ISO 9223

This standard classifies the corrosivity of an atmosphere based on measurements of time of wetness, and pollution categories (sulfur dioxide, airborne chlorides). The standard was not intended to be used in extreme service atmospheres such as those within chemical or metallurgical processing facilities or where there is direct contact with salt spray.

Only airborne chlorides and sulfur dioxide are considered in terms of classifying the pollution, this gives good coverage of rural, urban, industrial and marine atmospheres. Based on these measures an atmosphere is classified as being in one of five categories in terms of its corrosivity using two types of units, i.e. short term corrosion rate (CR) of steel as g m^-2 year^-1 (one year) or mm year^-1 (twenty years):

Category	Short term	Long term
	(g m^-2 year^-1)	(mm year^-1)
C₁	CR <= 10	CR <= 0.1
C₂	10 < CR <= 200	0.1 < CR <= 0.5
C₃	200 < CR <= 400	1.5 < CR <= 6
C₄	400 < CR <= 650	6 < CR <= 20
C₅	650 < CR	20 < CR

And convert these corrosivity ratings into short term corrosion rates (g m^-2 year^-1) according to the following table.

Category	Steel	Copper	Aluminum	Zinc
C₁	CR <= 10	CR <= 0.9	negligible	CR <= 0.7
C₂	10 < CR <= 200	0.9 < CR <= 5	CR <= 0.6	0.7 < CR <= 5
C₃	200 < CR <= 400	5 < CR <= 12	0.6 < CR <= 2	5 < CR <= 15
C₄	400 < CR <= 650	12 < CR <= 25	2 < CR <= 5	15 < CR <= 30
C₅	650 < CR	25 < CR	5 < CR	30 < CR

The use of the ISO guidelines and atmospheric data pertaining to these two environments have revealed the most important parameters affecting atmospheric corrosivity. In the case of the maritime Greenwood base, the time of wetness and chloride deposition rate are most important. In the rural environment, the time of wetness is likely to exert the strongest influence but chlorides transported from adjacent highways during the 'deicing' winter months can also be a factor.

ISO 9223

References