Although stainless steel is widely known as the ideal metal for beer brewing, it is still susceptible to corrosion and rust. The reason stainless steel doesn’t rust is because of the protective layer of chromium oxide on its surface. However, if this protective layer is removed through washing or reacting with bleach, the iron in the stainless steel will be exposed to air and begin to rust. Additionally, stainless steel can be damaged by contact with ordinary carbon steel, as the iron particles from the carbon steel can adhere to the stainless steel and cause rusting. Carbon steel can be found in various tools, food cans, and steel wool.
Stainless Steel and Rust
Steel is composed of a mixture of iron and carbon, with the carbon content usually around 0.5%. In contrast, stainless steel contains iron and a high percentage of chromium, typically ranging from 10% to 30%. The presence of chromium is what gives stainless steel its corrosion-resistant properties.
The chromium in stainless steel quickly reacts with oxygen, forming a protective layer of chromium oxide on the surface. This chromium oxide layer acts as a barrier, preventing rust and corrosion from occurring.
Why Does Stainless Steel Corrode?
In general, stainless steel brewing equipment is highly resistant to corrosion. However, if the protective layer of chromium oxide is somehow damaged or destroyed, and the iron is directly exposed to air, the stainless steel will begin to rust. Exposure to bleach or other strong cleaning agents can also harm the protective chromium oxide layer. Similarly, excessive scrubbing or the use of steel wool can remove the protective layer as well.
Passivated Stainless Steel
In brewing equipment, passivation refers to the process of chemically treating stainless steel to create an invisible protective layer or coating. Passivated stainless steel is resistant to corrosion and pitting caused by cleaning chemicals (such as acids, alkalis, and disinfectants) and carbon dioxide (CO2). Although stainless steel is inherently resistant to rust, it is not completely impervious to certain chemicals, especially chlorides found in table salt (NaCl), which can cause scratches and pitting corrosion if the stainless steel is not properly passivated. Low pH beer and CO2 can also cause pitting corrosion over time. Therefore, passivation of stainless steel is crucial, and it should be descaled and pickled before the passivation process.
What Are Descaling and Pickling?
Although descaling and pickling are often associated with passivation, they are actually performed prior to passivation to ensure the stainless steel is clean and free of any contaminants. Descaling and pickling involve the use of strong acids, such as hydrochloric acid (HCl), nitric acid (HNO3), hydrofluoric acid (HF), and sulfuric acid (H2SO4), to remove iron and oxide residues from the stainless steel surface. Without proper descaling and pickling, the passivation process cannot be effective.
The traditional method of passivation in stainless steel breweries involves using citric acid and nitric acid. Citric acid is a mild organic acid that is effective at removing iron particles, but it does not leave a protective coating on the metal surface. Nitric acid, on the other hand, is the most commonly used chemical for passivation. By using a high concentration of nitric acid (at least 20% active substance), an invisible layer of chromium oxide (Cr2O3) forms on the surface of the stainless steel after 24 hours of air drying, providing long-lasting protection against corrosion.
However, this traditional method has its drawbacks. It is not a permanent solution, and due to the dangerous nature of concentrated nitric acid, it is not commonly reused in breweries. This poses a challenge for brewers who need to deep clean and passivate their equipment at least twice a year. Finding a safer alternative to keep the metal clean becomes essential.
Newer and Safer Passivation Methods
Fortunately, there are alternative methods to maintain metal cleanliness, neutral taste, and shine without resorting to extremely hazardous chemicals. One such method involves using caustic rinses and acid rinses to remove protein residues. However, these methods may not effectively remove beer stone deposits, which can lead to microbiologically induced corrosion (MIC) and pitting if left untreated. Therefore, it is important to explore other options to keep the metal in good condition.
Passivation of Existing Equipment
If possible, rinse off any excess beer stone from the equipment. Then, use a mixture of nitric acid and phosphoric acid (1-2 ounces per gallon) with a pH below 2. Circulate the mixture through the spray ball for at least 15 minutes at temperatures below 140°F (60°C). Do not exceed 140°F (60°C) and ensure that the nitric acid remains in the solution instead of being released into the air. Drain the acid solution without rinsing, and depending on the severity of the beer stone, add a non-corrosive alkaline detergent (1-2 ounces per gallon) containing hydrogen peroxide or sodium percarbonate. Circulate the solution through the spray ball and heat exchanger at temperatures of 120-140°F (50-60°C) for 15-30 minutes. Drain the solution and immediately proceed to the next step. Rinse the equipment with hot water at the same temperature, checking the pH of the rinse water and container walls. The passivation process is complete when the pH is neutral.
Passivation of New Brewing Equipment
Assuming that the new stainless steel equipment has been passivated at the factory, it should already be in good condition. However, there may still be traces of oil, road dirt, dust, or debris on the metal surface. In this case, it is recommended to perform an alkaline hot alkaline cleaning to remove these contaminants. If any surface rust is observed, a citric acid wash can effectively remove it. Follow the six steps mentioned above for the passivation procedure of new brewing equipment:
1. CIP (Clean-in-Place) with a heavy-duty, non-chlorinated liquid-built caustic cleaner (2 oz per gallon of hot water at 140-180°F) for 15-30 minutes to remove machine oil, road grime, etc. Drain and rinse thoroughly.
2. If there is any surface rust, perform a CIP using a citric acid solution (2 oz per gallon of 120-130°F water) for 15-30 minutes, then rinse well.
3. Immediately after rinsing the citric acid, perform a CIP with a blend of phosphoric acid and nitric acid, followed by an oxygenated non-caustic cleaner, following the conversion coating passivation steps.