Why Corrosion Occurs In Pumps
One of the more prevalent problems for ANSI pumps is corrosion. Corrosion is “The unwanted wastage of metallic materials due to reaction with the environment. The effect includes the loss of strength of material, a change in appearance, change in surface heat transfer and fluid flow properties, contamination, seizure, electrical contact failure, leakage, and general surface damage.”1 Corrosion can be very difficult to prevent at times, however, if the correct material is used for a pump’s construction, then there will be little to no room for corrosion to occur.
The most common form of corrosion found in centrifugal pumps is known as “uniform corrosion”. Uniform corrosion is a “form of corrosion with nearly uniform material loss rate on the entire surface.”2 One form of uniform corrosion is rust. Typically, uniform corrosion is the result of water and… Fortunately, uniform corrosion is also the least damaging of all forms of corrosion, as it almost always extends to only the surface of a pump, and is easy to evaluate. Most of the time, uniform corrosion occurs in pumps of cast iron construction. Because rust is iron-oxide (FeO) which is produced when iron (Fe) reacts with oxygen (O) molecules in air and water, rust can be thus prevented when iron is not in contact with air or water. Uniform corrosion can thereby be prevented by utilizing “more resistant materials such as stainless steel or nickel-based superalloys.”3
Though more harmful and not as common as uniform corrosion, the second most common form of corrosion is known as “Galvanic corrosion”. Galvanic corrosion occurs relative to two surfaces in a pump: “Galvanic corrosion occurs when there are combinations of different bare metal surfaces (for example, stainless steel and copper) in the presence of an electrolytic medium such as an acid or alkali. This essentially sets up a battery with charge and material transferred between the different metal surfaces.”4 The best means of preventing Galvanic corrosion is to choose metals which are as similar as possible in the construction of a pump, as well as for the rest of the system.
The third most common form of corrosion is known as “pitting corrosion” (rephrase). Pitting corrosion is usually the result of a protective layer/coating on a metal being damaged, thus exposing and corroding the underlying metal. Cavitation, the source of many problems for pumps, can also cause pitting corrosion. One example of pitting corrosion could be stainless steel. Stainless steel is an iron-based alloy, containing a layer of chromium exterior to the iron in order to prevent rust. If the protective layer of chromium on the exterior of stainless steel is damaged in one area, then one area of the steel lacks protection and thus is subject to pitting corrosion. Pitting corrosion occurs “particularly in liquids with chlorides and especially on stainless steels and on aluminium alloys.”5 The best means of preventing & resisting pitting corrosion is to utilize metals with proper coatings for the construction of a pump. Some examples of metal alloys with high resistance to pitting corrosion are alloys with high nickel content: “Nickel helps to stabilize the microstructure, increasing SCC resistance. Manganese, in moderate quantities and in association with nickel, will perform many functions attributable to nickel and helps prevent pitting”6
Corrosion can seriously damage a pump if the proper materials for a pump relative to the fluid being pumped are not taken into consideration. The metals which the pump is constructed of need to be resistant to acids/bases of extreme pH levels in order to avoid uniform corrosion, as well as pitting corrosion. Also, the entire pumping system need to be as similar to one another as possible throughout the system in order to avoid Galvanic corrosion. Though there are other forms of corrosion, uniform, Galvanic and pitting corrosion are the three most common.
If you would like to learn more about corrosion-resistant metals for ANSI pumps, as well as all kinds of pumps, EMC can help! Contact EMC today to learn more about the recommended construction for your specific pump!
1 A Dictionary of Chemical Engineering by Carl Schaschke (Oxford, 2014), p. 84
4 Same as 3
5 Same as 2