Scanning auger microprobe study of hot-dipped regular-spangle galvanized steel: Part I. surface composition of As-produc
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Untreated bright and dull zinc coatings produced on a hot-dipped-coating pilot line from a zinc bath containing small concentrations of aluminum, antimony, and lead were examined with a scanning Auger microprobe. The results show that the entire surface of the zinc coating was covered by a very thin, continuous film comprised of aluminum and oxygen. The thickness of this film, of the order of 5 nanometers, was the same for both bright and dull coatings and independent of zinc spangle roughness. Beneath this film the zinc surface contained numerous small precipitates that occupied a considerable portion of the total coating surface area, perhaps as much as 25 pct for some spangles. In all, five distinct types of precipitates were observed in the coatings. Aluminum was common to each type except for one which was essentially pure lead. In the others the principal element combined with aluminum was oxygen in one case, antimony in the second, iron in the third, and zinc in the fourth case. Based on the results of Auger electron spectrometry, these appear to be A1203, A1Sb, one of the aluminum-rich iron-aluminum intermetallic compounds, and the zinc-rich solid solution of zinc in aluminum, respectively. The size of these various precipitate particles tended to be larger and their frequency greater in rough than in smooth spangles.
I.
INTRODUCTION
BECAUSE there is an increasing demand for zinc-coated steel with finish treatments, a study was made to develop an understanding of the composition and morphology of the surface of galvanized steel, the interaction between that surface and treatment films subsequently applied to it, and the factors that determine the effectiveness of those treatment films in meeting their intended objectives. Part I of this paper deals with the characterization of the hotdipped galvanized steel surface before any surface treatment is applied. Not all galvanized steel sheets have the same appearance. In some cases the coatings have an overall brilliance, and close examination shows that a large number of the spangles are smooth and very reflective. But in other cases the coatings have a dull appearance because most of the spangles are frosty and poorly reflective. The relationship between spangle topography and reflectivity has been studied by Jaffrey, Browne, and Howard tlJ using optical and electron microscopy. They concluded that there are four distinct spangle types: (1) mirror like--flat, featureless surface, high reflectivity, (2)feathery ~ dendritic pattern, high reflectivity, (3) dimpled--small regular hillocks without a preferred alignment, relatively low reflectivity, and (4) r i d g e d - small hillocks aligned in a system of ridges, low reflectivity. Poorly reflecting spangles not only detract from a zinc coating's appearance, they apparently also adversely affect its surface reactivity. Helwig, I21 for instance, has observed that chromate films applied to galvanized steel for stain resistance are most effective in zinc coatings that have a minimum of dull spangles. This observed
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