Corrosion Problems in Brewing by John J. Palmer (Article written for the March/April issue of Brewing Techniques) Beer is corrosive. Not only is beer acidic but it contains live microfauna which can cause bio-fouling and bio-corrosion. Beer can be corrosive to the tanks and fluid lines used in the brewing process, and it can be corrosive to the brewery building too. Several common problems, causes and solutions will be discussed here in the hope that this information will help both micro and homebrewers. Beer and Concrete. Let's start at the ground level with the concrete floors found in most commercial breweries. Beer acts 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 bio-fouling 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 non-stainless (mild) steel. It is where these different metals join that corrosion can be a frequent problem. Galvanic Corrosion All corrosion is basically galvanic (over-generalization). 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. Harken back to your high school chemistry class and I will explain. An electrolyte can be defined as any liquid containing dissolved ions ex. tap water. Each metal has an inherent electrical potential. These potentials are small, but provide for the ranking of the metals from the most passive (lowest potential) Platinum, to the most active (highest potential) Magnesium. See Table 1. Table 1- Galvanic Series in Seawater Magnesium Zinc Aluminum (pure) Cadmium Aluminum Alloys Mild Steel and Iron Un-passivated Stainless Steels Lead-Tin Solders Lead Tin Un-passivated Nickel Alloys Brass Copper Bronze Silver Solder Passivated Nickel Alloys Passivated Stainless Steels Silver Titanium Graphite Gold Platinum 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 intensity of that dissolution can be eyeballed from Table 1, but there are many variables (electrolyte, size, shape, degree of passivity, time, etc) that control a particular corrosion cell's rate. Okay, enough chemistry. 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 Copper is generally more acid resistant than it is alkaline resistant. Alkalines like Bleach, Ammonia 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, very useful for dissolving organic deposits, should be used with caution. Copper is not resistant to oxidizing acids like Nitric and Sulfuric and non-oxydizing-acid solutions that have oxygen dissolved into them. Copper is usually resistant to non-oxidizing acids like Acetic, Hydrochloric, and Phosphoric. 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. Stainless steel has better wear resistance for these purposes. Stainless Steel 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 stainless. These steels do have their Achilles Heel and that heel is Chlorine, which is common in cleaning solutions. Lets say we have an electrolytic solution containing chlorine ions, bleach water for instance. These chlorides are caustic or alkaline and cause the protective oxide layer 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 what if there were a deep scratch in the wall, or a rubber gasket against the steel creating a crevice? Well, 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 oxylate) 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. Procedures for the removal of beerstone were given in the article "Care and Feeding of Stainless Steel" in the July/August issue by Micah Millspaw. However, one of the procedures given 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. 1. Do not allow the stainless steel vessel to sit for extended periods of time (hours, days) filled with chlorinated water. 2. Use alloy specific buffered/inhibited cleaning solutions which reduce the amount of corrosion attack to the metal. (Homebrewers are familiar with B- Brite(tm) and Alconox(tm).) 3. Fill the vessel completely so that all surfaces are at the same potential. 4. Circulate or stir the water to eliminate local concentration/ de-oxidation. 5. After the cleaning or sanitizing treatment, rinse the vessel with de- ionized water to prevent evaporation concentration and either dry it completely or fill it with beer. These simple practices will preclude chlorine induced corrosion. Cathodic Protection for 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.) Alternative Metals 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(tm). 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(tm) alloys have high strength in very high and very low temperatures. These alloys are more corrosion resistant than austenitic stainless. In Closing Every solution has its problems and brewery corrosion is an enthusiastic participant in the game. Fortunately, discussions with several micro-breweries have indicated that the situation is not as dire as the literature search would lead me to believe. Most brewery planners and brewing equipment manufacturers have keyed in to using passivated stainless and waterproofing surfaces in contact with beer. The information presented here should help complete the picture for people who want to understand what's happening and help maintain their investment. Further Reading Some of the information presented in this article came from a chapter in ASM Metals Handbook, 9th Edition, Volume 13 - Corrosion, titled, "Corrosion in the Brewery Industry" by Edgar W. Dreyman of PCA Engineering, Inc. This chapter contains more specific information on some of topics concerning brewery equipment I mentioned above. I would invite you to read this work. [Shadow Box} Passivation of Stainless Steel per Federal Specification QQ-P-35C Passivation of stainless for enhanced corrosion protection in the aerospace industry is performed according to Federal Specification QQ-P-35C. It specifies several different solutions and regimens depending on the alloy type. For 304 and 316 alloy stainless, it specifies immersing in Solution Type VI or Type VII. The 400 series and Precipitation Hardening stainless steels should be passivated in Type II. See Table I. Passivation should be performed by full immersion of the parts (or complete filling in the case of tanks) to prevent severe etching that would otherwise occur at/above the waterline. Care must be taken not to greatly exceed the recommended times or temperatures as this risks damage to the parts. After passivation the parts should be thoroughly rinsed by spraying and/or immersion in tap water, followed by a rinse with de-ionized water and a warm air drying. Drying temperature should not exceed 140¡F. Passivated appearance will be lightly grayed. There should be no evidence of etching, pitting or frosting. Table I - Passivation Treatments Type Temperature Time, min. Sodium Dichromate Nitric Acid (¡F) (minutes) (% by weight) (% by vol.) II 120 - 130 20 2 - 2.5 20 - 25 VI 70 - 90 30 -- 25 - 45 VII 120 - 150 20 -- 20 - 25 [End Shadow Box]