If rust is a concern either use a metal that doesn’t corrode, or coat one that does. Metals that don’t rust – looking at you, stainless steel – carry a cost penalty, and often a performance one too. Coating lets you use a metal that’s less expensive and often easier to work. Paint is the first coating that comes to mind when looking to protect mild steel, but it doesn’t last very long. So that leads us to galvanizing, and the million-dollar question: Does galvanized steel rust?
The short answer is, yes, and also no. Galvanization is a zinc coating applied over the top of steel. It prevents rust and corrosion far longer than paint will, often for 50 years or more, but eventually that brown rot will set in. Want to know why? Keep reading to find out.
How Galvanizing Prevents Rust
Let’s start with some definitions. Corrosion is how metal breaks down as oxygen attacks the surface. Rust is the special type of corrosion experienced by iron. Oxygen creates iron oxide, which flakes away from the body of the metal, exposing fresh metal to oxygen.
Metals that don’t rust, like aluminum and stainless, form an oxide layer on the surface. This resists further corrosion. Another metal that oxidizes without rusting is zinc, and zinc is interesting because it bonds well to steel.
So, if you want to give steel a coating that lasts longer than paint, cover it with a layer of zinc. And that’s galvanization.
Zinc prevents oxygen and water from reaching the steel beneath. It does this by first forming a zinc oxide layer on the surface. When there’s moisture present that turns into zinc carbonate. This gives the metal a dull gray patina that’s insoluble in water and stops any further chemical changes.
So why doesn’t galvanized steel last forever? Keep reading.
Factors that Affect Galvanized Steel
Having said that zinc carbonate is insoluble, that’s not completely true. It will break down slowly under certain conditions. These include:
- Relative humidity above 60%. (Tropical environments, hothouses used for raising tropical plants)
- Sodium chloride (salt) in water or air. (Marine or coastal environments.)
- Situations where the coating is frequently wet or soaked. (A car wash would be an example.)
- Sulfur dioxide pollution in urban atmospheres.
- Exposure to hydrogen sulfide from volcanoes, hot springs, natural gas, and sewer gas.
- Strong alkalis such as plaster and cement, (especially Portland cement,) as these contain chlorides and sulfates.
- Acid rainwater runoff from roofs with wood shingles
- Moss and lichen, which will hold moisture and create a high humidity micro-climate.
Impact of Local Environmental Conditions
When you’re thinking about using galvanized steel it is important to consider what the local conditions will be like. More specifically, think about:
- Air. High or low humidity, exposure to salt, acids or industrial pollutants. (Desert air: not a problem. Tropical city: potential problem.)
- Soil. Burying galvanized steel in soil, (as the base of a fence post for example,) will expose it to more moisture. However, the size of the impact depends very much on the type of soil and overall conditions, (muddy and wet or sandy and dry.)
- Temperature. High temperature alone won’t break down zinc, providing it’s below the melting point of 787°F (420°C). It is however an accelerating factor when combined with corrosive factors like humidity and industrial pollution. Low temperatures have no effect on a galvanized coating.
How Long Do You Need it to Last?
So does galvanized steel rust? The answer is yes, but very slowly. In fact, slow enough that it’s usually not a problem. This is why galvanized steel has been in use for the last 2,000 years, and why it’s probably going to be okay in your application.