ISO 9224 pdf download – Corrosion of metals and alloys — Corrosivity of atmospheres -Guiding values for the corrosivity categories

ISO 9224 pdf download – Corrosion of metals and alloys — Corrosivity of atmospheres -Guiding values for the corrosivity categories.
In cases wt,ere it is important to estimate a conservative upper limit of corrosion attack after an extended exposure, the b value used in Equation (1) should be increased to account for uncertainties in the data. One way to do thés is to add two standard deviations to the average value to obtain a value at the upper 95% confidence level. For the four metals shown in Table 2, the standard deviations of the fi values111 are:
— Carbon steel: 0.0260
Zinc: 0,0300
Copper. 0.029 5
— Aluminium: 0,039 5
NOTE Estimation of a conservative upper limit of corrosion attack after an extended exposure is based on uncertainties in k This estimation does not take cto aixount uncertainties in rar. atiich are defined in LSO 9223
The 82 values in Table 2 include the two standard deviation additions and may be used where an upper limit of corrosion attack is desired when using the fiat panel data from the ISO CORRAG programme. Table 3 also provides calculated values for the function, t, up to 100 years using 82 values for h (see Clause 7 for exposures beyond 20 years).
Annex A provides maximum corrosion attack for the standard metals covered in ISO 9223 for exposures up to
20 years for the six corrosion categories. These calculations are made using the time exponents given in
Table 2.
6 Specific criteria for calculation of corrosion rates of structural metals
6.1 Steels
The protectiveness of rust layers on steels in atmospheric exposures is very strongly affected by the alloying elements in the steel Weathering steels, in particular, have specific alloying additions to promote the formation of a protective rust layer that develops during the exposure, Other carbon and low-alloy steels vary significantly in their performance in atmospheric exposures depending on their specific alloy content. The calculation procedure for corrosion rates of steels in regard to their composition is given in Annex C.
The 81 and 82 values in Table 2 are estimated for carbon steel with the composition mentioned in Table 1.
NOTE For more witormation on the use of zmc coatings br corrosion protection, see ISO 1471:3-1.
6.3 Copper alloys
Copper alloys, such as brasses (i.e. copper-zinc alloys), bronzes (Le. copper-tin alloys), nickel silvers (Le. copper alloys with zwic and nickel contents) and cupronickels. have airnosphenc corrosion rates similar to. or somewhat less than. pure copper(4151. The Bi 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 atrnospheres 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 ii 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 clad on the high strength alloy 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 some po4nt 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. Unfortunately, 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 in most cases, especially if the exposure is not much greater than 20 years.