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Preventing Corrosion In The Brewery

04/08/2015

By John J. Palmer (Brewing Techniques)

Corrosion in the Brewery: How to Get the Upper Hand

 

Corrosion on metal

 

Behind the heft of your brewery’s structure and the shine of the equipment lurks a menace that can ruin your beer and your financial investment. Understanding corrosion leads to preventive practices that protect and preserve your beers, today and well into the future.

 


 

Beer is corrosive. It is acidic, and it contains live microfauna that can cause biofouling and biocorrosion. Beer can corrode the tanks and fluid lines used in the brewing process, and it can corrode the brewery building as well. This article discusses common examples of corrosion in breweries, its causes, and practical solutions and preventive measures in the hope of helping both home and microbrewers better manage their breweries.

 

Starting at Ground Level: 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. Feeding off of the sugars that soak in, bacteria can grow in the pores of the concrete. Such biofouling can lead to spalling and cracking, particularly if the seepage reaches the steel rebar. Steel in contact with concrete will corrode rapidly when moisture is present.

The preventive solution is to coat the floors and rebar with waterproof epoxies. Several types of epoxies are available, including polyamide epoxies that will cure in high-humidity, cool-temperature areas. Odors from curing epoxies can be absorbed by beer, hops, and malts. Water-based epoxies have little curing odor and can be used when this is a concern. Glazed tile joined with epoxy grout is another alternative; it provides good wear characteristics in high-traffic areas.

Once bacteria become entrenched, they can be removed only by removing the contaminated concrete. This can be done by grit-blasting or acid-etching. If contamination is deep, however, several inches of concrete may need to be removed to eliminate the infestation and its accompanying stench.

 

Brewery Equipment and Metal Corrosion

 

The equipment investment of a brewery is considerable. Because it is important that any metal contacting the beer not react to produce off-flavors, stainless steels are commonly used. Stainless steels are acid-resistant and do not taint the product. Other common brewery metals are brass, copper, aluminum, and nonstainless (mild) steel. It is where these different metals join and contact each other that corrosion can be a problem.

Galvanic corrosion: All corrosion is basically galvanic (a slight overgeneralization, but useful for practical discussion). 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. Cleaning off the corrosion products does not solve the problem. The cause of the corrosion is usually the environment (electrolyte) or the metals themselves. To understand this phenomenon, all we need is a little high school chemistry.

An electrolyte can be defined as any liquid containing dissolved ions (tap water, for example). Each metal has an inherent electrical potential. These potentials are small, but they provide the basis for ranking metals from the most passive (lowest potential) platinum to the most active (highest potential) magnesium.

Place any two metals in a common electrolyte and a galvanic reaction takes place. The more active metal will dissolve (ionize) in preference to the more passive. The driving force of that dissolution can be gauged from the distance between the metals as shown in the box (the larger the distance, the greater the dissolution), but there are many variables (electrolyte, size, shape, degree of passivity, temperature, time) that control a particular corrosion cell’s rate.

OK, enough chemistry. Suffice it to say that beer is an excellent electrolyte and that if the brewer has mild steel contacting copper, the steel will corrode. If the brewer has copper contacting passivated stainless steel, the copper will corrode. Brass fittings and silver solder are right in the middle with regard to potential, but fortunately the difference is small and corrosion rates will therefore below.

One rule of thumb is that if the physical size of the cathode (the most passive metal) is considerably smaller than that of the anode (the most active metal), the rate of corrosion will be very small. As a practical illustration, passivated stainless steel rivets on a copper tank would cause minimal corrosion of the copper. Copper rivets on a passivated stainless steel tank, however, would soon be history.

Copper: Copper is generally more acid-resistant than alkaline-resistant. Alkalines, such as bleach, ammonia, and hydrogen peroxide, will quickly blacken copper and brass. The blackening is the result of the formation of black oxides. These oxides 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 such as nitric and sulfuric and non-oxidizing acid solutions that have oxygen dissolved into them. Copper is usually resistant to nonoxidizing acids such as acetic, hydrochloric, and phosphoric. Beer can be considered a non-oxidizing acid due to the low pH and lack of dissolved oxygen.

 

Galvanic Series in Seawater

Most active (anodic)

Magnesium

Zinc

Aluminum (pure)

Cadmium

Aluminum alloys

Mild steel and iron

Unpassivated stainless steels

Lead–tin solders

Lead

Tin

Unpassivated nickel alloys

Brass

Copper

Bronze

Silver solder

Passivated nickel alloys

Passivated stainless steels

Silver

Titanium

Graphite

Gold

Most passive (cathodic)

Platinum

 

Good commercial cleaning solutions contain buffering agents and inhibitors that prevent corrosion attributable to 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.

Corrosion resistance in 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 quite corrosion-resistant and when passivated are basically inert to beer. Passivation is a process in which oxidizing acids are used to build up a protective oxide layer. Passivation is what makes stainless steel stainless. But these steels do have their Achilles heel — chlorine, a common ingredient in cleaning solutions.

Crevice corrosion. Let’s say we have an electrolytic solution containing chlorine ions; bleach water, for example. These chlorides are caustic (alkaline) and deteriorate the protective oxide layer. 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 the wall has a deep scratch in it, or a rubber gasket against the steel creates a crevice? These areas can become electrically different from the surrounding area, creating a galvanic cell. On a microscopic scale, the chlorides 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 it corrodes. This is known as crevice corrosion.

 

Passivation as Performed in the Aerospace Industry

In the aerospace industry, passivation of stainless steel for enhanced corrosion protection is performed according to Federal Specification QQ-P-35C. It specifies several solutions and regimens depending on the alloy type. For 304 and 316 alloy stainless, it specifies immersing in Type VI or Type VII solutions (see Table I); the 400 series and precipitation hardening stainless steels should be passivated by immersion in Type II solutions.

Passivation should be performed by fully immersing the parts (or, in the case of tanks, completely filling) 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 may damage the parts or vessel.

After passivation, the parts should be thoroughly rinsed by spraying and/or immersing in tap water, followed by a rinse with deionized water and a warm air drying. Drying temperature should not exceed 140 °F (60 °C).

Passivated surfaces appear lightly grayed. No evidence of etching, pitting, or frosting should be present.

table i

Passivation Treatments

Type

Temperature

Time

Sodium Dichromate

Nitric Acid

(°F)

(°C)

(min)

(w/w)

(v/v)

II

120–130

49–54

20

2–2.5

20–25

VI

70–90

21–32

30

25–45

VII

120–150

49–66

20

20–25

 

The same thing can happen at the water’s surface if the container is only half full. In this case, the steel above the waterline is exposed only to air, and the passive oxide layer is stable. Beneath the surface, the oxide layer is at a different potential and less stable because of the chloride ions. Now the crevice is represented by the waterline: stable area above, less stable but very large area below, and crevice corrosion occurs at the waterline. Usually this type of corrosion manifests as pitting or pinholes. The mechanism described is accelerated by localization, so a pit is most often the result.

Biofouling and beerstone. Biofouling and beerstone scale (calcium oxylate) can also cause corrosion. The metal beneath the deposit becomes oxygen-depleted through biological or chemical means, and corrosion occurs. This is one reason why it is important to remove beerstone.

An article published in the July/August issue of BrewingTechniques presented procedures for removing beerstone. One of the procedures given, however, can lead to further trouble unless extreme care is taken. Muriatic acid is another name for hydrochloric acid (HCl). As you might surmise from the discussion thus far, very strong chlorides are the last thing you want contacting your steel. If you use this acid to remove beerstone scale, it is imperative that you thoroughly rinse the vessel afterwards. Phosphoric acid is a much preferred choice because it does not attack the steel.

Concentration. 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, the 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 surface 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 life cycle of the keg, that site will become deep enough for crevice corrosion to take over, and the pit will corrode through.

 

Practical Steps for Preventing Corrosion of Stainless Steel

 

Based on the chemistry of corrosion outlined above, we can develop practices that ensure that stainless steel surfaces are not attacked and pitted by the use of chlorinated cleaning solutions.

·         Do not allow stainless steel vessels to sit for extended periods of time (hours, days) filled with chlorinated water.

·         Use alloy-specific buffered/inhibited cleaning solutions that reduce the amount of corrosion attack to the metal (home brewers are familiar with B-Brite and Alconox).

·         Fill the vessel completely so that all surfaces are at the same potential.

·         Circulate or stir the water to eliminate local concentration/deoxidation.

·         After the cleaning or sanitizing treatment, rinse the vessel with deionized 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: Because corrosion is the result of a difference in electrical potential between metals and the subsequent ion exchange, one practical method to prevent it is cathodic protection. This kind of protection, which is used by breweries and petrochemical companies, 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.

This technique can be very effective when applied to such equipment as the bottling line pasteurizer. Most modern pasteurizers are continuous-feed systems in which the bottles are alternately sprayed by various temperature water jets. The water is highly corrosive because of dissolved gases from the high amount of aeration occurring in the spray. The water is a good electrolyte for the galvanic corrosion couples (the different alloy pairs) used in construction. In addition, within this warm, wet, and oxygenated environment are several sites at which bacteria and other biologicals can grow and create deposits. These sites can easily become oxygen-deprivation cells.

Cathodic protection works very well in preventing both types of corrosion. Several anode materials are available: resin-impregnated carbon, high-silicon cast iron, and platinum-coated niobium and titanium. The platinum electrodes are attractive because of their passivity and long service life.

One problem encountered when applying this technology to brewing is that oxygen can form as a by-product at the cathode; if the over-voltage is too high, oxygen is produced from the breakdown of the water. Such oxygen production is no problem for external (non–beer contacting) equipment but would lead to badly oxidized beer if it occurred in conditioning or lagering tanks. The solution is to use resin-impregnated carbon at the anode. In such cases, 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 electrocarbonated beer as the next big advertising campaign.)

 

Alternative Metals

 

Several alternative alloy systems can be used to combat various corrosion situations. Corrosion and cracking of 300-series stainless steel as a result of scaling or hard-water evaporation can be remedied by substituting type 444 or type 446 ferritic stainless steel for various fittings. These alloys are also more resistant to biofouling conditions than are 300-type alloys.

An alloy group that has been popular in both the aerospace and chemical production industries is that of the nickel–copper alloys, the Monels. These alloys are commonly used in corrosive-fluid systems for piping and pump fittings and in heat exchangers. Such systems are virtually immune to corrosion-assisted cracking.

Another, more expensive metal alloy system that is very useful for corrosion resistance is that of the nickel-chromium alloys. These Inconel alloys have high strength at very high and very low temperatures. They are more corrosion-resistant than austenitic stainless.

 

Protect and Preserve

 

Every solution has its problems, and brewery corrosion is an enthusiastic participant in the game. Fortunately, discussions with several microbreweries have indicated that the situation is not as dire as the literature search might suggest. Most brewery planners and brewing equipment manufacturers have keyed in to the practices of waterproofing surfaces that contact beer and using passivated stainless steel. The information presented in this article helps complete the picture for brewers who wish to maintain their investment over the long term.

 

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