COULOMETRIC STUDY OF OXIDES IN ALUMINIUM BRASS CONDENSER TUBES

摘要

Aluminium brass (alloy CuZn22Al2 or C68700, Al brass)tubes are commonly used for condensers and exchangers. Thequantitative speciation of oxides present in the inorganic layeradherent to the inner surface of tubes could be an useful parameterto quantify their corrosion resistance considering that theirchemical nature and concentrations like the superficial carbonblack film condition the final corrosion behaviour of the tubes (1,2).In this work, the coulometric reduction of oxides was consideredand applied to measure the quantitative distribution of copper(II), copper (I) and zinc (II) oxides present on the inner surfaceof aluminium brass tubes formed during the industrial productioncycles. Using the chronopotentiometry (constant currentcoulometry) that is a fast, accurate and not expensive technique,it is possible to have qualitative and quantitative informationon the oxides present on the surface of Al brass.This electroanalytical technique is commonly used to quantify theoxide level on different kind of copper products like copper tubes,capillary wires or special finished brown coloured copper stripsfor roofing (3-10). The reduction of Cu_2O, formed in differentconditions, can occur at different overvoltage (11-14). Copperalloys, in general, and brasses in particular were not intenselystudied showing, in some cases strong different and oppositeresults and the reduction of ZnO, in the brass, is reported onlyrecently (16, 17).Chronopotentiometric reduction of oxides can be performed by aconstant current density applied to the specimen, in differentdeaerated (3,18) 0.) M (1 M =) mole/litre) solutions of Na_2CO_3(4-7), Na_2SO_4 (20) or KCl (8-1O) avoiding acidic or complexingsolutions (19) that can partially dissolve the oxides and reducethe accuracy of the analysis.If a metal forms only one oxide, the potential-time curve couldhave a quasi-constant value during the oxide reduction. When theoxide is reduced, the cathodic potential drops rapidly to thecharacteristic potential of the hydrogen ion H_3O~+ reduction tohydrogen gas, H_2. During the reduction of very thin oxide layersthe potential does not remain constant during the reduction butdecreases slowly: the thinner is the layer the more rare theobservation of a constant potential during the oxide reduction is (21).The reduction potential of an oxide depends on the its chemicalnature and its stability state. An aged or thermally treatedoxides should be more difficulty reduced than the ones just formed.In other words, a greater overvoltage will be necessary to reducethis kind of oxides (11, 12,21,22). In the case of Al brass threeoxides CuO, Cu_2O and Zn0, can be cathodically reduced but Al_2O_3can not be reduced. Their reduction order will depend to theirthermodynamic and kinetic properties that can be expressed, inthis case, as cathodic overvoltage. On copper the formation ofcopper oxides occurs according to the classic thermodynamicconsiderations: Cu_2O will be formed as first and then CuO willgrow on it. Consequently, the firstly formed, Cu_2O will bereduced as last for thermodynamic and kinetic considerations.