Chemical State Analysis of Iron in Zinc Phosphate Coatings by Conversion Electron Mossbauer Spectrometry

Abstract

The surface of rimmed steel was chemically treated in a zinc phosphating bath and the physicochemical states of iron compounds in phosphate coatings were studied by means of conversion electron Mossbauer spectrometry, X-ray diffractometry, and scanning electron microscope. High spin iron(II) compound (I. S.: 1. 26 mm/s, Q. S.: 3. 40 mm/s, phosphophyllite: Zn2Fe∗ (P04)2∗4 H2O) and amorphous iron (HI) compound, which were estimated to lie underneath the hopeite [Zn3(PO4)2∗4 H2O] layer, were observed in zinc phosphate coatings formed by the immersion of rimmed steel in Bonderite 3008 solution. The ratio of iron(IT) to iron(M) in the coating increased with increasing the immersion time (15∼120 s). The respective addition of oxidizing reagents (H2O2, NaC1O3+NaNO2, NaNO2 and NaN02+NaN03) into a zinc phosphating bath decreased the ratio of iron(I[) to iron (HI) in the coatings in the order of magnitude of their oxidation-reduction potential. Both the higher concentration of zinc ion and the addition of nickel ion in the bath prevented the formation of phosphophyllite and favored the deposition of unidentified iron(I[) phosphate (presumed to be Fe3∗ (PO4)2,,3H2O) under the hopeite layer. Iron(J[) compound (I. S.: 1.26 mm/s, Q. S.: 2. 06 mm/s) was deposited in the leading product of seholzite [Zn2Ca(PO4)2-2 HaO] formed by the treatment in a calcium-zinc phosphating bath (XCa=Ca/(Ca + Zn)=0.8). The principal axis of the electric field gradient was approximately oriented at 60° to the steel surface. The magnetic orientation of the interfacial iron beneath the phosphate coating changed from being parallel to the steel surface to a randomly packed state with the increase of the immersion period in the phosphate bath and of the phosphate coating weight. © 1980, The Chemical Society of Japan. All rights reserved.