A thermometric study of the kinetics of acid dissolution of four copper alloys used in desalination plants

• Published in: Thermochimica acta 1985-07, Vol.89, p.171-185
• Main Authors: Arain, Rasheed A., El Din, A.M.Shams
• Format: Article
• Language: English
• Subjects: Applied sciences; Corrosion; Corrosion mechanisms; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Materials science; Metals, semimetals and alloys; Metals. Metallurgy; Other techniques and industries; Physics; Specific materials
• ISSN: 0040-6031; 1872-762X
• DOI: 10.1016/0040-6031(85)85493-9

The dissolution of four copper alloys, commonly used in desalination plants, in HNO 3 solutions of varying concentrations is studied by the thermometric technique. The difference between the initial and maximum temperature, Δ T, varies with the molarity of the acid according to Δ T= a( M−M 0), where a is a constant and M 0 is the concentration below which no detectable rise in temperature is recorded. High values of M 0 indicate better resistance to acid attack. The reaction number (RN) varies with the molar concentration of the acid as: RN = AM n , with A and n depending on the nature of the alloy. One and the same value of n is, however, obtained when the effective acid concentration ( M − M 0) is considered. This shows that all four alloys dissolve following the same mechanism. Plots of log RN vs. 1/ T m, where T m, the maximum temperature (K), is linear, allowing the calculation of the activation energies of dissolution. Additions of Cl −, HSO − 4 and H 2PO − 4 ions to copper alloys dissolving in nitric acid solutions lower their RN values. This inhibiting effect is attributed to the specific adsorption of the ions and their displacing NO 2 molecules from the surface, which causes the interruption of the autocatalytic cycle of dissolution in HNO 3. The uptake of the ions follows a Freundlich adsorption isotherm. The applicability of the thermometric technique in evaluating corrosion problems in desalination plants is further exemplifed by studying the action of Stannine LTP on the dissolution curves of the four alloys in HNO 3. This compound is used as inhibitor in the acid-cleaning of distillers. In all cases a decrease in RN is recorded, due to the adsorption of the additive on the metal surfaces.