Due to its versatility, durability and affordability stainless steel production continues to increase around the world year after year, but stainless steel is not a single alloy.
While stainless steel gets a large part of its corrosion resistance from chromium, there are near-endless combinations of various metals marketed as stainless steel today. Thus, understanding the different types available is essential to finding the right product for your boat and application.
There are two main labels you will find on most stainless-steel alloys: Families and Grades.
Families includes Austenitic, Duplex, and others. As that austenitic is the most frequent material you will find, it is the Grade that you look for.
The grade defines its content which will give its resistance to corrosion, and other physical properties. Example, if you plan to machine parts, some grades like 303 or 316F as sulphur and/or Phosphorus to help its machinability and maintain corrosion resistance. Note that 316L has a lower carbon content, helping its weldability.
A2 (AISI-304) Corrosion protection class CRC II has low corrosion resistance resistant to freshwater and rainwater. Its main application is for outdoors wood screw fastener and household plumbing.
A4 (AISI-316 / AISI-316-L) Corrosion protection class CRC III with medium corrosion resistance resistant to freshwater and rainwater resistant to saltwater and chlorines. With this level of protection, it will find application in marine industry (boat building) and for outdoor coastal areas and the food industry.
You may also find Duplex D6 with a corrosion protection class CRC VI for high corrosion resistance. It’s main areas of application is where aggressive media are used (e.g. salt and chlorine) and under extreme stress (e.g. chains).
These are only the three commonly used materials. A way to identify the screws is to check the stamping on top (See below). Not you can detect A2 from A4 by magnetism, the later being less sensible to a magnet.
Thus, next time you buy a Stainless-steel product or fastener, make sure you check for the grade, especially if you plan sailing in salt water because many hardware store, even chandlery do not carry A4 / 316L .