What can be done about corrosion in submersible pumps?

Here are a few tips for end-users about what to watch out for, plus a survey of the materials and techniques which can help counter corrosion risks.

A submersible pump's corrosion risk is, of course, determined by the environment it is exposed to and, specifically, by the medium it is required to pump. The following liquids can be expected to pose a corrosion risk: sea water, hydrochloric acid, some types of solvents, hydrogen sulphide, liquids with a high copper content, bases with a high pH value and some liquids containing a mixture of acids.

In addition to this general list of potentially corrosive liquids, a further rule of thumb should also be borne in mind. With metals, it is normally the case that the higher the concentration of corrosive substances in a liquid, the worse the corrosion will be. For rubber and plastics, however, long-term exposure to lower concentrations can also result in quite severe corrosion.

General corrosion of an aluminum pump after three months in sea water with low pH.

Counteractive strategies

Corrosion problems in submersible pumps can be counteracted in two basic ways. Firstly, the entire pump can be manufactured of a corrosion resistant material, such as stainless steel. Secondly, various other measures can be taken, such as coatings, anodes and/or the use of resistant materials for particular components which are especially exposed to corrosion risks. If we look first at the main material of manufacture, the vast majority of submersible pumping operations are carried out by cast iron pumps. This normally presents no corrosion problems when pumping liquids such as surface water and domestic sewage. The low oxygen content in raw sewage, in particular, lowers its corrosion effect to almost nothing.

In mining and construction applications, aluminum submersibles are usually preferred. This is, however, for reasons of weight, given that the majority of these pumps are portables, rather than from considerations about corrosion. The large, main pumps in mines, on the other hand, are usually made of cast iron. This material is robust, inexpensive and, as these pumps are stationary, weight considerations are not significant (and, in fact, in alkaline liquids, cast iron is a more resistant material, being able to tolerate pH values of up to 14, whereas aluminum is resistant only up to a pH value of around 8).

A test being carried out to develop regulating equipment for use with impressed current.

Stainless steel

Stainless steel is used as the main material in submersible pumps for two reasons: firstly, for use in acidic liquids and, secondly, where the purity of the liquid being pumped is a prime consideration, as in certain process industries, such as paper and paint production, where colour purity is important.

Coatings and anodes

A less expensive and more flexible alternative against salt water corrosion is to use a coating on a conventional cast iron pump. This is more flexible because the entire range of Flygt pumps can be coated. The most widely used coating is epoxy (a polymer material). Normally, zinc anodes are used in conjunction with epoxy coating because of the inevitability of post-production scratches occurring in the coating.

The use of anodes significantly extends the life of a coated pump. Between five to ten anodes are implanted at various points around the cast iron structure of the pump. Because zinc has a lower electric potential than cast iron, the contact between the two materials spontaneously generates a micro current. The presence of the current protects areas of cast iron exposed by the scratches, while the zinc anodes themselves are "sacrificially" corroded.

An epoxy coated, CS-3126 pump from Flygt.

Impressed current

An alternative to using sacrificial anodes is to supply the micro current by cable from an external power source. This is known as an "impressed current". In this method, a non-sacrificial anode is suspended in the liquid beside the pump. The anode is attached by cable to the pump and the micro current is introduced into this cable. The impressed current method, however, is complicated, expensive and requires a lot of monitoring to be effective. In addition, the extra cables required constitute obstructions in the pump well and often get damaged. Consequently, the implanted (sacrificial) anode method, being simpler and less expensive, is normally preferred.

Resistant components

Rotating shaft seals, rubber "O" rings, cables and cable bushings are crucial components of submersible pumps. They are often manufactured from resistant materials for use in corrosive environments. In conventional submersible pumps, nitrile rubber is used for the "O" rings, but for pumps designed for use in acidic liquids, or liquids containing solvents, the rings are made from fluoro-carbon rubber, known as "Viton"R. As a rule, Flygt submersible pumps manufactured from stainless steel are always supplied with corrosion-resistant "Viton"R, "O" rings.

The standard material for the outer rotating shaft seal is made of a special, corrosion resistant cemented carbide known as WCCR (Wolfram Carbide Corrosion Resistant). For use in acidic liquids the shaft seals can be made from two types of ceramic: aluminum oxide or silicon carbide. The latter is more expensive but has the advantage of also being more abrasion resistant.

Cable protection

Finally, the cable for a submersible pump also needs protection in corrosive liquids. Chlorinated rubber is used as cable sheathing for conventional pumps but Flygt has developed a special "HCR" (High Corrosion Resistance) cable, with sheathing made from a fluorinated ethylene plastic.

Tailored solutions

These corrosion resistance measures are used in various combinations depending on the exact nature of the corrosion potential the pump is likely to be exposed to. For example, in liquids containing solvents it is possible to use standard submersible pumps and simply insert corrosion resistant "O" rings and fit an "HCR" cable.

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