Galvanizing makes a huge difference in the longevity of steel. A coat of zinc applied in a hot dip at a professional galvanizing facility protects steel from oxidation and other kinds of corrosion for 50, 75, and sometimes even 100 years before the very first need for maintenance. But why does steel need to be coated with zinc to be galvanized? After all, copper doesn’t rust, either.
The reason the galvanizing process works isn’t all about whether a coating on steel rusts or doesn’t. The reason galvanizing with zinc works is that it protects steel in ways copper cannot.
All Metals Oxidize, But Some Metals Oxidize Faster Than Others
The reason that the galvanizing process uses zinc instead of other metals is that zinc oxidizes and experiences acid corrosion “sacrificially” to steel. That means that when zinc is in contact with steel, oxygen and acids will attack the zinc rather than the steel beneath it. And the formation of an alloy of zinc and steel in the intense heat of a galvanizing pot forms a protective layer of zinc on a steel construction member or a steel part that can’t be corroded away.
The idea behind galvanizing started out with some observations about how metals conduct electricity. Some metals conduct electricity better than others. That’s because they more readily give up electrons to the flow of electrical current than others.
When an atom of a metal gives up an electron, it takes on a positive charge. Acquiring a positive charge makes it attractive to chemicals that are negatively charged like the oxygen in water. Iron in steel and oxygen in water form ferric or ferrous oxide, also known as rust. Similarly, unprotected steel can interact with ions in air and water pollution to form compounds that weaken it, threatening cosmetic damage and then loss of structural integrity.
There’s a pecking order among metals that determines which metal reacts first if both metals are exposed to oxygen or other corroding substances. Some metals are so reactive it is completely impossible to use them for galvanizing. Sodium (the same sodium in sodium chloride, also known as table salt) is so reactive that dropping sodium turnings into water results in a “Whoosh” and an explosion. If somehow it were possible to use sodium to galvanize steel, it would prevent rusting, but the heat of reaction with water would melt it.
Other metals are not quite as reactive as sodium. The hierarchy of reactivity goes like this:
- Magnesium is more reactive than zinc.
- Zinc is more reactive than aluminum.
- Aluminum is more reactive than cadmium.
- Cadmium is more reactive than cast iron.
- Cast iron is more reactive than carbon steel.
- Carbon steel is more reactive than stainless steel.
- Stainless steel is more reactive than lead.
- Lead is more reactive than solder.
- Solder is more reactive than tin.
- And tin is more reactive than copper.
Metals that are more reactive protect metals that are less reactive by losing electrons first. Zinc is more reactive than any kind of steel, so it protects steel. But steel is more reactive than copper, so if you coated steel with copper, and the copper got scratched, the steel would corrode while the copper stayed intact. That’s not the effect architects, engineers, and fabricators are looking for.
Copper Can Be Used For Weathering Steel
That doesn’t mean that steel isn’t ever dipped in a copper coat. Copper is used to make weathering steel, also known as COR-TEN steel. The copper coating gives the steel beneath it a weathered appearance, which is sometimes architecturally or aesthetically desirable.
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