AS 4312:2019 Atmospheric corrosivity zones in Australia
2.5.2  Stainless steels
Stainless steels are steels containing at least 10.5 % of chromium and have a passive film, which provides very good corrosion protection in most atmospheres. As a result, the general atmospheric corrosion rate is so low as to be virtually unmeasurable. However, some atmospheric contaminants cause pitting or surface staining. The extent will depend on the grade of stainless steel and quality of the surface finish as well as the corrosivity of the atmosphere. The main contaminants of concern are chlorides and metallic iron dust. Where these are not present, such as interior and rural environments, any grade and finish is likely to remain free from rust stains. In marine environments, the basic ferritic grades will show rust staining, as will 304 grade. Industrial environments can cause staining, especially if exposed to pollutants such as chloride and iron dust. In a severe marine atmospheric environment, 316 grade is resistant, showing only slight spotting. With a smooth surface finish or regular washing, or both, 316 would normally be selected in marine and severe marine atmospheric environments.
NOTE  EN 1993-1-4:2006+A1: 2015 (Annex A of Eurocode 3 Part 1-4) provides a procedure for determining the environment for selection of stainless steels depending on the risk of exposure to chlorides (from salt water or de-icing salts), sulfur dioxide and the cleaning regime or exposure to rain washing. A corrosion resistance class may be determined, enabling selection of the appropriate grade of stainless steel. The procedure has been developed for European environments and may not be applicable for Australian environments.
2.5.3  Zinc
The atmospheric corrosion rate of zinc is generally assumed to be linear with time. In rural and other contaminant-free environments, the corrosion rate of zinc will be very low, around one-tenth to one-twentieth of that of steel in the same environment, and usually less than 1 µm/y. In industrial environments, zinc sulfate and other soluble salts will form, which will wash from the surface, and their corrosion rates are higher, 1 μm/y to 10 μm/y. These are also one-tenth to one-twentieth the corrosion rate of steel. In marine atmospheres, soluble zinc chloride will form and the corrosion rate, as with steel, is much higher. Corrosion rates of 2 µm/y to 15 µm/y are typical, depending on proximity to the coast. These are one-twentieth to one-hundredth the corrosion rate of steel. The corrosion rate of zinc tends to decrease with time in marine atmospheres. Table 2.1 gives corrosion rate of zinc according to ISO 9223 for the various corrosivity categories.