Laboratory simulation of formation of steel corrosion products with soluble salt contents

Abstract

It is common to find soluble salts, generally chlorides and/or sulphates, among the atmospherical corrosion products when steel has been exposed for some time to atmospheres contaminated with Cl⁻ and/or SO₂. In field studies, these salts are found heterogeneously distributed throughout the rust, accumulating preferentially in the innermost corrosion product layers. However, in laboratory studies, which have tried to simulate this surface condition of contaminated steel for different purposes (for instance, to study the effect of the presence of soluble salts at the steel/paint interface), the specimens used have consisted of initially clean steel substrates contaminated with the desired levels of soluble salts by even dosing over the metallic surface. This situation is thus significantly different (in terms of the absence of rust and uniform distribution of salts) to that occurs naturally in the atmosphere. In an attempt to approximate more closely to what happens in the atmosphere, the present work has established a methodology using accelerated corrosion cabinets to produce corrosion products containing similar levels of chlorides and sulphates to those found on steel surfaces in natural conditions. The results obtained show that by varying the exposure time in a salt fog cabinet (in the case of chlorides) and the volume of SO₂ introduced in a Kesternich cabinet (in the case of sulphates), it is possible to form corrosion products with similar contents of these salts to those formed naturally during 3 months of exposure to three atmospheres of different aggressivity in Spain, located at Cabo Vilano (marine), Avilés (industrial) and Madrid (urban).