title: Corrosion behavior of as-received and previously cast type III gold alloy.
abstract: OBJECTIVE
  The rationale for using gold alloys is based largely upon their alleged ability to resist corrosion, but little information is available to determine the corrosion behavior of recast alloys. This study characterized the elemental composition of as-received and recast type III gold alloy and examined the in vitro corrosion behavior in two media using a potentiodynamic polarization technique.
  
  
  METHODS
  Seventy-eight disk-shaped specimens were prepared from a type III gold alloy under three casting protocols according to the proportion of as-received and recast gold alloy (n = 26). (1) Group as received (100% as-received metal), (2) group 50% to 50% (50% wt. new metal, 50% wt. once recast metal), and (3) group recast (100% once recast metal). The surface structures of 20 specimens from each group were examined under scanning electron microscopy, and their elemental compositions were determined using X-ray energy-dispersive spectroscopy. Further, the potentiodynamic cyclic polarization between -1000 and +1000 mV (SCE) were performed for six specimens from each casting protocol in 0.09% NaCl solution (n = 3) and Fusayama artificial saliva (n = 3) at 37 degrees C. Zero-current potential and corrosion current density were determined. The data were analyzed with 1-way ANOVA and the Ryan-Einot-Gabriel-Welsch multiple-range test t (alpha= 0.05).
  
  
  RESULTS
  Elemental composition was significantly different among the casting groups (p < 0.001). The mean weight percentage values were 72.4 to 75.7% Au, 4.5 to 7.0% Pd, 10.7 to 11.1% Ag, 7.8 to 8.4% Cu, and 1.0 to 1.4% Zn. The mean values for Zero-current potential and corrosion current density for all casting protocols were not significant (p > 0.05); however, the difference between the electrolytes was significant (p < 0.001). Fusayama artificial saliva seemed to offer the most corrosive environment.
  
  
  CONCLUSIONS
  Type III gold alloy in any casting protocol retained passivity under electrochemical conditions similar to the oral environment. Moreover, high-gold type III alloys from reputable manufacturers and recasting protocol tested should produce acceptable corrosion-resistant castings.
authors.affiliation: Section of Restorative Dentistry, and Prosthodontics, College of Dentistry, University of Tanta, Egypt. ayadmf@hotmail.com
authors.forename: Mohamed F
authors.initials: MF
authors.lastname: Ayad
authors.forename: Ghada M
authors.initials: GM
authors.lastname: Ayad
mesh_terms: D003300:Copper / Q000032:analysis; D003343:Corrosion; D003734:Dental Casting Investment / Q000032:analysis / Q000737:chemistry*; D017743:Equipment Reuse; D006046:Gold / Q000032:analysis; D006047:Gold Alloys / Q000032:analysis / Q000737:chemistry*; D006801:Humans; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D010165:Palladium / Q000032:analysis; D011048:Polarography; D011199:Potentiometry; D012464:Saliva, Artificial / Q000737:chemistry; D012834:Silver / Q000032:analysis; D012965:Sodium Chloride / Q000737:chemistry; D013052:Spectrometry, X-Ray Emission; D013499:Surface Properties; D013696:Temperature; D015032:Zinc / Q000032:analysis
chemical_list: D003734:Dental Casting Investment; D006047:Gold Alloys; D012464:Saliva, Artificial; D012834:Silver; D012965:Sodium Chloride; D010165:Palladium; D006046:Gold; D003300:Copper; D015032:Zinc