Depth composition and chemical states of elements constituting the overlayers of Ni-xCr alloys (x = 0-30 at.%) passivated in borate buffer solutions (pH = 9.2) were determined as a function of the chromium bulk content of the alloy. Depth sputter profiling was performed using both Auger electron spectroscopy (AES) and low energy ion scattering spectroscopy (LEIS). Chemical bonding and (oxy-hydroxy) structures of alloying elements in the passive films were investigated by x-ray photoelectron spectroscopy (XPS). The electrochemical study mainly consists in establishing the cathodic reduction kinetics of the passive layers to characterize the resistance of the internal Cr,O, barrier to reduction.Very thin films (less than 2.5 nm) were obtained in those conditions showing duplex structures where minor external nickel ox y-hydroxide layers are depicted covering an inner protective barrier mainly composed of chromium oxide Cr20,. Only at high bulk chromium contents (> 15 at.%) are complete Cr,O, layers built at the interface with the metallic alloy. Beneath the film, in the underlying matrix, a metallic nickel enrichment combined with a chromium depletion is observed, which seems to confirm, as for Fe-Cr alloys, a mechanism by which Cr oxidizes preferentially in this medium during the first steps of the film growth. I N T R O D U C T I O NIn a recent paper,' we reported analytical results on the influence of the chromium bulk content [Crlh on the depth composition of the passive film grown on the surface of Fe-Cr (0 to 31.5 at.%) for 2 h at +0.3 V/SCE in borate solutions (pH = 9.2). A chromium 'enrichment', i.e. a Cr atomic density higher than in the bulk alloy, was displayed in the inner layers of the film at any studied [Crib. Beyond a critical [Crlh, namely between 11 and 16Cr at.%, the oxidized Cr concentration at the film/alloy interface nearly reaches the value of the bulk C r 2 0 3 and in acidic or neutral solutions this was correlated with an improved corrosion resi~tance.~-~ While the passivation of such Fe-Cr alloys has been widely studied, only a few results concerning Ni-Cr alloys have been published, and they are generally related to their gas-phase oxidation.6-8 As in Fe-Cr, the oxide film could be described as a bi-layer model, the inner part of the film being mainly composed of chromium oxide, whereas nickel oxide was detected in the outer part.As far as Ni-Cr-Fe alloys are concerned, we showed in a previous paper on the composition of passive layers grown in NaCl solutions," that very thin films were obtained, mainly consisting of some nearly complete chromium oxide layers disposed, as in Fe-Cr alloys, at the interface with the metallic alloy. Moreover, this internal chromium oxide layer was successively covered with Fe and Ni 'oxides'. This sequence is similar to the depth distribution reported during the room temperature oxidation of Ni-Cr-Fe" which was interpreted as the result of a selective oxidation of chromium followed by the progressive oxidation of other alloying elements (Fe and Ni) presentin...

The flank wear rate of cutting inserts grows in proportion as the force and cutting temperature in-crease which in their turn depend on the value of bevel edge wear. It is possible to reduce the influence of a tool contact area with the work piece upon processes accelerating wear by means of changing the flank form. The paper reports the methods for the estimation of wear resistance of flanks in cutters of different forms. There is offered a new design of replaceable tangential cutting inserts with a combined form of an end flank allowing the considerable increase of a tool life without decrease of work piece quality. The results of new cutting insert manufacturing testing in rail milling operations are presented

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