Brewing Metallurgy

This page was copied from the web site of John Palmer (http://www.howtobrew.com)

For routine cleaning of copper and other metals, percarbonate-based cleaners like PBW are the best choice. For heavily oxidized conditions, acetic acid is very effective, especially when hot. Acetic acid is available in grocery stores as white distilled vinegar at a standard concentration of 5% acetic acid by volume. It is important to use only white distilled vinegar as opposed to cider or wine vinegar because these other types may contain live acetobacteria cultures, which are the last thing you want in your beer.

Brewers who use immersion wort chillers are always surprised how bright and shiny the chiller is the first time it comes out of the wort. If the chiller wasn't bright and shiny when it went into the wort, guess where the grime and oxides ended up? Yep, in your beer. The oxides of copper are more readily dissolved by the mildly acidic wort than is the copper itself. By cleaning copper tubing with acetic acid once before the first use and rinsing with water immediately after each use, the copper will remain clean with no oxide or wort deposits that could harbor bacteria. Cleaning copper with vinegar should only occasionally be necessary.

The best sanitizer for counterflow wort chillers is Star San. It is acidic and can be used to clean copper as well as sanitize. Star San can be left in the chiller overnight to soak-clean the inside. Cleaning and sanitizing copper with bleach solutions is not recommended. The chlorine and hypochlorites in bleach cause oxidation and blackening of copper and brass. If the oxides come in contact with the mildly acidic wort, the oxides will quickly dissolve, possibly exposing yeast to unhealthy levels of copper during fermentation.

Cleaning Brass

Some brewers use brass fittings in conjunction with their wort chillers or other brewing equipment and are concerned about the lead that is present in brass alloys. A solution of two parts white vinegar to one part hydrogen peroxide (common 3% solution) will remove tarnish and surface lead from brass parts when they are soaked for 5-10 minutes at room temperature. The brass will turn a buttery yellow color as it is cleaned. If the solution starts to turn green and the brass darkens, then the parts have been soaking too long and the copper in the brass is beginning to dissolve, exposing more lead. The solution has become contaminated and the part should be re-cleaned in a fresh solution.

Cleaning Stainless Steel and Aluminum

For general cleaning, mild detergents or percarbonate-based cleaners are best for steel and aluminum. Bleach should be avoided because the high pH of a bleach solution can cause corrosion of aluminum and to a lessor degree of stainless steel. Do not clean aluminum shiny bright or use bleach to clean an aluminum brewpot because this removes the protective oxides and can result in a metallic taste. This taste-detectable level of aluminum is not hazardous. There is more aluminum in a common antacid tablet than would be present in a batch of beer made in an aluminum pot.

As with aluminum, the corrosion inhibitor in stainless steel is the passive oxide layer that protects the surface. The 300-series alloys (a.k.a. 18-8 alloys) commonly used in the brewing industry are very corrosion-resistant to most chemicals. Unfortunately, chlorine is one of the few chemicals to which these steels are not resistant. The chlorine in bleach acts to destabilize the passive oxide layer on steel, creating corrosion pits. This type of attack is accelerated by localization and is generally known as crevice or pitting corrosion.

Many brewers have experienced pinholes in stainless-steel vessels that have been filled with a bleach-water solution and left to soak for several days. On a microscopic scale, a scratch or crevice from a gasket can present a localized area where the surface oxide can be destabilized by the chlorine. The chlorides can combine with the oxygen, both in the water and on the steel surface, to form chlorite ions, depleting that local area of protection. If the water is not circulating, the crevice becomes a tiny, highly active site relative to the more passive stainless steel around it and corrodes. The same thing can happen at the liquid surface if the pot is only half full of bleach solution. A dry stable area above, a less stable but very large area below, and the crevice corrosion occurs at the waterline. Usually this type of corrosion will manifest as pitting or pinholes because of the accelerating effect of localization.

A third way chlorides can corrode stainless steel is by concentration. This mode is very similar to the crevice mode described above. By allowing chlorinated water to evaporate and dry on a steel surface, those chlorides become concentrated and destabilize the surface oxides at that site. The next time the surface is wetted, the oxides will quickly dissolve, creating a shallow pit. When the pot is allowed to dry, that pit probably will be one of the last sites to evaporate, causing chloride concentration again. At some point in the cleaning life of the pot, that site will become deep enough for crevice corrosion to take over and the pit to corrode through.

It is best to not use bleach to clean stainless steel and other metal. There are other cleaners available that work just as well without danger of corrosion. The percarbonate-based cleaners like PBW are the best choice for general cleaning. If you have a particularly tough stain liked burn malt extract then you may need something stronger. There are oxalic acid based cleansers available at the grocery store that are very effective for cleaning stains and deposits from stainless. They also work well for copper. One example is Revere Ware Copper and Stainless Cleanser, another is Bar Keeper's Friend, and another is Kleen King Stainless Steel Cleanser. Use according to the manufacturer's directions and rinse thoroughly with water afterwards.

Passivating Stainless Steel

A situation that often comes up is, Hey, my stainless steel is rusting! Why? What can I do to fix it?

Stainless steel is stainless because of the protective chromium oxides on the surface. If those oxides are removed by scouring, or by reaction with bleach, then the iron in the steel is exposed and can be rusted. Stainless steel is also vulnerable to contamination by plain carbon steel, the kind found in tools, food cans, and steel wool. This non-stainless steel tends to rub off on the surface (due to iron-to-iron affinity), and readily rusts. Once rust has breached the chromium oxides, the iron in the stainless steel can also rust. Fixing this condition calls for re-passivation.

Passivating stainless steel is normally accomplished in industry by dipping the part in a bath of nitric acid. Nitric acid dissolves any free iron or other contaminants from the surface, which cleans the metal, and it re-oxidizes the chromium; all in about 20 minutes. But you don't need a nitric acid bath to passivate. The key is to clean the stainless steel to bare metal. Once the metal is clean (and dry), the oxygen in the atmosphere will form the protective chromium oxides. The steel will be every bit as passivated as that which was dipped in acid. The only catch is that it takes longer-- about a week or two.

To passivate stainless steel at home without using a nitric acid bath, you need to clean the surface of all dirt, oils and oxides. The best way to do this is to use an oxalic acid based cleanser like those mentioned above, and a non-metallic green scrubby pad. Don't use steel wool, or any metal pad, even stainless steel, because this will actually promote rust. Scour the surface thoroughly and then rinse and dry it with a towel. Leave it alone for a week or two and it will re-passivate itself. You should not have to do this procedure more than once, but it can be repeated as often as necessary.

Galvanic Corrosion

All corrosion is essentially galvanic. The electrochemical difference between two metals (when wet) causes electrons to flow and ions to be created. These ions combine with oxygen or other elements to create corrosion products. What this means to a brewer is that cleaning off the corrosion products will not solve the problem. The cause of the corrosion is usually the environment (brewing) and the metals themselves.

Each metal has a small inherent electrical potential; it's what allows you to make batteries out of potatoes. The electricity does not come from the potato, but from the difference in potential of metals that you stuck into it - like copper wire and an iron nail. All metals have a particular potential and a ranking of the metals from the most passive (lowest potential - platinum), to the most active (highest potential - magnesium), is shown below. See Table 1.

Table 19 - Galvanic Series in Seawater

Place any two metals in wet contact with one another and a galvanic reaction takes place. The more active metal of the two will dissolve (ionize). The farther apart the two metals are on the galvanic series, the greater the difference in potential and the stronger the dissolution will be. Size also makes a difference - if the more active piece of metal is smaller than the more passive, the corrosion will be enhanced but if more passive metal is smaller than the more active, the corrosion will be diminished.

Okay, enough chemistry. What this means to the brewer is that if he has copper or brass fitting in contact with passivated stainless steel, the copper will corrode over time. Brass fittings and silver solder have a potential that is close to copper and behave the same way relative to stainless steel. In a wort chiller situation (copper, brass and solder), the silver solder is the most passive and it has the smallest area, so very little corrosion takes place.

With the relatively short usage times that homebrewing equipment sees, corrosion between metals is not a big problem. I am presenting this information so that if you do experience some corrosion, you will hopefully understand what is causing it and can take care of the problem.

Soldering, Brazing, and Welding Tips

Soldering with a propane torch is the easiest way to join copper and brass. You can even use solder to join copper or brass to stainless steel, you just need the proper flux. But there are a couple tips to keep in mind to make it work right the first time:

• Use a liquid flux instead of a paste flux. The paste flux tends to leave tacky residue that is difficult to clean off. If you must use a paste flux, use it sparingly.
• Use plumbing (silver) solder only. Do not use electrical or jeweler's solder because these often contain lead or cadmium. These are toxic metals.
• Apply solder separately to each of your parts before joining them. This practice is known as "tinning" and makes joining the parts easier. Heat the parts, not the solder. Play the flame all around the joint to get it good and hot before you apply the solder. This allows the solder to flow evenly over the joint.

Brazing is like soldering but it is done at higher temperatures and is applicable to more metals. It can readily join stainless steel to itself, and is an alternative to welding. The recommended filler rod for brewing service is AWS type BAg-5, and its temperature range 1370-1550°F (743-843°C). While brazing can provide a stronger joint, the high brazing temperatures can be bad for stainless steel. At those temperatures, carbon in the stainless steel can form chromium carbides which takes the chromium out of solution, making the steel non-stainless near the joint. This area is prone to rust and cracking after it is in service. The problem cannot be fixed by re-passivation so it is best to avoid excessively heating the parts during the braze and keep the total time at temperature to four minutes or less. Propane torches are usually not adequate for brazing. You will need to use MAPP gas or acetylene. Welding is the best methods for joining stainless steel, but it takes skill to make a good joint.

There are two welding processes that will work- MIG (auto-wire feed type) and TIG (tungsten electrode type). TIG welding allows the best control for these small joints. Your best bet is to look in the Yellow Pages of your phonebook for a stainless steel welder to do the job for you. The cost should be minimal, $20-50 depending on the amount of welding needed. I had pipe nipples welded onto 3 converted kegs for 20 dollars. If you want to do it yourself, or you have a friend that welds but has not done stainless before, here is what you need to know:

Ideally, the backside of the weld should be purged with the argon gas to prevent heavy oxidation. But, most welders don't bother, so the backside of the weld joint should be ground/sanded down afterwards to expose clean metal. Do not use steel wool! To clean off the black/blue-ish oxides that could initiate corrosion in the heat affected zone around the weld or braze joints on stainless steel, use the oxalic acid based cleansers and procedures mentioned above in the passivating section.