Entrapment of corrosion inhibitors in silane coatings to improve bronze corrosion protection

The corrosion protection of a coating depends largely on the intrinsic barrier properties of the polymer film, but can be reinforced by the presence of corrosion inhibitors trapped in the coating itself. In the case of bronze artworks exposed outdoors, commercial coatings such as Incralac contain benzotriazole that operates in the dual function of inhibitor of bronze corrosion and anti-UV substance. In this case, the inhibitor is soluble in the electrolyte (the rain) that permeates through the coating, so producing an inhibited solution that can exert its protective action at the coating / metal interface. In general, the addition of inhibitors in coatings may be limited by solubility problems. Moreover, rainfalls may induce a rapid inhibitor leaching, which determines a fast drop of protection performances. To overcome these problems, in recent years great efforts were devoted to encapsulation of inhibitors in suitable carriers, which may be constituted by porous particles or nanocapsules containing inhibitors, coated by polyelectrolytes, or particles with lamellar structure containing intercalated corrosion inhibiting species. Such particles dispersed in the coating may release corrosion inhibiting species due to variations of solution pH linked to local onset of corrosion phenomena or due to ion-exchange phenomena at the arrival of aggressive ions. In addition, also β-cyclodextrin is known to be capable of encapsulation by complexing inhibiting organic molecules of small dimensions, which can be progressively released into the aggressive solution inside the coating, when the free corrosion inhibitor is subtracted from the complexation equilibrium at the onset of corrosion phenomena. The introduction of the inhibitor-containing carriers in the coatings can lead to a significant increase in the durability of the coating protectiveness. For this reason, as part of the M-ERA.NET European project called B-IMPACT, some organic substances containing a mercapto group, which proved to be good corrosion inhibitors of bronze in concentrated synthetic acid rain (ARX10), were added to silane coatings in the form of β-cyclodextrin complexes. The performances of the obtained coatings were compared to those of plain coatings or coatings containing only β-cyclodextrin, during both 20 day immersions in ARx10 and exposures to wet&dry cycles with synthetic AR spray. During immersion tests, the corrosion behavior was monitored by electrochemical impedance spectra, followed by the recording of cathodic and anodic polarization curves. On the coated samples exposed to cyclic acid rain spray a cross cut was present at which the corrosion attack was particularly intense in the absence of inhibitors. The best results were achieved in the presence of β- cyclodextrin complexes which improved the coating protectiveness.