Stainless steel is mostly preferred over other kinds of materials because it can resist corrosion in rough environments although it is not fully resistant to rust/corrosion. Since it is somewhat resistant to corrosion, hence the name, stainless steel.
When stainless steel comes in contact with corrosive fluids or high humidity/high salinity conditions, the topmost layer of chromium oxide that protects the steel from getting corroded gets thinner and thinner with time which can cause the stainless steel screw to rust. Rust is not just bad because it looks unpleasant to the eyes but it also weakens the structural integrity of the instrument thereby affecting the overall performance and lifespan of the structure.
The topmost layer of steel that protects it from corrosion can only work if the concentration of chromium is at least 12%. Anything less than that causes the layer to weaken and thus the steel