Corrosion by Beer
Beer and Concrete
Let's start at the ground level with the concrete floors found in most commercial breweries. Beer, and particularly wheat beer, can act as a weak acid, dissolving the lime in the concrete. Bacteria can grow in the porosity of the concrete feeding off the sugars that soak in. Once bacteria becomes entrenched, it can only be removed by removing the contaminated concrete. This can be done by grit blasting or acid etching but if the contamination is deep, several inches of concrete may need to be removed to get rid of the infestation and accompanying stench. This biofouling can lead to spalling and cracking of the concrete, particularly if the seepage can reach the steel rebar. Steel in contact with concrete will rapidly corrode in the presence of moisture. In both cases, the solution is to coat the floors and rebar with waterproof epoxies. There are several types of epoxies available, including polyamide epoxies that will cure in high humidity, cool temperature areas. Other water-based epoxies have been developed that have little curing odor, which could be adsorbed by the beer, hops or malts. Glazed tile joined with epoxy grout is another alternative which provides good wear characteristics in high traffic areas.
Beer and Brewery Equipment
The equipment investment of a brewery is considerable. Any metal contacting the beer should not react to produce off flavors. It is for this reason that stainless steel is so commonly used. These steels are acid resistant and do not taint the product. Other common brewery metals are brass, copper, aluminum and mild steel. It is where these different metals join that corrosion can be a frequent problem through what is known as galvanic corrosion.
The electrochemical difference between two metals in an electrolyte causes electrons to flow and ions to be created. These ions combine with oxygen or other elements to create corrosion products. What this means is that cleaning off the corrosion products does not solve the problem. The cause of the corrosion is usually the environment (electrolyte) or the metals themselves. Place any two metals in an electrolyte in contact with one another and a galvanic reaction takes place. The more active metal will dissolve (ionize) in preference to the more passive. The severity of the problem can be estimated from the distance between these metals in a galvanic series, but there are many variables (electrolyte, size, shape, degree of passivity, time, etc) that control a particular corrosion rate.
What this means to the brewer is that if he has mild steel in contact with copper, the steel will corrode. Beer is an excellent electrolyte. If the brewer has copper in contact with passivated stainless steel, the copper will corrode. Brass fittings and silver solder are right in the thick of things with regard to potential, but fortunately the difference is small and corrosion rates would be quite low. One rule of thumb is that if the cathode size is much smaller than the anode size, then the rate of corrosion will be very small. As a practical illustration, stainless steel rivets on a copper tank would cause minimal corrosion of the copper. Copper rivets on a stainless steel tank would soon be history.
Copper is generally more acid resistant than it is alkaline resistant. Oxidizers such as bleach and hydrogen peroxide will quickly cause blackening of copper and brass due to the formation of black oxides. These oxides will rub off, exposing new metal to corrosion. For this reason alkaline cleaners such as ammonia and oxidizers, very useful for dissolving organic deposits, should be used with caution. Copper is not resistant to oxidizing acids like nitric and sulfuric acids and non-oxidizing-acid solutions containing dissolved oxygen. Copper is usually resistant to non-oxidizing acids, e.g. acetic, hydrochloric, and phosphoric acids.
Commercial cleaning solutions should contain buffering agents and inhibitors to prevent corrosion from the solution. Thinning of copper vessels has been observed where water sprays and abrasive cleaners are routinely used.
The corrosion inhibitor in stainless steel is the passive oxide layer that protects the surface. The 300 series alloys commonly used in the brewing industry are much more corrosion resistant and when passivated are basically inert to the beer. Passivation is a process in which oxidizing acids are used to build up the protective oxide layer. Its what makes stainless steels stainless. These steels do have their Achilles heel and that heel is chlorine, a common ingredient in cleaning solutions.
Active chlorine present in bleach solutions can cause the protective oxide layer on stainless steels to deteriorate. If a stainless steel container is completely full of this electrolyte, every surface is at the same electrical potential and nothing happens. But if there is a scratch in the wall, or a rubber gasket against the steel creating a crevice, then these areas can become electrically different from the surrounding area and a galvanic cell can be created. Inside the crevice, on a microscopic scale, the chlorides can combine with the oxygen, both in the water and on the steel surface, to form chlorite ions, thus depleting that local area of oxygen. If the bleach water is still, not circulating, then that crevice becomes a tiny highly active site relative to the more passive stainless steel around it and corrodes. This is known as Crevice Corrosion. The same thing can happen at the water's surface if the keg is only half full. In this case, the steel above the waterline is in air and the passive oxide layer is stable. Beneath the surface, the oxide layer is at a different potential and less stable due to the chloride ions. Now the crevice is represented by the waterline. Stable area above, less stable but very large area below, crevice corrosion occurs at the waterline. Usually this type of corrosion will manifest as pitting or pinholes. The mechanism described is accelerated by localization so a pit is most often the result.
Bio-fouling and beerstone scale (calcium oxalate) can cause the same corrosion phenomena. The metal underneath the deposit becomes oxygen depleted via biological or chemical means and corrosion occurs. This is one reason why the removal of beerstone is important. However, one of the procedures used can lead to further trouble. Muriatic acid is another name for Hydrochloric Acid (HCl). As you would surmise from this discussion, these very strong chlorides are the last thing you want contacting the steel. It is imperative to thoroughly rinse the vessel if this acid was used to remove the scale. Phosphoric acid is a much better choice as it does not attack the steel.
A third way that chlorides can cause corrosion of stainless 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 change the electrical potential of the surface at that site. The next time the surface is wetted, corrosion will immediately take place, creating a shallow pit. The next time the keg is allowed to dry, that pit will probably be one of the last sites to evaporate, causing chloride concentration again. At some point in the cycle life of the keg, that site will become deep enough for crevice corrosion to take over and the pit will corrode through.
By using the above information to understand what is happening to the steel, we can develop usage practices to ensure that the stainless is not attacked and pitted by the use of chlorinated cleaning solutions.
Cathodic Protection of Equipment
As mentioned earlier, corrosion is the result of a difference in electrical potential between metals causing ion exchange. A practical method to prevent this that is used by breweries and petro-chemical companies is cathodic protection. This kind of protection works by applying a direct current voltage that is equal and opposite to the voltage difference between the two metals. Applying this voltage to the metal structure removes the driving force for corrosion and the otherwise-more-anodic metal is protected.
Applying this technique can be very effective in such equipment as the bottle line pasteurizer. Most modern pasteurizers are continuous feeds where the bottles are alternately sprayed by various temperature water jets. The water is highly corrosive due to the high amount of aeration occurring in the spray. The water is a good electrolyte for galvanic corrosion couples from the different alloys used in construction. In addition, within this warm, wet, and oxygenated environment are several sites where bacteria and other biologicals can grow and create deposits. These sites can easily become oxygen deprivation cells as previously discussed. Cathodic protection works very well in preventing both types of corrosion. Several anode materials are available for use: resin impregnated carbon, high silicon cast iron, or platinum coated niobium and titanium. The platinum electrodes are attractive because of their passivity and long service life.One problem when applying this technology to the brewery industry is that oxygen can form as a byproduct at the cathode. The oxygen comes from a breakdown of the water if the over-voltage is too high. This is not a problem for external equipment but would lead to badly oxidized beer if used in conditioning or lagering tanks. The solution in these cases is to use resin-impregnated carbon. In this case, if and when oxygen is formed, it immediately combines with the carbon to form carbon dioxide. (We can only hope that this does not lead to Electro-Carbonated Beer becoming the next big advertising campaign.)
The are several alternative alloy systems available which can be used to combat different corrosion situations. Corrosion and cracking of 300 series stainless steel resulting from scaling or hard water evaporation can be remedied by substituting type 444 or 446 ferritic stainless for various fittings. These alloys are more resistant to bio-fouling conditions than 304.
An alloy group that has been popular in both the aerospace and chemical production industries are the nickel-copper alloys, the Monels. These alloys are commonly used in corrosive fluid systems for piping and pump fittings, as well as heat exchangers. This system is virtually immune to corrosion assisted cracking.
Another more expensive metal alloy system that is very useful for corrosion resistance are the nickel-chromium alloys. These Inconel alloys have high strength in very high and very low temperatures. These alloys are more corrosion resistant than austenitic stainless.
"Corrosion in the Brewery Industry" by Edgar W. Dreyman of PCA Engineering, Inc. ASM Metals Handbook, 9th Edition, Volume 13 - Corrosion, 1987, pages 1221-1225.
Beer has a pH of about 4 when fresh, but this can drop to 3.5 or below if the beer is exposed to oxygen such that it sours, as is inevitable in a traditional cask after dispense. Fresh ciders may have a pH as low as 3.3 and, when oxidized, even below 3. Stainless steel is generally impervious to these levels of acidity, but the oxide layer with which aluminum alloys protect themselves from corrosion is attacked by any pH less than about 4 or over about 9. Aluminum alloy containers are therefore internally lined at manufacture by a sequence of steam-sealing, anodizing and epoxy lacquering. However, if that lacquer lining is broken down (such as may be caused by impact to the keg during handling), then not only can flakes of lacquer get into and jam the extractor valves but also the keg itself can be corrosively attacked. This is most prevalent at exposed welds and can threaten the structural integrity of the container.
Acidity is generally reported in terms of lactic acid, though a great number of other organic and inorganic acids are involved. The acidity figure should be reasonably constant, though slight fluctuations can be expected. Various strains of yeast, as well as lack of proper wort aeration, can affect acidity. (reference)
Abnormally high acidity can be an indication of bacterial infection of the wort and/or beer. Ordinarily it is not a sensitive type of test, because long before titrable acidity creeps up, the palate detects the microbial disturbance; indeed infected beers often show but a small increase in measurable acidity. Some brewers claim that lower acidities of, say, 0.12 percent produce a smoother beer than, say, a beer acidity of 0.17 per cent; much depends on the character of the product.
The pH value of a beer is of considerable importance. Increased pH can, in some cases, bring about an undesirable effect in the palate appeal of the product. Often improper or unnecessary acidification of the mash can increase the pH of finished beer by increasing the production of nitrogenous degradation products and phosphates which act as buffers. The age of the malt and the sulfuring conditions can influence the pH.
The main benefit of adjusting mash pH is to simplify processing under modified malt. Lower mash pH does not typically translate into lower beer pH, because the lower mash pH also works to promote the activity of phosphatase enzymes (which work to improve the buffering mechanism in the wort). Working with your mash pH will tend to improve your amino acid levels, and increase fermentable sugars. Lower pH in combination with softer water (lower residual alkalinity) will improve also the polyphenol balance, which could improve your flavor stability (could is the key word). A reasonable target mash pH is 5.6.
pH tends to rise through the lauter. Depending on the type of beer being brewed, the thickness of the mash, the proportion of dark malts, etc, it may go as high as 6.0 by the end of the runoff. Lighter beers benefit from a lower lauter pH, darker beers can tolerate a higher pH.
Because many of the physical and chemical processes that happen in the boil are pH dependent (hop acid isomerisation negatively affected, protein coagulation positively, etc.) lowering the pH too early in the process will have unwanted side effects. Food grade phosphoric is a good choice, and a pH of 5.2 is a good target. The best way to calculate dosing rates is experimentation.