engleski

Inhibiting Effect of 4-Methyl-1-p-Tolylimidazole to the Corrosion of Bronze Patinated in a Sulfate Medium

Bronze statues exposed at urban out-door atmospheres suffer a corrosion process induced by acid rain. A mitigation method to preserve these artworks of cultural heritage shall be found: use of corrosion inhibitors is one of the most convenient and cost-effective techniques. Benzotriazole (BTA) is known as particularly efficient inhibitor but this compound is toxic and suspected to be carcinogenic. Among nontoxic imidazole based inhibitors on copper and its alloys, we found in former work that 4-methyl-1-p-tolylimidazole (TMI) is efficient, though slightly less than BTA. First, with bare Cu-6Sn (wt-%) bronze, inhibiting efficiency in artificial acid rain in urban environment was evaluated by the Tafel extrapolation, the polarization resistance and the electrochemical impedance spectroscopy (EIS) methods. This comparative study allowed the validation of the EIS technique. The experimental EIS exhibited two capacitive loops, one in high frequency domain allocated to the charge transfer resistance in parallel with the double layer capacitance, and another in low frequency, whose origin was attributed to the faradaic processes. The bronze coupons were dipped in 15 mM CuSO4 for a couple of weeks. A rather homogeneous and well adhered green patina was obtained. However, two areas could be distinguished by SEM imaging ; dark areas covered by heaps of crystal rods and white smooth areas. EDS analyses and micro-Raman spectroscopy showed that the crystals are composed of brochantite (Cu4SO4·(OH)6), while the smooth area is composed of cuprite (Cu2O) or of cuprous hydroxide (Cu(OH)2) with tin oxide (SnO2). EIS data were collected every hour during 24 hours, and they showed, though badly separated, three capacitive loops. The third one which appeared at the highest frequency domain is attributed to the dielectric property of the patina layer (Cf – Rf). However, since Cf is too high compared with the patina layer observed by SEM it is considered that this layer can be split into an inner compact layer exhibiting dielectric properties, and an outer coarse and porous crystal layer. The impedance modulus at 10 mHz, the lowest frequency applied in the present study, is much higher in presence of TMI. Besides, the impedance modulus determined in presence of TMI increased markedly with immersion time, revealing a good protective effect of this non-toxic corrosion inhibitor.

Corrosion protection; Inhibitors; EIS; Bronze; Atmospheric corrosion

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