ISO 9223 pdf download – orrosion of metals and alloys -Corrosivity of atmospheres -Classification, determination and estimation

ISO 9223 pdf download – orrosion of metals and alloys -Corrosivity of atmospheres -Classification, determination and estimation.
6 Classification of corrosivity of the atmosphere
The corrosivity of atmospheric environments shall be classified either by determination of the corrosivity in accordance with Clause 7 or, where this is not possible, by estimation of the corrosivity in accordance with Clause 8. Both methods of the corrosivity evaluation represent a generalized approach and are characterized by some uncertainties and limitations.
A corrosivity category determined from the first-year corrosion loss reflects the specific environmental situation of the year of exposure.
A corrosivfty category estimated from the dose-response function reflects the statistical uncertainty of the given function.
A corrosivity category estimated using the informative procedure based on the comparison of the local environmental conditions with the description of typical atmospheric environments can lead to misinterpretations. This approach is to be used if experimental data are not available.
Annex A defines uncertainties related to the determination and normative estimation of atmospheric corrosivity categories.
Detailed classification of low corrosivity of indoor atmospheres covering the corrosivity categories Cl and C2 in terms of this International Standard is specified in ISO 11844-1, ISO 11844-2 and ISO 11844-3.
A.4 Sources of uncertainty
Regarding the determination of corrosivity categories based on exposure of specimens, it should first be clarified that the levels given in Table A.1 are expressed for an average value calculated from three individual values and not for an indMdual corrosion value.
The uncertainty levels given in Table A. 1, both for determination and estimation, are based on the exposure of materials at many different test sites, but only for one exposure period. Thus, the values should have a general validity, but the variation in corrosion attack that can occur from year to year, depending on natural variations in climate, is not included in the values in Table A.1.
Regarding the estimation of corrosivity categories based on the dose-response functions, the total uncertainty consists of two parts, uncertainty in the dose-response functions and uncertainty in the measurements of environmental parameters. Of these, the uncertainty in the dose-response functions is dominant. Aiso, the values in Table Al are based on an average uncertainty over the range of parameters used in the function. As for all regression functions, this uncertainty is lowest in the middle range, corresponding to corrosivity category C3, and higher at the lower and upper ranges, corresponding to corrosivity categories Cl and C5. Uncertainty for category CX is the highest and it is not covered by this calculation.
In sheltered positions, only dry deposition occurs. The cumulative effect of pollutants, induding particulates, should be considered. Specific problems of atmospheric corrosivity of low aggressive indoor atmospheres are characterized in ISO 11844-1, ISO 11844-2 and ISO 11844-3.
The wetting of surfaces is caused by many factors, for example dew, rainfall, melting snow and a high humidity level. The length of time during which the relative humidity is greater than 80 % at a temperature greater than 0 °C is used to estimate the calculated time of wetness. z of corroding surfaces. The time of wetness is underestimated by the time that the temperature exceeds 0 °C and 80 % RH in frigid zones (freezing point depression).
Information on calculated time of wetness is helpful for informative atmosphesic corrosivity estimation. Table B.1 presents time of wetness characteristics for different exposure conditions.
The most important factor within a particular temperature-humidity complex is the pollution level caused by sulfur dioxide or airborne salinity. The pollution level shall be measured in accordance with the specifications of ISO 9225.
Other kinds of pollution can also exert an effect [oxides of nitrogen (NOJ. nitric acid (HNO3) and industrial dust in populated and industrial zones) or the specific operational and technological pollution of microclimates [chlorine (Cl2), hydrogen sulfide (H2S). organic acids and de-icing agents]. These types of pollution have not been used as classification criteria.
According to this International Standard, the other kinds of pollution should be considered as accompanying ones [e.g. oxides of nitrogen (NOt) in urban atmospheres] or specific operational ones (e.g. vapours of acids in operational microclimates). Concentrations of the most important pollutants in different atmospheric environments are listed in Table B.2.