Abstract
The bonding properties of zirconium- and titanium-based conversion coatings were evaluated using model conversion solutions of H2ZrF6 and H2TiF6 with addition of various organic additives (PAA, PVA, PVP). Macroscopic testing techniques such as contact angle and pull-off adhesion measurements were performed on galvanized steel sheets. Complementary to this, molecular studies were performed on model zinc substrates using ATR-FTIR in the Kretschmann configuration. The macroscopic and molecular approaches showed a good correlation demonstrating ATR-FTIR in the Kretschmann configuration to be a valuable tool to gain fundamental insights in metal oxide-polymer interfacial phenomena. Zirconium-treated galvanized steel substrates were shown to have a higher bonding affinity for the polyester coil coat primer than titanium-treated galvanized steel substrates. The presence of organic additives did not further improve the bonding properties. Yet, organic additives initially improved the interfacial stability of titanium-treated substrates. However, on the long term, organic additives are shown to be detrimental for polyester coil coat adhesion. This adverse effect of organic additives on the long term was assigned to its selective dissolution during immersion and was most pronounced for titanium-treatments. The limited effect of organic additives in case of zirconium-treatments was attributed to the higher portion of chemical interfacial bonds, as well as its tendency for crosslinking reactions causing entanglement of polymeric compounds in the zirconium oxide structure.
Highlights
Recent restrictions on the use of carcinogenic hexavalent chromium, as well as ecological concerns on the use of extensive use of phosphates lead to the development of a new generation of surface treatments. [1] Currently, zirconium- and/or titanium-based fluoroacid treatments are considered as viable alternatives since they improve both corrosion resistance [2,3,4,5,6,7] and paint adhesion [8,9,10,11,12] on both ferrous and nonferrous substrates [1]
X-ray photoelectron spectroscopy (XPS) and Field emission auger Electron spectroscopy (FE-AES) study The formation of a zirconium- and titanium-based conversion film on galvanized steel (GI) was studied using XPS
Based on the zirconium-based conversion mechanism proposed by Lostak et al [19] it is suggested that the zinc matrix is more noble compared to the aluminium impurities
Summary
Recent restrictions on the use of carcinogenic hexavalent chromium, as well as ecological concerns on the use of extensive use of phosphates lead to the development of a new generation of surface treatments. [1] Currently, zirconium- and/or titanium-based fluoroacid treatments are considered as viable alternatives since they improve both corrosion resistance [2,3,4,5,6,7] and paint adhesion [8,9,10,11,12] on both ferrous and nonferrous substrates [1]. During the anodic dissolution of native metal (hydr)oxides, cathodic counter reactions create local surface alkalization, which lead to precipitation of zirconium- and titanium oxide [15,16,17,18,19,20]. In the case of titanium oxide, spontaneous dehydration via oxolation reactions leads to TiO2 which crystal structure (rutile or anatase) depends on the acidic and temperature conditions. [14[21,23] the high coordination number of zirconium (NZr = 8 vs NTi = 6) and resulting geometry (associated with its larger ionic radius) does not allow the formation of compact condensation products.22] Instead, amorphous oxy hydroxides are being formed. Verdier et al showed that during the conversion of AM60 magnesium alloy, titanium occurred only in its oxide form (TiO2), whereas zirconium, depending on the solution composition, was found as oxide (ZrO2), oxyhydroxide (ZrO2-xOH2x) and hydroxyfluoride. Verdier et al showed that during the conversion of AM60 magnesium alloy, titanium occurred only in its oxide form (TiO2), whereas zirconium, depending on the solution composition, was found as oxide (ZrO2), oxyhydroxide (ZrO2-xOH2x) and hydroxyfluoride. [23] This confirms the formation of amorphous zirconium oxyhydroxide phases, which have been reported to have variable compositions depending on the experimental conditions. [22] As such, the pH, Zr
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