Contrary to what its name suggests, stainless steel can rust.
When conditions are tough enough (chlorine, salts, chemicals …), corrosion can occur. This usually occurs at damage to the oxidation layer or by implantation of other metals in the surface or where dirt is concentrated in cracks and crevices. Also condensation (acid rain) may provide long-term rust. Therefore it is advisable to regularly clean exterior railings with a damp wash cloth.