Improvement of barrier properties of Cr-free pretreatments for coil-coated products

Effect of Hot Dip Galvanized Steel Surface Chemistry and Morphology on Titanium Hexafluoride Pretreatment

“…Pretreatment solutions based on titanium or zirconium hexafluorides are widely used in the color coating industry for hot dip galvanized steel. The uniformity of a titanium hexafluoride pretreatment layer on the HDG surface has been shown to correlate with the humidity resistance of coated panels [14].…”

Multiscale and Multi-Technical Approach to Characterize the Hot-Dip Galvanized Steel Surface and Its Consequence(s) on Paint Adhesion and Tendency to Blistering

“…In the second case, the loss of adhesion is attributed to the accumulation of water at the interface which induces swelling and internal stress. If the presence of remaining native aluminum oxide on the surface hindered the correct formation of the pretreatment layer as depicted in the literature [8,[12][13][14][15][16][17], it can be supposed that it can lead to an incomplete layer and thus to a weaker interface with reduced stability towards water. In addition, the native aluminum oxide on the surface of HDG steel is known to be inert and to form weaker bonds towards organic molecules [2,8,12,14,41].…”

“…The important quantity of blisters for all conditions and thus the consequent loss of adhesion could be linked to the homogeneity of the pretreatment primer layer. In this work, the pretreatment primer layer is based on a Cr-free chemistry, and it is known that the homogeneity of such a layer is of upmost importance for its protective ability [11][12][13][14][15][16]. Indeed, Cr-free pretreatments do not have a self-healing ability and rely mostly on a barrier type of protection.…”

“…In addition, to affect the adhesion properties of the complete system, the surface state of HDG steel will also impact its performance related to corrosion resistance and its tendency to blistering. Recent studies have highlighted the impact of the distribution of the pretreatment layer on the tendency to blistering and were able to partially link it to the presence of the native aluminum oxide film at the surface [12][13][14][15][16][17]. Furthermore, studies have shown, that in the case of phosphate-based and chromate-based conversion layer, the presence of the native aluminum oxide film at the surface could delay the formation of the conversion layer and thus reduce its thickness [2,4,16,18].…”

“…Usually, before application of a pretreatment layer, HDG steel strips are alkaline cleaned. This step is performed to remove any contamination at the surface, such as oils, to improve the wettability of the surface and is also known to dissolve aluminum phases and remove the native oxide film [6,8,13,19,20]. Indeed, the hydroxide ions in the etchant react with the metal-oxide surface and cause the dissolution of the oxide layer and the underlaying metal.…”