Not All Stainless Is Created Equal

In modern distilleries, the presence of stainless steel is de rigueur. Everything from fermentors to pumps to tools as simple as shovels are often made from stainless steel. The reason for this is simple: The material is both tough and relatively cheap, making it the obvious choice to endure the harsh environments found in distilleries.

Considering stainless steel’s ubiquitousness, it’s wise for a distiller to know all about the metal, its different types, and the strengths and weaknesses of those types. Unfortunately, that’s not always the case. Many distillers don’t know much about this material they use every day, and as a result, they sometimes misuse their tools.

Best combined with knowledge on how best to maintain your stainless, here are some fundamentals to help you ensure consistent product and a long life for your equipment.

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Why Stainless Works

First, it helps to know more about exactly what stainless steel is.

Credit for the invention of stainless steel, sometimes called “rustless” steel, goes to Leon Guillet in France in 1904—though there is evidence that others were experimenting with it as early as 1821. Guillet found that by adding a small amount of chromium to regular iron, he could greatly increase the alloy’s resistance to corrosion and stress.

That’s partly because chromium is a transition metal that naturally resists chemical corrosion, and it is incredibly hard. It’s also because having chromium present in a metal allows for the creation of something known as a passivated layer.

Passivated layers are thin surface layers of chromium oxide that occur in steel that contains at least 10.5 percent chromium. Passivated layers are often only a few molecules thick; although invisible to the naked eye, these layers are incredibly robust. A stainless-steel surface that’s been fully covered in chromium oxide can resist rust and attack from either acidic or basic chemicals. Passivated layers are also flavor-neutral—critical when storing high-proof spirits or other liquids that can pick up off-flavors.

Despite what some distillers may believe, passivated layers occur naturally on any stainless steel that’s exposed to oxygen. The reason we often perform chemical passivation is simply to remove any free iron or other contamination from the metal’s surface. That ensures that the oxygen can react exclusively with chromium across the entire surface of the metal.

Although passivated layers are resistant to chemicals and rust, their thinness makes them highly susceptible to damage from abrasion, such as by scrubbing or brushing. They’re also susceptible to heat stress, as the metal expands and contracts with temperature changes. That’s why you should re-passivate your stainless steel regularly, either through chemical treatments or by simply allowing the stainless to fully air-dry after cleaning. That will help to extend its lifespan.

Although this passivated layer takes some awareness and upkeep, stainless steel’s combination of corrosion resistance and flavor neutrality makes it invaluable for maintaining cleanliness and consistency.

Types of Stainless

There are hundreds, if not thousands, of different types of stainless steel available to distillers, and they all have their own strengths and weaknesses.

In North America, we identify most stainless using the Society of Automotive Engineer’s (SAE) three-digit identifier system, though some companies also use the ASTM International six-digit system. It’s important to check which grading system a company uses, especially when buying stainless steel from outside the United States; certain grades often do not line up exactly with their American counterparts.

Here we’ll highlight the three most common stainless-steel grades used in distilleries, using SAE nomenclature.

304 Stainless

The most common type of stainless found around the world may be 304 stainless. Used in everything from fermentation tanks to kitchen sinks, it’s relatively durable and easy to work with.

304 stainless is a type of austenitic steel, a family of stainless first developed in Germany in 1912. Austenitic steels contain a mixture of chromium and nickel in proportions designed to specially harden the steel while keeping it ductile and resistant to corrosion. For standard 304 stainless, the composition is 18 percent chromium and 8 percent nickel—that proportion creates a strong yet workable steel that can be formed into almost any shape or thickness. The production process for most 304 stainless also makes it nonmagnetic, though it’s possible to buy specially produced 304 that maintains its magnetism.

There are many sub-varieties of 304 stainless, but most distillers will only ever run into either standard 304 or 304L. The “L” in the latter denotes a lower allowable carbon content in the finished stainless. That reduced carbon content helps to reduce carbide precipitation during welding, making it easier to work with and modify. However, it also lowers the overall strength of the stainless, leaving it more susceptible to stress fractures. That can be problematic for stainless equipment that may be exposed to extreme environments.

The availability and reliability of 304 has made it the go-to stainless for many distillers when building fermentors and holding vessels. However, it does have one important weakness: 304 stainless and other austenitic steels are highly susceptible to what is known as stress corrosion cracking (or SCC), a phenomenon that can irreparably damage the metal, rendering tanks and vessels completely useless. That’s why 304 stainless isn’t commonly used for equipment exposed to extreme-temperature or high-chloride environments.

316 Stainless

The second type of stainless steel that distillers are likely to encounter is 316 stainless. Like 304 stainless, 316 is an austenitic steel that includes chromium and nickel as added constituents—but it uses more nickel and less chromium to increase its hardness.

However, what really sets 316 stainless apart is the addition of a small amount of molybdenum in its manufacturing. Molybdenum is important because it is another transition metal that doesn’t expand or contract greatly with temperature changes, and it’s also highly resistant to chemical corrosion. The inclusion of as little as 2 percent molybdenum in the overall makeup of 316 stainless greatly increases its operating strength and resistance to temperature stress.

Comparable to 304, 316 stainless also has several different sub-varieties, but the standard type is by far the most common in distilleries. The increased strength and durability of 316 have made it the preferred stainless for use in corrosive environments, and it’s often chosen for chemical skids or stills.

The main drawback to 316 stainless is its cost—often as much as 40 percent more than that of 304. That’s why most distillers use it only for select equipment that they know will see increased wear and tear. It’s also important to note that while 316 stainless does provide increased protection from SCC, it is still susceptible.

440 Stainless

The third type of stainless steel that distillers may see, although it’s less common, is known as 440 stainless.

Along with other types in the 400 class, 440 stainless belongs to a special family known as ferritic stainless. Ferritic stainless steel is magnetic and doesn’t contain nickel. Instead, besides iron, it includes only chromium and trace amounts of other metals, such as titanium, that help to increase the hardness of the metal.

Ferritic steels such as 440 stainless have become popular in some distilleries because of their strong resistance to SCC and ability to handle extremely corrosive environments. Indeed, Sea-Cure—a specialized type of ferritic steel—is capable of withstanding high chloride environments, and it’s become popular for equipment that may be exposed to seawater.

Unfortunately, the process of making ferritic steel also limits its thickness, making it unsuitable for many heavy-duty constructions, and it’s still relatively rare in distilleries. It can also sometimes be more expensive than 304 or 316 stainless, which means most distillers use it only if the conditions require it.

Know Your Stainless

Stainless steel plays an incredibly important role in distilleries big and small around the world. Without it, it would be much more difficult to maintain consistency and clean effectively.

As science and metallurgy continue to advance, there’s little doubt that even better and cheaper types of stainless steel will become available. For now, though, it’s important to understand the types of stainless that we have in our facilities, be aware of their weaknesses, and know how to use their strengths appropriately.