Stainless steel has a protective element to the steel which helps resist corrosion. Stainless is available in A2 Grade (sometimes known as ASTM 304 or 18-8) and A4 Grade (commonly known as marine grade). Whilst both have anti corrosion properties to them, A4 is far greater and is generally used in extremely corrosive situations such as chemical environments and on vehicles.
The most common finish for Fixings is BZP (Bright Zinc Plated)
This is commonly chosen for its rust and oxidation prevention with indoor use, it will not prevent rust and oxidising within an outdoor environment.