Methods: Two Ag-based Cu-free brazing alloys (Ag-12Ga and Ag-20In) and one commercially available (Dentaurum Universal silver Solder Ref.380-604-50, Dentaurum, Germany), were studied. The experimental alloys were fabricated by inductive melting of the pure metals and pellets of all the materials were conventionally ground and polished. The corrosion behaviour was investigated by Electrochemical Impedance Spectroscopy (EIS) and potentiodynamic anodic scan (PDS) in Ringer's solution employing a mini-cell system. Measurements were carried out within a 70KHz-10MHz frequency range and 15mV wave amplitude at a sampling rate of 10 points per decade. PDS was realised from –1V up to +1.2V (versus SHE) potential range and 10mV/s scan rate. A saturated calomel electrode was used as a reference electrode (Eo=241 mV vs.SHE) and sampling area was 0.008cm2. Data were collected and analysed using Corrware, Corrview, VoltaMaster and Origin software. The equivalent circuits emerged using Zview software.
Results: From the Nyquist and Bode plots, two time constants are identified. The equivalent circuits of the experimental alloys are composed of the electrolyte resistance, the coating and the double layer resistances and constant phase elements (cpe). The Nyquist plot of Dentaurum contains a capacitive loop followed by an inductive one. The equivalent circuit contains the electrolyte and coating resistance, the coating and double layer capacitance. From PDS the Icorr values were calculated. The experimental alloys (IcorrAg-12Ga = 2.82 *10-7 A/cm2 , IcorrAg-20In=2.15 *10-7A/cm2) demonstrated lower Icorr values comparing to Dentauruum (IcorrDentaurum=2.40 *10-4 A/cm2) thus better corrosion behaviour.
Conclusions: The developed alloys form a non-homogenous oxide coating on their surface and are less vulnerable to corrosion than the conventional Ag alloy.