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Corrosion Problems in Brewing

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

<Most Active/Anodic>
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
<Most Passive/Cathodic>

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]

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