# Evaluation on the Corrosion of the Three Ni-Cr Alloys with Different Composition

Authors: Srinivasa B. Rao; Ramesh Chowdhary
Journal: International Journal of Dentistry
 (2011)

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## Abstract

Dental casting alloys are widely used in contact with oral tissue for many years now. With the development of new dental alloys over the past 15 years, many questions remain unanswered about their biologic safety. Concepts and current issues concerning the response to the biologic effects of dental casting alloys are presented. In this paper, samples of three commercially available nickel-chrome (Ni-cr) casting alloys (Dentaurum, Bego, Sankin) were taken to assess their   corrosion   behavior, using  potentiodynamic  polarization method (electrochemical method) with fusayama artificial saliva as an electrolyte medium to check for their biocompatibility. The parameters for corrosion rate and corrosion resistance were obtained from computer-controlled corrosion schematic instrument, namely, potentiostat through corrosion software (power CV). The results obtained were analyzed by classic Tafel analysis. Statistical analysis was done by Student'st-test and ANOVA test. It was concluded that Dentarum and Bego showed satisfactory corrosive behavior, with exception of Sankin which depicted higher corrosion rate and least resistance to corrosion. Thus, the selection of an alloy should be made on the basis of corrosion resistance and biologic data from dental manufactures.


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## Body

## 1. Introduction

Dental casting alloys are widely used in dentistry,  especially in contact with oral tissues for many years.   It is of paramount importance to understand and get familiar with biocompatibility  of  casting  alloys  for  their  long-term  success  in  rendering  successful treatment for the patients. One  of  the  most  relevant properties of a casting alloy to its biologic safety is its corrosion resistance [1]. Research in this area has generated as many questions as it has answered, and much more needs to be known about the biocompatibility of these alloys.In view of the extensive use of base metal alloys as an alternative to conventional gold-based alloys [2], an effort should be made to evaluate the corrosion properties. This ultimately answers the question of performance for a longer period of time in oral cavity. Alloys, such as copper-aluminium [3], copper-silver [4], and copper-Zinc [5], were subsequently rejected because of their higher corrosive property. Nickel-chrome (Ni-cr) casting alloys were developed as an alternate, because of their superior properties  in relation to porcelain-fused-to-metal (PFM) applications, high strength for crowns and fixed partial denture, better elongation percentage, and high elastic modulus for removable partial denture [6].  Electrochemical studies have shown  that Ni-cr casting  alloys   do  corrode  in  physiological  solution  such  as balanced salt, protenacious  solution,  artificial  saliva, and human  saliva [7].     Contrary  to this, it is also reported that there is good corrosion resistance for Ni-cr casting alloys in the oral cavity [8].   It has been suggested that 16%–27% chromium provides an adequate protective oxide film for these nickel-based alloys. Alloys with lower chromium content may not be able to adequately develop oxide films for corrosion resistance in the oral milieu [9].To evaluate the corrosion behavior of biomaterials, various in vitro methodologies have been used quantitively to check for their biocompatibility. Electrolytes like fusayama’s artificial saliva, 1% Nacl, darvells solution, cell culture medium, and so forth [10], with methodologies like electrochemical techniques and potentiodynamic polarization method, are used to evaluate the corrosion rate and corrosion resistance [11]. Surface analysis (X-ray photoelectron spectroscopy) [12] and other methods like cytotoxicity and analysis of elements released into the solution [13], atomic absorption spectrophotometry [14], and zonal coulometric analysis [15] have also been used.Potentiodynamic polarization method, chosen for this study, is an acceleration test, which helps manufacturers screen dental casting alloys more rapidly, and its theory provides new formulations to the clinician, more expediently [16]. In this study, it was planned to evaluate the corrosive behavior of three different commercially available Ni-cr casting alloys, by using potentiodynamic polarization method.


## 2. Materials and Methods

An in vitro study was carried out  for the evaluation of corrosive behavior, that is, corrosion rate and corrosion resistance of different commercial manufacturers of Ni-cr casting alloys, using potentiodynamic polarization method. The Ni-cr alloys selected for this study were from Wiron 99 (Bego, Bremen, Germany), Remanium cs, (Dentaurum, Springen, Germany), and    CB Soft (Sankin, Dentsply, York, USA) (Table1).Table 1
Details of the metal alloys used.
Base metal alloyManufacturing companyProduct nameCompositionNickel chromiumDentaurumRemanium CSNi-61 wt%Cr-26 wt%Mo-11 wt%Si-1.5FeCeA1Co < 1Nickel chromiumBegoWiron 99Ni-65Cr-22.5Mo-9.5No:1Si −1Fe-0.5Ce-0.5C:0.02Nickel chromiumDentsply SankinCB SoftNi-72.8Cr-4.9Cu-12.3Other −10%The samples of commercial Ni-cr alloys of 10.0 × 2.0 × 1.0 mm dimensions in size were polished incrementally from 400 grit emery paper to 1000 grit emery paper and then washed with acetone and distilled water in an ultrasonic cleaner. Prior to the experiments, samples were placed in airtight plastic bottles to prevent contamination.Fusayama artificial saliva was used as an electrolyte medium (Table2). Fusayama artificial saliva solution constituents closely resemble those of natural saliva.  During the study, the artificial saliva solution temperature was maintained at room temperature of 25°C [16].Table 2
Composition of fusayama artificial saliva solution.
0.96 grams (gms)Kcl  Potassium chloride0.674 gmsNacl  sodium chloride0.0405 gmsMgcl2·6H2O magnesium chloride0.117 gmsCacl2·2H2O calcium chloride0.091 gmsK2HPO4 potassium di hydrogen phosphate0.11 gmsmethyl parahydroxybenzoate8.0 gm70% sorbitolFive  samples,  each  of 3  different  companies’  alloys,  were made (Remanium  CS  (Dentaurum), Wiron 99 (Bego), and CB Soft (Sankin)) in total 15 in number. All samples were individually soldered to copper wire to pass an electric current with a Tygon tubing which is attached to glass tube. Ni-cr casting alloy test sample was selected as working electrode, platinum as a standard electrode, and saturated calomel electrode as reference electrode. These sample electrodes were placed in a cell with a few millimeters (mm) apart in artificial saliva (fusayama) as an electrolyte medium. All the electrodes: the working electrode—Ni-cr casting alloys, and reference electrode standard electrode—platinum, saturated calomel electrode  of 1 square centimeter (sqcm), were dipped in electrolyte medium—artificial saliva.


### 2.1. Potentiodynamic Scan

Potentiodynamic scan is designed for electrochemical applications that require relatively large current and high-compliance voltage such as battery studies, corrosion, electrolysis, and electroplating. The power range is ±2 A, and the compliance voltage is ±25 V. The potential of a corroding metal, often termedEcorr, is probably the single most useful variable measured in corrosion studies as well as during the corrosion monitoring of complex field situations. It is readily measured by determining the voltage difference between a metal immersed in a given environment and an appropriate reference electrode. The scan was performed at the corrosion potential (Ecorr). That is, the rate of oxidation exactly equals to the rate of reduction.  Scanning is performed at rate of 1 mv/second up to +1 volt (anodic polarization), to determine “a” as anodic Tafel slope constant, and up to −1 volt (cathodic polarization), to determine “c” as cathodic Tafel slope constant, and it then reverses back to Ecorr corrosion   potential. The  result  of  particular  experiment  processed by microcomputer  is  displayed  as polarization curves. The resultant graphs were analyzed using classic “Tafel analysis” which is displayed as log of current on X-axis and potential on Y-axis.


### 2.2. Classic Tafel Analysis

Classic Tafel analysis helps in the interpretation of polarization curves and is performed by extrapolating the linear portions of a log current versus potential to their intersection at an anodic and cathodic current that determines theIcorr(1)Corrosionrate(mpy)=0.13Icorr(EW)K/d,whereEW=22.4 is equivalent weight of corroding specimen, d=8 is density of corroding specimen, Icorr is corrosion current density   A/cm2, and K is corrosion constant 1.288×105.


## 2.1. Potentiodynamic Scan

Potentiodynamic scan is designed for electrochemical applications that require relatively large current and high-compliance voltage such as battery studies, corrosion, electrolysis, and electroplating. The power range is ±2 A, and the compliance voltage is ±25 V. The potential of a corroding metal, often termedEcorr, is probably the single most useful variable measured in corrosion studies as well as during the corrosion monitoring of complex field situations. It is readily measured by determining the voltage difference between a metal immersed in a given environment and an appropriate reference electrode. The scan was performed at the corrosion potential (Ecorr). That is, the rate of oxidation exactly equals to the rate of reduction.  Scanning is performed at rate of 1 mv/second up to +1 volt (anodic polarization), to determine “a” as anodic Tafel slope constant, and up to −1 volt (cathodic polarization), to determine “c” as cathodic Tafel slope constant, and it then reverses back to Ecorr corrosion   potential. The  result  of  particular  experiment  processed by microcomputer  is  displayed  as polarization curves. The resultant graphs were analyzed using classic “Tafel analysis” which is displayed as log of current on X-axis and potential on Y-axis.


## 2.2. Classic Tafel Analysis

Classic Tafel analysis helps in the interpretation of polarization curves and is performed by extrapolating the linear portions of a log current versus potential to their intersection at an anodic and cathodic current that determines theIcorr(1)Corrosionrate(mpy)=0.13Icorr(EW)K/d,whereEW=22.4 is equivalent weight of corroding specimen, d=8 is density of corroding specimen, Icorr is corrosion current density   A/cm2, and K is corrosion constant 1.288×105.


## 3. Results

Quantitative values of corrosion rate (mpy) were ranked and compared using Student’st-test and ANOVA test.   The values for corrosion rate were recorded (Table 3).Table 3
Mean values for corrosion rate (MPY).
Alloys10th day20th day30th dayDentaurum Remanium (CS)17.013050647.127785216.61719Bego (Wiron 99)51.72680244.161395682.0454818Sankin (CB Soft)26.830393661.8402746293.692578According to test sequence, which is obtained fromEcorr (corrosion current potential) values (Table 4),  Icorr (corrosion current density) values (Table 5) were measured, using classic Tafel analysis. Even values for corrosion resistance (K cm−2) were recorded (Table 6).Table 4
Mean values for corrosion potentialEcorr (mV).
10th day20th day30th dayDentaurum Remanium (CS)431.18441.78492.58Bego (Wiron 99)470.86493.28744.08Sankin (CB Soft)567.58587.32740.58Table 5
Mean values of corrosion current densityIcorr  (A).
10th day20th day30th dayDentaurum Remanium (CS)3.513.7363.092Bego (Wiron 99)4.953.8184.622Sankin (CB Soft)5.9986.5543.376Table 6
Mean values for corrosion resistance (K cm−2).
10th day20th day30th dayDentaurum Remanium (CS)20.3814.3418.18Bego (Wiron 99)21.9812.3316.77Sankin (CB Soft)15.9515.9512.12Statistical analysis for corrosion rate of different company samples, namely Remanium CS (Dentaurum), Wiron 99 (Bego), and CB Soft (Sankin) were analyzed at various intervals of time using Student’st-test (Figure 1). For Remanium CS (Dentaurum), the “t” calculated values between the 10th and 20th, 20th and 30th, and 10th and 30th days are 0.51, 3.42*, and 1.52, respectively. For Wiron 99 (Bego) and the “t” calculated values between the 10th  and 20th, 20th   and 30th, and 10th and 30th days are 0.59, 3.20*, and 1.73, respectively. Finally for CB Soft (Sankin), the “t” calculated values between the 10th and 20th, 20th and 30th, and 10th and 30th days are 1.98, 2.86*, and 2.91, respectively.Figure 1
Comparison of corrosion rate (t-values) of casting alloys at various intervals of time.Statistical analysis of Ni-cr casting alloys at various intervals of time is calculated by  using  ANOVA  test  and  is  tabulated.  By the assessment  of  corrosion  rate  (mpy),  the calculated “f” values on the 10th, 20th, and 30th days are 2.79, 1.21, and 13.5*, respectively (Figure 2).Figure 2
Comparison of corrosion rate of nickel-chrome casting alloys at various intervals of time.


## 4. Discussion

Corrosion  of dental  alloys  is  a complex  process,  depending  not  only on alloy’s  composition  and  structure,  but  also  on  many  other  factors  such  as  surface treatment, environmental  conditions  around  the  alloy,  and  composition  of  surrounding electrolyte selected for the study [1, 17–20].However, for specific environment, corrosion depends on the structure and composition of the alloy [1, 21]. The structure of the alloy, whether in single or multiple phases, is an important factor for its corrosion rate [1]. On the other hand, some alloying elements are very prone  to  enhance  the  behavior  of  corrosion,  resulting  in  the release  of  elements into     the electrolytes and thus increasing or decreasing the corrosion rate [1, 22–24]. Compromising    these  physical  properties  leads  to an increase  in  biological irritation.Remanium CS resulted in lower corrosive rate. This can be explained by its higher percentage of chromium. Chromium as chromium oxide (Cr2O3) and molybdenum as molybdenum oxide (Mo3) provide the initial stability to prevent dissolution of metal ions and thus provide resistance to corrosion and lesser corrosive rate. Wiron 99 was the next best among the Ni-cr casting alloys. This can be explained by its low percentage   of chromium  22.5 wt%  and  molybdenum  9.5 wt% compared  to  Dentaurum’s Remanium CS. Chromium as (Cr2O3) and molybdenum as (MO3) help in the formation of stable surface oxide film. Sankin’s CB Soft showed the highest corrosion rate amongst all the samples selected for the study. There is a huge variation in its chemical composition when compared to the rest of the alloy samples. Less amount of chromium content, that is, 4.9 wt% and the absence of molybdenum element in CB Soft (Sankin) resulted in the absence of surface oxide passive film formation onto the metal surface.Thus, the composition and integrity of the surface oxide film on Ni-cr casting alloy are critical for their performance as dental restoration.   The results showed that Ni-cr casting alloys with a higher chromium and molybdenum content have much higher passive range and are immune to corrosion. As demonstrated by the results of Al-Hiyasat et al., Remanium CS had the least cytotoxicity and CB Soft the most [18]. This depends not only on the chemical composition but also on the characterization of passive film on the alloys [14].A  study  conducted  by  Leung  and Darvell  mentioned  that  fusayama  artificial  saliva solution provides only theinorganic components, that is, NaCl 15.33, K-5.37, Ca 540, Po4,  4.23, Na25-15.34, P207 0.01, and Cl 23.02 (concentration of components are in mmoL/L) and does  not  permit  the  simulation  of  the effects  of  organic  components, however, this electrolyte has a response close to natural saliva [25].  The actual conditions connected with the chemical and physical nature of the corrosive milieu are very complex, or even difficult to simulate the composition environment of oral milieu [14].A study by Geis-Gerstorfer  et al. [16] mentioned that, Ni-cr casting alloys do corrode and show average substance loss, varying between 0.540 and 3.26/mg/cm2after 35 days [7]; contrary to it, it is reported that there is good corrosion resistance for Ni-cr casting alloys in the oral cavity [9], but, in an in vitro study conducted by Chen et al. [19], mentioned that the Ni-cr casting alloys presented high resistance to corrosion. Thus, it can be mentioned that corrosion resistance is inversely proportional to corrosion rate; the more the value of corrosion rate is the least will be its corrosion resistance [26].


## 5. Conclusion

Within  the  limitations  of  this  study,  in  correlation  with  literature,  it  can  be concluded that Dentaurum’s Remanium CS and Bego’s Wiron 99 showed satisfactory corrosion behavior, with the exception of CB Soft of Sankin which depicted higher corrosion rate and least resistance to corrosion.In the future development of alloys, an effort should be made to gain a better understanding of the interactions between the surface of the metal and its environment; a particular interest should be given to those between the physical and chemical state of alloy surface and its corrosion behavior.



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*Source: 397029-2011-03-03.xml*