AS 4312:2019 pdf free.Atmospheric corrosivity zones in Australia Section 1 Scope and general.
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 pm/y for steel) but are higher (of the order of 20 1m/y to 30 .tm/y for steel) in damp but non-polluted environments. Corrosion rates are very high, often over 100 im/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.
The rapid change in corrosivity for steel with distance inland is shown in Figure 2.1. There is significant scatter in these results as factors such as local variations in weather, winds and local topography all affect the amount of salt in the air and the amount deposited at a given site. The marine influence is not consistent although the results do tend to divide into distinct groups:
(I) Near rough seas or surf beaches, for example Newcastle, the corrosivity figures are very high, falling away rapidly moving inland. At about 0.5 km to 1 km inland, the figures generally tend to flatten out, but still remain quite high, averaging around 50 im/y for steel up to 3 km or more inland.
(ii) Around sheltered coastal waters, for example Melbourne, results are completely different. Right on the shoreline corrosivity is high, reaching 60 jtm/y or 70 im/y, but significantly less than figures obtained near ocean surf. The results level out moving inland more than 500 m or so in such an environment, suggesting no further marine influence.
(iii) In tropical environments, corrosion rates can be very high right at the shoreline but drop away rapidly moving inland. On sheltered tropical coasts, such as along the reef coast in Queensland, corrosivity falls away quickly to levels similar to temperate sheltered bays about 1 km inland. Fewer surveys have been carried out in such environments, and scatter in results is far greater.AS 4312 pdf download.