ISO 9224-2012 pdf download – Corrosion of metals and alloys一 Corrosivity of atmospheres一Guiding values for the corrosivity categories.
CreviCes and sheltered areas not exposed to rain impingement have been observed to experience significantly higher corrosion damage than predicted by Equation (1) in extended exposures. In addition, designs using weathering steels or unprotected carbon steels should anticipate that rain run-oft leaves rust deposits on surfaces exposed to this run-off and permanent staining can occur to concrete, stone, masonry and other porous materials,
Steels that have been hardened to produce tensile strengths above about 1 000 MPa can suffer environmentally assisted cracking as a result of atmospheric corrosion.
6.2 Zinc materials
Zinc alloys aiso vary significantly in thew atmospheric performance. The Bi values in Table 2 are obtained from commercially pure zinc alloys, but other zinc alloys have shown higher b values m atmospheric exposuresPi. Electroplated zinc coatings, mechanically plated zinc coatings, and hot-dipped zinc coatings all have unique behaviours, and using Equation (1) with the 81 or B2 values might not accurately predict their performance. Zinc materials are particularly susceptible to attack from sulfur dioxide, and environments with high levels of this gas (sulfur dioxide range P3) probably corrode at higher rates than predicted by Equation (1). In these cases, it is prudent to assume a corrosion rate that is linear with time, that Is the h value is 1,0.
NOTE For more information on the use of zinc coatings for corrosion protection, see ISO 14713-1.
6.3 Copper alloys
Copper alloys, such as brasses (i.e. copper-zinc alloys), bronzes (i.e. copper-tin alloys), nickel silvers (i.e. copper alloys with zinc and nickel contents) and cupronickels. have atmospheric corrosion rates similar to, or somewhat less than, pure coppor(4J(5J. The 81 and 82 values In Table 2 are adequate for all of these materials. Brasses with zinc contents above about 20 % can experience dezincification in aggressive atmospheres. Two-phase brasses are most susceptible to this type of attack. It should also be noted that strain-hardened copper alloys can experience environmental cracking in natural atmospheres if their degree of strain hardening is high enough.
6.4 Aluminium alloys
Aluminium alloys experience both uniform and localized corrosion in natural atmospheres. As a result, the attack calculated by the above-mentioned methods can seriously underestimate maximum penetrations that occur. In addition, high strength, age hardening alloys that contain significant copper or copper-zinc levels can experience exfoliation corrosion. Aluminium products having a layer of galvanic protective alloy dad on the high strength aloy generally have much improved corrosion resistance in atmospheric exposures. Specific tempers have also been developed for high strength, age hardenmg alloys containing significant copper-zinc levels in order to prevent exfoliation or stress corrosion cracking. Alloys with good long-term corrosion behaviour used for structural, marine and building applications are covered in specific aluminium standards.
7 Long-term exposures
Equation (1) has been observed to be valid for exposures of up to 20 years’ duration for the metals covered by this International Standard. However, Equation (1) is based on the fact that the corrosion product layers increase in thickness and degree of protection during the exposure. At seine point in time beyond 20 years, the layer stabilizes and, at this point, the corrosion rate becomes linear with time, because the rate of metal loss becomes equal to the rate of loss from the corrosion product layer. Unforlunatety, there are no experimental data that show when this might occur and there is no method of predicting this time. The use of Equation (1) beyond 20 years Is probably justified wi most cases, especially if the exposure is not much greater than 20 years.
However, an approach that yields the maximum estimate of attack is to assume that the corrosion rate becomes linear at 20 years of exposure. In this case, the corrosion rate may be calculated using Equation (2).