Stainless Steel Screws

Stainless Steel: When to Use It,
and What Do the Numbers Mean?

Stainless Steel was developed about 90 years ago to solve the rusting problems of standard grades of steel. It is best used where the eventual corrosion of plated products would create a real problem — salt-water boats or docks (above water), and redwood siding quickly come to mind.

Only a few of the many engineered grades of Stainless Steel are suitable for fasteners — those that can be “cold headed” easily, are corrosion resistant, and have adequate torsional and shear strengths. Of these, the 300 series of stainless steels is most popular. These alloys contain about 18% chromium and 8% nickel for good corrosion resistance, but make them non-magnetic and non-heat-treatable. Differences between 304 and 305 series stainless steel are slight, but 316 stainless adds molybdenum, which increases corrosion resistance, strength, and cost. Stainless Steel must be “Passivated” for use below the water in marine applications. Non-Passivated screws may corrode quickly if they don’t maintain contact with the flowing oxygenated water which is required in order to form the chromium oxide film which protects them against galvanic corrosion and eventual failure.

When a bit of corrosion resistance can be sacrificed for strength and reduced cost, 410 stainless steel is used. It contains about 12% chromium, no nickel, and sufficient carbon to allow heat treatment, making it mildly magnetic. Because it can be heat-treated, it is used extensively for self-drilling fasteners. It has a slightly “coated” surface finish appearance.