A precise disc model 15 HC DIAMOND  was used to obtain the test specimens of Ag– Pd– Au– Cu alloy with area comprehending only  the  welding area, and an ISOMET  1000-BUEHLER  machine was used to separate the base metal from the welding area after the laser process。 The exposed     geometric

Table 1

Chemical composition of the alloy

Au Ag Pd Cu

wt。%   1。25 F 0。57   64。78 F 4。35   24。80 F 2。16   9。16 F 1。80

areas of the welding cord and of the base metal  were

0。057 cm2。 The metallographic analysis of the exposed surface of the base metal and the welding area was done with optic microscopy, after polish with emery cloth from 180 to 1000 mesh, alumina with granulation 1 and 0。3 Am and nitro-muriatic acid application [8]。 The work electrodes were prepared from the test specimens used on the metallographic analysis。 Measures of open circuit potential versus time were used in the electrochemical essays, as well as potentiodynamic polarization and electrochemical impedance。 An electrochemical cell containing  NaCl

0。15 mol l — 1  (0,9%) airy solution with three electro-

des was also used, with the saturated calomel elec- trode (SCE) as reference system and a graffiti cylinder as auxiliary electrode。

Electrochemical measures of corrosion were done with a potentiometer Solartron SI1287。 Potentiody- namic polarization curves were observed at 0。001  V s — 1 immediately。 Impedance measures were done with the analyzer of frequency response, Solartron 1255, connected to an electrochemical interface, Solartron 1287, and an amplitude of 10 mV was applied to a frequency channel that varied from 100 kHz to 6 MHz, obtaining five points for each frequency decade, con- trolled by the software Zplot [9]。 The software Zveiw [10] was responsible for the adjustments。

3。 Results and discussion

Fig。 1 presents a coarse biphasic fusion micro- structure in the base metal area。 Fig。 2 illustrates a refined dendritical microstructure in the laser weld area, deriving from the high speedy cooling imposed by the laser weld because of a located fusion process, followed by a quick cooling during the welding, which does not allow the microstructure to return to its initial biphasic structure。

Fig。 3 shows the open circuit potential versus time curves for the base metal and laser weld areas of the Ag– Pd– Au– Cu alloy。 The stabilization of the poten- tial was observed 3 h after immersion for both areas, and the laser weld presented a stabilization   potential

50 mV higher。 Some AgPd alloy researchers have observed that, usually, an alloy open circuit potential increases with the increase of the noble metals con- centration [1]。

1890 M。L。 Santos et al。 / Materials Letters 57 (2003)  1888–1893

Fig。 1。 Micrograph of the Ag – Pd– Au – Cu alloy base metal。

Fig。 3。 Open circuit potential versus time curves to Ag – Pd– Au– Cu alloy, in 0。9% NaCl airy solution: (a) base metal; (b)  laser。

The polarization curves on Fig。 4 present differ- ences on the anodic behavior, with the occurrence of an area corresponding to the first transpassive region close to + 0。07 V (SCE) on the laser weld。 The numbers obtained for the corrosion potentials, Ecor, indicate that the laser weld area presents higher corrosion resistance。

The impedance responses originated in the open circuit potential, obtained in the steady state for the base metal area, present the occurrence of one distorted semicircle at high  frequencies  (Fig。  5)。 The equivalent electrical circuit model better adjust- able to the characteristics of the resulting spectrum is composed of a parallel association of  RTC  and CPE, which represents the electrochemical behavior of the interface in the high frequencies area,    includ-