The first images below are what the current (2006~present) South Korean 10-Won coin looks like. It's the smallest-value circulating coin in the country. In exchange rate, it's equivalent to 8/10ths of a U.S. Cent, or .92 Japanese Yen. It still costs 30 won to make this "new" 10 won coin (ugh!), but it was also considered to be about 20 won cheaper than the cost of manufacturing an old 10 won coin, which was 65%Cu-35%Zn in composition. The 10-Won coin is 18mm in diameter, 48% Cu (outer layer) and 52% Aluminum (core). I believe this is the first South Korean clad coin. The Koreans claim it's the world's first such copper-coated aluminum coin, or the first manufactured in this way. A zoomed-in edge-view: Below are images of this 10-Won coin after submersion in water or run through a clothes washer. Reports in Korea say that the two metals (copper and aluminum) are in direct contact with each other by pressing copper on aluminum. In this case, a so-called 'galvanic corrosion phenomenon' occurs in which an electrolyte such as water accelerates the corrosion. At South Korea's Incheon International Airport, there is a fountain which contains coins from all over the world, and Korean reporters found these 10-Won coins there all corroded as bubbles were rising around the outside of them as they lay in the fountain. They claim that the copper on the surface can easily fall off and become unacceptable as money. Here's an explanation being bandied about in the Korean blogosphere: "...both copper and aluminum are very vulnerable to 'chloric acid'. Chloric acid may be unfamiliar, but it is a component of our common laundry detergent and pool water. In fact, a simple experiment showed that the new 10-won coin quickly decays in water with detergent" And another: "[This is] caused by galvanic corrosion, a phenomenon that occurs when two different metals with different ionization tendencies are attached. In particular, the corrosion of aluminum parts is extremely fast, because aluminum is much more ionized than copper, and aluminum is oxidized first instead of copper." First, I'd like to know if the above explanations are true. Perhaps @BadThad or @desertgem can help? Has such phenomena taken place with other coins in the world? Do Zincolns have this problem? After being in a washing machine, immersion in water, etc:
Zincolns only have this problem when a hole in the copper layer exposes the zinc core. But since the copper layer is so thin, this happens very easily. The coins above look like they come from the mint with the core already exposed. Both zinc and aluminum are more active than copper. As a result, when you bond copper to zinc or aluminum, then expose them to a liquid with dissolved salts, the zinc or aluminum tends to dissolve away (displacing hydrogen from the water, which forms those bubbles if it happens fast enough). Zinc and aluminum are also amphoteric, meaning they can be attacked by either acid or alkaline solutions. (Many other metals are only attacked by acid solutions.) Detergent and bleach are both commonly alkaline. Dishwasher detergent is especially alkaline, which is why aluminum pans can come out dull and grey if you put them through the dishwasher. We actually had a couple of aluminum baking pans develop pinholes from this before we caught on to what was happening.
mlov43 nice photos. I believe I have an Israeli copper coated aluminum coin. I really enjoy (as you may recall) South Korean coins. Especially the "Turtle Boat" designs. Do you have any N. Korean coins? If so can you share a few photos? Gary in Washington state
Zincolns, definitely. The zinc and aluminum have have higher ionization potentials than the copper or bronze, causing the ion migration to the higher potential metal making it effectively a sacrificial anode. Sacrificial anodes are typically used to prevent the corrosion. By attaching a zinc anode to, say, a ship hull, the zinc anode will corrode before the steel hull only costing a cheap zinc and preventing costly repairs.
Exactly. And the Mint has effectively built a ship with a zinc hull thinly plated with steel, guaranteeing that the hull will rot away in short order if it gets wet and scratched. Fortunately, a cent is a lot cheaper than a ship, and makes less of a mess when it rots. Unfortunately, the three hundred billion cents that have been minted since 1982 are actually a good bit more expensive than a ship, and the mess they're making is scattered over the whole country.
So, the aluminum in this 10-Won coin is acting as a sacrificial anode (in relation to the copper) because the edge is exposing the aluminum?
Do the terms "chloric acid" and "galvanic corrosion" make sense? I think "galvanic corrosion" makes sense.
No. Chloric acid exists, but it's not involved here. They might mean "hydrochloric acid", or perhaps just "chloride ion". I'm not about to attempt translating those terms into Korean, though, so I won't snark at the person or process trying to translate them to English. "Galvanic corrosion" is correct.
Exactly. And having a thick sacrificial anode inside, and a thin shell outside, is a terrible design, precisely because this is the guaranteed result.
