Passivated Stainless Steel Brewing Equipment
Stainless steel is commonly used to make home beer brewing equipment. It has several benefits over other kinds of metal. Its corrosion resistance and long durability makes it a very popular material for craft beer equipment. However, stainless steel can eventually corrode when exposed to certain contaminants, such as iron and sulfur compounds in water. To prevent this from happening, a regular maintenance regime must be followed.
Although stainless steel is known as the perfect metal for beer brewing, it can also corrode or rust. Stainless steel does not rust because of the protective chromium oxide on the surface. If the protective layer of chromium oxide is removed by scouring or reaction with bleach, the iron in the stainless steel is exposed to the air and rusts.
Stainless steel is also vulnerable to normal carbon steel, the metal material will remain in stainless steel jars (because of the affinity of iron to iron) and is very susceptible to rusting. Once the rust destroys the chromium oxide coating, the iron in the stainless steel will also rust.
Carbon steel can be found in a variety of tools, food cans, and steel wool. Let’s take a look at why stainless steel is corroded? And how to passivate stainless steel equipment to protect beer from metal or other odors.
Stainless Steel and Rust
Steel is made from a mixture of iron and carbon, with a carbon content of only about 0.5%. In contrast, stainless steel is manufactured from iron and chromium. The content of chromium in stainless steel is as high as 10-30%, and it is a key element that makes stainless steel resistant to corrosion.
Chromium in stainless steel reacts very quickly with oxygen, and after the reaction, a protective layer of chromium oxide is formed on the surface of stainless steel. Chromium oxide prevents stainless steel from rusting and corroding.
Why does stainless steel corrode?
Your stainless steel brewing equipment is usually very corrosion resistant. Stainless steel will rust when the protective layer of chromium oxide is destroyed for some reason, exposing the iron directly to the air. However, if you expose stainless steel to bleach or other bleach cleaners, it can damage the protective chrome oxide layer. Of course, over-washing or scrubbing stainless steel jars with steel wool can also damage the stainless steel’s protective chromium oxide coating.
Passivated stainless steel
Passivation in brewing equipment refers to the process of chemically treating stainless steel with an invisible protective layer or coating. Passivated stainless steel is immune to corrosion and pitting caused by cleaning chemicals (acids, caustic, disinfectants) and carbon dioxide (2). Stainless steel literally means steel that won’t rust, but it’s not indestructible. Certain chemicals, especially chlorides (such as table salt) are very hard on stainless steel, and if they are not passivated, they can scratch the stainless steel and cause pitting. Even beers with lower values and 2 can cause pitting in stainless steel over time. In this way, passivation of stainless steel is very important, but before passivation of stainless steel, it needs to be descaled and pickled.
What is descaling and pickling?
Although descaling and pickling are often confused with passivation, descaling and pickling must precede passivation to obtain clean, bare metal. Although synonymous with passivation, descaling and pickling remove iron and oxides from stainless steel for effective passivation. Stainless steel will not passivate without first descaling and pickling. Descaling and pickling are done using very strong acids including hydrochloric acid ( ), nitric acid ( 3 ), hydrofluoric acid ( ) and sulfuric acid ( 24 ).
Traditional passivation is the two most common chemicals used for passivation in stainless steel breweries: citric acid and nitric acid. Citric acid is a mild organic acid that is very good at chelating iron, but by itself does not leave a protective coating to prevent metal passivation (from chemical attack). The use of high concentrations of nitric acid (about 20% or more active) is the most widely known passivation method. After 24 hours of air drying, an invisible layer of chromium oxide (23) forms on the stainless steel surface to protect the metal.
However, the problem with this method is that it is not sexual, and because of the dangers of using nitric acid in its dangerous concentration, few people will reuse it in breweries. This poses a question for brewers who at least deep clean and repassivate metal every year – how do you keep the metal clean safely?
Newer, Safer Passivation
There are ways to keep metals in good condition, taste neutral and shiny without having to use extremely dangerous chemicals and concentrations. Using a caustic rinse, an acid rinse, while a tried and true method for removing protein stains, tends to be poor on beer stones and does not passivate the metal properly. Over time, beer stones can build up and the metal can develop microbial induced corrosion (). If left unchecked, these deposits can pit the metal. So what should be done with the metal to keep it in good shape?
Passivation of existing equipment
The following are passivation procedures for existing equipment (breweries and fermentation vessels):
1. If possible, please rinse the excess beer stone;
2. Add 1-2 ounces of the nitric/phosphoric acid mixture per gallon and make sure the value is 2 or less. Circulate through the spray ball at a higher temperature of 140° for at least 15 minutes. Do not exceed 140° (60°) to keep the nitric acid in solution rather than venting to the air;
3. Drain the acid solution, but do not rinse;
4. Add 1-2 ounces per gallon of a non-corrosive alkaline cleaner containing hydrogen peroxide or sodium percarbonate, depending on the beer stone. Circulate at 120-140° (50-60°) for 15-30 minutes using a spray ball and heat exchanger;
5. Drain the solution. Then immediately go to step 6;
6. Rinse the equipment with water of the same temperature as the hot water. You also need to check the rinse water and the value on the container wall, when the value is neutral, the passivation step is complete;
Passivation of new brewing equipment
Assuming the metal has been passivated at the factory, when the new stainless steel arrives at your brewery, it should be in pretty good shape. However, oil, road grime, dust, grime and debris may remain on new metal. In this case, alkaline hot alkaline cleaning is recommended to remove oil and debris.
If any surface rust is observed, a citric acid rinse works well to remove surface rust. Follow the six steps mentioned above.