Verfahren zur Beseitigung der sichtbaren Kornflaechenstruktur

In the hot dip coating of a ferrous metal strip with molten Zn, visible spangle is suppressed by inducing a multitude of closely spaced spangles which are invisible or barely visible to the naked eye by applying to the freshly coated strip, at a point where the temperature of the molten Zn coating on the strip is from just above its solidification temperature to 40 DEG F. there above, a water solution of a reactive inorganic salt which is thermally decomposed on contacting the molten Zn coating, the salt being one which will thermally decompose at a temperature within the range of 175 to 550 DEG F. and release radicals reactive with metallic Zn or a salt which will hydrolize when added to water to form a reactive inorganic salt which is decomposable within the temperature range, the salts and their decomposition products being non-toxic and leaving no harmful, difficulty removable residues on the coated strip. Specified inorganic salts which decompose at 175 to 550 DEG F. are dibasic ammonium phosphate, NaHCO3, sodium dihydrogen orthophosphate, ammonium borate, ammonium molybdate and ammonium sulphate. Hydrolyzable salts which may be used are sodium carbonate and trisodium phosphate. The Fig. shows solution containing a reactive inorganic salt being sprayed on Zn coated strip 13a from spray heads 18 extending entirely transversely across the strip. The height of the spray heads 18 above a molten Zn bath 11 is automatically adjusted such that the metal coating is molten as enters the spray but solid as it leaves. A radiation pyrometer measures the strip temperature and by a signal to an electronic device 20 controls an electric hoist motor which raises or lowers the spray head and pyrometer.