Abstract
The sulfidation of copper is a common phenomenon but there is considerable uncertainty about the mechanisms involved in this process. The aim of this work is to study the copper behavior in atmospheres containing low hydrogen sulfide concentration (100 ppbv) and low relative humidity (up to 30%) by using electrolytic reduction. Four experiments were run according to a 2 2 factorial statistical design. The independent variables were the exposure time and the relative humidity level and the response was the thickness of the corrosion species formed (Cu 2 O, CuO, Cu 2 S). The test was performed in a tubular mixed flow gas chamber at 30°C and the hydrogen sulfide concentration was kept in 100 ppbv using a permeation tube device. In each experiment, three copper specimens were withdrawn from the center of the chamber to undergo electrochemical evaluation. The coulometric reduction was performed with a cathodic current density fixed at 0.05 mA/cm 2 and the reduction time of each chemical species formed was measured. A voltage-time plot was obtained (E vs. t) for each copper specimen and data was then submitted to a Fortran computer program to obtain the differential curve (dE/dt vs. t) that allows to establish the reduction time of each constituent. The thickness of each chemical species present in the tarnish product was obtained from the Faraday's Law and a multiple regression analysis was made to estimate, simultaneously, the effects of time and the relative humidity level on film's thickness. The results showed that in contrast to what is observed for high relative humidity levels, in atmospheres with low humidity levels and low H 2 S concentration, an increase in the humidity does not enhance total and individual tarnish film thickness. The electrochemical reduction method can therefore be used for atmospheres with very low levels of aggressiveness where gravimetric techniques can not be used.
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