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Appendix B - Brewing Metallurgy
B.2 Galvanic Corrosion
All corrosion is essentially galvanic. The electrochemical difference between two metals (when wet) causes electrons to flow and ions to be created. These ions combine with oxygen or other elements to create corrosion products. What this means to you is that cleaning off the corrosion products will not solve the problem. The cause of the corrosion is usually the environment (brewing) and the metals themselves.
Each metal has a small inherent electrical potential; it's what allows you to make batteries out of a potato, a nail, and copper wire. The electricity does not come from the potato, but from the difference in potential of metals that you stuck into it - like copper wire and an iron nail. All metals have a particular potential and a ranking of the metals from the most passive (lowest potential - platinum), to the most active (highest potential - magnesium), is shown below. See Table 1.
Table 19- Galvanic Series in Seawater
Mild Steel and Iron
Un-passivated Stainless Steels
Un-passivated Nickel Alloys
Passivated Nickel Alloys
Passivated Stainless Steels
Place any two metals in wet contact with one another and a galvanic reaction takes place. The more active metal of the two will dissolve (ionize). The farther apart the two metals are on the galvanic series, the greater the difference in potential and the stronger the dissolution will be. Size also makes a difference - if the more active piece of metal is smaller than the more passive, the corrosion will be enhanced but if more passive metal is smaller than the more active, the corrosion will be diminished.
Okay, enough chemistry. What this means is that if you have a copper or brass fitting in contact with passivated stainless steel, the copper will corrode over time. Brass fittings and silver solder have a potential that is close to copper and behave the same way relative to stainless steel. In a wort chiller situation (copper, brass and solder), the silver solder is the most passive and it has the smallest area, so very little corrosion takes place.
With the relatively short usage times that homebrewing equipment sees, corrosion between metals is not a big problem. I am presenting this information so that if you do experience some corrosion, you will hopefully understand what is causing it and can take care of the problem.