The influence of bronze composition on the protective properties of phosphonic acid films

The objects of cultural heritage (CH) are often made of bronze, whose composition varies with time and the location where they were cast. For this reason, it is desirable that the coating intended for corrosion protection of bronze CH is effective on different types of bronzes. The aim of this work was to examine the influence of the chemical composition of bronze substrate on the possibility of its corrosion protection by 16-phosphonohexadecanoic acid (COOH-PA) films. Studies were conducted on the three types of bronzes and pure metals composing bronzes (copper, tin, and zinc). The self-assembled films of COOH-PA were characterized by spectroscopic measurements (Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy), X-ray diffraction, scanning electron microscopy and electrochemical (linear polarization, and electrochemical impedance spectroscopy) measurements. Electrochemical studies were conducted for two weeks in a simulated acid rain solution in order to determine the protection efficiency and stability of organic film.Protective phosphonic acid films formed on all studied substrates but their protective properties depended on the substrate’s composition and corrosion resistance and on the formation of metal–COOH-PA complexes inside the phosphonic acid layers. The least protective film formed on Zn due to its low corrosion resistance, as the substrate began to dissolve shortly after immersion in the acid rain solution. After two weeks of exposure to acid rain solution, the highest level of corrosion protection was observed on quaternary bronze, which was partially ascribed to the improved corrosion resistance of the substrate due to the dissolution of more reactive elements from the surface. Additionally, spectroscopic measurements revealed the formation of carboxylates and phosphonates that increased the stability of COOH-PA films on the bronze surface.