Behaviour of Rust and Rusted Steel Surfaces

Abstract

The effect of the presence of rust on steel on its further corrosion during outdoor and laboratory exposure to SO₂ has been examined. More aggressive initial conditions favour the formation of non-protective, loose rust which-flakes off from the metal surface and increases the subsequent yearly corrosion values. The initial rust present increases the rate of attack during the first 2–3 months, but there is then a general falling off of the corrosion rate with time. SO²-contaminated rust is able to promote corrosion on clean (rust-free) steel, but the amount of corrosion is not dependent on the amount of SO² adsorbed by the rust. The order of severity of attack on steel by different electrolytes differs from that caused by rust produced by the same electrolytes. Rust contaminated with FeSO₄ or FeCl₃ enhances the rusting of steel, whereas rust converters (such as phosphates) have beneficial effects. In the further rusting in SO₂ of prerusted steel specimens it was found that the duration.of the pre-rusting period determines the subsequent rate of rusting, but the ‘age’ of the rust, rather than its thickness, determines its protective effect. In interrupted SO₂ exposure, in which the pre-rusted specimens were first exposed to SO₂ and then to humid SO₂-free air, it was found that a temperature (18°e) and humidity (50% R.H.) which are too low to initiate the formation of incipient reaction product (FeSO₄, 4H₂O) on clean metal, allow sufficient formation of these products on rusted specimens to enable rusting to continue in moist SO₂-free air.