title: Electrochemical corrosion and surface analyses of a ni-cr alloy in bleaching agents.
abstract: OBJECTIVE
  The aim of this study was to evaluate the corrosion behavior of a Ni-Cr dental casting alloy subjected to 10% hydrogen peroxide (HP) and 10% carbamide peroxide (CP) bleaching solutions and to determine the composition of the surface oxide layer formed on the alloy specimens.
  
  
  METHODS
  Ten cylindrical specimens (4 mm in diameter × 25 mm in height) were cast from a Ni-Cr alloy (Wiron 99) and divided into two groups (n = 5). A potentiodynamic polarization test was used to compare the corrosion rates of specimens in HP and CP (pH = 6.5). Before cyclic polarization tests, all alloy specimens were allowed to reach a steady open circuit potential (Ecorr ) for a period of 1 hour. Then tests were initiated at 100 mV versus standard calomel electrode (SCE) below Ecorr and scanned at a rate of 1 mV/s in the anodic direction until reaching 1000 mV over the Ecorr value. The scan then was reversed back to the Ecorr of the specimens. The open circuit potentials (Ecorr ) and the current densities (Icorr ) were determined using the anodic Tafel regions extrapolating from the curves. Differences in Ecorr and Icorr were determined using one-way ANOVA (α = 0.05). In addition, corrosion rates were calculated from these curves. Before and after polarization tests, a scanning electron microscope (SEM) examination accompanied by energy dispersive X-ray spectroscopy (EDS) was used to analyze the surface morphology. The surface characterization of the passive film formed on alloy specimens was also performed by using X-ray photoelectron spectroscopy (XPS).
  
  
  RESULTS
  In this study, bleaching agents had an effect on the anodic process for two groups. Although no statistical difference was identified between the groups for both corrosion parameters, results indicated that the effect of CP on the corrosion behavior was less than that of HP. These results agreed with the SEM observations. XPS data showed that oxide layers formed on all groups contained mainly Cr2 O3 , NiO, and MoO3 , and the amounts of oxides formed on CP-treated specimens were higher than HP treated ones. Also, molybdenum rates were increased with CP application compared to HP.
  
  
  CONCLUSIONS
  The comparison of the effects of the two bleaching agents at 10% showed that the alloy suffered less corrosion with CP than HP. This result was also confirmed by the SEM and XPS data. The presence of Mo on the oxide layer affected the oxide layer, leading to lower corrosion rates.
authors.affiliation: Research Associate, Department of Prosthodontics, Gazi University, Faculty of Dentistry, Ankara, Turkey.
authors.forename: Evşen
authors.initials: E
authors.lastname: Tamam
authors.forename: A Kevser
authors.initials: AK
authors.lastname: Aydın
authors.forename: Semra
authors.initials: S
authors.lastname: Bilgiç
mesh_terms: D000537:Aluminum Oxide / Q000737:chemistry; D000077463:Carbamide Peroxide; D002858:Chromium Alloys / Q000737:chemistry*; D017608:Chromium Compounds / Q000032:analysis / Q000737:chemistry; D003343:Corrosion; D003734:Dental Casting Investment / Q000737:chemistry*; D045662:Dental Etching / Q000379:methods; D003775:Dental Polishing / Q000379:methods; D055664:Electrochemical Techniques; D006801:Humans; D006861:Hydrogen Peroxide / Q000737:chemistry*; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D008982:Molybdenum / Q000032:analysis / Q000737:chemistry; D009532:Nickel / Q000032:analysis / Q000737:chemistry; D010087:Oxides / Q000032:analysis / Q000737:chemistry; D010545:Peroxides / Q000737:chemistry*; D011199:Potentiometry; D011897:Random Allocation; D013052:Spectrometry, X-Ray Emission; D013499:Surface Properties; D058205:Tooth Bleaching Agents / Q000737:chemistry*; D014508:Urea / Q000031:analogs & derivatives* / Q000737:chemistry
chemical_list: D002858:Chromium Alloys; D017608:Chromium Compounds; D003734:Dental Casting Investment; D010087:Oxides; D010545:Peroxides; D058205:Tooth Bleaching Agents; C082290:molybdenum trioxide; D000077463:Carbamide Peroxide; D009532:Nickel; D008982:Molybdenum; D014508:Urea; D006861:Hydrogen Peroxide; C028007:nickel monoxide; D000537:Aluminum Oxide; C023600:chromic oxide
keywords: Ni- Cr alloys; XPS; bleaching agents; corrosion; oxide layer