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Cathodic Protection for Equipments
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 steel 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 steel.
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 steel and waterproofing surface 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.
Related References:
1. Duplex Stainless Steel Pipe
2. Duplex Stainless Steel
3. Super-Duplex Stainless Steel
4. Principle of Duplex Stainless Steel
5. How the Austenite Ferrite Balance Achieved
6. Corrosion Resistance of Duplex Stainless Steel
7. Stress Corrosion Cracking SCC of Duplex Stainless Steel
8. Barrier to Using Duplex Stainless Steel
9. Duplex Stainless Steel Grades Comparison Table
10. S32101 | S32205/S31803 | S32304 | S32750 | S32760
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