A thermometric study of the dissolution of some CuZn alloys in acid solutions

Abstract

The dissolution of Cu and Zn, and of three CuZn alloys in solutions of different concentrations of HNO3 was studied by the thermometric technique. The variation of ΔT (i.e. Tm − Ti) and the reaction number (R.N.) with the concentration of the attacking acid followed the relations:ΔT=A1(C−C0) andR.N.=A2Cn, successively. Based on the values of the constants A1, Co, A2 and n, the rate of dissolution of the different materials was found to increase in the order: Cu < Alloy I (70.5%Cu) < Alloy III (58.00%Cu) < Alloy II (62.00%Cu) < Zn. A break was noted in the thermometric curves of Alloy I. This was due to the primary attack on the Zn-component of the alloy, and was confirmed by studying the effect of the initial temperature, Ti, and the area/volume ratio on the shape of the curves, as well as by chemically analysing the corroding solution for both Cu2+ and Zn2+. The effect of the concentration of HCl, H2SO4, H3PO4 and of their salts on the reaction number of the alloys in 4N HNO3 was examined in detail. The adsorption of the foreign acid anion on the metal surface retarded dissolution. Inhibition increased in the succession: Cl− < HSO4− < H3PO4−. The alloys were not affected by cold 4N HCl, H2SO4 or H3PO4. Attack could be initiated in the case of the first two acids through the addition of KNO3. The thermometric curves were characterized by long incubation periods, by sharp temperature maxima and by the rapid decrease of temperature thereafter. This supported the conclusion that attack was of the pitting rather than of the general type. Because of stronger anion adsorption, attack in 4N H2SO4 was much lower than in 4N HCl. No attack was recorded in 4N H3PO4. Parallelism between corrosion assessment by the thermometric technique and by the weight loss method was established.