Effect of Relative Humidity on Copper Corrosion by Acetic and Formic Acid Vapours

A comparative study is made of the copper corrosion rate and corrosion products originated by acetic and formic acid vapours at 40%, 80% and 100% relative humidity (RH) using gravimetric, scanning electron microscopy, X-ray powder diffraction and X-ray photoelectron spectroscopy techniques. Five acetic and formic acid vapour concentration levels (10, 50, 100, 200 and 300 ppm) were tested for a period of 21 days at 30 °C. The copper corrosion rate in the presence of acetic acid vapour was up to ∼0.002 mg/cm2 day (mcd) for 40% RH, ∼0.07 mcd for 80% RH and ∼0.23 mcd for 100% RH. Formic acid vapours yielded a copper corrosion rate of up to ∼0.005 mcd for 40% RH, ∼0.03 mcd for 80% RH and ∼0.13 mcd for 100% RH. The main compounds found were cuprite (Cu2O), copper hydroxide (Cu(OH)2), copper acetate (Cu(CH3COO)2) and copper formate (Cu(HCOO)2).