Effectiveness Assessment of Corrosion Inhibitors of Bronze Surfaces, an Application of the Thin Layer Activation Technique to Materials of Interest in the Field of Cultural Heritage

In the frame of a European project (Eureka S!2210 “Eurocare-Bronzart”), the effectiveness of several corrosion inhibitors for bronze surfaces: benzotriazole derivatives and 3-mercapto-propyl-trimethoxy-silane, was comparatively evaluated by the Thin Layer Activation method. Bronze surfaces were irradiated with a 11.5 MeV proton beam, generated by a MC-40 energy variable cyclotron. The tracer radio nuclide 65Zn (t1/2 = 244 days) was produced by the following nuclear reaction: 65Cu (p,n)65Zn. Activated surfaces of different bronze alloys, whose composition is suitable for production of outdoor sculptures, were coated with the inhibitors and exposed to natural and artificial corrosive environments. After determination of the activity versus depth functions, the thickness loss of the activated samples, due to the effects of corrosion, was calculated by measurements of the 65Zn g- activity at 1115 KeV (Ig = 0.50), carried out with a multi channel analyser equipped with a Ge detector. In evaluating the sample thickness loss two different procedures were applied. The first one consisted in the quantification of the activity of 65Zn on the solid sample, prior to exposure and after stripping. The second one consisted in a step by step electrolytic dissolution of the solid sample, after stripping the corrosion layer. The 65Zn activity was successively measured in the pickling solution and in each solution arising from the electrolytic dissolution of the solid sample. The last method, because of its high sensitivity, was applied to evaluate the thickness loss at sub micrometer scale, as in the case of a mild corrosion.