AS 4312 pdf download

AS 4312 pdf download.Atmospheric corrosivity zones in Australia
1.1  Scope
This Standard provides guidelines for the classification of atmospheric corrosivity zones in Australia and their effect on the corrosion of steel and other metals. The guidelines in this Standard use the corrosion rate classifications defined in ISO 9223 to delineate atmospheric corrosivity zones. Environments that are not atmospheric, such as immersed, underground and splash zones, are not considered in this Standard. For these environments, refer to AS 2159 (piling) or AS/NZS 2041.1 (buried corrugated metal structures).
1.2  Application
This Standard is intended to be used by designers and specifiers of corrosion control methods to assist in determining the correct atmospheric corrosivity zone in Australia. The influence of micro climates is also addressed and should be considered when specifying the necessary corrosion protection that is required. In general, a knowledge of atmospheric corrosivity is necessary in the following corrosion mitigation activities: (a) Selection of protective coatings such as paint coatings according to AS 2312.1 or hot dip galvanized coatings according to AS/NZS 2312.2. A more severe environment will normally require a higher performance or thicker coating system, or both. (b) Selection of in-line pre-coated products and suitable metals. The durability of in-line pre- coated products and metals such as copper and stainless steels depends on the corrosivity of the environment. (c) Maintenance of protective coating systems. In severe environments, maintenance is more difficult and usually required more frequently than in less severe environments. (d) Design of products and components such as fasteners. The importance of minimizing corrosion through careful design and fabrication procedures becomes more important as the environment becomes more corrosive. NOTE  AS 2312.1 and AS/NZS 2312.2 provide details of selection and maintenance of protective coatings, and include design features for minimizing corrosion.
2.1  General
Atmospheric corrosion of metals requires the presence of oxygen and an electrolyte, usually water (moisture). In such an environment, corrosion rates are controlled by the percentage of time that the surface is wet. Soluble contaminants such as salts are hygroscopic and have a major influence on corrosion rates, by absorbing moisture and creating a layer of electrolyte when none would otherwise exist, or by increasing the effective conductivity of the electrolyte. Corrosion rates are very low in dry, non-contaminated environments (of the order of 1 µm/y for steel) but are higher (of the order of 20 µm/y to 30 µm/y for steel) in damp but non-polluted environments. Corrosion rates are very high, often over 100 µm/y for steel, in the constant presence of moisture when combined with salt or other contaminants.
2.2  Macro-climatic factors
2.2.1  Primary factors The two most important factors affecting the corrosion rates of metals in Australia are: (a) Time of wetness, which is the period of time during which a metallic surface is covered by adsorptive and/or liquid films of electrolyte that are capable of causing atmospheric corrosion. Moisture is mandatory for atmospheric corrosion to occur and, all things being equal, the longer the period of time that a layer of moisture is present on a metallic surface, the greater is the amount of corrosion. Time of wetness is influenced by a range of factors, for example metal type, the shape, mass and orientation of the object, any sheltering and pollutants on the surface. The length of time for which the relative humidity (RH) exceeds 80 % at a temperature greater than 0 °C has been used to estimate time of wetness. (b) Airborne salt, which is a major stimulant of atmospheric corrosion near the coast, causes the most damage to infrastructure in Australia as most of the population live within 50 km of the coast. Many common metals are rapidly attacked near surf beaches. The deposition rate of sea salt on exposed surfaces is directly related to its concentration in the atmosphere. Deposition, and therefore corrosion, usually drops off rapidly with distance travelled inland.