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

Abstract

Titanium hexafluoride pretreatments are known to improve paint adhesion and function as a barrier between the coating and the hot dip galvanized (HDG) steel surface. Interactions at the zinc/pretreatment interface are of utmost importance for the formation of pretreatment layers and the corrosion resistance of color coated hot dip galvanized steels. Removal rate of inert aluminum oxide from HDG steel samples by chemical dissolution was studied. XPS measurements showed that the surface Al 2 O 3 layer thickness decreased rapidly already at mild alkaline cleaning, while complete removal of Al required severe etching. Low reactivity of an Al 2 O 3 -rich surface was confirmed by impaired formation of a titanium hexafluoride pretreatment layer. Grain boundaries and deformation twinnings were shown to be of importance for the reactivity of the HDG surface and for the precipitation of the pretreatment chemical. Helium ion microscopy images and electron probe microanalysis (EPMA) of a pretreated sample showed accumulation of the pretreatment chemical at the grain boundaries. Al removal rate was fast at the deformation twinnings at the grain plateaus. Slow Al removal was observed at dendritic valleys and grain boundaries. The results increase understanding of the reactivity of hot dip galvanized steel surface.

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

“…After alkaline cleaning, the content of aluminum dropped, and the bulk value was reached after 300 nm. These results are in accordance with the literature showing an aluminum dissolution due to the alkaline cleaning step [6,[14][15][16]37].…”

“…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].…”

“…These results confirmed that a lesser extent of aluminum at the surface was beneficial in terms of adhesion. From a pure adhesion point of view, the improvement of the adhesion with a low concentration of aluminum at the surface could be explained by the inert behavior of the native oxide film on the surface after the galvanizing process [2,8,12,14]. According to the literature, the native oxide film is composed of γ-Al 2 O 3 or amorphous aluminum oxide and it is known that for adhesion, the hydroxide form and most particularly the hydroxyl fraction are beneficial in particular for aluminum [2,[43][44][45].…”

“…Indeed, it was shown by Saarima et al [8,12] that the reactivity of the HDG surface was lowered by the presence of native aluminum oxide at the surface. In addition, most of the pretreatment layer formation on HDG steel relies on their reactivity towards zinc as their formation mechanism goes through a dissolution-precipitation process [4,[14][15][16]. Thus, as the presence of native aluminum oxide at the surface of HDG steel can slow down the formation of the pretreatment layer, it is possible that an incomplete (in thickness or structure) pretreatment layer could result in lowered adhesion.…”

“…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].…”