Diphosphonate thin films on zinc: Preparation, structure characterization and corrosion protection effects

The reaction of water-based solution of 1,5-diphosphono-pentane (DPP) and 1,7-diphosphono-heptane (DPH) with high purity polycrystalline zinc surface was investigated at room temperature. XRD and XPS studies evidenced the formation of crystalline zinc–phosphonate film on the metal surface. The formed layers give hydrophilic properties to the surface. A conversion-type interaction of diphosphonic acid compounds with the oxidized zinc surface was unambiguously shown by XPS analysis. Conclusive results were obtained by synthesized zinc–diphosphonate model compounds imitating the ones developed on the zinc surface, revealing a simple, closely 1:1 ratio of the phosphonate groups with zinc. Reactions with both diphosphonates resulted in significant protective effect of zinc against corrosion, although the structure and quality of the formed layers exhibit marked differences. The in-depth distribution of the composition and dissimilarity of the layer thickness were determined by glow discharge optical emission spectrometry (GD-OES). The corrosion inhibition was explained by the formation of insoluble zinc–phosphonate salt on the zinc surface, blocking the zinc dissolution process.