the passive layer has been removed, the bare metal surface is exposed to the corrosive material. If the passive layer cannot be regenerated quickly enough, significant damage can be seen. Fluids that contain suspended solids are often times responsible for 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.
Stress corrosion can result from the combination of an applied tensile stress and a corrosive environment. In fact, some materials only become susceptible to corrosion in a given environment once a tensile stress is applied. Once the stress cracks begin, they easily propagate throughout the material, which in turn allows additional corrosion and cracking to take place. The tensile stress is usually the result of expansions and contractions that are caused by violent temperature changes or thermal cycles. The best defense against stress corrosion is to limit the magnitude and/or frequency of the tensile stress.
EnduroSolv® products are available to combat the ravages cooling tower and boiler corrosion such as Uniform Attack and Pitting.
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