You won’t be able to find a self-drilling or self-tapping fastener in HDG and HDG is not recommended in coastal applications. Hot-dip galvanized (HDG) finishes have been a workhorse for corrosion protection throughout the years but they are not ideal in all situations. If a fastener is going to be outside in the elements it will need more than zinc plating. You might expect a zinc-plated fastener to reach anywhere from 15 hours to 120 hours to red rust in a salt spray test. The longevity of a zinc-plated fastener depends a lot on the thickness of the zinc plating and the how it was applied. If you need a little more protection a zinc-plated finish will help. In a nice dry indoor application, a black oxide or phosphate finish might be enough. The test can run for hours, days, weeks or months, and is measured in hours. The test pieces are left in the chamber and observed to see when the first sign of red rust or corrosion is visible. It basically consists of a chamber with a salt solution that sprays periodically. The best we can do is compare different finishes and materials using the common ASTM B117 salt spray test.
We cannot predict with any accuracy how long a fastener may be in service before some corrosion develops. Different finishes have different levels of corrosion protection. The vast majority of fasteners are steel and have a protective finish applied. The environment in which a fastener is exposed to effects the rate of corrosion immensely.Ĭorrosion is accelerated in coastal regions or highly industrial areas. Ugh! What can we do? First let’s try to work on mitigating general corrosion.įirst, we should think about where the fastener will live. Putting two incompatible metals together, even if they are both corrosion resistant on their own, can lead to galvanic corrosion. There is a little more chemistry involved with galvanic corrosion and once you learn a little about it, it can be a little scary. That’s where the lesser-known second bucket, galvanic corrosion comes in. The good news is that there are metals in the world like zinc, aluminum and stainless steel that can protect themselves from corrosion. Unprotected steel is quick to rust in normal environmental conditions. When we think about general corrosion we often think about steel and rust. We know that it is accelerated by salt and harsh chemicals. General corrosion is the one everybody knows about and is caused by air and water. For the most part corrosion can be placed into two buckets, general corrosion and galvanic corrosion.
Since a completely corrosion-free fastener does not exist, our goal should be to select a fastener that is as durable as the members it is fastening. It can literally be the impetus for a catastrophic collapse of a structure.
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