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I have a recipe for a false cider that requires the addition of lemon juice after secondary fermentation prior to bottling/kegging. I've done this before in the bottle with good success but I'm wondering -- should I be concerned about the corrosion potential of citric acid against the stainless steel of the keg?

I'm expecting to use the juice from about 4-5 lemons.


Basic Understanding

Stainless steel is not immune from corrosion.

Corrosion and stainless steel? It's stainless, so it should be free from the risk of rust, pitting, and wear, right? Wrong. Although the 300-series stainless steels used in brewing equipment are normally highly resistant to corrosion, their resitance can be compromised in several ways. For example, some of the cleaning and sanitization techniques commonly used with glass and plastic are hazardous to stainless steel, and you can wreck your stainless equipment if you use them.

Some metals such as aluminum actually benefit from interaction with citric acid.

Acids are often used to passivate stainless steel in low concentrations. Passivation is the process by which the clean surface builds up an oxide layer which protects it to corrosion.

Citric acid can be used for cleaning & passivating stainless steels, as an alternative to nitric acid.

The typical acidity range of beer and cider is safe for stainless.

Beer has a pH of about 4 when fresh, but this can drop to 3.5 or below if the beer is exposed to oxygen such that it sours, as is inevitable in a traditional cask after dispense. Fresh ciders may have a pH as low as 3.3 and, when oxidized, even below 3. Stainless steel is generally impervious to these levels of acidity, but the oxide layer with which aluminum alloys protect themselves from corrosion is attacked by any pH less than about 4 or over about 9.

Based on my findings I'm convinced that short-term contact is completely fine if not even beneficial but I'm curious about the effect of long-term contact of high citric acid concentrations against stainless steel.

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Your corney keg will be fine.

The issue of corrosion comes into play if there are unpassified areas, usually from cleaning with an abrasive.

You can repassify to restore the oxide layer with a mild acid like starsan or bar keepers friend. Soak and let air dry.

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