Hybrid coating on steel: ZnNi electrodeposition and surface modification with organothiols and diazonium salts

Sacrificial electrodeposited ZnNi is currently studied for replacing chromate conversion coatings (CCC) in anticorrosion applications. The present-day performances of ZnNi are still away from those of CCCs and the additional organic layers such as polymers and paints are still permeable and cannot prevent the corrosive species to reach the metal. Suitable adhesion primers could improve the situation by minimizing the access of the corrosive species to the polymer/metal interface.As a contribution to this interface problem, the present work provides a comparison of the protective properties of two structurally related molecules (4-nitrothiophenol and 4-nitrobenzenediazonium) grafted on a ZnNi coating electrodeposited on steel. Films of 4-nitrophenyl have been prepared according to the self-assembly process while films of 4-nitrobenzene have been obtained by electrochemical grafting, n-dodecanethiol being used as model system.The adsorption of these molecules as well as the resulting organic film is characterized by X-ray photoelectron spectroscopy (XPS) and polarization modulation-infrared reflection absorption spectroscopy (PM-IRRAS). The protective properties of the organic films against corrosion are investigated by linear sweep voltammetry (LSV), cyclic voltammetry (CV) and scanning vibrating electrode technique (SVET).