These Phillips flat wood screws are a 316 marine grade stainless steel with rolled threads. Rolled threads have a constant shank diameter that tapers at the point. Wood Screws are a thread forming screw with a coarse pitch thread. 316 marine grade stainless steel is the choice for severe enviroments that require a high level of corrosion resistance. Type 316 stainless steel is the preferred material used in the marine enviroment and boating industry due to its resistance to pitting corrosion. 316 stainless steel has high resistance to corrosion from sea sealt water and industrial chemicals.