Stainless steel or chrome-vanadium?

Tabelle 1

Tabelle 2

Table1

Table2

Different materials in comparison: Materials used in cleanrooms not only have to withstand mechanical strains but also offer a maximum of security regarding sterility and absence of germs or other product risks. And concerning this matter there are striking differences between stainless steel and chrome vanadium.

The experience always shows that many companies in the pharmaceutical and food industry still work with chrome-vanadium- alloys even in cleanrooms or hygienically sensible areas, whereas only a small fraction works with stainless steel tools. But is that really important? Are the materials so different? There is a clear answer: yes. Thus for exemple in matters of hardness of the tools and consequently the torque capacity as well as the abrasion and wear resistance. In view of product protection as well as disinfectability and sterilisability chrome-vanadium and stainless steel also differ regarding their thermic capacity, the corrosion and the resistance against mechanical strains.

“Of course there are not only differences between chrome-vanadium and stainless steel alloys but there are also differences within the class of stainless steel,” says Steffen Hild, managing director of the cleanroom specialized CAT Clean Air Technology GmbH in Stuttgart. In the case of stainless steel or, when using the correct terminology, rust-resistant steel there are five alloying structures which are in turn dependent on the components of steel like chrome, carbon, nickel and molybdenum:

- austenitic rust-resistant steel
- martensitic rust-resistant steel
- ferritic rust-resistant steel
- precipitation-hardening rust-resistant steel.

The following comparison is limited on chrome-vanadium as well as on the martensitics and austenitics. Besides similar alloying components as they are given regarding the austenitics martensitics have an excellent characteristic which they almost predestine  to be used as material for tools: They can be thermally hardened which is not the case regarding the austenitics.

Hardness of the steel

The hardness of a tool is relevant especially regarding two reasons. Thus for exemple regarding the question what a tool shall accomplish. In most cases it fulfills the task to transfer a torque or a force on another component. If the tool deforms hereby because it cannot withstand the necessary forces, one can instantly dispose of it – thus for exemple often used tools like fork-ring-keys or screwdrivers. The abrasion hardness and wear resistance are also very important. As in sensitive production areas metal particles due to abrasion should not develop or even get in touch with the end product. This is essential particularly in the pharmaceutical and food sector but also in microelectronics – namely under quality aspects as well as regarding the process-risk-analysis (see table 1).

Cleaning, disinfection and sterilisation

The industrial requirement regarding the cleaning of tools is at least a cleanliness on the basis of the visual effect. Contamination adheasions on the surface have to be removed considering the product protection. In the case of higher microbiological requirements like in the food or pharmaceutical sector it is impossible to avoid disinfection and sterilisation of the tools (see table 2).

Corrosion and autoclaving

What leads to corrosion in the case of chrome-vanadium alloys is an electrochemical reaction or rather a liquid which serves as an ion exchanger. The top layer of the material has a moderate corrosion protection as long as it is undamaged. But already the mechanical strain on the tools leads to micro-fissures in the surface which can then quickly corrode. Later on it often happens that the surface peels off at least partially. The disinfection with aggressive mediums also promotes the process of corrosion, because the popular mediums are a “better” ion exchanger.

Rust-resistant steel sorts like martensites and austenites have a clear advantage in this matter because of the immanent passive layer of the material as this layer encloses the material like a regenerative protective cover. Similar to aluminium this protection is an oxide layer, more precisely a chrome-oxide-layer. If the protection layer is damaged, the material below it oxidates with oxygen and generates a new layer. The passive layer is comparatively inert and thus offers an effective protection against corrosion.

Looking finally at the process of autoclaving there is a further disadvantage of chrome-vanadium. As this material withstands only temperatures of nearly under 100 degrees Celsius. If the temperatures are higher, the surface of the material peels off what inevitably leads to corrosion.

Conclusion

Product protection and life cycle of cleanroom tools are extremely dependent on the material selected. In fact the respective material has to meet the mechanical requirements of a tool, but in areas critical for production the cleanliness and hygiene requirements are also of decisive importance. “As tools used in cleanrooms have to be regularly disinfected and sterilised, tools made out of martensite steel sorts are to be favoured over austenites and more than ever over chrome-vanadium in the sense of an effective product protection,” recommends Steffen Hild, general manager of CAT.