The effect of airborne nutrients on atmospheric corrosion

Many extensive atmospheric corrosivity surveys have been undertaken covering cities, regional areas and sometimes countries in order to plot corrosion maps and identify possible sources of higher infrastructure risk. Classically, the surveys employ mild steel, copper steel and/or zinc coupons and corrosivity is derived by determining mass loss of the exposed specimens. Higher areas of corrosion loss are generally ascribed to airborne chlorides from marine sources and/or sulphur and nitrogen compounds from industrial fallout. Bacterial influence on corrosion has been well documented in buried and marine conditions and more recently recognized as a major factor in tidal conditions notably as accelerated low water corrosion. Hence it follows that if bacteria play a part in immersed, tidal and buried conditions, they should also be involved with atmospheric corrosion. To elucidate this hypothesis a trial has been designed and implemented in which mild steel coupons have been deployed in ten different environments to monitor the effect of airborne compounds. Chlorides, nitrogen and sulphur compounds can be considered as pollutants but equally can be thought of as essential nutrients for bacterial growth. Locations have been chosen ranging from extreme to moderate for chloride deposition and for differing particulate nitrous and sulphur compounds. In addition, a sub-alpine and a desert environment far removed from contamination are part of the project. The present paper reports the results for mass losses and corrosion products in the first year of exposure and compares these observations with material already available in the literature.