Why do we use stainless steel (SS) in our breweries?
SS is an alloy of iron, chromium and nickel. It has been preferred in the brewing industry to solve problems caused by other traditional materials:
- Copper (brewhouse): hard to keep clean and shows less structural strength.
- Wood (fermentation/maturation): Sanitation issues, prone to bacterial contamination.
The chromium in SS reacts with oxygen present in the air to form a protective surface layer of chromium oxide that will prevent the oxidation of iron present underneath, or in other words: it will prevent rust.
The amount of chromium and nickel in the alloy will determine its resistance to corrosion. The most common type of stainless steel used in the brewing industry is the 304 type which is composed of 18% chromium and 8% or 10% nickel.
The chromium oxidation will occur spontaneously and thus will naturally form this protective “passive film” of chromium oxide. It is however recommended to chemically assist this process to guarantee a homogeneously protected stainless steel and avoid corrosion due to contaminants left behind after manufacturing.
After a thorough caustic clean of new equipment and the total removal of eventual industrial residual, acids such as nitric or citric acid should be used to effectively remove any free iron from the surface and therefore leave a higher chromium proportion which will favor the creation of the passive layer while preventing potential iron oxidation from starting.
Regular acid cleans are important to prevent beerstone build up in tanks and to maintain a healthy passive layer. Nitric acid being both oxidising and hydrolysing it is great at degrading proteins in beerstone and dissolve minerals while maintaining surface chromium oxide and rid of any eventual iron contamination.
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