Multi-analytical investigation of stainless steel grade AISI 420 in simulated food contact

Martensitic stainless steel grade AISI 420 intended for cutlery knives and scissor blades has been investigated in an attempt to correlate microstructure and surface finish (polishing procedure) to metal release rates of the main alloy constituents, iron and chromium. Metallographic investigations proved the knife steel and, in particular the scissor steel to have very inhomogeneous microstructures. The knife steel contained bands of ferrite in the martensitic structure while the scissor steel revealed large amounts of tempered martensite and inclusions of manganese sulphides and silicon oxides. Samples of different surface finish were immersed in 3 vol% acetic acid at 40 °C from 1 to 10 days for metal release testing, simulating food contact. The largely inhomogeneous microstructure of the scissor steel results in high and greatly varying metal release rates, despite a fairly high chromium content of the surface oxide. Areas of inclusions seem to act as initiation points for accelerated metal release and the forming of a surface oxide depleted in chromium, but rich in copper. The generation of high surface temperatures during polishing was found beneficial from a metal release perspective as a result of the formation of a surface oxide of improved passivating properties.