Stainless steel pumps
The stainless steel pump group is normally considered to be pumps which are manufactured from austenitic stainless steels from the Groups A2, A4 or superior. These figures relate to the components which come into contact with the media. External components such as bearing supports are often made of deviating materials e.g. grey cast iron.
Stainless steel pumps should ensure that the components are not attacked by corrosion without additional coatings such as paintwork being necessary. This is achieved by a passivity of the surfaces. This effects from oxygen from the atmosphere, or also special passivity media form a layer on the surface of stainless steel, a layer which therefore shields the base material so that a corrosion resistance is achieved against a variety of media.
Depending on the material quality, different resistance levels for stainless steel pumps arise. The common term "rust free" is therefore rather misleading. Depending on the medium, stainless steel pumps can also be attacked by corrosion and therefore "rust".
Stainless steel pumps are utilised for a variety of applications. In the food industry, pumps made of stainless steel are utilised due to the hygienic requirements so that the term foodstuff pump and stainless steel pump are often used interchangeably as a synonym in this industry. The production of stainless steel pumps requires that particular attention is implemented. It must be avoided that stainless steel components are affected by ferritic materials. Foreign particles from ferritic materials incorporated in the surface finish of stainless steels will inevitably lead to corrosion attack. An initial but mainly visual problem may eventually lead to damage.
A black/white separation is required for the production of stainless steel pumps. Mild steel must be processed separately from stainless steel to safely prevent impurities. Welded joints must be post-treated at all times. Critical is the unprocessed cast stainless steel surfaces. Despite deviations in the requirements, it may be that stainless steel castings are processed with ferritic material blasting due to a lack of time. These impurities cannot always be removed completely.
Therefore in the foodstuff industry, cast surface finishes on stainless steel pumps must be prevented. The most common pump principles are also available on the market as stainless steel pumps. If the common austenitic stainless steels proves not to be sufficient to ensure a permanent resistance, then there is the possibility to utilise higher-quality materials such as Duplex, 1.4539 or also Hastelloy.
For seawater-based applications, bronze is ideal and plastic pumps are available for very aggressive media. Although stainless steel is rather more ductile than hard, stainless steel pumps can also be utilised for conveying abrasive media. The most important factor is a low flow Speed, which exists for twin screw pumps as an example. Often the decisive key components for these application methods are also diffusion hardened. Without dimensions and form alterations and without affecting the corrosion resistance, this can be achieved by a diffusion hardening at 1000-1200 HV.
A diffusion hardening also prevents galling of components made of stainless steels in the context of unintentional contact during operation. This means that nickel-based alloys with reduced resistance can be utilised when considering cleaning agents based on nitric acid.