Most access cover and grating products manufactured from grey or ductile cast irons are coated in paint (often bitumen based) prior to dispatch, in order to provide an aesthetic finish. Indeed, those products supplied to BS EN:124, BS 5834 or BS 750 all receive this barrier paint coating, as standard, unless otherwise specified by the customer.
This coating itself acts as a barrier coat for transportation and short-term aesthetic purposes only and provides no electrolytic / galvanic corrosion protection to the underlying iron.
In light of this and the fact that paint coatings can be porous to gases and fluids, particularly when applied to sand-cast surfaces, once the coated product has been exposed to the atmosphere or subject to abrasion from the effects of trafficking, it can easily be compromised. Where it is compromised and the coating protection effectively lost, a natural defensive layer then begins to form on the iron surface which involves the formation of a thin oxide film, usually visible as light orange/brown corrosion deposits (rust). As the formation of these surface oxides effectively slow further oxidation of the ferrous substrate, the corrosion process is consequently also slowed unless the oxide layers are removed. E.g by mechanical means (trafficking, etc...).
In the case of products not trafficked but still exposed to atmosphere, light oxidation (corrosion) can still occur due to the porous nature of common paint coatings. Indeed, even in low levels of humidity where the iron surface is effectively dry, surface corrosion can still occur due to the hydroscopic nature of any iron oxide already present. Specifically, below about 60% relative humidity only air-formed oxide film occurs, but as the humidity increases above 80% (from hydroscopic absorption and surface wetting), the corrosion can accelerate to appear as characteristic rusting.
Corrosion can also be aggravated by the presence of corrosive gases and airborne particles (in particular, hydroscopic salts likely to induce electrolytic corrosion. e.g. sodium chloride), which can gain direct contact with the metal surface via the porous coating layer.
Atmospheric sulphur dioxide gas is especially harmful as it can be absorbed and concentrated in soot particles which can settle on the product surface to form mild, but corrosive, sulphurous acid where moisture is present. Therefore, due to the higher concentration of soot particles and sulphur dioxide in industrial areas, the rate of iron corrosion is usually higher in these regions. Similarly, rates of corrosion in marine or coastal environments are usually higher than in non-coastal areas due to the presence of airborne sea salt (sodium chloride).
It therefore follows that where products are exposed to only low volumes of dust particles, moisture or impurities, surface rust formation is significantly inhibited. Instead, the air-formed oxide-film-only grows slowly in thickness, forming a progressively more protective corrosion layer.
