Our compression springs used on our older style roof hatches are made out of chrome silicon steel. That would apply to both -OSS (ferrous) option and non -OSS (non-ferrous).
For our -OSS stainless steel hardware option the compression spring features a E/M 6226 High Performance Fluorocarbon to provide extra protection from corrosion.
We do apply grease on the compression spring housing. We would be OK using an un-coated spring, however it may not last as long as a spring that is coated. The grease helps with lubrication and surface corrosion.