Determination of Surface Topography and Composition of Cr-Free Pretreatment Layers on Hot Dip Galvanized Steel

Abstract

Topography and composition of Cr-free, titanium-based pretreatment layers on hot dip galvanized steel were studied with scanning electron microscopy, atomic force microscopy, time of flight secondary ion mass spectrometry and Auger electron spectroscopy. A layer within the target coating weight range (4-10 mg Ti/m 2 ) for industrial coil coating processes contained a micro-structure with hillocks and valleys, showing significant topographical variations. A local maximum film thickness of about 50 nm was detected for a sample containing 5.0 mg Ti/m 2 . The hillocks were composed of metal complexes and phosphates, formed as a result of rapid zinc dissolution and metal hydroxide/phosphate precipitation reactions. During the layer formation also the polymer component of the pretreatment chemical becomes embedded within the structure. The structure composed of hillocks and valleys may be highly beneficial for paint adhesion, increasing the surface contact area for primary and secondary chemical bonding.

Characterisation and anti-corrosion property of Ti conversion coating on manganese surface

“…In recent years, with the increasing environmental problems, the use of K 2 Cr 2 O 7 is gradually banned. Therefore, a great deal of researches had been carried out to replace the chromate with the use of molybdates, titanates, salts of rare earth metals and other materials [10 -17]. Hydrolisable silicon alkoxides, sodium vanadate solution combined with colloidal silica were used to form a hydrophobic sol–gel coating on zinc surface to improve corrosion resistance in a patent from the U.S. [18].…”