A Mössbauer study on gray stains in electrogalvanized steel

M. ZAPPONI; T. PÉREZ; C. P. RAMOS; G. POLLA; C. SARAGOVI; D. COOK

Virginia

Simposio; Int. Symposium on the Industrial Applications of Mössbauer Spectroscopy (ISIAME 2000).; 2000

Electrogalvanized steel is being used increasingly for the manufacture of automobiles, domestic appliances and other products. Both one-side and two-side electrogalvanized steel sheets are being produced. The one-side coated product is particularly desirable in applications that require superior protection on the inside of the low carbon steel panel, where corrosion usually starts. The steel also contains one high quality surface that can be formed, joined and painted in the usual way. Such parts, needing a good superficial finish, require an uncoated surface with properties as close to that of cold rolled sheet as possible. Plating only one side, turning off the plating current to the anodes facing the side that remains uncoated, produces one-side electrogalvanized sheets. As the steel strip travels through the plating section, the uncoated side is attacked by the acid electrolyte. This pickling action leads to highly active surface, appearing sometimes ?gray stains?.  The degree of stain formation depends upon many variables such as history and grade of steel, pretreatment practices and plating process conditions. It is of economical importance to know the origin of these defects and their effect on the final performance and esthetics of the material. With the aim of identifying the origin of the stain defects, we performed studies of the steel surfaces using Mössbauer spectroscopy (MS) and X-ray diffraction (XRD).  Powders were removed from the ?stained? and ?non-stained? areas of the uncoated steel surfaces, by tape, scraping and ultrasonic bathing. They were analyzed by TMS. Spectra were compared with those obtained from CEMS on the sheets where the powder was left attached. Results show that a Mössbauer inactive coating was covering both areas, being that over the stained area wider and in amorphous state. The results suggest that this coating formed after the production processing thereby not effecting the performance of the steel. The combined Mössbauer analysis showed that a layering of iron-bearing species was present in both areas; a-Fe and Fe3C being present at larger depths and Fe-C more superficially. XRD did not identify the presence of any Zn-bearing species.