Electrochemical simulation of vapour side corrosion of Cu–Ni condenser tubes in multistage flash evaporation distillers

Abstract

The corrosion of 70–30 Cu–Ni condenser tubes in the vapour of multistage flash evaporation distillers occurs within a thin water film adhering to the metal surface. This, together with the fact that corrosion involves the alternate formation and dissolution of copper oxides, justifies the application of electrochemical techniques in aqueous solutions for the study of vapour side corrosion. Open circuit potential (OCP) transients were used to establish the effects of vacuum and of O₂, CO₂, and mixtures thereof on the corrosion process. The study was carried out in 4%Na₂ SO₄ solution at ambient temperatures. The state of vacuum in the distiller was simulated by purging the solution with N₂ gas. The OCP of the working electrode tended towards more negative values, revealing the instability of the air formed oxide. This changed slowly to a Cu₂O film. Introduction of O₂ into the cell resulted in the instantaneous shift of potential towards less negative values. Cuprous oxide was oxidised to hydrated cupric oxide. Reintroduction of N₂ gave rise to a clear potential step related to the slow decomposition of Cu(OH)₂ to fresh Cu₂O. The latter oxide thickened with time. Flushing of the solution with CO₂, following deaeration, also caused the OCP to change to more positive values. The change was, however, a result of the solution turning acid, causing slow attack on Cu₂O. This was gleaned from the results of N₂ purging, in which a gradual, linear decrease of potential to the Cu₂O value occurred. A 1: 1 mixture of O₂ and CO₂ caused the OCP of the electrode to move in the positive direction. Both Cu₂O and Cu(OH)₂ dissolved in the acid solution. Introduction of N₂ caused the OCP to shift to the Cu/Cu₂O potential. This stage was, however, short lived and the potential moved once more towards less negative values. This behaviour is attributed to the occurrence of the autocatalytic reaction Cu²⁺ + Cu = 2Cu+. The conclusion finds support in the results of a second cycle, in which the potential fluctuated between those of oxide formation and metal dissolution. Comparison is made between the results of corrosion in aqueous solutions and in the vapour phase. Open circuit potential measurements in solution confirmed and extended the results of weight loss determinations in the vapour phase.