Compatibility between cataphoretic electrocoating and silane surface layer for the corrosion protection of galvanised steel

The compatibility between a cataphoretic electro-coating and a silane layer applied on galvanized steel was evaluated by performing electrochemical impedance measurements on coated and uncoated samples. During electro-deposition, the water electrolysis induces hydrogen production. This process can induce degradation or destruction of the silane layer. This process was simulated by reproducing the application conditions of electro-coating in an aqueous solution of same pH(6) and conductivity (1600µS) than the electro-coating bath, but without any pigments and binder. A current of 2mA/cm2 was applied between the sample and the counter-electrode during 10 and 20 s. These conditions are representative of the mean real application conditions just before the coating formation. The loss of the barrier effect offered by the silane layer was evaluated by EIS before and after simulation. This simulation shows whether it is possible to conveniently design the properties of the silane layer to maintain its protection and adhesion promotion properties after polarization. The barrier properties and the water uptake of the electro-coated samples were evaluated by EIS as a function of immersion time in a sodium chloride solution (0.1 M). The coated silane pre-treated samples show a good behaviour compared to the samples coated without pre-treatment. By properly managing the deposition conditions of sol–gel films it is possible to obtain cataphoretic coating with improved corrosion resistance. Silane sol–gel films of different thicknesses and curing temperature were produced. It was demonstrated that a 120nm thick silane sol gel film cured at 180◦C ensures a very good compatibility with the electro-coat. In fact, this system shows a very high corrosion resistance even after 50 days of immersion in a sodium chloride solution. Also the resistance in the salt spray chamber of the electro-coated thin silane layer cured at 180 ◦C is remarkable. The results confirm that, if conveniently designed, silane sol–gel film properties, the silane layer is a good adhesion promoter of the cataphoretic coating on galvanized steel and this property is maintained for long exposure times.