Abstract
Immersion tests of 0.004% C steel were carried out at 460-540°C. Vacuum melted specimens in the form of 3×40×40mm were abraded, degreased in acetone and dipped into liquid zinc held in a magnesia crucible placed in an electric furnace. After soaked for 24hr, specimens were pickled in 10% HCl solution and weighed to measure iron loss. Metallographic examinations were also carried out. Iron loss increases with increasing temperature and has a maximum at about 500°C. In accordance with the equilibrium diagram alloy layers, consisting of Γ, δ1C, δ1P and ζ layer, are formed during the reactions between iron and molten zinc. Compact and well-adhering alloy layers protect iron against the attack by the melt at 460°C. At 500°C ζ layer disappears. δ1P layer is completely broken up and loses its protective property. Alloy layers are stable again at 540°C. Scanning electron micrographic studies revealed that a compact δ1P layer formed at 460°C is destroyed at grain-boundaries on etching in a nital solution. From this observation it can be concluded that δ1P layer is fragile at grain-boundaries, and breaks up readily, when it reacts with molten zinc. Since a continuous layer ζ prevents it from direct contact with zinc, it remains compact at 460°C. At 500°C due to the absence of ζ layer it is attacked and disrupted into fragments at 500°C, which makes it possible for liquid zinc to penetrate through it. As a result enhanced corrosion occurs. This break-up is caused not only by the chemical reactions with the melt, but also by the shear stresses at grain-boundaries arising from the formation of δ1P crystals from δ1C crystals. These stresses decrease with increasing temperature to give stable alloy layers at 540°C. Since δ1P crystals are built up only at δ1P/δ1C phase boundaries, expansion forces in a direction parallel with the surface develope in alloy layers at the corners of a specimen, as alloy crystals grow outwards. These expansion forces fracture δ1P layer to bring about intensified attack at the corners.
