Industrial processes across the globe require pumps to operate reliably and efficiently. The latest pump designs and coating technologies offer significant improvements in the long term performance of industrial pumps. By minimizing the effects of corrosion and erosion, users can enhance productivity and reduce running costs. Continued research into the processes that degrade pump performance is being matched by the development of better application techniques for protective coatings. By gaining a better understanding of both the pumping process and the factors that affect it, end users can make significant improvements in their maintenance strategies. Almost every industrial process involving liquids will include a pump at some point. From deep sea oil and gas to DNA sequencing, pumps are required to perform a vast range of tasks. However, no matter what the design or the size of the pump, central to every application is reliability and efficiency – minimizing down time and running costs is essential to modern industry.

For those working with large industrial pumps, often operating in harsh environmental conditions, maintaining pump performance in the face of a continuous threat from corrosion and erosion can be a particular challenge. With increased knowledge of these processes and the techniques used to tackle them, it is possible to implement a more cost effective pump refurbishment program. Corrosion is commonly defined as a chemical reaction between the component surface and the reacting fluid passing through a pump. In general a distinction is drawn between general or uniform corrosion and localized corrosion like pitting and crevice corrosion. Non-stainless materials suffer mainly from uniform corrosion whereas metals forming oxide layers that adhere to and passivate the surface are prone to localized corrosion. Flow accelerated corrosion (FAC) describes the removal of the protective oxide layer on a metal. The speed of this process is affected by the oxygen content, the flow velocity and, to some extent, the chloride content. The formation of a calcareous layer due to high carbonate hardness of the water reduces or even prevents FAC.

The influence of oxygen can be seen in the following example: Water with an oxygen content of less than 20 ppb (parts per billion) and a flow velocity around 15 m/s will typically see a corrosion rate around 0.01 mm/year. However, increased oxygen content can see the corrosion rate rise to several mm/year, which will present a significant challenge to the process. Fortunately FAC only poses a real issue for low carbon steels and cast iron. Increasing the chromium content or using stainless steel will largely eliminate the vulnerability to flow accelerated corrosion. Pumps that are used to transfer fluids containing abrasive substances, such as sand, can experience significant levels of erosion, especially in areas with high flow velocities. This can be seen in the oil and gas industry where injection pumps are employed to force water back into the oil field and thus maintain the pressure which is needed to lift the oil to the surface. The entrained sand particles act as an abrasive and the high working pressures only serve to compound the issue.

In operating conditions where both erosion and corrosion are present, the degradation mechanism can become very complex and depends on the type of substrate and the fluid chemistry. Corrosion may create oxide layers with low adherence to the substrate which is prone to erosion, or erosion may damage the passive layer, leading to an activation of the surface which accelerates corrosion. In this case surface protection regimes are often the best and sole option. Most commonly seen on the pump impeller, cavitation is caused by a pressure difference, either on the pump body or the impeller. A sudden pressure drop in the fluid causes the liquid to flash to vapor when the local pressure falls below the saturation pressure for the fluid being pumped. Any vapor bubbles formed by the pressure drop are swept along the impeller vanes by the flow of the fluid. When the bubbles enter a region where the local pressure is greater than saturation pressure, the vapor bubbles abruptly collapse, creating a shockwave that, over time, can cause significant damage to the impeller and/or pump housing.

For pump manufacturers, the key is to mitigate the corrosion problems by using the most appropriate base material in the construction of the pump. For applications where the use of carbon steel or cast iron is preferred due to cost reasons, the corrosion rate can be estimated very accurately. Based on the mutually accepted corrosion rate per year, the service life of the pump can be anticipated and factored into the maintenance costs of the application. If the expected corrosion rate is not acceptable the pump materials have to be upgraded to stainless steels which leads to higher costs. In cases where this cost increase is prohibitive, the alternative is to use advanced coatings that can be tailored to suit each application. If stainless steel is selected for an application, the expected service life is much longer, in some cases infinite. However, this is only true as long as the appropriate stainless steel grade has been chosen for the specific application, it has been produced carefully and is used within the agreed fluid specifications. Special care is required as soon as particles are introduced into the fluid.

In this case even stainless steel becomes susceptible to corrosion due to the passive layer being damaged and the base material becoming activated, which then starts to corrode. Normally the passive layer can be re-established, but if the chloride content is too high or the pH level is too low, the material may remain in an active state and the corrosion continues. Another frequent cause of corrosion in stainless steel pumps are stagnant conditions caused by process interruptions or intermittent operation. A further threat for stainless steel is chlorine, which is used to combat biological growth in the pump or the connected pipelines. Low level concentrations, around 2 ppm, will have little impact on stainless steel, but it is important to understand how and where the chlorine is introduced into the water flow, to avoid spot concentrations that will damage the protective layer. Unexpected corrosion can easily negate the anticipated improvement in durability of stainless steel compared to the much cheaper carbon steel variant.