Stainless Steel 10107/16/2012
The Care and Feeding of Stainless Steel
The unique properties of stainless steel make it a material of choice among knowledgeable brewers. This primer on stainless steel reviews its composition and properties, discusses methods of cleaning and sanitization, and provides important tips for anyone fabricating or modifying stainless steel equipment.
By Micah Millspaw
As the art of home brewing advances, so does the technology of home brewing. Home brewers often borrow a page or two from commercial brewers’ standard practice to enhance their own breweries. Many home brewers now use discarded stainless steel beer and soda kegs, which are fairly easy to come by and are readily adaptable to many creative brewing systems.
Stainless steel is perceived as the ultimate in bulletproof, easy-to-sanitize equipment, and many home brewers aspire to the goal of an all stainless steel home brewery. Although stainless is very durable, it is possible to abuse and damage it beyond reasonable repair. As for sanitization, properly maintained stainless surfaces are easily sanitized, although getting those same surfaces clean can be quite a chore. This article presents the basics of proper care and feeding of stainless steel equipment.
My personal venture into stainless steel brewing equipment began many years ago with the desire for a large brew kettle. At the time, I had no exposure to other home brewers’ techniques or to any literature about home brewing, so I was on my own. Fortunately, I worked for a company that did stainless steel fabrication work, and they allowed employees to use their facilities after hours.
A few well-placed bribes of homebrew and the application of elbow grease netted me an excellent brew kettle. Later came the stainless fermentors and peripheral equipment. And about six months after that came the problems.
The main problem was that surface residues would not wash or scrub away. These residues began to affect heat transfer in the kettle and introduced undesirable flavors in the fermentors. I spoke with a local microbrewery about this, only to find that they were encountering similar problems themselves.
One day, while looking for some stainless steel ball valves, I stopped by a dairy equipment supplier and not only found the items I was looking for but also discovered that dairies have equipment and cleaning problems similar to those of breweries. In fact, the cleaning and sanitization procedures used in the milk industry are much more stringent than those used in the commercial brewing industry and are certainly more stringent than those used in the world of home brewing. Dairy suppliers, as it turns out, are a good and inexpensive source of information and chemicals for cleaning and sanitization.
TYPES OF STAINLESS STEEL
Stainless steel is an alloy steel with a bright, long-lasting, silvery finish. The alloy has a 11–26% chromium base, with various percentages of nickel added to increase toughness and titanium added to increase weldability.
Many types of stainless steel exist. Most are magnetic tools steels known as 400-series stainless steels, which are unsuitable for use in brewing. The type of stainless steel used in brewing and fermentation equipment is the nonmagnetic 300-series.
Several varieties of the 300-series exist. Those most common to brewing are 304 and 316 stainless steel. Kegs are usually made from these materials. Both 304 and 316 have very good corrosion-resistance properties and are easily welded. Other 300-series metals are to be avoided for brewery use, especially 303. The 303 variety has much less corrosion resistance and is much more difficult to weld.
Beware of imposters: If you are going to use recycled beer kegs, be certain that the kegs you use are actually stainless steel and not aluminum. Although the majority of American kegs are stainless (usually 304), many kegs from Europe and those made in the United States during the late 1950s and 1960s are aluminum. The cleaning procedures discussed in this article will severely damage aluminum. Also be aware that many valves and fittings that appear to be nice shiny stainless steel are actually nickel or chrome-plated brass.
The importance of proper care and feeding: Corrosion and stainless steel? It’s stainless, so it should be free from the risk of rust, pitting, and wear, right? Wrong. Although the 300-series stainless steels used in brewing equipment are normally highly resistant to corrosion, their resitance can be compromised in several ways. For example, some of the cleaning and sanitization techniques commonly used with glass and plastic are hazardous to stainless steel, and you can wreck your stainless equipment if you use them.
CLEANING AND SANITIZATION
Chemical agents: Bleach. Although bleach (potassium hypochlorite) may be the trusted sanitizing standby for home brewers with plastic and glass fermentors, it is a poor choice for sanitizing stainless steel. Prolonged contact with bleach solutions, especially heated ones, can pit the surfaces of stainless steel containers.
Caustic. Caustic (sodium hydroxide), an alkaline cleaning agent commonly used in the clean-in-place (CIP) systems of commercial breweries, is quite effective for removing organics from stainless surfaces, but it is a poor choice for home use. Sodium hydroxide is very dangerous. It is available in liquid and dry form, and both can cause severe skin burns. The proper storage and use of caustic in a home setting is difficult and risky. The use of caustic in cleaning solutions has another downside: Although it is effective for dissolving organic carbon-based compounds, it tends to increase the occurrence of calcium-based deposits on stainless steel. Also, caustic solutions must be used carefully because in high concentrations and with lengthy contact, caustic (like acids) can damage the surface of the stainless steel.
Acids. Some acids can be used for a variety of stainless steel cleaning chores. Phosphoric acid and muriatic acid are very effective in removing beer stone (see below). They are often used to neutralize the effects of caustic cleaning solutions. An added benefit of using acids to clean stainless fermentation equipment is that the pH of any residue on the vessel’s surface can be more closely matched to that of the incoming beer, thus reducing shock to the beer and to the yeast in it. Food-grade acids are usually available in liquid form and should be stored and used with caution. Excessive concentrations and contact times can also damage a stainless steel surface finish.
Nitric acid is often used to passivate the surface of stainless equipment (see below) to improve its sanitary surface finish. The acid mildly etches the surface. It is important not to damage passivated surfaces (see the section on passivation, below).
Iodaphors. Iodaphors, which have recently become quite popular with home brewers, are excellent sanitizing solutions. Iodaphors are made up largely of iodine, phosphoric acid, and alcohol. Iodaphor is also available without phosphoric acid for use with acid-sensitive materials. A concentration as low as 12.5 ppm with a 2-min contact time is adequate for most home brewing needs. Iodaphors can also be used to sanitize a plethora of items either by soaking or recirculating the solution. Store your iodaphor solutions in a dark place because sunlight will break them down and greatly lessen their effectiveness.
Most of these chemicals are available from various industrial and agricultural suppliers. When purchasing any of these chemicals (or similar ones), ask for a material safety data sheet (MSDS). OSHA requires that suppliers provide you an MSDS, but you nevertheless will often have to ask for one. When you get the MSDS, read it and make sure you understand it. Most of these compounds, if mishandled or abused, can be hazardous to you and the environment.
The beer stone problem: The most common surface deposit on stainless steel brewing equipment is beer stone, or calcium oxylate. Beer stone usually appears as a dull brownish to brownish-white film on the metal’s surface. It is most likely to occur in stainless steel brew kettles, hop jacks, counter-flow heat exchangers, and primary fermentors. Prolonged contact with hot wort will result in the greatest amount of beer stone buildup. It is necessary to remove the beer stone before it affects the flavor, or, worse, becomes a sanitization problem.
The reason this calcium-based buildup occurs is that stainless steel, unlike copper, is not wetted by hot wort; a thin layer of air remains between the steel and the hot wort, allowing precipitate to literally bake onto the stainless surface. Perhaps this is an additional reason why many breweries continue to use copper brewhouses or even install new ones. It should be noted that the problem of nonwetting occurs with aluminum, which is even more difficult to clean.
Plastic abrasives: The best way to clean stainless steel equipment in a home brewery is by hand, with a plastic abrasive. Scrubbing pads such as Scotch Bright are inexpensive and perfectly suited for removing brewing residues from stainless surfaces. The grades of plastic abrasives range from coarse to very fine. A good scrubber, some elbow grease, and hot water should effectively clean most of your stainless steel hardware. Never use steel wool, which can induce rust, or abrasive pads that contain soaps inside of them. A dairy brush, which can be found at janitorial supply houses and dairy suppliers, can also be quite useful.
Cleaning soda kegs: Many home brewers store their beer in soda kegs. These containers come in sizes ranging from 3 to 10 gal, and they are ideal for fermenting, dispensing, bottling, and filtering beer. They are also easy to come by and inexpensive. Although several articles describing the use of these containers have appeared in the home brewing literature, to date none has covered the long-term care of soda kegs.
As mentioned above, bleach should not be used for cleaning soda kegs, because it will eventually pit the interior of the keg. Heated bleach solutions will hasten the pitting. Trisodium phosphate (TSP), however, can effectively clean soda kegs, especially when used the keg is soaked in it, because it will not harm the stainless steel or rubber components. Plastic abrasives and some elbow grease also work well.
Eventually, beer stone will begin to build up in the keg. The beer stone can be loosened by soaking the keg for 2–6 h in a 1.7–2.0 pH solution of phosphoric acid at a temperature of 120–130°F 49–54 °C). After the acid soak, the beer stone should be easy to remove using plastic abrasives.
Rust prevention: Although stainless steel is a nickel alloy and does not ordinarily rust, iron-based rust can occur if stainless steel comes into contact with steel containing a high level of iron. The rust often appears as a reddish-brown stain, which seldom is very penetrating and can often be removed by polishing. The longer the stainless steel has been in contact with steel with a high iron content, however, the greater the damage. Deep pitting can occur; in fermentation equipment, these pitted areas can prove difficult to keep clean.
Welding: Don’t weld your own stainless steel. The rust-resistant properties of stainless steel can be compromised also by excessive heat from welding and grinding during the fabrication process. Although much home brewing equipment can be built by the amateur craftsperson and the free labor of helpful friends, several things merit attention if you build your own stainless steel brewery.
Many of the attributes of stainless steel that brewers prize are the result of the crystalline structure of the metal itself. This structure is not a naturally occurring one for steel. Excessive heating can alter the crystalline structure of the stainless steel, causing it to lose its corrosion resistance.
Welding stainless steel should be done only by skilled persons with the proper equipment. Otherwise, the welding can turn into a hideous nightmare. The 300-series stainless steels have a low critical temperature compared with conventional steels. During welding and grinding, care must be taken not to overheat these metals. Damage from overheating is often localized but can nonetheless cause considerable trouble. In some cases, overheated spots that were previously nonreactive to magnetic material will become responsive to magnetic forces. Heavy machining of stainless steel can also cause this effect.
Use the right tools: Other sources of rust induction that often can be traced to fabrication are wire brushes and grinding equipment that has been previously used on conventional steels. After the weld is made, it is usually necessary to use a wire brush on the welds to remove surface impurities. Use only brushes made of stainless steel or brass wire.
Grinding wheels or disks that have been used to grind conventional steels should also be avoided. Grinding with a used wheel can introduce rust into deeper scratches, which can easily result in sanitization problems. Using the right tools when building your stainless steel home brewery will prevent long-term problems and will reduce the number of unsightly blemishes on the outside of your shiny vessels.
The resistance of stainless steel to corrosion and discoloration is, in part, due to the existence of a film of surface oxidation that protects the metal. Improper or excessive use of chlorine-based sanitizers or contact with steel wool, steel tools, or steel parts can damage this film.
A technique known as passivation can be used to provide a chemically clean surface that will aid in the reformation of the surface oxide layer. The oxide film forms naturally on clean surfaces exposed to the atmosphere, but contact with acid mixtures containing oxidizing agents can enhance its formation. An acid wash also serves the important function of dissolving any free-iron contamination on the surface of the stainless steel. Passivation is therefore recommended as a cleaning procedure to remove rust spots and free-iron deposits (the accompanying box outlines the basic procedure). It should be used on sanitary surfaces of stainless equipment after any fabrication work.
FIRST STEPS, BUT SURE STEPS
This information should help home brewers overcome some of the problems encountered when using stainless steel equipment. Although much more can be learned about stainless steel, the detail provided in this article will take most brewers a long way toward successful brewing.
Passivation (National Association of Dairy Equipment Manufacturers, now the Dairy and Food Industry Supply Association, Rockville, Maryland).
“The Crystalline Structure of Metals,” M. Millspaw, research paper, 1991.
Metalwork Technology and Practice, 7th ed., (Repp/McCarthy/Ludwig, Bloomington, Il, 1982).
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