But in the severe environment, industrial and marine, the
coupons with lower Cr in bulk and Cr in the film
would corrode faster. In the environment with Cl, Cl
destabilized the film and attacked the metal under the
film. Once passive film breakdown locally, pit
formed and propagated. If Cr content in the bulk was
low and environment was severe, the reproduction of
the film might not be possible or as good as the initial
state. Pits grew bigger and deeper. Then corrosion
product or rust would spread out and stain the surface
around the pits. Rust-stain could be very large
comparing to the size of the pit. RN value was the
way to identify rusted area and to quantify
degradation of stainless steel in the atmosphere