ISO and ASTM Standards

The International Corrosion Research Group conducts its atmospheric coupon exposure program in accordance with published guidelines of the International Organization for Standardization and the American Society for Testing and Materials. The standards most closely related to atmospheric exposure of test coupons are listed below.

Corrosivity of Metals and Alloys - Corrosivity of Atmospheres.

ISO 9223 Corrosivity Classification

The nature and rate of atmospheric corrosion of a metal or alloy depend on the properties of surface formed electrolytes, related to the level and type of gaseous and particulate pollutants in the atmosphere and to the duration of their action on the metal surface. Data on the corrosivity of the atmosphere are essential for development and specification of optimized corrosion resistance for manufactured products. The corrosivity categories (C) may be assessed in terms of the most significant atmospheric factors influencing the corrosion of metals and alloys i.e. time of wetness (J)  and pollutant levels, specifically sulfur dioxide (P) and/or air-borne salinity (S).

Time of Wetness (TOW, J) in hours per year, is calculated as the time the relative humidity is greater than 80% at an air temperature greater than 0 ^oC. Experimental TOW can also be determined directly by various measuring systems often located on or nearby the exposed coupon. Time of Wetness is divided into 5 categories as given in Table 1.

Atmospheric pollution is divided into two categories: pollution by sulfur dioxide, SO₂, and by air-borne salinity (Cl^-) . These two types of pollution are representative for rural, industrial and marine atmospheres and are generally measured at in exposure site in the vicinity of the coupon exposure racks, (refer ISO 9225). The pollution by sulfur dioxide (P), and by air-borne salinity (S) are each divided into 4 categories as listed in Table 2.

Classification of corrosivity. Corrosivity (C), of the atmosphere is divided into categories which are broadly described in Table 3. The corrosivity category for individual metals may be determined from environmental data using time of wetness and pollution categories of Tables 1 and 2. These atmospheric corrosivity categories are listed in Table 4. Additionally, the corrosivity category may be determined from corrosion rates (r_corr) for first year of exposure of standard specimens. These categories are listed in Table 5. The corrosion rate (r_corr) is determined from mass loss measurements as specified in Internationl Standard ISO 9226.

Table 1: Wetness classification based on time of wetness.

Table 2: Sulfur dioxide and chloride classes.

Table 3: Categories of corrosivity.

Table 4: Corrosivity categories from environmental data.

Table 5: Corrosivity categories from first year exposure data.

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ISO 9224: Guiding values for the corrosivity categories.

ISO 9225: Measurement of pollution.

ISO 9226: Determination of corrosion rate of standard specimens.