Pumps are manufactured in an immense variety of sizes and types and are applied in numerous fluid-handling applications. Pumps often provide service in severely corrosive or severely abrasive environments – and sometimes in both such environments simultaneously. To maximize pump life, the utilization of proper materials in pump construction is vital. Namely, the pump should be composed of materials with the required mechanical properties, corrosion resistance and abrasion resistance.
It is important that the alloys used in the design and construction of a pump for a hostile environment be made from the correct type of stainless steel. If chosen correctly for its environment, the stainless steel will resist corrosion and offer strength and toughness with a reasonable margin of ductility in extreme temperatures.
What is stainless steel?
Chromium, a hard and brittle transition metal, is a chemical element that – when added to steel – makes steel stainless. Immunity to atmospheric corrosion requires a chromium content of at least 11%, and steel containing 11 or more percent of chromium is considered chromium stainless steel.
In chromium stainless steel, a tightly-adherent chromium oxide film forms over the surface of the steel. This protective film is so thin that it is invisible even under a microscope, yet it cannot be broken if oxygen is present, maintaining its protective function.
Nickel, when added to chromium stainless steel in excess of 6%, produces a series of strong, ductile alloys of improved corrosion resistance called chromium-nickel stainless steel.
Other elements, such as sulfur, columbium and molybdenum, are sometimes added to give stainless steel other special properties.
Stainless Steel Grades & Castings
Stainless steels are sorted into three grades that help describe certain properties of the alloy based on chemical composition and properties. In general, the more chromium present, the more corrosive-resistant the alloy.
Chromium grades are magnetic and can be either martensitic (when hardenable by heat treatment) or ferritic (non-hardenable by heat treatment).
Chromium-nickel grades are normally non-magnetic and their structure is austenitic (non-magnetic and non-hardenable by heat treatment). Most chromium-nickel grades are highly ductile and can be subjected to more drawing and stretching before breaking than any other steel. Additionally, chromium-nickel grades are exceedingly strong; even in an annealed condition, they have a tensile strength nearly two times that of low-carbon steel.
Group I: Martensitic Straight Chromium (Hardenable)
The stainless steels in this group contain 11.5% to 18.0% chromium as the principal alloying element. Nickel is present in two of the types.
Martensitic Straight Chromium Castings (400 Series):
Group II: Ferritic Straight Chromium (Non-Hardenable)
The difference between ferritic straight chromium and martensitic straight chromium is the difference in chromium percentage. Ferritic straight chromium contains 14.0% to 17.0% chromium and the lower carbon percentage makes these alloys non-hardenable. Nickel is present only in traces.
Group III: Austentic Chromium-Nickel (Non-Hardenable)
The largest percentage of castings poured are in this group. These alloys contain 16.0% to 26.0% chromium and 6.0% to 22.0% of nickel.
Due to these compositions, these alloys cannot be hardened by heat treatment. They can, however, be annealed by rapid cooling, which increases toughness and resistance to corrosion. Varied specifications include adding elements such as molybdenum, titanium, columbium, selenium and sulphur for special properties.
Austentic Chromium-Nickel Castings (300 Series):
Alloy 20 is a special austenitic stainless steel that offers superior corrosion resistance to certain substances, particularly sulfuric acid. Even though the physical properties of the 18-8 type alloys are retained in alloy 20, its corrosion resistance is considerably greater due to its higher percentage of nickel (27.0% to 30.0%).
Pump Application Considerations
After determining the required pump hydraulics, the primary determinants are the anticipated service environment and its effects on the pump’s construction materials. To select the correct materials for proper pump application, the salesperson or sales engineer much know the environment in which the pump will operate and the characteristics of the materials and alloys available. Every type of stainless steel reacts differently to corrosive environments, so it is important to select the best type for individual applications.
There are many types of corrosion with which the salesperson or sales engineer should be familiar, including the following.
General corrosion is the uniform removal of metal that occurs as a result of electro-chemical and chemical reactions between the metal and its surrounding environment. Usually, resistance to general corrosion increases as the alloy content of the stainless steel increases.
Localized & Specific Area Corrosion
Localized corrosion is often more problematic than general corrosion because it cannot be compensated for through the use of thicker materials, as can be done for general corrosion (unless previous duplicate applications allow for an adjusted pump casting to thicken this area).
Localized corrosion includes pitting, crevice corrosion, erosion corrosion, cavitation corrosion, intergranular corrosion and stress corrosion cracking. While each type of localized corrosion is an individual problem, a misapplied pump may be subjected to some or all of these conditions simultaneously.
Pitting & Crevice Corrosion
Pitting and crevice corrosion can occur when the alloy is exposed to halogen ions, particularly chlorides, and the resistance to pitting and/or crevice corrosion is dependent on the amounts of molybdenum and chromium present in the stainless steel.
Typically seen in areas where there is relatively little fluid flow, pitting and crevice corrosion is best corrected through equipment design (i.e. designs that limit the number of crevices). Designing the pump to eliminate stagnant or dead areas lowers the chances of the positive buildup of solids under which crevice corrosion can occur. Additionally, it’s important in the application design that the flow rate (velocity) is maintained to prevent build-up and that the application prevents vapor pockets where a liquid/vapor interfaces. This type of interface permits a concentration of halogen salt, which increases the likelihood of pitting.
Finally, increasing the chromium and molybdenum generally improves resistance to pitting and crevice corrosion. Welding instead of mechanical fasteners is also helpful in minimizing crevice corrosion.
Erosion/corrosion is a localized type of corrosion that typically occurs in selective areas of the pump and is frequently described as a combination of general corrosion and mechanical abrasion in a localized area. The related mechanical abrasion usually results from solid particles impinging on a part or from turbulence in the fluid stream.
Material of Construction & Flow Rate
Both material of construction and flow rate must be considered when anticipating or facing a pump erosion problem.
The general corrosion resistance of the alloy and the velocity of the fluid becomes very important in pump materials that rely on the chromium-rich oxide film (or another protective film) on the stainless steels. At a certain level of corrosion, increased hardness of the alloy is beneficial in combating erosion/corrosion. Increasing hardness, however, is not always the correct answer.
In general, the harder the alloy, the greater the corrosion resistance, including resistance to erosion/corrosion. On the other hand, the streamlining of the flow pattern into and through the pump to prevent impingement is just as important as material selection.
Typically caused by pump misapplication (poor flow conditions), cavitation corrosion occurs when a combination of mechanical and electrical material deteriorations allows the formation of bubbles in the surface of a component, usually the impeller. Once a bubble forms on a component, it will break with enough force to rupture the protective film on the surface of the stainless steel – instantaneously exposing the component to a corrosive environment and general corrosion. Even though the film will rapidly regenerate upon contact with oxygen, a fine amount of general corrosion will occur before regeneration takes place.
Limiting and controlling cavitation corrosion is best achieved via system design changes and proper pump application that can eliminate the formation of bubbles. The selection of the pump is essentially the same alloy consideration as it is for erosion/corrosion.
Stress Corrosion Cracking
This type of corrosion occurs when a piece of component under tensile stress is subjected to a corrosive environment. A particularly troublesome type of localized corrosion, stress corrosion cracking is usually unpredictable. This is because when stress corrosion cracking occurs, the part typically fails through the brittle cracking of a material with very little evidence of general corrosion.
Choosing the Right Stainless Steel Pump
While pump success may be definitely predicted in certain applications, impurities in chemicals and chemical compounds frequently alter effectiveness under actual service conditions. As some impurities inhibit corrosion and others accelerate it, the actual service conditions in terms of corrosion and abrasion should be fully understood before choosing the best available materials of construction that meet the hydraulic requirements.