Effects of magnetic fields on the phosphating process

Abstract

The influence of permanent magnetic fields (MF) 0.5 Tesla (T) on the morphology and corrosion protection (‘drop’ method) of phosphate coatings on high purity Zn (99.9) surfaces has been studied. Two types (i) of Zn phosphating and (ii) ZnCa phosphating solutions have been investigated. The weights of the phosphate coatings without MF differ, approximately twice (P Zn phosphate ∼ 8 g/m 2, while P ZnCa phosphate ∼ 4 g/m 2. ZnCa phosphate coatings are better defined and compact. The number of crystallites ranges from 3.3 10 −2/cm 2 to 3.3 10 −3/cm 2. The amount of crystallites increases in the case of ZnCa phosphating. Three variants of MF were applied: (i) phosphating in perpendicular MF, (ii) in MF parallel to the surface; (iii) phosphating solutions were pre-treated by MF for 10 min and then phosphating process was carried out. A zinc phosphating process in Zn phosphate solution is less influenced by MF in comparison with ZnCa phosphating solution. In this case, phosphate coating is fine grained and compacts, and the number of crystallites increases. The weight reduction of ZnCa phosphate coating reaches to 25%. Corrosion protection increases several times. The fact that the influence of MF remains when solutions are pre-treated in MF and then phosphating is carried out (‘memory’ effect), facilitates its industrial use.