Corrosion protection of outdoor bronze: historical quaternary alloy and modern alloy with artificial patination

Outdoor bronzes suffer corrosion phenomena that generate partially protecting layers called patina. In urban atmosphere, the action of the environment makes these patinas partially leached away by rainwater. Moreover, the geometry of exposure influences the corrosion behaviour: in particular, different kinds of patinas are formed in “sheltered” or “unsheltered” areas due to the action of acid rain. These two kinds of patina were simulated through dropping (runoff) and wet&dry (stagnant rain) testing devices in order to obtain representative substrates for the application of protective coatings. However, when considering representative substrates for the application of protective coatings, also artificial patinas such as “liver of sulphur” (based on the use of sulphide solutions), frequently applied by artistic foundries, should be taken into account. The present work reports on chemical and morphological characterisation of (i) historical quaternary bronze (Cu-Sn-Zn-Pb), pre-patinated by artificial ageing (both by dropping and wet&dry tests) and (ii) K2S artificially pre-patinated modern bronze (Cu-Si-Mn), with and without silane-based coatings formulated during the EU-project B-IMPACT.