Metallurgy for Homebrewers
Every homebrewer is faced with equipment choices, whether they make it themselves or buy it from a shop. That decision usually involves determining the best combination of cost, performance and anticipated maintenance for that item. There are brewers that live by the mantra, “simpler is better.” There are other brewers that live by, “cheaper is better.” And there are a few that unequivocally state, “only the best will do.”
But which is which? How can you decide? First you need to have a basic knowledge of the available materials, and then you can apply the strengths and weaknesses of those materials to the requirements of the application.
Corrosion
All corrosion is basically galvanic (that is, related to electrical currents that are induced when two metals are in contact with an electrolyte). While this statement may be an overgeneralization, it helps explain how and why corrosion occurs. The electrochemical difference between two adjacent metals creates a battery. If there is an electrolyte present, such as water or beer, then an electric current will flow and the more active of the two metals will ionize. These metal ions will readily combine with oxygen to form oxides or other corrosion products. Corrosion can also occur between two adjacent areas on the same piece of metal, if the presence of dirt, a chemical or a scratch can make the two areas seem electrically different from each other.
The relative surface area of the two metals also affects the corrosion rate. If the more passive metal has a larger surface area than the active metal, the corrosion of the active metal will be increased (and vice versa).
All metals are electrochemically different from one another, covering the spectrum from very active (e.g. magnesium, zinc, aluminum) to very passive (e.g. titanium, gold). The more active metal will corrode in preference to the more passive. This property is often used for corrosion protection. Anodic protection is where an active metal (e.g. zinc) is plated onto a more passive metal (e.g. steel) part to protect it. The zinc corrodes instead of the steel. The problem with anodic protection in brewing is that the active metal is released to the environment (i.e.,
our beer).
Cathodic protection is where a more passive metal is plated onto a more active, such as gold plating onto a steel part. The problem with cathodic protection is that a breach in the plating will cause the more active metal underneath to corrode rapidly. Nickel and chromium platings are often used on brass to protect the brass from corrosion and provide a more aesthetic appearance. Brass, copper, stainless steel, and silver solder are close enough together on the galvanic series that there is not much potential for corrosion between them.
Metals can also be protected from corrosion by building up a uniform oxide film. The efficacy of the oxide film varies and may not be proof against all corrosion. Some oxides are very passive and inhibit almost all corrosion, such as the chromium oxides that protect stainless steel. Other oxides are more reactive, like red rust and heat tint on stainless steel, and do not inhibit further corrosion at all.
Stainless steel is referred to as being “passivated” when the protective chromium oxide surface layer is unbroken. If this oxide layer is breached by iron (from a wire brush or drill bit) or dissolved by chemical action (like bleach) or compositionally altered by heat (brazing or welding), it will rust. The problem with stainless steel corrosion is usually not an off-flavor, but more often a hole in a valuable piece of equipment.
Most other oxides are somewhere in between, and can be used to protect the metal from specific environments. The oxides of copper, brass and aluminum fall in this category. Copper and brass will develop a dull stable oxide over time that is resistant to corrosion in wort, but scouring the metal shiny bright will remove the passive film. To encourage a passive film on aluminum, copper and brass, wash the item thoroughly, dry it thoroughly, and then put it in your oven (dry) at 350 °F (177 °C) for about 10 minutes. This will help the anhydrous oxide layer to thicken.
The passive oxides that prevent corrosion also interfere with metal joining and need to be removed before soldering, brazing or welding.
Specific Metals
Aluminum
Aluminum is easy to form, machine and can be welded with the proper equipment. The thermal conductivity of aluminum is good, about half that of copper. The aluminum alloys most commonly used for cookware are alloys 3003 and 3004, which have very good corrosion resistance. Under normal brewing conditions, aluminum (by itself) will not corrode and should not contribute any metallic flavor to your beer. Do not clean the metal shiny bright between uses or you will increase your chance of getting a metallic off-flavor.
Aluminum will corrode if placed adjacent to another metal like copper in wort or beer, but the short contact time during a typical brewday is not a problem. Aluminum and copper/brass couples should not be used for long term storage of beer. Percarbonate-based cleaners like Straight A and PBW, or unscented dishwashing detergent, are recommended for general cleaning. Do not use bleach or caustic because these will cause pitting.
Copper
Copper has the highest heat conductivity, is easy to form and was traditionally used for making the brewing kettles or “coppers.” Copper can be readily soldered, brazed and welded with the proper equipment. Soldering and brazing should be more than adequate for most brewery uses.
Copper is relatively inert to both wort and beer. With regular use, it will build up a stable oxide layer (dull copper color) that will protect it from any further interaction with the wort. Only minimal cleaning to remove surface grime, hop bits and wort protein is necessary. There is no need to clean copper shiny-bright after every use or before contact with your wort. It is better if the copper is allowed to form a dull copper finish with use.
However, you need to be aware that copper can develop a toxic blue-green oxide called verdigris. Verdigris includes several chemical compounds — cupric acetate, copper sulfate, cupric chloride, etc. — and these blue-green compounds should not be allowed to contact your beer or any other food item because they are readily soluble in weakly acidic solutions (like beer), and can lead to copper poisoning (i.e., nausea, vomiting). To clean heavy oxidation (black) and verdigris, use vinegar or oxalic acid-based cleansers like Revereware Copper and Stainless Steel cleanser.
For regular cleaning of copper and brass, unscented dish detergent or sodium percarbonate-based cleaners are preferred. Cleaning and sanitizing copper wort chillers with bleach solutions is not recommended. Oxidizers like bleach and hydrogen peroxide quickly cause copper and brass to blacken; these oxides do not protect the surface from further corrosion, and are quickly dissolved by the acidic wort. Copper and other trace metals are beneficial nutrients for yeast, but the amounts that are dissolved from non-passive oxides can be detrimental to
the batch.
Copper counterflow wort chillers should not be stored full of sanitizer or water. Any biological deposits can lead to corrosion in both water or sanitizer. Copper should be rinsed thoroughly with clean water and allowed to drain
before storage.
Brass
Brass is a group of alloys made from copper and zinc, with some lead thrown in for machinability. The lead percentage varies, but for the alloys used in plumbing fittings, it is 3% or less. It is this lead that can be dissolved off by the wort. While this teeny, tiny amount of lead is not a health concern, most homebrewers would be happier if wasn’t there at all. (See the sidebar on page 61 for a method to remove surface lead from brass)
Brass can be readily soldered and brazed to copper and stainless steel. The thermal conductivity of brass is similar to aluminum, but the corrosion resistance of brass is more similar to copper. The reason that brass fittings are not commonly used in commercial breweries is that the clean-in-place (CIP) systems and chemicals that are commonly used with stainless steel are too corrosive to copper and brass. As homebrewers, we don’t have to use such strong chemicals, nor are our parts in service 24/7, so corrosion is greatly reduced.
Stainless Steel
Stainless steels are iron alloys containing chromium and nickel. The most common type of stainless steels used in the food and beverage industry are the 300 series, typically containing 18% chromium and 8% nickel. The specific alloys that are most often used are AISI 304 and 316, which are very corrosion resistant and are basically inert to beer. The 200 series alloys use manganese instead of nickel and cost much less than the 300 series. They have similar corrosion resistance and machinability, but are not weldable.
Stainless steels are the most durable of brewing metals, but also the most expensive. Stainless steel can be readily formed, but is more difficult to machine than the other metals. The thermal conductivity of stainless steel is about 10 times less than aluminum.
The key to achieving a passive surface is getting the steel clean and free of contaminants. The easiest way to do this at home is to use a sponge or soft scrubby and kitchen cleanser made for cleaning stainless steel cookware. Three examples are Bar Keepers Friend, Kleen King and Revereware Stainless Steel cleansers. The active ingredient in these cleansers is oxalic acid and it serves the same cleaning purpose as nitric acid. Once the surface has been cleaned to bare metal, the passive oxide layer will reform immediately. These cleansers are an effective method for repassivating stainless after cutting, grinding, soldering or welding, and also work very well for cleaning copper.
Do not use steel wool or even a stainless steel scrubby; they will cause rust. Stainless steel is not invulnerable, any breach in the oxide layer by another metal, or formation of non-passive oxides due to soldering or welding, can initiate corrosion, especially in the presence of chlorides. To be blunt, chlorides are bad for all metals.
Bio-fouling (trub deposits) and beerstone scale (calcium oxylate) can also cause corrosion. The metal underneath the deposit can become oxygen depleted via biological or chemical action and lose passivity, becoming pitted. A two step procedure is most effective for removing beerstone. Beerstone is a combination of protein buildup and mineral deposit, so removal works best if the protein is broken up with a caustic, like sodium hydroxide or PBW, and then the remaining lime can be dissolved by an acidic cleaner like CLR (Calcium Lime Rust Remover).
Stainless steel plate chillers should not be stored full of water or sanitizer due to the possibility of galvanic or biologically-induced corrosion. Plate chillers should be rinsed thoroughly with clean water after cleaning and allowed to drain before storage. Blowing the chiller dry with compressed air will surely help prevent any chance of corrosion during storage.
Interactions with Liquids
The more pertinent question that brewers probably want answered is, “How do these metals affect my beer?” As noted earlier, many metals are necessary nutrients or co-factors for good fermentation but some metal interactions can be a problem. First is the well-known blood-like flavor from iron. The source of the iron can be well-water, rust on stainless steel or from exposed carbon steel in a chipped porcelain-enameled brewpot.
The role of aluminum in Alzheimer’s has been thoroughly discounted, but concerns still come up occasionally when homebrewers consider getting an aluminum brewpot or a turkey fryer setup to brew bigger batches. However, a metallic off-flavor would be noticed long before a toxic level of aluminum could be ingested. Don’t clean your aluminum shiny bright, let it turn dull and you will not have any metallic off-flavors.
Copper is a double-edged sword in brewing. It is beneficial before fermentation, but detrimental afterwards. Copper ions react with the hydrogen sulfide produced during fermentation and reduce it to insoluble copper sulfide, which is left behind with the trub and yeast cake. Switching to all stainless steel brewing equipment can lead to noticeable quantities of hydrogen sulfide and sulfur off-flavors and aromas in the beer. The use of copper wort chillers will provide all the copper necessary, as will including a short piece (1 inch) of copper tubing in the boil.
Copper is a problem post-fermentation because it catalyzes staling reactions, including the production of hydrogen peroxide and can oxidize the alcohols to aldehydes. Finished beer should not be stored in contact with copper, although serving beer with copper tubing in a jockey box should not be a problem, because of the short contact time immediately before serving.
Copper pennies will also work in the boil to supply necessary copper, as will brass, but the caveat with both pennies and brass is the zinc. While zinc is an important nutrient for yeast growth, it can be too much of a good thing. Corrosion of brass can cause increased acetaldehyde and fusel alcohol production due to high yeast growth when zinc concentrations exceed 5 ppm. Excess zinc can also cause soapy or goaty flavors. But like copper, brass is usually stable in wort and will turn dull with regular use as it builds up a passive oxide layer.
Brass should be treated like copper for normal cleaning.
Equipment Issues
Now let’s turn our attention to specific brewery equipment. What material characteristics should you look for in a brewing pot, fluid fittings and tubing? Some of the considerations might be heat conductivity, ease of cleaning, cost and customization. Some brewers might consider cost to be the biggest concern, but short term cost may need to be weighed against long term durability or adaptability. Each material choice should be considered to the application, and how that application may change over time.
In terms of cost, porcelain-enameled steel may seem like the best option, but finding 8-gallon (30-L) or larger pots may be difficult, and the life expectancy due to chipping of the porcelain is often just a couple years. It is nearly impossible to drill holes in porcelain steel for weldless fittings without chipping the coating. Porcelain steel also has poor conductivity and tends to develop hot spots that can scorch the wort.
Aluminum is the next most expensive material after porcelain steel. It has good conductivity and good cleanability. Aluminum is difficult to solder or braze, but can be welded by an experienced welder. However, aluminum is so easy to machine that it is often easiest to add a ball valve by simply drilling a hole and using a weldless bulkhead fitting. A large thick-walled aluminum pot will last forever with gentle cleaning (no caustic) and is typically half the cost of stainless steel.
Stainless steel does not have the conductivity that copper or aluminum has. Still the scorching of wort as a result of this is rarely a problem, and its cleanability is excellent. The 300 series of stainless steel can be readily welded and machined. Large stainless steel pots can be quite expensive, especially if they have aluminum clad bottoms for better heat distribution. A good stainless steel brewing pot will last forever.
One very useful feature on large brewpots is a ball valve, allowing easy transfer of hot liquid to a chiller or another vessel. Ball valves are available in brass, nickel-plated brass and stainless steel. A plain brass valve will last a very long time with proper maintenance. If regular cleaning and maintenance are somewhat difficult, then nickel-plated or stainless steel valves are much more maintenance-free.
Nickel-plated valves and fittings are about twice the cost of plain brass, but 304 stainless steel valves and fittings can be three times the cost of the plain brass. The 200 series alloys cost less than the 300 series and are being increasingly used where weldability is not needed. Stainless steel fittings may buy peace of mind, but honestly a little attention to the recommended cleaning of brass will buy nearly the same performance.
With the cost of copper skyrocketing right now, copper tubing is now more expensive than stainless steel tubing, but copper is easier to find at your local hardware store. Brass is usually only available as rigid pipe or pipe nipples, not tubing. Copper tubing for wort chillers is still the best choice for overall performance, but if you are building a jockey box or similar, you may want to trade the slower thermal conductivity for better corrosion and staling resistance by adding a few more coils of the stainless steel.
Summary
Each material has its advantages and disadvantages depending on the product form and where you want to use it in the brewery. Aluminum and brass are often the most economical choices, but require more attention and gentler cleaning regimens. Copper has long been a clear favorite for do-it-yourselfers because of its easy fabrication, high thermal conductivity and expense, but the cost has now increased to the point where stainless steel is often more economical. Stainless steel has always been the gold standard for brewing equipment due to its excellent corrosion resistance and durability, but the cost of an all-stainless setup used to be prohibitive. The availability of less expensive stainless alloys for valves and fittings is changing that. Weigh your wants and needs and use the information in this article to build a brewery that works best for you.
