Choice of stainless steel finish and cleaning of surfaces is crucial to protect against bacteria and viruses.
Stainless steel offers superb hygienic and anti-corrosion properties for food, beverage and pharmaceutical industries mainly because of its ability to repair its protective chrome oxide surface when exposed to oxygen.
With over 500 grades and finishes to choose from, there is a solution for every product and process, provided you have the right surface finish to start with and the correct cleaning protocols during operation.
Most engineers and fabricators working with stainless steel have some knowledge of different grades and their properties, but finishes – which are equally important for hygiene and corrosion resistance, – are not quite as well known.
SURFACE FINISH
Surface finish of equipment plays a vital role in the production of foodstuffs.
- The smoother the surface, the more even the protective chrome oxide surface.
- A smooth surface reduces the propensity for solids to attach to the surface. Solids on the surface exclude oxygen thus limiting the steel’s ability to re-passivate.
- Bacteria and viruses find rough areas where they are protect from factors such as cleaning agents.
One measure of the texture of the surface is roughness, most commonly expressed as Ra.
Calculations involving several factors are required to establish the suitable surface finish for each application. This is necessary for almost all processes, particularly food and beverage.
Examples of stainless steel surface finishes
No 1 Hot rolled annealed, pickled. This is a dull, slightly rough surface.
2D Cold rolled, annealed and pickled. Smoother than a No1 finish, with a dull appearance.
2B Cold rolled, heat treated, pickled skin passed. A shinier appearance.
BA Bright annealed. This finish resembles chrome plating.
In addition, there are Bead blasted and Electro-polished finishes.
Surface Roughness – Some general comments
A few important points:
- A stainless steel sheet with a specific Ra value can differ in looks and service
- Finishes can and do vary from mill to mill.
- Incorrect surface treatment can result in irreversible damage. For example, there is no repair procedure if pickling acid is spilt onto a vessel manufactured from a 2B sheet or coil.
- Scratches on the surface of a tank or vessel can and should be blended out but the affected area will have a different appearance
CLEANING PROTOCOLS
Cleaning of equipment to minimise contamination.
One of the big advantages with stainless steel is its cleanability.
The cleaning products used and the frequency of cleaning are both important.
Various chemicals will kill off viruses and bacteria such as Listeria or Covid-19, but the cleaning chemical used should not corrode stainless steel and also not be dangerous to health. For instance, sodium hypochlorite is an effective cleaning agent, but not recommended as it can cause pitting on stainless steel.
The choice of surface finish is just one of the issues dealt with by our Technical Team at NDE Stainless Steel where we offer in-house technical advice and support to customers in many industries from hygienic to mining. Whether you are purchasing stainless steel for equipment manufacture, building a plant or project, solving maintenance or breakdown issues or upgrading your plant or equipment, we assist customers with our expertise in many areas of stainless steel solutions for hygiene and corrosion resistance.
- Call Heinrich Vollgraaff, Sales Manager at NDE Cape in Montague Gardens with your queries on 082 413 5350 or www.nde.co.za
