Formation and stability of organophosphonic acid monolayers on ZnAl alloy coatings

The formation and stability of octadecylphosphonic acid (ODPA) layers have been studied on ZnAl alloy coatings. The passive film covered alloys were characterized by means of scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), time-of-flight second ion mass spectroscopy (ToF-SIMS) and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) before and after the adsorption of ODPA layers. The thickness and ordering of adsorbed ODPA films as well as their stability in aqueous solutions were further studied by means of static water contact angles reflecting the hydrophobicity of the surface. It was observed that self-assembled monolayers (SAMs) of ODPA form on ZnAl alloys with higher Al-content (≥ 5%) such as Zn–5%-Al and Zn–55%-Al. For small concentrations of Al (Zn–0.5%-Al) the formed Al-oxide containing passive film was not stable enough to prevent dissolution and precipitation of a phosphonate salt film. The results show that organophosphonic acids can potentially be used for temporary corrosion protection, and as molecular adhesion promotors or inhibitors on ZnAl alloy coatings with Al-concentrations (≥ 5%) covered with of Al-oxide rich passive films.