Influence of pH-Variation with Two Different Acids of 0.9% Saline Solution on the Corrosion Behaviour of Two Experimental Dental CoCr Alloys with the Same PRE

Introduction: Through food and drink, various acids reach the oral cavity. In biofilms, lactic acid is produced, and gastric acid secretion leads to an extremely strong lowering of the pH. These changes in pH can influence corrosion processes in the oral cavity. The aim of this study was to simulate such pH conditions and to compare the electrochemical corrosion behaviour of 2 experimental CoCr alloys with same pitting resistance equivalent (PRE) value in 0.9% saline solution (pH 6.3) which was additionally adjusted with 2 different acids (0.02 M HCl and lactic acid) to the pH-values of 5, 4, 3 and 2. Material & Methods: The experimental dental alloys Co-28Cr-5Mo-1.5Si-Mn and Co-28Cr-10W-1.5Si-Mn with PRE=44.5 were used. From each alloy, 6 specimens were prepared. 0.9% saline solution was used as a reference electrolyte. With the use of a pH-meter (691, Metrohm), the pH-values of the saline solution were adjusted to pH 2, 3, 4 and5 either with 0.02 M HCl or lactic acid. With each alloy and electrolyte, electrochemical corrosion measurements according to ISO 10271 were performed, which consist of an open-circuit-potential measurement over 2 h followed by anodic polarisation (-150 mV~Ecorr to 1 V, speed 1 mV/sec). From each measurement, the parameters Ecorr after 2 h, Ez, Ep, ip, i(300 mV), i(300mV+Ez), icorr, Rp, E(-6) and E(-5) were determined. Results: Generally, the corrosion behaviour was different for both alloys and the acids used. With the exception of the calculated parameter Rp, higher corrosion resistance could be determined with decreasing pH in a clear ranking for Co-28Cr-5Mo-1.5Si-Mn with 0.02 M HCl; however, in lactic acid only to pH 3. For Co-28Cr-10W-1.5Si-Mn, decreasing corrosion could be measured in both acids only to pH 3; a clear ranking was found only for the calculated potential parameters. The most prominent differences could be seen at pH 2 adjusted with lactic acid: The icorr value was 71 ± 25 in lactic acid and 24 ± 5 nA/cm² in 0.02M HCl for Co-28Cr-5Mo-1.5Si-Mn, and respectively 2556 ± 1698 and 55 ± 28 nA/cm² for Co-28Cr-10W-1.5Si-Mn. Conclusion: The Mo-containing alloy showed good acidic corrosion resistance for all pH values used which were adjusted with 0.02 M HCl. This alloy is superior at a low pH-value adjusted with lactic acid compared to Co-28Cr-10W-1.5Si-Mn. Acknowledgement: Special thanks to Dentaurum (Ispringen, Germany) for casting the specimens. Keywords: electrochemical corrosion measurements, experimental nonprecious alloys, pitting resistance equivalent (PRE), organic and anorganic acids