If you're designing steel outdoor furniture or infrastructure, your go-to material would be stainless steel—if your client has deep pockets. If they don't, you'd step down to galvanized steel, which adds a protective layer of zinc, offering the corrosion resistance at a lower cost.

Konture Studio's Gravitas line of outdoor furniture

There are actually multiple types of galvanized steel that designers should be familiar with. Here we'll explain the three most common types: Cold galvanized, electro-galvanized and hot-dip galvanized. (Note: When laypeople say only "galvanized steel," they typically mean cold galvanized or electro-galvanized. Designers and manufacturers will be more specific with each other.)

Cold Galvanizing

Cold galvanizing isn't really galvanizing, in the scientific sense; cold galvanizing is simply the process of applying zinc-rich paint to the steel, forming a mechanical bond. This is the cheapest of the three options, and has the advantage that it can be done on-site (i.e. wherever the workpiece is, as opposed to bringing the workpiece to a production facility). It is also the least durable, with the zinc wearing away the fastest versus electro- and hot-dip galvanizing.

Unsurprisingly, manufacturers and retailers of cold galvanizing paints claim that it lasts just as long as the other methods. It doesn't; ask a scientist, a construction expert, or someone with expertise who is not trying to sell you the paint.

Electro-Galvanizing

Electro-galvanizing, commonly used by the automotive industry, is the process of electroplating a piece of steel to form the zinc layer. The workpiece is immersed in a zinc salt solution, and electricity is applied. This produces an electrochemical reaction which results in the steel workpiece being coated with a layer of protective zinc oxide.

The finish is shiny; as you'll see further down, that shininess is how you can visually distinguish electro-galvanized vs. hot-dipped.

Electro-galvanizing, unlike cold galvanizing, has to be done at a facility; the bath isn't something you can throw into the back of a pickup truck. Ditto for hot-dip galvanizing.

Hot-Dip Galvanizing

Hot-dip galvanizing produces the most durable results of the three. As the name suggests, heat is used rather than electricity. The workpiece is immersed in a bath of molten zinc at extremely high temperature. Once it's out of the bath, the zinc coating meets oxygen and forms zinc oxide. Then it meets carbon dioxide, and forms zinc carbonate. This protective layer, chemically bonded to the steel, is thicker than the layer produced by electro-galvanizing and will last the longest.

Hot-dip galvanizing produces an easy-to-spot visual effect. You will have seen this look on outdoor infrastructure or in my case, farm buckets:

Image: TMg, CC BY-SA 3.0

Image: TMg, CC BY-SA 3.0

That's called "spangle." It's the telltale sign that the workpiece has been in a zinc bath at 840 degrees Fahrenheit, is coated in zinc carbonate and will last a good long time.

How long? Accounts vary. First off, with all galvanization methods, the steel can and will still rust; but the zinc will rust before the steel does, essentially serving as a sacrificial layer and warning indicator. It visually signals, before the rust has reached the steel, that the piece needs attention.

As for estimates, it's impossible to get a hard figure on how long cold galvanizing methods will protect the piece for, as it depends on the specific zinc content of the paint and how well it was applied. For electro-galvanized, 25 to 50 years is a reasonable expectation; for hot-dip, 50 to 80 years. But these figures must be taken with a grain of salt, or perhaps a bucket of saltwater; a seaside environment will hasten the onset of corrosion.

It's also worth noting that some manufacturers of hot-dip galvanized products do offer lifetime anti-rust warranties. (Perhaps the fine print reads "will not rust in your lifetime.")