Atmospheric corrosion initiation on steel from predeposited NaCl salt particles in high humidity atmospheres

Abstract

Corrosion initiation on steel from predeposited NaCl particles was observed in situ using an optical microscope. Corrosion did not initiate from small droplets (D<&sim45 μm) formed by the deliquescence of predeposited NaCl particles upon exposing to high humidity (RH>80%). Corrosion occurred under larger NaCl droplets and was accompanied by the formation of secondary droplets outside of the perimeter of the initial droplets. Sometimes, corrosion that initiated from the large NaCl droplets propagated in the form of filiform corrosion. Elemental mapping using energy dispersive X-ray analysis showed that Cl was concentrated at the filament tip, while Na migrated to the outside region of the secondary droplets. Under large NaCl droplets, there was evidence that corrosion initiated from slag based inclusions.