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Steel strip reactivity towards galvanising zinc baths, can vary substantially, depending on surface chemistry and the conditions under which recrystallization annealing is carried out prior to hot-dipping. On-line electrolytic nickel precoating of the strip in a high current density cell, was envisaged as a tool to equalize the behaviour of the bath-steel substrate system. To determine the precoating thickness that is strictly necessary to efficiently isolate the steel from the zinc bath, a theoretical anisothermal diffusion model was developed for the iron-nickel system. Galvanizing nickel precoated steels results in the formation of Zn-Ni or Zn-Ni-Fe intermetallics, depending on whether the initial precoating thickness is over or below 20 g/m{2}. It was observed that the growth rate of Zn-Ni compounds is independent of both annealing protective atmosphere dew point and steel chemistry. The adhesion of zinc on initially nickel precoated steels is usually poor: the coating flakes and peels off, thus leading to inconsistent weight losses on cylindrical cup drawing tests (cup-tests). After galvannealing, corresponding weight losses are about twice those of classical products.

Additional information

Authors: LAMBERIGTS M, Centre de recherches métallurgiques, Liège (BE);SERVAIS J-P, Centre de recherches métallurgiques, Liège (BE)
Bibliographic Reference: EUR 16655 FR (1997) 42pp., FS, ECU 8.50
Availability: Available from the (2)
ISBN: ISBN 92-828-0745-2
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