Hello N.E.W. Ales readers, Wes here, let’s take some time to discuss something we have been doing a lot of at the brewery. It’s something that’s often not done completely right or sometimes skipped all together. Of course, everyone will be excited to learn I’m talking about passivation of stainless steel.
Anyone that has looked at the price of even basic homebrewing equipment will tell you how expensive stainless steel is. Its easily the largest asset of any brewery and something we should all take care of.
Before I get ahead of myself, let’s talk about what makes stainless steel so special and why it appears in bulk at every food and beverage place that I have ever visited or worked for. When you look at normal carbon steel, you have the potential for large amounts of corrosion. For example, go look at the brake rotors on your car after sitting for a day or two. You wil notice rust buildup very quickly, especially if the moisture in the air is high or it has rained recently. This is due to the steel oxidizing and forming iron oxide on the surface. If the surface rust is left alone and not treated or cleaned, then you have the potential for large holes of rust to form. Makes me think of my old Jeep Cherokee that lost weight due to the amount of rust on every single body surface.
Now, if you have a business that plans to run a beverage made of water, and don’t want your water tasting like rust, then steel is out of the question. A much better option is plastic, unless you’re also worried about the hygiene of the liquid in contact with plastic, then your only option is stainless steel. Yes, stainless steel is the holy grail of any manufacturing process. It’s very friendly to cleaning and disinfection chemicals, it can withstand a huge range of temperatures, it can be polished to a mirror finish, and it won’t rust like normal steel.
Now you may ask, what makes stainless different? Well, that has to do with the addition of one more element called Chromium. The amount of Chromium allows the steel to have all the properties we desire from stainless steel. The best part about Chromium is its ability to form a passive (remember we’re talking about passivation) layer on the metal itself.
This passive layer is microscopically thin and is formed when the oxygen in the air touches the stainless steel. This passive layer on the steel’s surface prevents corrosion of the metal by blocking oxygen, and thus oxidation (rust).
So, after all that we have a shiny metal that won’t rust because it has a small layer of film on the surface that’s produced when the steel touches the air. So, what is passivation and why should you care? Honestly, it sounds like stainless kind of takes care of itself, right?
The major reason we are passivating so much is because all of our tanks were bought new. When the supplier is making tanks for there customer, they need to cut, weld, grind, buff, sand, and scratch the surface when making things out of the steel. When the fabrication process is taking place, the steel loses its passive barrier, and Iron in the steel gets tossed to the surface to infect your shiny new tank. If left alone and the tank is placed into production, you will start to see pitting or rust form on the surface. If you’re making beer, then off flavors will start to show up because of the exposed metal, and you have given microbes a place to hide in your tank that’s very hard to clean out.
Therefore, your next question should be, “Wes, I just got some new tanks and I don’t want my beer to taste like pennies or get infected with wild yeast. How do I perform passivation to save my investment?”
Don’t worry, I’m going to lay it out there for anyone to copy. I’ll keep the reasoning of each step to myself or save it for another post, but if you follow the following steps, you should be good as gold. The only thing I will stress is the use of PPE (personal protective equipment), I’m talking about gloves and goggles at a minimum. These chemicals can be nasty at a professional level and still pack a punch with “safer” homebrew chemicals.
Step 1: Pre-rinse your tank to heat everything up and remove any large soil deposits.
Step 2: Get yourself a good caustic cleaner. I have experience with Ecolab, Chemstation and AFCO that can all offer you a good option. If you’re at the homebrew level, then something like PBW from 5-star should work just fine. I would recommend a concentration in the range of 4-10% caustic. You want a high pH in order to remove organic soil from the surface of the stainless steel. A lot of oil is left over from the manufacturing of the steel, and this step is vital to removing it. Wash the tank for a minimum of 40 minutes at 140 degrees F.
Step 3: Rinse out your chemical. This should be self-explanatory, but it needs to be said. Use the same temp water as your wash and flush till a titration shows no caustic, or the pH of rinse water matches your process water.
Step 4: Time for an acid wash! This is the important step of passivation that removes iron from the surface of the steel and gives you a 100% passive layer on the entire tank. Talk to your chemical rep for a good recommendation. A very heavy dose of star-san will also work, however, I’m a fan of granular citric acid. Use a 50% mix and run the wash for 20 minutes at ~100 F.
Step 5: Rinse again. Make it hot water and rinse until all the acid is out of the system using the pH or a titration kit to verify absence of the chemical used.
Step 6: You’re basically done, however, I wanted to add that you should open your tank up as much as possible to let the air contact the steel surface. This is what actually forms the passive layer. Let the tank air out for a minimum of 24 hours or more (I go for three days).
After that, your tank should be good to go into production without any worry of rust. Contrary to popular belief, this process only needs done once for the life of the tank as long as you acid wash your tank every once in a while (quarterly). Of course, that does not apply if you cut or weld at all. If you do cut or grind, then remember your removing that passive barrier to corrosion and need to re-passivate.
That wraps up my talk about the passivation process, and I hope everyone learned something about how to care for a brewery’s most valuable asset. Happy brewing!