The NZ Metal Roof and Wall Cladding Code of Practice is a comprehensive design & installation guide, and a recognised related document for Acceptable Solution E2/AS1 of the NZ Building Code.
Pitting corrosion is a highly localised corrosive attack that forms pits which have a very small surface area, but which can be quite deep.
Pitting occurs on non-ferrous metal when the protective passive film breaks down, or where it has been weakened or damaged by contamination. When the break-through occurs in the passive film, the actively corroding pit constitutes the anode and the large passive film surrounding the pit acts as a cathodic surface.
The rate of dissolution of the metal is strongly influenced by the ratio between anode and cathode areas, consequently the "driving force" behind pitting attack can be very strong and deterioration can spread quickly.