Be Sure to Match the Type to the Task
The aisles of your lumberyard, hardware store, or home center are overflowing with a wide range of choices among screws and fasteners. For galvanized products, there are three ratings—G90, G185, and hot-dip galvanized (HDG)—and each is stronger than the next in resisting corrosion. But there are also epoxy-coated fasteners and stainless options, too. It’s a common assumption that all stainless screws and fasteners are alike, but they are not. Like galvanized, there are common grades for wood work and construction: Type 304, Type 305, and Type 316.
What’s the difference? “The reason there is a range of stainless steel choices is that different stainless steels are required for different applications,” said corrosion specialist Dr. R. Winston Revie, who is an expert on stainless steel. Dr. Revie worked for 33 years at the CANMET Materials Technology Laboratory.
To understand stainless steel, Dr. Revie points out that the required properties of stainless steel are actually obtained in three different ways: A) through the composition of the stainless steel, where elements are added to create a metal alloy; B) through the processing of stainless steel with heat treatment; and C) through “work hardening,” which is the manufacturing process of rolling out the steel—like making a pie pastry—which makes the steel harder.
At the lower end of the quality scale, Dr. Revie explains that Types 304 and 305 have similar corrosion resistance. “But Type 305 has a higher nickel content than 304, and this higher nickel content decreases the required work hardening, which saves on the energy required for manufacture.” That said, they are essentially the same in terms of anti-corrosion performance.
“For greater anti-corrosion protection, look to Type 316. Type 316 contains 2% to 3% molybdenum,” Revie said. Molybdenum is a chemical element that forms hard, stable carbides in metal alloys like stainless steel. “Because of the molybdenum contained in Type 316, it has a superior corrosion performance by comparison. The molybdenum in Type 316 creates a passive film [a corrosive-resistant barrier] that imparts superior corrosion resistance to chloride-containing environments, such as around seawater,” Revie said.
If you are building for products to be used near the sea, or where there are corrosives in the air (for example, near pools that use chlorine), Type 316 is the desired grade of fastener choice, because of the extra corrosion resistance. “Type 316 is also particularly resistant to crevice corrosion,” Dr. Review points out, “the type of corrosion that tends to form under screw heads. If water were to leak in around the screws, and if the water contained a bit of salt, crevice corrosion could develop.”
For the best corrosion resistance in areas where a product is going to be used around/installed around pools, marinas, large bodies of water, and areas that encounter salt water or salt air, Type 316 is the clear choice. Plus, Type 316 has an added benefit. Not only do its anti-corrosive properties provide a long life for the fastener, but if you are building with materials that are prone to staining from traditional steel fasteners, Type 316 will help prevent that staining as well. This is especially important when you’re working with expensive woods, like mahogany, cedar, redwood, but it’s also true of PVC, capstock, and composites. The use of Type 316 will add to the service lives of these materials, and preserve the reputation of you, the installer.
So, the next time you reach for a stainless steel fastener, remember that the grades are not all the same. Check the label and match the type to the task. Opt for Type 316 when there is any doubt about corrosion resistance.