Avoiding corrosion to zinc compositions

Preserving Collectibles column from March 30, 2015, edition of Coin World
By , Special to Coin World
Published : 03/15/15
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Not long ago I received an email from a collector of zinc coinage.

He said that a number of his coins had what looked like white powder on the surface. He asked what was happening and what he should do.

Zinc corrosion

The white powder the reader is referring to is a zinc corrosion product. All zinc corrosion products are white. Scientific analysis would be necessary to identify which corrosion product it is.

A newly minted zinc coin appears lustrous. This luster will be maintained provided the coin is not in a corrosive environment. The reason for this is that zinc reacts with atmospheric oxygen to produce a protective layer of zinc oxide that is thin enough to be transparent.

High humidity will cause zinc to loose its luster producing either zinc hydroxide or a corrosion product referred to by conservators as “white rust.” Both of these corrosion products are fairly stable and adhere well to the surface of the metal, provided thestorage environment is unpolluted.

However, pollutants such as acetic and formic acid react with these protective layers to form corrosion products such as zinc acetate and zinc formate.

Readers will remember that both acetic and formic acids are released from poor quality paper, cardboard, nonarchival adhesives (e.g., polyvinyl acetate emulsions), and wood and wood composites (e.g., plywood and “chipboard”).

Acid and alkaline

The acids in these materials react with atmospheric moisture to create an acidic environment: essentially an acid rain. Higher temperature and relative humidity (RH) speed up the reaction.

To protect metal coins, tokens or medals from the acids and chlorides in our skin, I advise wearing gloves for handling a collection.

Chlorides react with zinc to create zinc chloride. Zinc chloride is hygroscopic, meaning that it will absorb and hold atmospheric moisture. Zinc chloride will do this at quite a low levels of RH, around 10 percent. The moisture that it absorbs provides the water needed for corrosion to occur.

Zinc also reacts with ammonia, a common ingredient in household cleaners.

Zinc is an amphoteric metal, meaning that it is corroded under both an acidic and alkaline environment. Its corrosion products are soluble below pH 6 (acidic) and above pH 12 (alkaline).

So, what is the reader to do?  First, I recommend looking at how and where coins are stored. Are they in archival-quality holders and containers?  Or is exposure to atmospheric pollutants such as acetic or formic acid and ammonia possible? Were gloves always worn when the coins were handled?

Upgrading the storage environment is fairly straightforward. Unfortunately, undoing poor handling is not possible. A fingerprint etched into the surface of a coin is permanent.

Avoid internet recipes

Should the coins be cleaned? I feel that cleaning these coins is a job for a conservator. By no means should a collector be tempted by some of the “recipes” on the Internet.

One website recommended removing stubborn dirt and grime from zinc coins by sprinkling the coin with table salt and then rubbing it in with a cloth.

Apparently, the salt was to act as an abrasive scrubbing away the dirt and grime. Clearly, knowing what we know about zinc and chlorides, this is a technique that should be avoided.

Another site recommended removing corrosion by first soaking a coin in a dilute solution of sulfuric acid, followed by a rinse in a dilute solution of sodium hydroxide to neutralize the acid, followed by a rinse in water.

Again, avoid this technique! Sulfuric acid is quite acidic with pH values ranging from pH 1 to 3 depending on the molarity of the solution. Sodium hydroxide has a pH of 14. Both are clearly in the pH ranges that accelerate the rate at which zinc corrodes.

Lastly, I would advise a reader to document the current condition of these coins, to monitor their condition over time.

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