Microscopic local elements in the phosphating process

D. Zimmermann

doi:10.1016/S0013-4686(03)00385-2

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Microscopic local elements in the phosphating process

2003-09-01

PRE

D. Zimmermann

A.G. Muñoz

J.W. Schultze

DOI：10.1016/S0013-4686(03)00385-2

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Abstract

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The addition of Ni2+ to the phosphating bath improves the corrosion stability as well as the adhesion of coating layers. This is caused by the formation of Ni deposits on the base of pores of the phosphate layer. They do not only catalyse the surface reactions but generate a more corrosion resistant surface as well. The deposition of Ni during the phosphating of electrogalvanised steel was studied in aqueous solutions by simultaneous measurement of the rest potential and electrode capacity in dependence on time, temperature and bath composition. The electrochemical response of the system was corroborated with SEM pictures, gravimetric measurements and X-ray fluorescence analysis. Rest potential patterns under different solution and initial surface conditions were interpreted by microscopic local elements. During the deposition of the phosphate layer, the cementation of Ni on the initially corroding Zn gives rise to the formation of surface Ni-Zn alloys, providing cathodic centres where the reduction of H+ and accelerators (NO2-, NO3-) is favoured. The process matches with a quasi-instantaneous displacement of the rest potential from that corresponding to the Zn/Zn2+ couple towards the corrosion potentials of cementated Ni-rich alloys. The appearance of a simultaneous characteristic capacity peak was analysed in terms of the surface changes due to Ni-cementation together with an increment of the growth rate of phosphate crystals and the formation of oxide films. Microscopic local Ni deposits have an extension of some micrometer and a thickness of few hundred nanometer up to 1 mum. (C) 2003 Elsevier Ltd. All rights reserved.

Keywords:

phosphating

Ni deposition

Zn-coated steels

local elements

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