Stainless steel is a go-to metal for equipment used by a multitude of industries, including distilling. It’s relatively cheap, easy to work with, and—most importantly—corrosion-resistant. It’s a great metal for storing reactive chemicals, such as high-proof ethanol.
However, some distillers take the words “corrosion-resistant” to mean “corrosion-proof,” which stainless steel is not. There are still many situations in which stainless steel can corrode, and knowing how to prevent that is an important part of being a successful distiller.
Here, we introduce three of the most common types of corrosion found within the distillery—and how to prevent them.
Galvanic Corrosion
Galvanic corrosion is perhaps the most common type of corrosion found on stainless steel in distilleries. Luckily, it’s also one of the easiest to avoid.
Sometimes referred to is as bimetallic corrosion, galvanic corrosion occurs when positively charged metals are exposed to negatively charged ones in the presence of an electrolyte. When that happens, the electrolyte—which can be any medium that allows for electron transfer—begins to transfer electrons from the positively charged metal to the negatively charged metal. That causes the corrosion of both metals.
Stainless steel is an anodic metal, which means it is strongly positively charged. If it’s installed near a cathodic, or negatively charged, metal such as zinc or aluminum, all it needs is an electrolyte for corrosion to take place. In distilleries, that electrolyte often comes in the form of water; spilling some water on both metals would allow the transfer of electrons. In fact, there’s no need for spilled water for this transfer: Moist air and condensation are often enough to create an electrolytic connection, if the two metals are close enough together.
Avoiding galvanic corrosion is mostly a matter of awareness. If you notice corrosion, the first thing you should do is locate anywhere in your facility where stainless steel may be in contact with another metal. That’s commonly found where tanks are anchored to the ground or where different types of pipes connect to each other. However, keep an eye out for mismatched pipe supports and many other unexpected places.
Once you’ve identified the connection points, you can consult a metallurgical chart to identify whether the connecting metal has a different enough charge from stainless steel to cause corrosion. For example: Copper and brass are both mildly anodic metals that can be safely used in connection with stainless steel; tin, which is mildly cathodic, cannot.
If you spot a problem between stainless steel and another metal, remove the connection or replace it with a metal that won’t corrode. If that’s not possible, install an insulator between the two metals, and plan a regular schedule of checks to ensure that the insulator is functioning correctly.
Stress-Corrosion Cracking
A second type of corrosion often found on a distillery’s stainless steel is known as stress-corrosion cracking, or SCC.
SCC is corrosion caused by a mixture of tensile stress, temperature, and exposure to corrosive chemicals, especially chlorides. The most common source of SCC is stainless that’s recently been welded, which can create small pits or crevices from the stress. If those crevices are then exposed to a corrosive environment at high enough temperatures, a fracture can begin to form. As that fracture deepens, corrosive ions are drawn to it and begin to interact with the atomic lattice that makes up the stainless steel’s metal matrix. These further stress the fracture, causing it to grow until it eventually forms a large crack surrounded by brittle metal, rendering the vessel unusable.
SSC is of particular interest to distillers because 300-series stainless steel, which includes the most common types used in distilleries, is especially susceptible to it. Indeed, SCC has been observed in Type 304 stainless at temperatures as low as 125°F (52°C), well below the temperatures that most distillers use for cleaning. That makes SCC a serious potential problem for any distiller working with stainless steel. Note that Type 316 stainless, also popular in distilleries, has a reputation for being hardier—and that’s somewhat true, but even this type has been observed undergoing SCC at 150°F (66°C), which is still below the standard cleaning temperature for most distillers.
Preventing SCC can be difficult because cracks often happen suddenly and without warning. The main thing to do is avoid high-temperature welding on stainless steel because it often increases tensile stress. You should also identify points where stainless pipes or tanks may need additional bracing and talk with engineers or tank manufacturers about possible trouble spots. Finally, you should minimize the use of chlorides or other corrosive chemicals for cleaning, taking special care to never leave any chemical residue in a stainless vessel for extended periods of time. Those steps will help to minimize the chances of SCC.
There are some stainless types, such as ferritic stainless, that are more resistant to SCC, but they’re very expensive and still not completely immune. Furthermore, once SCC has occurred on a piece of stainless, it’s very difficult to repair, as it makes the surrounding metal brittle and prone to further breakup. That is why your focus should be on prevention.
Microbiologically Influenced Corrosion
Finally, a third type of stainless-steel corrosion to be aware of is microbiologically influenced corrosion, or MIC.
Issues with MIC are often associated with biofilms, and they tend to occur when stainless steel is exposed to slow-moving or stagnant nutrient-rich liquids. The causes of MIC can vary; bacteria, fungi, and even microalgae are all known sources. Because of that variability in source, the mechanisms behind MIC are highly dependent on which microbes have colonized the stainless steel, and it would be impossible to cover them all in this article.
Regardless, it’s helpful to be aware of two common causes of MIC.
The first cause comes from iron-oxidizing-bacteria, or IOBs. IOBs derive energy from the oxidation of ferrous iron into its ferric state, and they can rapidly break down stainless steel through a complex series of oxygen-reduction reactions. They are extremely effective at colonizing open water sources, and they need very little to survive. That makes them a common cause of MIC in stainless that’s open to the atmosphere.
The second common cause, and perhaps a more concerning one, comes from anaerobic bacteria known as sulfur-reducing bacteria, or SRBs. These are specialized bacteria that can use sulfur instead of oxygen in their metabolism, and they thrive in the anaerobic conditions often found in fermentors. A by-product of their metabolism is hydrogen sulfide, a strongly acidic compound that interacts with the passive layer on stainless steel, eventually breaking it down and forming small pits. If those pits are allowed to grow, it becomes impossible to fully wash the hydrogen sulfide out of them, and the process can become self-sustaining—eventually resulting in the complete deterioration of the stainless steel, regardless whether the SRBs are removed from the system.
Because MIC can happen under both anaerobic and aerobic conditions, and it’s very hard to detect visually, it’s important to do everything you can to prevent it. Good equipment design and cleaning practices are the best defense against MIC:
- After every cleaning cycle, inspect your vessels for standing water or dirty surfaces that might allow microbes to grow.
- Inspect pipes for dead legs or low points where debris might accumulate and become food for unwanted microbes.
- Finally, periodically take some water that’s passed through important systems and have it tested for biocontamination.
Those steps will help to ensure that cleaning procedures are still effective and that no errant microbes have invaded the system.
Now you know: Despite stainless steel’s excellent resistance to corrosion, it is not impervious. This is only a brief overview of some of the most common types of corrosion your distillery’s stainless steel may face. It’s critical to learn and understand the dangers that exist to stainless steel, so you can take preventative steps to reduce them. In turn, that will help ensure that you get the maximum useful life out of any your equipment—be it a simple length of pipe or a 10,000-gallon fermentor.