Article
Stainless TLC: Tips from the Pros
John Palmer, author of How To Brew (Brewers Publications) and co-author of Brewing Classic Styles (Brewers Publications). In addition to homebrewing, John holds a degree in metallurgical engineering from Michigan Technological University.
Stainless steel relies on surface chemistry uniformity for its corrosion resistance. You might think, “Hey, it’s stainless, I don’t have to clean it right away. It will be fine if I leave it for a couple days. I’ll just pour some water in and let it soak.” But if the pot is half full, then it is not uniform and corrosion could occur at the waterline over time. The propensity for corrosion will depend on what is in the water — salts, minerals, cleaning agents, etc. It is generally resistant to most household chemicals with one main exception — chlorine. Chlorine and chlorides can break down the protective oxide layer and start corroding the steel beneath. Once that happens, that breach is no longer stainless compared to the surrounding area and corrosion will accelerate. Biological contamination can also cause an area to be compromised. A chunk of trub can provide a haven for bacteria to hide under, and as they eat the trub, their waste products (often acidic and containing sulfur) can cause corrosion under there away from the oxygen that makes the protective oxides.
Another big problem is too much heat from blowtorch propane burners. Never let stainless steel get cherry red as those temperatures will cause chromium carbide formation that takes the chromium out of the alloy and makes it un-stainless. Lower heat can still be a problem if it causes discoloration. The protective stainless steel oxides are transparent but heat tint (yellow, brown, blue and purple oxides) is not protective and will initiate further corrosion. Heat tint should be removed from the interior of a brewing vessel where exposure to liquids can cause corrosion. Heat tint on the outside of the pot is not desirable, but is less likely to be a problem.
If the stainless gets scratched it can lead to crevice corrosion, so buff or grind out deep scratches. If your thumbnail doesn’t get caught in it, then it is probably too small to worry about. For best results, do not use angle grinders or coarse sandpaper to remove scratches. Use fine-grained sandpaper (320 or higher) to smooth out the scratch, and follow up with kitchen cleanser to give it a good polish. Always store your stainless steel clean and dry. Scratches in stainless are really not a big deal for homebrewers because they are more accessible and easier to monitor. In professional breweries using CIP (clean-in-place) systems, it is difficult to actually clean the bottom of scratches.
John Blichmann, owner and founder of Blichmann Engineering. John has a BSME (Mechanical Engineering) from Iowa State University and worked for Caterpillar’s engine division for fifteen years. He “retired” from an engineering supervisor position to start Blichmann Engineering in 2002.
Stainless steel is simply carbon steel with a mixture of chromium (Cr) (10–18% is common) and most of the time Nickel (Ni) where 8–14% is common. Chromium quickly and readily reacts with oxygen to form chromium dioxide (CrO2), which is an extremely stable and corrosion resistant compound. It is that very property that makes stainless steel resist corrosion. When Cr is exposed to oxygen, a very thin layer of CrO2 is instantly formed preventing further penetration of chemicals through the layer. When this layer forms, the stainless is deemed “passive.” The key in keeping your stainless a shrine of coolness is to properly maintain that protective layer. With that in mind, chlorine, most commonly bleach, is very harmful to stainless. Chlorine will chow through that layer like a teenage boy through a box of Cheerios and begin to cause pits. In these small pits, oxygen can’t get back in to replenish the layer and over time this pit can lead to a hole in your equipment. In the case of Iodophor, using the recommended concentration for no more than the prescribed time isn’t a problem, but never soak for extended periods. The key thing to maintaining your passive layer is to clean thoroughly and dry completely.
Be very particular about who you let weld on your equipment and what equipment you choose to have welded. Boiling/mashing vessels are not as sensitive to weld quality issues since exposure time is short. But storage and fermenting vessels are exposed for weeks/months, so weld quality is paramount. A tiny bit of weld porosity, cooling crack or oxidized weldment can lead to corrosion and a site for bacterial contamination, so choose a welder wisely. That is a reason we make our conicals weld-free — to avoid issues with weld quality.
Ashton Lewis, Master Brewer at Springfield Brewing Company and is also the Technical Editor for Brew Your Own and the author of BYO’s “Mr. Wizard” column.
In small breweries and home breweries where stainless steel is manually cleaned with abrasive pads and brushes with metal tips, damage is often manifested in the form of scratches to the material. Scratches can be especially problematic when the scratch is made by carbon steel or an abrasive contaminated with carbon steel. These scratches rust because the surface is contaminated with “free iron,” meaning that the iron is not part of the stainless steel crystalline alloy structure.
I suggest cleaning stainless equipment with alkaline cleaners, such as caustic and sodium metasilicate, to remove organic soils like yeast and trub. Acid cleaners work well for removing mineral scale. One of the handiest acids to use at home is white vinegar, and periodic descaling is sufficient in most areas. If you brew with really hard water more frequent descaling may be required. Very hard water can leave mineral scale and scale can lead to crevice corrosion, but you really have to neglect your equipment at home to end up with this problem.
Many sanitizers can safely be used on stainless steel, including iodophors, quats, peroxyacetic acid, hot water and bleach. The key with using bleach is to keep the pH high (over 10) by not mixing with acids or using after acid cleaning with minimal or no rinsing of the acid. Stainless equipment is commonly damaged by the improper use of chlorinated cleaners and sanitizers.
Stainless steel can be polished to remove scratches. The best way to do this is to manually polish the surface with an abrasive intended for stainless steel. At Mueller we use a variety of abrasives that are made using silicone dioxide (glass) for the grit. When polishing stainless you need to make sure to polish in the direction of the grain; just like wood, stainless has a grain structure that is clearly visible by observing the surface.
Stains can usually be removed with acid since stains are caused by something on the surface. If you have rust stains the acids that are effective are also dangerous. Nitric acid and hydrofluoric are both excellent for rust removal, but are also both strong, nasty acids. Whenever these types of compounds are considered for use the MSDS should always be read to understand the handling issues.
Stainless is a great material of construction for brewing as evidenced by its widespread use as the preferred material for all brewing equipment in breweries around the globe. Keep it clean and don’t scratch; this makes for happy brewers!
Written by Betsy Parks
Issue: November 2009
