ANODIC FILM STUDIES ON NICKEL UNDER HIGH RATE TRANSPASSIVE DISSOLUTION CONDITIONS M. DATTA, H. J. M~THIEU and D. LANDOLT Materials Department, Swiss Federal Institute of Technology, Lausanne, Switzerland (Receiued 1 February 1979) Abstract - Coulometry and Auger Electron Spectroscopy (AES) were employed to study thickness and composition of anodic films formed on nickel under high rate transpassive dissolution conditions. Nickel anodes werepolarized at constant current densitiesup to 30 A/cm’ in alkaline nitrate electrolytes of different nitrate and hydroxyl ion concentration using a flowchannel cell with a constant electrolyteflow velocityof lOm/sec. Results show that with increasing currant density film thickness goes through a maximum. Nitrogen is detected at the apparent filmmetal interfacein the current region wheremetaldissolution occurs. No correlation between anodic film thickness and dissolution efficiency is found. The data, together with previous observations, suggest that high rate transpassive dissolution takes place from film free sites. INTRODUCTION Transpassive dissolution of iron and nickel in passivat- ing electrochemical machining (ECM) electrolytes has been investigated by a number of apthon[l-lo]. It is well known that when applied current density in the transpassive potential region is increased the relative importance of oxygen evolution decreases and that of transpassive metal dissolution augments. Eventually the metal dissolves at a current efficiency close to loo”/, yielding generally the same valence state as under active dissolution conditionsr91. The described be- havior indicates that the pass~~tilm which normally protects the anode becomes non-protective as current density is increased. Little is known at present about the processes responsible for the change in film properties or about the nature of the electrode surface under high rate transpassive dissolution conditions. Anodic films formed in stationary ECM electrolytes on iron and nickel have been investigated by J. P. Hoare and co-workers[l, 2,4-61 using potentiostatic polarization in conjunction with coulometry. Ob tained results showed film thickness to increase with potential in the passive potential region, then to decrease upon going from the passive to the trans- passive region. The observed decrease was attributed to fiIm dissolution due to an “ion exchange me- chanism” originally proposed by T. P. Hoar[ll]. According to this, under the influence of a high electric field, electrolyte anions may be incorporated into the film lattice by exchanging positions with oxygen ions and thus facilitate film dissolution. No direct proof for the existence of such a mechanism has been given up to now, however. Furthermore, the described experi- ments were performed in stationary electrolytes and at relatively low current densities where oxygen evol- ution is the predominant reaction under transpassive conditions. The question of how current efficiency for high rate transpassive metal dissolution under forced convection conditions is related to passive film thick- ness, therefore, needs further study. Recently, the l Supplier: C&al Tee, Grenoble Cedex, France. present authors investigated high rate transpassive nickel dissolution in sodium nitrate solutions and found evidence for local attack and pitting[8]. This suggests that mechanisms other than the one described may play a role in governing transpassive metal dissolution etliciency. The present study was initiated to learn more about anodic films formed under high rate transpassive dissolution conditions and about their role in governing metal dissolution efficiency in passivating ECM electrolytes. Nickel dissolution in sodium nitrate was chosen for study since this reaction had been investigated previously in our laboratoryp, 8, lo]. Coulometry and Auger Electron Spectroscopy were employed for characterizing anodic films formed at current densities up to 30A/cm2 under well con- trolled hydrodynamic conditions. EXPERIMENTAL Galvanostatic experiments were performed using single crystal nickel* (99.95 %, orientation[lll]) an- odes in a flow channel cellr71 through which the electrolyte was pumped at a c&&ant l&ar velocity of lOOOcm/sec. Cylindrical nickel anodes, 14 mm in diameter, which could be easily removed from the electrode holder for AES studies were used. Only a part of the anode corresponding to the width of the channel (3 mm) was exposed to the electrolyte while the remaining part was insulated by the teflon spacer constituting the flow channel. Nickel samples were first chemically polished in a solution containing a mixture of nitric (150 cm’), sulfuric (50 cm3), or- thophosphoric (50cm3) and acetic (25Dcm3) acids heated at 95°C. Chemically prepolishedmmples were then carefully polished on a cloth using 1 m diamond paste, rinsed with soap and distilled water and finally cleaned and dried with acetone. Anode potentials were recorded by means of a digital oscilloscope using a mercury sul&e reference electrode connected to the channel by a backside capillary. Potentials, however, are reported against normal hydrogen electrode. Electrolyte solutions were prepared from analytical grade chemicals and distilled water. Alkaline solutions 843