If you’re working on a job that calls for galvanized steel you want to be sure that’s what you’re using. Just out of the hot-dipping process, galvanized steel has a bright appearance that quickly changes to gray. It’s not necessarily obvious though when steel has been galvanized, which is why, on a surprisingly regular basis, we’re asked how you can tell.
Why It Matters
Galvanizing is the best way of protecting steel against corrosion. It lasts much longer than paint and ensures the steel underneath withstands the rigors of life outdoors for 50 years or more. If you’re fabricating a steel structure that needs to last, it’s important the material you use is galvanized.
It may also be that you want to assure your customers you’re using galvanized steel. Perhaps you’re making the claim in your advertising, or they are in theirs. Alternatively, maybe you have some steel stock on hand but aren’t sure if it’s been galvanized. (It may just have been painted.) Either way, you don’t want to be caught out in the rain, so let’s talk about how to tell.
Three Ways to Determine Galvanization
The galvanizing process bonds a layer of zinc onto steel. There are three main ways to tell if this has been done:
- Thickness measurement
- Electron paramagnetic resonance (EPR)
- Laboratory testing
Here’s a look at the pros and cons of each.
Coating thickness is easily measured with a magnetic probe, providing the coating itself is non-magnetic, which of course zinc is. The beauty of this kind of measurement is that it’s quick and nondestructive. That makes it easy to test material in multiple locations.
The downside of thickness measurement is that it’s not actually confirming the presence of a zinc coating. It just measures separation of probe from the base steel. All it tells you is that the steel has been coated with something non-ferrous.
Electron Paramagnetic Resonance (EPR)
Sounding like something from a sci-fi movie, EPR is a technique for revealing what metals are present on a surface. In simplistic terms, a probe is brought into contact with the surface, and a signal sent that starts some of the surface electrons vibrating. By identifying the frequency responsible for each resonant response it’s possible to deduce which materials are present.
There are two downsides to using EPR as a galvanization test. First, it’s really just going to tell you there’s zinc present on the surface. It could have been applied as paint rather than through galvanizing. And second, EPR kit is still very specialized and not inexpensive.
This is the gold standard of galvanization testing. It might start with EPR or another spectroscopy technique to determine if zinc was present but would be followed by sectioning. These samples then go under a microscope to be examined for evidence of an intermetallic layer at the zinc-steel boundary. This is where iron diffuses into the zinc and it’s only visible under magnification. Seeing layer confirms that the steel is galvanized.
Laboratory testing is the only way to be absolutely sure steel has been galvanized, but it has at least two drawbacks. It’s slow, because sectioning takes time, and it’s also a destructive process that can’t be used on a finished fabrication.
Not an Easy Question to Answer
As we’ve explained here, it’s surprisingly difficult to tell if steel has been galvanized. Appearance is a clue and non-magnetic behavior indicates the presence of a coating, but only with laboratory testing can you be 100% sure.