title: Corrosion behavior of as-received and previously cast high noble alloy.
abstract: BACKGROUND
  The rationale for using high noble alloys is based largely upon their alleged ability to resist corrosion. However, combining previously cast metal with new alloy might have a detrimental effect on the corrosion behavior of a high noble alloy.
  
  
  OBJECTIVE
  The purpose of this study was to characterize the elemental composition of an as-received and recast high noble alloy and to examine the in vitro corrosion behavior in 2 media, using a potentiodynamic polarization technique.
  
  
  METHODS
  Disk-shaped specimens, 6 mm in diameter and 3 mm thick, were prepared from a high noble alloy (Ney-Oro-B2) under 3 casting protocols, according to the proportion of as-received and recast gold alloy (n=26); the groups included an as-received (100% as-received metal) group, 50% to 50% group (50 wt% new metal, 50 wt% once-recast metal), and recast group (100% once-recast metal). The surface structures of 20 specimens from each group were examined under scanning electron microscopy (SEM), the elemental compositions were determined using x-ray energy-dispersive spectroscopy at 3 sites on the specimen, and the data were averaged. Further, the potentiodynamic cyclic polarization between -1000 and +1000 mV (SCE, or saturated calomel electrode) was performed for 6 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 and 2-way analysis of variance and the Ryan-Einot-Gabriel-Welsch multiple-range t test (alpha=.05).
  
  
  RESULTS
  Elemental composition was significantly different among the casting groups (P<.001). The mean weight percentage values were 72.7% 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 (ZCP) and corrosion current density (I(CORR)) for all the casting protocols were not significant (P=.67 and P=.51, respectively). Moreover, the mean values were not significant for corrosion current density with the electrolyte effect (P=.45). Only zero-current potential had a significant electrolyte effect (P<.001). Furthermore, the interaction between casting protocols and electrolyte were not significant among all corrosion parameters.
  
  
  CONCLUSIONS
  High noble alloy in all casting protocols evaluated retained passivity under electrochemical conditions similar to the oral environment.
authors.affiliation: Section of Restorative Dentistry and Prosthodontics, College of Dentistry, University of Tanta, Tanta, Egypt. ayadmf@hotmail.com
authors.forename: Mohamed F
authors.initials: MF
authors.lastname: Ayad
authors.forename: Stanley G
authors.initials: SG
authors.lastname: Vermilyea
authors.forename: Stephen F
authors.initials: SF
authors.lastname: Rosenstiel
mesh_terms: D003300:Copper / Q000032:analysis; D003343:Corrosion; D003735:Dental Casting Technique; D004563:Electrochemistry; D004573:Electrolytes; D006046:Gold / Q000032:analysis; D006047:Gold Alloys / Q000737:chemistry*; D008855:Microscopy, Electron, Scanning; D010165:Palladium / Q000032:analysis; D011048:Polarography; D011199:Potentiometry; D012464:Saliva, Artificial; D012834:Silver / Q000032:analysis; D013052:Spectrometry, X-Ray Emission; D015032:Zinc / Q000032:analysis
chemical_list: D004573:Electrolytes; D006047:Gold Alloys; D012464:Saliva, Artificial; D012834:Silver; D010165:Palladium; D006046:Gold; D003300:Copper; D015032:Zinc