Nothing rivals the certainty of death and taxes like corrosion. We have all seen rusty screws, nuts, bolts and rivets. We all have probably seen some today. Occasionally rust and corrosion may fit a certain aesthetic, but most of the time it is a major nuisance that if undetected or unremedied can be dangerous.
Mitigate both general and galvanic corrosion in fasteners
It can literally be the impetus for a catastrophic collapse of a structure. 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. For the most part corrosion can be placed into two buckets, general corrosion and galvanic corrosion.
General corrosion is the one everybody knows about and is caused by air and water. We know that it is accelerated by salt and harsh chemicals. When we think about general corrosion we often think about steel and rust. Unprotected steel is quick to rust in normal environmental conditions. The good news is that there are metals in the world like zinc, aluminum and stainless steel that can protect themselves from corrosion.
Problem solved, right? Not quite.
That’s where the lesser-known second bucket, galvanic corrosion comes in. There is a little more chemistry involved with galvanic corrosion and once you learn a little about it, it can be a little scary. Putting two incompatible metals together, even if they are both corrosion resistant on their own, can lead to galvanic corrosion. Ugh! What can we do? First let’s try to work on mitigating general corrosion.
Fastener Environment
First, we should think about where the fastener will live. The environment in which a fastener is exposed to effects the rate of corrosion immensely.
Corrosion is accelerated in coastal regions or highly industrial areas. The vast majority of fasteners are steel and have a protective finish applied. Different finishes have different levels of corrosion protection. We cannot predict with any accuracy how long a fastener may be in service before some corrosion develops. The best we can do is compare different finishes and materials using the common ASTM B117 salt spray test. It basically consists of a chamber with a salt solution that sprays periodically. The test pieces are left in the chamber and observed to see when the first sign of red rust or corrosion is visible. The test can run for hours, days, weeks or months, and is measured in hours.
In a nice dry indoor application, a black oxide or phosphate finish might be enough. If you need a little more protection a zinc-plated finish will help. The longevity of a zinc-plated fastener depends a lot on the thickness of the zinc plating and the how it was applied. You might expect a zinc-plated fastener to reach anywhere from 15 hours to 120 hours to red rust in a salt spray test.
If a fastener is going to be outside in the elements it will need more than zinc plating. Hot-dip galvanized (HDG) finishes have been a workhorse for corrosion protection throughout the years but they are not ideal in all situations. You won’t be able to find a self-drilling or self-tapping fastener in HDG and HDG is not recommended in coastal applications. There are other options for long-life fasteners.
A number of companies offer proprietary organic coatings and finishes that do well in normal environments and reach 500 to 1,000 hours in salt spray testing. Harsher environments like industrial and coastal areas require fasteners that can go beyond that 1,000-hour threshold. You can choose a fastener with a zinc or stainless steel cap. The exposed caps are highly corrosion resistant. For the harshest environments, 304 or 316 stainless steel offer the highest corrosion resistance.
Galvanic Corrosion
Now let’s work on galvanic corrosion. Galvanic corrosion occurs when two dissimilar metals are immersed in a conductive solution and are electrically connected. What does that mean? That means that if you have two different types of metals, they have direct contact and there is a salty liquid present, galvanic corrosion might occur. But if you can eliminate any one of those three components you prevent the galvanic reaction. If the metals are similar, you can separate the fastener with a non-conductive sealing washer. You can keep the connection dry and you will not have a reaction.
To help in our selection, the Metal Construction Association and some fastener suppliers publish fastener compatibility guides for attachment of various types of metal roof and metal wall panels. Ask your favorite fastener supplier for one. These guides are a great way to help you select the best fastener material for corrosion protection for your application and put your mind at ease.
Josh Krohn is engineering services manager at Triangle Fastener Corp., Cleveland. To learn more, call (216) 351-9933 or visit www.trianglefastener.com.
