There are a number of materials used for constructing machine screws, including steel, plated steel, brass, and nylon.
Standard Machine Screws
Standard Machine Screws are made from steel, usually galvanized with zinc for an anti-corrosion coating, extremely important in machine screws where, if they are used with nuts, rusting will soon make it almost impossible to loosen the nut. These are cheap, easily available, and suited for many tasks, but they are still prone to rusting, over time
Stainless Steel Machine Screws
Stainless steel machine screws are more expensive, but they are almost immune to corrosion, making them popular for external use and for precision parts. They are commonly used for fixing guttering, boatbuilding, and precision engineering, where their anti-rusting properties come to the forefront.
Stainless steel is not as strong as mild steel, so try to use a slightly longer and thicker machine screw wherever load-bearing capability is a genuine concern.
Stainless steel machine screws are often graded, generally with a three digit SAE Stainless Steel Grade number beginning with ‘3.’ For most uses, 305 or 306 stainless steel machine screws are adequate, but 316-grade stainless steel is common for marine engineering and precision manufacturing. Other standards for stainless steel grading are given by numbers such as 16/10 or 18/0, which give the percentages of chromium and nickel in the alloy.
Brass Machine Screws
Brass is a much more common material for machine screws than for larger bolts or wood screws. This is partly because, at smaller scales, such as machine screws, the lesser strength of brass machine screws is much less of a problem, and it has the added advantage that it does not rust as intensely as steel, so there is much less of an issue with the nut permanently rusting to the nut.
Brass is often used as inlays for wood, circuit boards, and other tapped holes, so only brass machine screws should be used with those.
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