Monitoring of Silver Alloy Tarnishing in Sulphides by Electrochemical Noise Measurements: Application of Statistical and Recurrence Plot Analysis

Silver historical artefacts were manufactured using Ag‒Cu alloys; these can tarnish in environments containing sulphides (H2S, COS), causing changes in their aesthetics and, occasionally, complete loss. This creates problems for their conservation and restoration. In this study, the tarnishing process of Ag-Cu alloys (0.925, 0.800, and 0.720) immersed in (NH4)2S for 48 hours was evaluated by electrochemical noise (EN) measurements. To relate the EN to the corrosion process, the EN data were subjected to direct examination and statistical and recurrence analyses. The Cu anodic behaviour controlled the icorr, the process was chaotic at the beginning (localized corrosion), and at longer immersion periods, the process became stochastic (passive condition). Based on the maximum line and the determinism percentage, a localized corrosion stage occurred at the beginning, followed by passivation that controlled the corrosion mechanism of the silver alloys.