Μm Diamond Paste Research Articles

Effect of Selenium on Dissolution of Manganese Sulfide Inclusions in Stainless Steel

The pit initiation sites for commercial stainless steels have been attributed to sulfide inclusions such as MnS. The chemical composition of the inclusions affects the pitting corrosion resistance of stainless steels.1,2 It is known that the inclusion solubility in aqueous solutions affects pitting corrosion resistance. The dissolution of sulfide inclusions produces S species and exposes the bare surface of the steel matrix, resulting in high pitting susceptibility near the inclusions. Selenium is known to improve the machinability of stainless steels. However, the effect of selenium on the dissolution and pit initiation of MnS inclusions in stainless steels remains unclear. In this study, a microelectrochemical technique was applied to analyze the effect of Se on the dissolution and pit initiation of MnS inclusions in stainless steels.Two re-sulfurized stainless steels were prepared by vacuum induction melting: one was Se-free, and the other was Se-added. The specimens were heat-treated at 1373 K for 30 min and quenched in water. After heat treatment, the specimens were polished with 1 µm diamond paste. Polarization curves were measured in 0.1 M NaCl and 0.1 M Na2SO4. An optical microscope and a scanning electron microscope equipped with an energy dispersive X-ray spectroscopy system were used to capture images of the specimen surfaces.

Elucidation of the Growth of Filiform Corrosion of AZ91D Mg Alloy in NaCl Solution

Magnesium (Mg) is the lightest structural metallic material with high specific strength. Mg alloys are highly expected to be applied in electronic, automotive vehicles and aerospace, etc. The most severe drawback of Mg and its alloys is corrosion susceptibility. When Mg alloys are used in chloride solutions, filiform corrosion sometimes occur. Being a common practice in the research of Mg corrosion, galvanostatic tests allow for easy comparison of the rate of hydrogen evolution with the applied current. But there are scarce studies reported about the in situ observation of filiform corrosion behavior of Mg alloys under galvanostatic polarization. The purpose of this study is to elucidate the growth of filiform corrosion of AZ91D Mg alloy in NaCl solutions by in situ observation.An ingot of AZ91D die-cast was cut into 25 mm × 30 mm × 5 mm sheets. The specimen surface was polished to 2000 grit with SiC paper and finished with a 0.25 µm diamond paste, followed by ultrasonically cleaning in ethanol. To confirm the metallographic structures, the sample was etched with a mixed reagent of 3 mL HNO3 and 80 mL of ethanol for 3 s. Galvanostatic polarization was performed using a micro electrochemical system with an in situ observation function. The electrolytes used were naturally aerated 0.1 M and 0.01 M NaCl (pH 8.0, adjusted with NaOH). The area of the electrode was about 0.01 mm2 or larger, the surfaces of the sample was covered with a silicone sealant except for the working electrode area. To start the experiments, an objective lens was put down to be immersed in the solution and clear view of the electrode area was attempted to obtain. After polarization, the specimen surface was assessed using OM, AES, FE-SEM and STEM.The AZ91D die-cast Mg alloys mainly consist of an α phase matrix (Mg solid solution) and β phases (Mg17Al12 intermetallic compounds). Lamellar (α + β) structures are also observed in the adjacent of β phases. Among these lamellar structures, some black dots are Mg-Al-Mn compounds. In the video of corrosion process, pits were observed in the boundaries of lamellar (α + β) structures and β phases. After polarization, EDS maps were taken. It was confirmed that filiform corrosion propagated in lamellar (α + β) structures and α phase matrix.

Visualization of pH Distributions inside the Crevice of Stainless Steel in Corrosive Environments

Stainless steels are known for its excellent corrosion resistance. However, stainless steels sometime suffer from crevice corrosion in chloride environments. The distribution of pH inside the crevice is recognized to be the important factor in crevice corrosion of stainless steels [1, 2]. This study focused on visualization of pH inside the crevice in corrosive environments.An imaging plate was prepared by a sol-gel method. Ethyl violet was used to assess the pH values. The fabricated imaging plates were calibrated by 0.01 M NaCl solution at pH 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0. The imaging plate was immersed in the 0.01 M NaCl solutions, and the color change was observed by an optical microscope. Type 304 stainless steel (Fe-18mass%Cr-8mass%Ni) was used as the specimens. The size of the specimen for the crevice corrosion test is 3 mm × 3 mm×17 mm. The specimen surfaces were polished with a 1 µm diamond paste. The scan rate of potentiodynamic polarization measurements was set to 23mV min- 1. The specimen for the polarization measurements is 17 mm × 25 mm × 3 mm in size. The electrode area was 1 cm2. The electrolyte used for the potentiodynamic polarization was 0.01 M NaCl (pH 3.0). The electrolyte was deaerated before the polarization measurements. The reference electrode was Ag/AgCl (3.33 M KCl). The counter electrode was Pt. The crevice corrosion tests were performed under a potentiostatic polarization. The applied potential was 0.3 V. The crevice was made between the steel surface (3 mm × 3 mm) and the imaging plate. Optical microscope images of the surface were taken every 30 seconds. The pH values inside the crevice were calculated from the R (red), G (green), and B (blue) values of each pixel in the images.In calibration, the color of the imaging plate changed from violet to yellow, suggesting that G values of imaging plate mainly responded to pH. The pitting potential of the specimens was 0.4 V. Therefore, 0.3 V was suitable for the crevice corrosion test. Because 0.3 V (Ag/AgCl (3.33 M KCl)) was lower than the pitting potential. The visualization of pH in the crevice corrosion was conducted. The pH decreased during the incubation, initiation, and growth of crevice corrosion. The pH was the important factor in the initiation and growth of crevice corrosion.[1] T. Kaji, T. Sekiai, I. Muto, Y. Sugawara, N. Hara, Visualization of pH and pCl Distributions: Initiation and Propagation Criteria for Crevice Corrosion of Stainless Steel, J. Electrochem. Soc., 159 (2012), C289-C297.[2] M. Nishimoto, J. Ogawa, I. Muto, Y. Sugawara, N. Hara, Simultaneous visualization of pH and Cl- distributions inside the crevice of stainless steel, Corros. Sci., 106 (2016), 298-302.

Elucidation of the Initiation of Pitting Corrosion and the Growth of Filiform Corrosion of AZ91D in Aqueous NaCl Electrolyte

1 IntroductionMg-Al-Zn alloys such as AZ91 (Mg-9% Al-1% Zn) are very lightweight, and expected to be used in transportation equipment (e.g.: automotive and aerospace). Mathieu et al. studied the effect of elements and phase compositions of AZ91 alloy on corrosion behavior 1)-4), and they revealed that the content of aluminum and the distribution of Mg17Al12 have the significant effect on the corrosion resistance of the alloy. However, the corrosion mechanism is still not perfectly figured out. In this study, the aim is to elucidate the initiation of localized corrosion of die-cast AZ91D.2 Experimental2.1 SpecimenDie-cast AZ91D was cut into 25 mm× 30 mm × 5 mm sheets, mechanically ground with SiC paper through a 2000 grit, finished with 0.25 µm diamond paste, and finally cleaned ultrasonically with ethanol. To ascertain the grain structure, the specimens were etched in a mixed reagent of 3 mL HNO3 and 80 mL ethanol.2.2 Macro-scale electrochemical measurementsOpen circuit potentials and potentiodynamic polarization curves were measured at 298 K. The specimen surfaces were coated with an epoxy resin and subsequently with paraffin. Measurements were performed in a conventional three-electrode cell: the counter electrode was a Pt wire, and the reference electrode was Ag/AgCl (3.33 M KCl). The electrolyte used were naturally aerated 0.1 M and 0.01M NaCl (pH 8.0, adjusted with NaOH). The size of the electrode area was ca. 1cm2.2.3 Micro-scale electrochemical measurementsTo elucidate the pit initiation sites, potentiodynamic polarization curves for a small electrode area for AZ91D die-cast were measured. The electrode area was ca. 0.25 mm2 or ca. 0.01 mm2. The specimen surfaces, with the exception of the electrode area, were masked with a silicone sealant, and the sealant was dried for 24 h. To preserve the electrolyte, a small acrylic cell was put on the specimen. Before the potentiodynamic polarization, the reference and counter electrodes were connected with a small droplet of the electrolyte. The electrolyte used were naturally aerated 0.1 M NaCl (pH 5.46) and 0.01M NaCl (pH 8.0, adjusted with NaOH). After polarization, corrosion morphology was characterized using a laser microscope, optical microscope, and FE-SEM.3 ResultsIn macro-scale measurements, the open circuit potential was approximately -1.5 V but oscillating during the immersion. The oscillations were due to the competition of corrosion initiation and repassivation. After immersion, it was observed that filiform corrosion occurred.In micro-scale polarization, regardless of the size of the sample area, pitting tended to occur underneath the coating. The discoloration occurred on the eutectics. Some eutectics discolored, and some did not transform. The discolored eutectics might be close to the grain boundaries. The initiation of pitting was seen at the eutectic phase adjacent of β phase. Pitting caused the initiation of filiform corrosion. After the pit initiation, hydrogen gas bubbles were generated. During the growth of filiform corrosion, hydrogen was also continuously produced.

Effect of Nb and Cu Additions on High Temperature Oxidation Behavior and Mechanical Properties of Al2O3-Forming Austenitic Alloys

The addition of Cu to austenitic steels was found to be effective for decreasing the critical Al content and promoting protective alumina scale formation from our recent studies. It was also reported that Nb addition to the alloy promotes the formation of alumina. These two elements are also known to improve the mechanical properties of steels by forming fine precipitates of Cu and intermetallics such as Laves and γ” phases, thus combined addition of these elements is expected to improve both the oxidation and mechanical properties of steels, however, there is no systematic study on the effects of the combined addition of these elements on the oxidation resistance and mechanical properties of austenitic heat-resistant alloys. The aim of this study was to investigate the effects of Nb and Cu on the oxidation behavior and mechanical properties of austenitic heat-resistant alloys containing Nb and Cu.Fe-25Ni-17Cr-(5~8)Al-(0, 0.5, 1)Nb-(0, 3, 5)Cu (in at.%) alloys were prepared by Ar-arc melting pure constituent metals. The ingots were cold-rolled by 90% and heat-treated at 1200ºC for 1 h, followed by aging at 800 ºC or 600 ºC for up to 500 h. Oxidation samples were cut from the cold-rolled plates and polished down to a 3 µm diamond paste. Oxidation tests were performed at 800°C in a box furnace in air for up to 100 h. The alloys were also subjected to a Vickers hardness test at room temperature and tensile tests at room temperature and 600ºC. The tensile test was performed in Ar at a strain rate of 0.005mm/s. Samples were analyzed by SEM, STEM, TEM, EPMA, GD-OES and XRD.Fig. 1 shows the oxidation kinetics and room temperature hardness of the alloys with/without different Nb and Cu contents. The oxidation kinetics of the alloys with/without different Nb and Cu contents shown in Fig. 1(a) and (b) reveals that the oxidation mass gain of the alloys without Cu decreased significantly by the addition of Nb and it further decreased with increasing Nb content. The oxidation mass gain of the 1Nb doped alloys tended to increase slightly with increasing Cu content. From the STEM-EDS analysis, it was confirmed that the protective alumina scale was formed on the Cu and/or Nb added alloys. An α-alumina was confirmed to develop on the alloys with Nb addition at about 600 ºC during heating, which is much lower temperature than that reported for formation of an α-alumina, suggesting that Nb addition promotes α-alumina formation. Vickers hardness of all alloys at room temperature increased with aging time, however, the maximum hardness was higher for the alloys aged at 600 ºC than 800 ºC. The hardness of the 5Cu alloy was higher than 0Cu alloy, which might be attributed to a formation of very fine Cu precipitates confirmed by TEM observation. Figure 1

Pit Initiation Behavior at Sulfide Particles in Sintered Stainless Steels

The pit initiation sites for commercial stainless steels have been attributed to sulfide inclusions such as MnS. The chemical composition of the inclusions affects the pitting corrosion resistance of stainless steels. Systematic investigation on the relationship between the chemical composition of the inclusions and the pitting corrosion resistance is required. However, it is difficult to prepare a series of the steel specimens of which the chemical composition of the inclusions is precisely controlled. In this study, spark plasma sintering was used to fabricate stainless steel specimens containing sulfide particles, and a microelectrochemical technique was applied to analyze the effect of the chemical composition of the sulfides on the pitting corrosion resistance of stainless steels. A type 304 stainless steel powder was mixed with a MnS powder to prepare the sintered specimen containing 0.03 mass% S. The particle sizes of the stainless steel and the MnS powder were –100 mesh and –325 mesh, respectively. The mixed powders were loaded into a cylindrical graphite die and were compressed using a hydraulic press. The pressed compact was spark plasma sintered in vacuum, under uniaxial pressure at 30 MPa. The sintered specimen was ground with SiC papers and then was polished with an 1 µm diamond paste. Potentiodynamic polarization was conducted in naturally aerated 0.1 M NaCl (pH 5.5) at 298 K. The specimen surface was masked to make an electrode area of ca. 100 µm × 100 µm containing a sulfide particle. All the potentials reported in this study refer to the Ag/AgCl (3.33 M KCl) electrode (0.206 V vs. standard hydrogen electrode at 298 K). The potential scan rate was 3.8 × 10-4 V/s. The anodic polarization curve was measured in 0.1 M NaCl to characterize the pit initiation behavior at the MnS particle in the sintered stainless steel. Polarization was started at –0.2 V and was stopped after the large increase in the current density was measured. The electrode area was observed using an optical microscope after polarization. The MnS particle partly dissolved, and the pit was found at the boundary between the particle and the steel matrix. This suggests that the large current increase was attributed to the pit initiation at the particle. It is expected that the sintered specimen can be used to investigate the pit initiation behavior at the sulfides in stainless steels.

Surface preparation effect on oxidation kinetics of Ni-base superalloy

In the present study an oxidation behavior of commercially available Ni-base superalloy IN 713C with different surface finishing, namely polished up to 1 μm diamond paste (resulting in Ra = 0.023) and ground by SiC paper with 80 grit (Ra = 1.23), were investigated at 950°C in air. It was found that ground sample developed very thin, protective α-Al2O3 scale. Moreover, locally formation of θ-Al2O3 in the forms of spikes was observed. For the sample with polished surface formation of outer chromia scale accompanied with internal oxidation of aluminum was observed. Moreover, for specimen which does not form continuous alumina scale or sub-scale, the GD-OES depth profiles showed an enrichment of boron at the oxide scale/alloy interface. It was proposed that the formation of protective alumina scale on ground samples is caused by introducing the defects into the near-surface region of the material, which are an easy diffusion paths for the elements forming a protective oxide scales.

Remote Imaging of the Buried Interface Using AFM-IR

The degradation of corrosion-resistant organic coatings continues to present an analytical challenge to corrosion scientists. This is because failures in corrosion protection are localised events, whereas coating analysis is predominately performed using bulk methodologies (e.g., vibrational spectroscopy, thermal analysis). This is compounded by the paucity of non-destructive techniques capable of detecting and characterising localised coating failure in the early stages (i.e., able to pinpoint the appearance of buried pitting sites, de-adhesion processes or blistering). Ex-situ microscopy has conventionally been used to characterise corrosion phenomena beneath organic coatings. However, physical sectioning precludes meaningful chemical analysis of the polymer at failure sites. Emerging non-destructive approaches that allow direct detection of corrosion at the buried interface include: acoustic microscopy, local electrochemical techniques (e.g. scanning Kelvin probe microscopy, the scanning vibrating electrode and local electrochemical impedance measurements) and can be employed to monitor corrosion of a buried substrate in situ. Whilst these can examine buried corrosion phenomena, little or no information is obtained about the overlying coating. Alternative techniques with improved resolution utilise scanning probe microscopy (SPM) methods (e.g. Kelvin probe force microscopy, KPFM) whilst detailed topographic images of the overlying coating may be simultaneously collected. More recently, SPM has been coupled to vibrational spectroscopy and allows direct nanoscale mapping of functional group chemistry. AFM-IR is a particularly versatile approach, whereby sub-diffraction limit detection of infrared absorbance is achieved by monitoring the deflection of an AFM probe in response to photo-thermal expansion of the sample. During AFM-IR measurements, the sample is illuminated by a rapidly pulsed tuneable infrared source, (an optical parametric oscillator, 10 ns pulses at a repetition rate of 1 KHz.) Upon absorbance of the infrared radiation, transient thermal expansion of the sample induces a contact-mode AFM probe to oscillate at its resonant frequency. A plot of the amplitude of oscillations as a function of infrared wavelength yields spectra that closely match conventional transmission-mode FTIR. Here we examine the degradation of model epoxy-phenolic can coatings on electrolytic chrome-coated steel (ECCS). Substrate materials were mechanically abraded successively to 1200 grit, then polished to 0.25 µm diamond paste to achieve a mirror-like surface finish. Stoichiometric epoxy-phenolic coatings were prepared by adding 1,1,1-tris(4-hydroxyphenyl) ethane to bisphenol-A diglycidyl ether (epoxide equivalent weight 172-176 g mol-1) using tetrabutylphosphonium bromide as catalyst. Coatings were spin-coated onto the substrates at 2000 rpm then cured at 150 °C for 60 min. The final coating thickness was around 7 µm. AFM-IR was performed on a NanoIR2 system (Anasys Instruments). In order to induce corrosion the polished ECCS samples were contaminated by spraying with a saturated sodium chloride solution in ethanol prior to coating. Corrosion was then initiated by placing a droplet of deionised water on the coating surface for 5 minutes, after which it was dried. Examination of the sample by optical microscopy subsequently revealed isolated patches of underfilm discolouration discernible through the coating (a). However, no corresponding change in the coating morphology was observed in AFM height images (b), and infrared amplitude maps obtained at 1260 cm-1 (c) and 1604 cm-1 (d) also remained featureless, indicating that significant deformation of the coating had not taken place. In contrast, infrared maps gathered at 916 cm-1 (e) and 1800 cm-1(f) showed an enhanced signal at the locus of discolouration. Preliminary results using AFM-IR mapping has been shown to detect the steel-polymer interface buried beneath an epoxy-phenolic coating. Generated infrared absorbance maps are sensitive to the substrate composition and morphology, opening up a new avenue for the detection of buried corrosion phenomena. Detection of the interface is dependent upon both the wavelength and power of incident infrared illumination. This is a result of the variable penetration depth of the radiation, in turn dependent on the infrared wavelength and laser power employed. Thus, judicious selection of infrared parameters permits the selective probing of the infrared absorbance of the coating alone, or a combination coating heterogeneity and interfacial reflectance. Figure 1

Effect of Ferritic Phase Content on Pitting Corrosion Behavior of Cast Duplex Stainless Steel

1. Introduction Duplex stainless steel (DSS) has been increasingly used in seawater desalination plants, oil & gas-related facilities, and chemical plants. This is because it has high corrosion resistance and high strength compared to austenitic stainless steels. It has been known that the phase ratio between the ferritic (α) and the austenitic (γ) phases can greatly affect the corrosion resistance of DSS. However, the data for the corrosion behavior of the cast DSS with different α/γ phase ratio is limited. The objective of this study is to elucidate the effect of α phase content on pitting corrosion behavior of the cast DSS. 2. Experimental procedures 2-1. Material The chemical composition of the cast DSS is shown in Table 1. The DSS was annealed isothermally to control α phase content in the DSS. The specimen whose α phase content was 32 vol% (1100 °C for 48 hours, WQ), 43 vol% (1250 °C for 48 hours, WQ) and 50 vol% (1300°C for 48 hours, WQ) was prepared from 25%Cr-8.4%Ni-2%Mo DSS (TP No. "1B+Ni" in Table 1), and the the specimen whose α phase content was 50 vol% (1100 °C for 48 hours, WQ), 58 vol% (1150 °C for 48 hours, WQ), 70 vol% (1275 °C for 48 hours, WQ) and 77 vol% (1300°C for 48 hours, WQ) was prepared from 25%Cr-5.6%Ni-2%Mo DSS (TP No. 1B in Table 1). The α phase content of the prepared specimens were measured by an image analyser from the optical micrographs (magnification: x200). 2-2. Pitting potential measurement The surface of the specimens was finished with 1 μm diamond paste. The pitting potential measurement was carried out in a concentrated artificial seawater (TDS: ca. 4.7%) at 80±2 °C under deaerated atmosphere by purging argon. Potentiostatic polarization treatment was performed at -0.65 V vs. an Ag/AgCl (Saturated KCl) electrode (SSE) prior to the test, then the corrosion potential of the specimen was measured for 10 minutes. Potentiodynamic anodic polarization was performed at a sweep rate of 20 mV/min. The potential when the current density reached 1 A/m2 was defined to be the pitting potential (V'c,100). Throughout this paper, the potential values are expressed in V vs. SSE. 2-3. Potentiostatic polarization measurement The surface of the specimens was finished with 1 μm diamond paste. 25 wt% NaCl aqueous solution at pH= 0 was used as the test solution to simulate a severe environment in a pit. The solution temperature was 40±2 °C. The test was conducted under deaerated atmosphere by purging argon. Potentiostatic polarization treatment was performed at -0.65 V prior to the test, then the corrosion potential of the specimen was measured for 10 minutes. The potentiostatic polarization was conducted. The potentiostatic conditions were -0.30 V, -0.25 V, -0.20 V, and -0.15 V. After the corrosion test, surface observation by a scanning electron microscope (SEM) and the depth profile measurement in the corroded area by a contact probe profilometer were conducted. 3. Results The pitting potential with different α phase content are listed in Table 2. The pitting potentials of the specimen prepared from 25%Cr-5.6%Ni-2%Mo DSS were approximately 0.17 ~ 0.18V and the pitting potentials of the specimen prepared from 25%Cr-8.4%Ni-2%Mo DSS were approximately 0.22 ~ 0.25V. The pitting potential of the cast DSS in the concentrated artificial seawater (TDS: ca. 4.7%) does not depend on the α phase content. The dependence of the preferentially dissolved phase on the holding potential, which determined by the SEM observation after the potentiostatic test, are also listed in Table 2. γ phase was preferentially dissolved at the higher holding potential and α phase was preferentially dissolved at the lower holding potential. With decreasing α phase content, γ phase was tendency to preferential dissolution at the lower holding potential. After the potentiostatic polarization test, SEM observation and the depth profile measurement in the corroded area were conducted to estimate the corrosion rate of each phase. The corrosion rate of the α phase tended to increase with increasing α phase content. In particular, as for the specimen in the α phase content of 58 vol% or more, the corrosion rate of α phase at higher potential range was significantly increased. The results of the SEM/EDS analysis in corroded area suggest that increase in corrosion rate of the α phase with increasing α phase content was caused by the Cr nitride formation in the α phase. Figure 1

Effect of Cr Content on Pit Initiation Behavior at MnS Inclusions in Fe-Cr Steels

It is well known that Cr improves the pitting corrosion resistance of stainless steels. In chloride-containing environments, MnS inclusions act as a preferential site for pit initiation of stainless steels. In this study, a microelectrochemical approach was applied to elucidating the effect of Cr addition on the corrosion resistance of pit initiation at MnS inclusions. Fe-5Cr, Fe-12Cr, and Fe-18Cr steels, containing 0.027 mass% S, were prepared by vacuum induction melting. Heat treatment was performed at 1123 K for 600 s. After the heat treatment, the steels were water-quenched. Their surfaces were polished down to 1 μm diamond paste. Since the MnS inclusions are darker than the steel matrix, they are clearly visible in the as-polished condition without etching, and it was confirmed that the inclusions consisted of Mn, S, and a small amount of Cr. This type of inclusion was simply referred to as MnS inclusion. In order to ascertain the dissolution potential of the MnS inclusion, the potentiodynamic polarization curves for the small areas with the inclusion were taken in a naturally aerated 0.1 M Na2SO4 solution at 298 K. As shown in Fig.1a, the increase in current density around 0.05 V was a characteristic common to all the specimens. After the polarization, the inclusions were observed by a field emission scanning electron microscope equipped with an X-ray energy-dispersive spectrometer, and it was concluded that the current increase around 0.05 V was due to the dissolution of the MnS inclusion. Figure 1b shows the potentiodynamic polarization curves for the small areas with the inclusion in a naturally aerated 0.1 M NaCl solution at 298 K. In the cases of Fe-5Cr and Fe-12Cr, the large current increases were observed around 0.05 V. It was suggested that the MnS dissolution directly brings about the initiation of pit (Fig1c-e). On the other hand, no stable pit was initiated on Fe-18Cr. Moreover, even the initiation potential of meta-stable pit was higher than the inclusion dissolution potential. The addition of Cr in the steels was shown to improve the repassivation ability of the steel matrix. Figure 1

Corrosion Behaviors and Metal Ion Releases of Metallic Biomaterials in Simulated Body Fluid

[Introduction] One of the major problems associated with the use of metallic biomaterials is their corrosion. Corrosion reactions, which are unavoidable in metals, cause not only deterioration or fracture of the products but also serious harm to living tissue. Nevertheless, metallic biomaterials are commonly used in the fields of medicine and dentistry because of their superior mechanical properties: an excellent combination of strength and ductility, resulting in high fracture toughness can not be substituted by other materials. Therefore, highly corrosion-resistant metals and alloys such as Ti, Ti alloys, Co-Cr alloys, and stainless steels are used for implant devices. All of them show superior corrosion resistance results from the formation of a passive film on the surface of the metal. However, very small quantities of the metallic ions actually dissolve through the passive film on these metallic biomaterials. There are already many literatures about the corrosion resistance of metallic biomaterials about the breakdown of the passive film. However, very limited information is available concerning the corrosion rate and the change in the amount of the released metallic ions as a result of the slight corrosion reaction with stable passive film. Therefore, in this study, several corrosion tests and surface analyses were performed in a simulated body fluid to clear the difference in the corrosion behaviors of various metallic biomaterials. [Materials and Method] Disk shaped specimens of pure Ti, Ti-50.8mol%Ni superelastic alloy (Ti-Ni), Co-20Cr-15W-10Ni alloy (L605, Co-Cr), type 316L stainless steel (316 SS), and pure Zr were prepared. The specimens for corrosion tests were mechanically ground to #800 grit SiC abrasive paper. The specimens for XPS were additionally polished and mirror-finished with 9 μm diamond paste and 0.04 μm colloidal silica. The specimens were then ultrasonically cleaned in acetone and ethanol. In this study, the simulated body fluid, 5.85 gL-1 NaCl and 10.0 gL-1lactic acid, as a corrosion test solution was prepared. The pH of the solution was checked in advance and was confirmed to be within the range of 2.30 ± 0.05 just after preparation. This solution was intended to be an accelerated body fluid for rapid testing of corrosion measurements of dental metallic materials prescribed by ISO 10271. An anodic polarization measurement was performed with a potentiostat and a function generator. A saturated calomel electrode (SCE) and a Pt-black electrode were used as a reference and a counter electrode, respectively. The surface area of the working electrode contacting the electrolyte was 0.38 cm2. After immersing the specimens into the test solution at 37°C, the open circuit potential (OCP) was measured for 10 min. Thereafter, the gradient anodic potential was applied at a constant sweep rate of 1 mVs-1from OCP. EIS measurement was performed with an electrochemical measurement system. A couple of the specimen disks were prepared and embedded in cold mounting epoxy resin. After grinding the surface, the electrodes were immediately immersed in the test solution and placed face-to-face at a constant distance. EIS measurement was started with an alternative potential of 10 mV in amplitude just 10 min after immersion. The measurement was continued for 48 h. The dissolution test was performed to evaluate the corrosion rate and amounts of the released metal ions during the immersion in the test solution. Twenty mL of the test solution was then put into the glass bottle, followed by the specimen. The bottle was completely sealed and kept for 7 d. The concentrations of the metal ions were measured with ICP-AES. [Results and Discussion] The polarization curves of all specimens except 316L SS showed clear passive region. Ti-Ni and Co-Cr showed transpassive region from 1.3 and 0.8 VSCE, respectively. Zr occurred pitting corrosion at 0.5 VSCE. On the other hand, 316L SS did not passivate and actively dissolved in the test solution. The amounts of the released ions from 316L were much higher than those of another specimens. However, Ti-Ni released certain amount of Ni ions, which are very hazardous for metal allergy. On the other hand, the amounts of the ions from Co-Cr and Zr were almost same level as detection limit of ICP-AES. It indicates that the protective performances against mass transfer of Co-Cr and Zr are superior to those of other biomaterials. Curve fittings could be performed using a conventional equivalent circuit models. The corrosion rates of all specimens decreased during the immersion period. However, the declines in the corrosion rates of Co-Cr and Zr were much larger than those of Ti, Ti-Ni, and 316 SS. It agrees with the results from the dissolution test.

The Effect of Change in Solution Chemistry of Bulk Solution on Crevice Corroson Propagation of Stainless Steel

Once crevice corrosion initiates, the hydrolysis of dissolved metal ions occurs to induce the acidification of solution inside the crevice and the migration of Cl- ions into the crevice from solution bulk. This Cl- ions migration promotes subsequent active dissolution and a further decrease of pH in the crevice. Therefore, the reduction of the concentration of Cl- ions is expected to be effective for suppressing crevice corrosion propagation. In this study, the influence of changes in solution chemistry on the propagation of crevice corrosion of stainless steel was investigated. After crevice corrosion was induced, in a chloride-containing solution the test solution was exchanged to chloride-free or chloride-containing solutions and corrosion propagation and repassivation behavior were monitored under the potentiostatic control. An austenitic stainless steel (0.017%C, 0.58%Si, 0.84%Mn, 0.028%P, 0.001%S, 12.15%Ni, 17.43%Cr, 2.08%Mo) was used to fabricate a crevice specimen schematically shown in Fig. 1. The steel was cut into 20 mm × 20 mm × ca. 5 mm, and the surface was polished down to 1 μm diamond paste. A through-hole with a diameter of 6 mm was formed at the center of the surface. Ultrasonic cleaning was carried out in ethanol, and then the specimen was passivated in 30 mass% HNO3 at 323 K for 1800 seconds. After that, the electrode surface area of the passivated specimen was polished down to 1μm diamond paste again. A lead wire was soldered to the electrode. An artificial crevice was formed between the electrode surface and a 20 mm × 20 mm × 2 mm polycarbonate sheet fixed by a bolt and a nut. The specimen was covered by insulating rubber except for the electrode surface. During the fabrication of the crevice, the electrode surface was kept being in contact with a NaCl solution (pH 5.0) dearated by N2 gas. The concentration of Cl- was adjusted to 20000 ppm. Potentiostatic polarization measurement at 0.3V (vs. Ag/AgCl) was conducted for 1000 seconds in a 20000 ppm Cl- containing NaCl (pH 5.0) at 298K to initiate crevice corrosion of the specimen. After that, the polarization was stopped and the test solution was exchanged to a chloride-free solution (test A) or a chloride containing-solution (test B), respectively. A Na2SO4 solution was selected as a chloride-free solution. The concentration of SO4 2- was adjusted to 20000 ppm. A chloride containing-solution was the same as the test solution used for the initiation of crevice corrosion. Subsequently, the potentiosstatic polarization at 0.3V was restarted and the change in current was monitored. After the test, the specimen surface was rinsed with deionized water and observed using an optical microscope. Figure 2a shows the time variation of current measured in the crevice corrosion tests. Figure 2b shows an enlarged view of the first 1000 seconds, that is, the current transient of the specimens in the NaCl solution. During this initial period, the currents gradually increased up to c.a. 1000 μA. It is therefore presumed that crevice corrosion occurred and propagated under the potentiostatic polarization at 0.3 V in the NaCl solution. After the solution was changed to the Na2SO4 solution, the current value increased to c.a. 700 μA until 4000 seconds passed and then decreased gradually to c.a. 30 μA. On the other hand, after the solution was exchanged to the NaCl solution, the current increased rapidly and remained at a high level until the end of the crevice corrosion test. These results suggest that the removal of Cl- ions from the bulk solution suppressed the propagation of crevice corrosion of stainless steel. Caption list: Figure 1 Schematic illustration of the specimen for crevice corrosion tests. Figure 2 (a) The time variation of current measured during the crevice corrosion tests at 0.3 V (b) An enlarged view of the first 1000 seconds of the crevice corrosion test. Figure 1

Microelectrochemical Investigation of Pit Initiation Site on Austenitic Cast Stainless Steel

Austenitic cast stainless steels are widely used as valves, pumps, and connecting tubes but sometimes suffer from pitting corrosion in chloride-containing environments, such as sea water. The corrosion behavior of cast stainless steels is being studied by immersion tests in acidic FeCl3 solutions; however, electrochemical investigations in near-neutral solutions are need to ascertain the corrosion mechanisms in the actual environments. For austenitic cast stainless steels, the primary ferrite solidification mode is effective for reducing the susceptibility of solidification cracking 1, so that the steels generally contain delta-ferrite phases. In this case, the solidification starts with the formation of primary delta-ferrite, and then the ferrite transforms to gamma-phase (austenite) during cooling. It is known that phosphorus and sulfur segregate at the delta/ gamma grain boundaries, and the segregation tends to cause corrosion, such as intergranular corrosion and pitting. Microelectrochemical techniques are successfully applied to investigate the pit initiation site on stainless steels.2-4 A small electrode area is suitable for studying the initiation site and morphology of corrosion, since the ratio of the dissolution to background currents is relatively high, and the polarization can be stopped at the very beginning of the corrosion processes. In this study, a microelectrochemical approach was applied to elucidating the initiation site of pitting for austenitic cast stainless steels. The effect of grain boundary segregation on pitting was also studied. A 20-kg ingot of austenitic cast stainless steel (0.007%C, 0.81%Si, 0.82%Mn, 0.028%P, <0.001%S, 8.1%Ni, 18.7%Cr, <0.01%Mo, <0.01%Cu, 0.001%Al, 0.0042%N, 0.0029%O was prepared by vacuum induction melting. To eliminate the effects of sensitization and sulfide inclusion on pitting, carbon and sulfur contents were lowered. The volume of delta-ferrite was around 15% at room temperature, and a small amount of Al-Mn oxide inclusion existed. The specimens (15 × 25 × 5 mm blocks) were taken from the center of the ingot and were used as-cast without any heat-treatment. The specimen surface was polished down to 1 μm diamond paste. The potentiodynamic anodic polarization curves were measured in a naturally aerated 0.1 M NaCl solution at 298 K. The electrode area was changed from 0.02 mm2 to 100 mm2. The pitting potential of the steel was 0.35 V (vs. Ag/AgCl, 3.33 M KCl), when the electrode area was 1 cm2. In the macroscopic measurements, the initiation site of pit was not identified, since a large pit with a diameter of ca. 100 μm was generated. The electrode area was then reduced to 0.02 mm2 (ca. 150 μm square); however, no pit was observed even when the electrode area contained the delta/gamma grain boundaries. It was suggested that the initiation sites for pitting were sparsely distributed in the steel. On the basis of the size of the delta-ferrite, the electrode area was adjusted to 1 mm2. Figure 1 shows the anodic polarization curves and the surface appearances of the sites of pitting before and after the polarization. A meta-stable pit was generated. In Fig. 1, the pit was initiated at the oxide inclusion located at the delta/gamma grain boundary. It was confirmed in the separate experiments that the metastable pit was also initiated at the delta/gamma grain boundary at which no inclusion was located. A scanning electron microscope (SEM) and a scanning transmission electron microscope (STEM) equipped with an energy-dispersive X-ray spectroscopy(EDS) system were employed to analyze the phosphorus and sulfur segregation at the grain boundary. Figure 2 exhibits the SEM/STEM images and EDS analysis of the grain boundary at which the metastable pit was initiated (Fig. 1). It was clear that phosphorus and sulfur were segregated at the delta/gamma grain boundary. Figure 3 shows the results of segregation analysis of the delta/gamma grain boundary at which no pit was initiated. No segregation of phosphorus and sulfur was detected in this case. It was clear that the pit initiation sites of austenitic cast stainless steel were the grain boundaries of the delta/gamma phases, and the segregation of phosphorus and sulfur was proven to cause the pit initiation in chloride environments. References; 1. V. Kujanpää, N. Suutala, T. Takalo, and T. Moiso, Weld. Res. Int., 9, 55 (1979). 2. I. Muto, Y. Izumiyama, and N. Hara, J. Electrochem. Soc., 154, C439 (2007). 3. A. Chiba, I. Muto, Y. Sugawara, and N. Hara, J. Electrochem. Soc., 159, C341 (2012). 4. A. Chiba, I. Muto, Y. Sugawara, and N. Hara, J. Electrochem. Soc., 160, C511 (2013). Figure 1

Three-dimensional characterization of ODS ferritic steel using by FIB-SEM serial sectioning method.

Considerable attention has been paid to the research of the electron tomography due to determine the three-dimensional (3D) structure of materials [1]. One of the electron tomography techniques, focused ion beam/scanning electron microscopy (FIB-SEM) imaging has advantages of high resolutions (10 nm), large area observation (μm order) and simultaneous energy dispersive x- ray microanalysis (EDS)/ electron backscatter diffraction (EBSD) analysis. The purpose of this study, three-dimensional EBSD analysis of ODS ferritic steel which carried out cold work using FIB-SEM equipment was conducted, and it aimed at analyzing the microstructure obtained there. The zone annealing tests were conducted for ferritic steel [2,3], which were produced through mechanical alloying and hot-extrusion. After zone annealing, specimens were mechanically polished with #400∼4000 emery paper, 1 µm diamond paste and alumina colloidal silica. The serial sectioning and the 3D-electron backscattering diffraction (3D-EBSD) analysis were carried out. We made the micro pillar (30 x 30 x 15 µm). The EBSD measurements were carried out in each layer after serial sectioning at a step size and milling depth was 80 nm with 30 slices. After EBSD analysis, the series of cross-sectional images were aligned according to arbitrarily specified areas and then stacked up to form a volume. Consequently, we obtained the 3D-IPF maps for ODS ferritic steel. In this specimen, the {111} and {001} grains are layered by turns. In addition, the volume fraction value of both plane are similar. The aspect ratio increases with specimen depth. The 3D-EBSD mapping is useful to analysis of the bulk material since this method obtain many microstructure information, such a shape, volume and orientation of the crystal, grain boundary.

Corrosion Behavior of Engineering Materials in Flow Field

In this study, the effect of fluid flow rate, surface roughness and strain level on the corrosion behavior of magnesium alloy AZ31 was characterized in a custom test bench. Specimens were prepared by mechanical polishing and subject to flow in a simulated body fluid at 37°C for 24 hrs. Compared to a specimen pre-strain of 0%, mass loss was shown to increase by approximately 6% with a specimen pre-strain of 10%. Similarly, mass loss increased by approximately 13% when the fluid flow rate was increased from 250ml/min to 500ml/min. Surface roughness had a significant influence on corrosion behavior. Compared to a specimen polished with a 1 µm diamond paste, the mass loss for a specimen polished with #600 sandpaper was 28% greater.

Assessment of the Oxidation Behavior of a Pt-Based Alloy for High Temperature Applications

Pt-based alloys are being developed as a possible future replacement for Ni-based superalloy components in the hottest section of turbine engines. The critical properties of these alloys are their ability to withstand higher thermal and mechanical stresses as well as to resist aggressive corrosive and oxidizing environments in applications. Oxidation properties of these alloys were investigated between 1150 and 1350 °C. The surface roughness of the as-polished samples was determined using atomic force microscopy, while the microstructures of both the as-polished and oxidized samples were examined using scanning electron microscopy. The alloy was found to be composed of a two-phase gamma/gamma prime microstructure, while the average surface roughness decreased from 5.78 nm after 1 μm diamond paste polishing to 4.13 nm with 0.25 μm diamond paste polishing. Microstructure examination of the oxidized alloy samples revealed the formation of compact and protective external oxide scale composed of α-alumina, as confirmed by the XRD and Raman spectroscopy. The results also showed that the oxide scale thickens with increased exposure time and temperatures according to parabolic kinetics. It was concluded from the results that the Pt-based alloys possess good oxidation resistance and thus will be suitable for high temperature applications, such as turbine engines.

The Influence of Surface Standardization of Lithium Disilicate Glass Ceramic on Bond Strength to a Dual Resin Cement

In vitro studies to assess bond strength between resins and ceramics have used surfaces that have been ground flat to ensure standardization; however, in patients, ceramic surfaces are irregular. The effect of a polished and unpolished ceramic on bond strength needs to be investigated. Sixty ceramic specimens (20×5×2 mm) were made and divided into two groups. One group was ground with 220- to 2000-grit wet silicon carbide paper and polished with 3-, 1-, and ¼-μm diamond paste; the other group was neither ground nor polished. Each group was divided into three subgroups: treated polished controls (PC) and untreated unpolished controls (UPC), polished (PE) and unpolished specimens (UPE) etched with hydrofluoric acid, and polished (PS) and unpolished specimens (UPS) sandblasted with alumina. Resin cement cylinders were built over each specimen. Shear bond strength was measured, and the fractured site was analyzed. Analysis of variance (ANOVA) and Tukey post hoc tests were performed. PE (44.47 ± 5.91 MPa) and UPE (39.70 ± 5.46 MPa) had the highest mean bond strength. PS (31.05 ± 8.81 MPa), UPC (29.11 ± 8.11 MPa), and UPS (26.41 ± 7.31 MPa) were statistically similar, and PC (24.96 ± 8.17 MPa) was the lowest. Hydrofluoric acid provides the highest bond strength regardless of whether the surface is polished or not.

Effect of finishing and polishing techniques on the surface roughness of a nanoparticle composite resin

Aim: To evaluate the surface roughness of the resin Filtek Z350 (3M ESPE) after different finishing and polishing techniques. Methods: Sixty specimens of 7x2 mm were made and distributed in 6 groups (n=10), according to the technique employed: G1 (control) – polyester strip – no finishing or polishing; G2- multi-blade burs; G3- diamond burs 3195F and 3195FF; G4- Diamond Pro Discs (FGM); G5- Sof-Lex Discs (3M ESPE); G6- Robinson bristle brushes with pumice paste for 20 s and felt disc with 2-4 µm diamond paste for 30 s. The specimens were stored in artificial saliva at 37°C for 7 days. After the finishing and polishing techniques, surface roughness (Ra, µm) was measured using Surf-Corder profilometer SE 1700. Data were subjected to one-way ANOVA and Tukey’s test at 5% significance level. Results: G3 presented the highest surface roughness mean value (0.61). G5 presented the lowest surface roughness mean value (0.15), but it was not significantly different from G1, G4 and G6. Conclusions: According to the obtained results, Z350 composite resin presented the lowest surface roughness when finishing and polishing systems were used (Sof-lex and Diamond Pro discs and Robinson bristle brush with pumice plus Diamond® felt disc with Diamond Excel® paste). The use of diamond burs (G3) resulted in the highest composite surface roughness. There was no significantly different between G1, G4, G5 and G6.

Ultra-structure characterization of self-etching treated cementum surfaces

to evaluate the effect of different conditioning treatments on surface roughness and topography of dental cementum. Extracted human canines were used for the present study. The mesial surface from the cervical third of the roots were ground flat with wet 600-grit silicon carbide paper. They were polished (up to 1/4 µm diamond paste) and treated as follows: 1) No treatment, 2) 35% H3PO4 during 15 s, 3) Clearfil SE Bond primer (SEB), 4) One-Up Bond F (OUB). The adhesive systems were applied following manufacturer 's instructions. SEB primer and OUB were removed from surfaces by washing and ultrasonic agitation with ascending ethanol solutions. Digital images of treated surfaces (5x5 and 15x15 µm) were obtained by means of an atomic force microscope (AFM) analysis. The average surface roughness (Ra nanometers) of the scanned areas was assessed. Data were analyzed by ANOVA and SNK multiple comparisons tests (p <0.05). phosphoric acid treatment produced the highest mean roughness value, at all scan sizes. At 5x5 µm AFM images, for self-etch adhesive systems no differences in roughness were detected. At 15x15 µm, when One-Up Bond F was employed the lowest value was obtained. When phosphoric acid treatment was applied, cementum surface roughness increased and a strong demineralization with exposed collagen fibers could be observed.

Initial bacterial adhesion on resin, titanium and zirconiain vitro

PURPOSEThe aim of this in vitro study was to investigate the adhesion of initial colonizer, Streptococcus sanguis, on resin, titanium and zirconia under the same surface polishing condition.MATERIALS AND METHODSSpecimens were prepared from Z-250, cp-Ti and 3Y-TZP and polished with 1 µm diamond paste. After coating with saliva, each specimen was incubated with Streptococcus sanguis. Scanning electron microscope, crystal violet staining and measurement of fluorescence intensity resulting from resazurin reduction were performed for quantifying the bacterial adhesion.RESULTSSurface of resin composite was significantly rougher than that of titanium and zirconia, although all tested specimens are classified as smooth. The resin specimens showed lower value of contact angle compared with titanium and zirconia specimens, and had hydrophilic surfaces. The result of scanning electron microscopy demonstrated that bound bacteria were more abundant on resin in comparison with titanium and zirconia. When total biofilm mass determined by crystal violet, absorbance value of resin was significantly higher than that of titanium or zirconia. The result of relative fluorescence intensities also demonstrated that the highest fluorescence intensity was found on the surface of resin. Absorbance value and fluorescence intensity on titanium was not significantly different from those on zirconia.CONCLUSIONResin specimens showed the roughest surface and have a significantly higher susceptibility to adhere Streptococcus sanguis than titanium and zirconia when surfaces of each specimen were polished under same condition. There was no significant difference in bacteria adhesion between titanium and zirconia in vitro.