A study on corrosion products and processes of patinated tin bronze in formic acid

The corrosion processes of bare bronze and three types of patina-bearing tin bronze in the presence of formic acid have been explored through laboratory exposure. Following exposure time of 4, 14 and 28 days, the morphology and composition of the corrosion products on the sample were characterized to investigate their evolution by using SEM, XRD and Raman. The results indicated that Cu2O was first generated on the bare bronze surface, then Cu(OH)2 was formed at the early stage of the corrosion and as the exposure time increased, Cu(OH)(HCOO) and Cu(HCOO)2 were further generated. Bronze with Cu2O layer was destroyed by formic acid and converted into Cu(HCOO)2, which was similar to the conversion process of corrosion products of bare bronze, except that, the account of products of bronze patinated with Cu2O was more than that of bare bronze, indicating that the Cu2O layer on the sample surface was destroyed and further transformed. The main corrosion product of bronze with CuCl patina was Cu2(OH)3Cl, which was consistent with the reaction process of powdery rust. For bronze with mixed patina, two corrosion processes of Cu2O patina and CuCl patina proceed at the same time, and the corrosion products were Cu(HCOO)2 and Cu2(OH)3Cl, which were basically consistent with the corrosion behavior of bronze covered with a Cu2O and a CuCl patina.