Corrosivity of atmospheres in relation to ambient air quality

Abstract

Atmospheric corrosion is not a very clearly defined subject. It occupies the territory between immersed corrosion and dry oxidation, since metals may be exposed to damp atmospheres or may be subjected to the full force of the weather. It is usually taken to include packaging and storage, painting and preparation for painting, and the effects of climate and air purity. Metals exposed to uncontrolled “normal” atmospheres may corrode more rapidly and by different mechanisms than those kept in pure, dry air – even if they are not exposed to rain. In dry atmospheres, the growing oxide film usually protects the underlying metal, giving rise to a logarithmic or square-root time law. In uncontrolled atmospheres, the rate usually remains constant for a period, but may fall off when the film has grown to an appreciable thickness. If the metal is exposed to rain, it may corrode while it is wet at the rate appropriate to immersion in impure, well-aerated water, but the rate will fall when it dries. Equations for predicting rates of atmospheric corrosion must therefore contain a term for “time of wetness” as well as for average temperature, average relative humidity, atmospheric purity and so on. These equations are of limited value because it is almost impossible to specify the local conditions sufficiently precisely. Detailed results are often difficult to rationalise but in general it is clear that persistent wetness, high temperatures and contamination by strong electrolytes are responsible for the highest rates of corrosion. Rates of rusting in impure controlled atmospheres without the complication of rain are low if the relative humidity is kept low, and remain fairly low even at 100% RH in the absence of strong electrolytes. The rate increases sharply at some RH in the region of 60-80% if the surface is contaminated with particles of sodium chloride, or ammonium sulphate, or if the atmosphere contains sulphur dioxide. Vernon showed that there was an increase in rate at about 60% relative humidity even in pure air, and a further sharp increase at 80% in the presence of 0.01% of sulphur dioxide. The increase was larger and the corrosion product less protective if the surface was contaminated with particles of charcoal. More recent evidence suggests that the SO2 content in this work was unrealistically high, and that higher RH is needed to initiate rapid corrosion with 1 ppm SO2 or less. In general, it seems that the “critical” relative humidity is set by the vapour pressure of some salt hydrate in the corrosion product, and that chlorides and sulphates are the most effective corrosive agents. The effect of moist atmospheres is to set up droplets of some strong electrolyte with the classical pattern of corrosion with a small anode in the centre of the drop acting as a source of ferrous ion and oxygen-reduction cathodes at the edges producing hydroxyl ion. Such droplets become covered by a transparent skin, presumably of ferrous hydroxide, which darkens and thickens and eventually runs through the range of hydrated ferric oxides up to Goethite and lepidocrocite. According to atmospheric conditions, these droplets may remain as discrete scabs, or may spread, or produce tracks that wander over the surface. When the rust layer is completed, the metal surface may become starved of oxygen, and it is quite common for a layer of Fe3O4 to form under the hydrated rust. The presence of corrosive salts will, however, prevent this layer from becoming very protective. Vernon showed that a carefully cleaned steel specimen would not rust even in a normal laboratory atmosphere if it was enclosed by muslin stretched over a wire frame. The protective oxide film continued to develop, and if the specimen was removed from the enclosure after, say, a year, it would remain unaffected by dust particles, perhaps for several weeks, before spots of rust appeared. Factors conducive to rusting of iron and steel are therefore: - Sensible moisture High RH (above 70-80%) - Salt mist - Surface contaminants (dust, sweat residues, soldering fluxes, etc) - Atmospheric contaminants (SO2, HCl, organic acids) - High temperature.