Views: 16 Author: Site Editor Publish Time: 2022-10-12 Origin: Site
Although stainless steel is known as the perfect metal for beer brewing, stainless steel can also be corroded or rusted. Stainless steel does not rust because of the protective chromium oxide on the surface. If the protective layer of chromium oxide is removed by washing or reacting with bleach, the iron in the stainless steel will be exposed to the air and rust. Stainless steel is also susceptible to damage from ordinary carbon steel. The metal material will remain in the stainless steel tank (because of the affinity of iron and iron), and it is very easy to rust. Once the rust damages the chromium oxide coating, the iron in the stainless steel will also rust. Carbon steel can be found in various tools, food cans, and steel wool.
Stainless Steel And Rust
Steel is made of a mixture of iron and carbon, and the carbon content is only about 0.5%. In contrast, stainless steel is made of iron and chromium. The content of chromium in stainless steel is as high as 10-30%, which is a key element to make stainless steel corrosion-resistant.
The chromium in stainless steel reacts very quickly with oxygen, and a protective layer of chromium oxide is formed on the surface of the stainless steel after the reaction. Chromium oxide can prevent stainless steel from rusting and corroding.
Why Is Stainless Steel Corroded?
Your stainless steel brewing equipment is usually very corrosion resistant. When the chromium oxide protective layer is destroyed for some reason, and the iron is directly exposed to the air, the stainless steel will rust. But, if you expose stainless steel to bleach or other bleach cleaners, it will damage the protective layer of chromium oxide. Of course, excessive scouring or using steel wool to scrub the stainless steel cans will also destroy the chromium oxide protective layer of the stainless steel.
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 will not be affected by corrosion and pitting caused by cleaning chemicals (acids, caustic alkalis, disinfectants) and carbon dioxide (CO2). Stainless steel means steel that will not rust, but it is not indestructible. Certain chemicals, especially chlorides (such as table salt, NaCI) are very hard on stainless steel. If the stainless steel is not passivated, they will leave scratches on the stainless steel and cause pitting corrosion. Even beer with low pH and CO2 will cause pitting corrosion of stainless steel over time. In this way, the passivation of stainless steel is very important, but it is necessary to descaling and pickling before passivating the stainless steel.
What Are Descaling And Pickling?
Although descaling and pickling are often confused with passivation, descaling and pickling must be performed before passivation to get clean, bare metal. Although synonymous with passivation, descaling and pickling can remove iron and oxides in stainless steel to achieve effective passivation. If the stainless steel is not descaled and pickled first, the stainless steel will not be passivated. Descaling and pickling are done using very strong acids, including hydrochloric acid (HCI), nitric acid (HNO3), hydrofluoric acid (HF), and sulfuric acid (H2SO4).
Traditional passivation is the use of the two most common chemicals 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 it does not leave a protective coating to prevent metal passivation (not chemically attacked). The use of high concentrations of nitric acid (approximately 20% or more of the active substance) is the most widely known passivation method. After 24 hours of air drying, an invisible layer of chromium oxide (Cr2O3) will form on the surface of the stainless steel to protect the metal.
But, the problem with this method is that it is not permanent, and because the use of nitric acid at its dangerous concentration is dangerous, few people will reuse it in the brewery. This poses a problem for brewers who deep clean and passivate the metal at least twice a year-how to safely keep the metal clean?
Newer And Safer Passivation
There are many ways to keep metals in good condition, neutral in taste, and shiny, without having to use extremely dangerous chemicals and concentrations. Use caustic rinse, acid rinse is a well-tested method to remove protein dirt, but its effect on beer stone is often very poor, and can not properly passivate metal. Over time, beer stone will accumulate and the metal will produce microbiologically induced corrosion (MIC). If left unchecked, these deposits can pit the metal. So what should be done with the metal to keep it in good condition?
Passivation Of Existing Equipment
（1）If possible, please rinse the excess beer stone;
（2）Add 1-2 ounces of nitric acid/phosphoric acid mixture per gallon and ensure that the pH is 2 or lower. Circulate through the spray ball for at least 15 minutes at the most temperature of 140°F. Do not exceed 140°F (60°C) and keep the nitric acid in the solution instead of being discharged into the air;
（3）Drain the acid solution, but do not rinse;
（4）According to the condition of beer stone, add 1-2 ounces per gallon of non-corrosive alkaline detergent, which contains hydrogen peroxide or sodium percarbonate. Use spray ball and heat exchanger to circulate at 120-140°F (50-60°C) for 15-30 minutes;
（5）Drain the solution. Then immediately execute step 6;
（6）Use water flushing equipment with the same hot water temperature. You also need to check the pH value of the rinse water and the container wall. When the pH value is neutral, the passivation step is completed;
Passivation Of New Brewing Equipment
Assuming that the metal has been passivated in the factory when the new stainless steel arrives at your brewery, it should look pretty good. But, oil, road dirt, dust, dirt, and debris may remain on the new metal. In this case, it is recommended to use alkaline hot alkaline cleaning to remove oil and debris. If any surface rust is observed, citric acid washing can well remove the surface rust. Follow the six steps mentioned above. This is the passivation procedure for new brewing equipment:
（1）To remove any machine oil, road grime, etc., CIP with a 2 oz. heavy-duty, nonchlorinated liquid-built caustic CIP Cleaner per gallon of hot water (140-180°F) for 15-30 minutes. Drain and then rinse well immediately;
（2）If there is any surface rust, CIP using a 2 oz. citric acid per gallon of 120-130°F water CIP for 15-30 minutes, then rinses well;
（3）Immediately after rinsing the citric acid, CIP with a phosphoric/nitric acid blend followed by an oxygenated noncaustic cleaner using the conversion coating passivation steps;