Improvement of Corrosion Resistance of TiO2 Layers in Strong Acidic Solutions by Anodizing and Thermal Oxidation Treatment

By anodization and thermal oxidation at 600 °C, an oxide layer on Ti with excellent corrosion resistance in strong acid solutions was prepared. The structural properties of TiO2 films before and after thermal oxidation were investigated with methods of Scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XRD) and X-ray diffraction (XPS). The electrochemical characterization was recorded via electrochemical impedance spectroscopy, potentiodynamic polarization and Mott–Schottky methods. XRD results show that a duplex rutile/anatase structure formed after oxidation, and the amount of anatase phase increased as the treatment time was prolonged from 3 to 9 h. XPS analysis indicates that as the thermal oxidation time increased, more Ti vacancies were present in the titanium oxide films, with decreased donor concentration. Longer thermal oxidation promoted the formation of hydroxides of titanium on the surface, which is helpful to improve the passive ability of the film. The anodized and thermally oxidized Ti samples showed relatively high corrosion resistance in 4 M HCl and 4 M H2SO4 solutions at 100 ± 5 °C. The passive current density values of the thermally oxidized samples were five orders of magnitude under the testing condition compared with that of the anodized sample. With the oxidation time prolonged, the passive current density decreased further to some extent.

Tribocorrosion Behaviour of Ti6Al4V Produced by Selective Laser Melting for Dental Implants

Additively produced Ti6Al4V implants display mechanical properties that are economically infeasible to achieve with conventional subtractive methods. The aim of the present research work was to characterize the tribocorrosion behaviour of the newly produced Ti6Al4V, also known as titanium grade 5, by a selective laser melting (SLM) technique and compare it with another specimen produced by a conventional method. It was found that the tribological properties were of the same order, with the wear rate being k= 6.3 × 10−4 mm3/N·m and k = 8.3 × 10−4 mm3/N·m for respectively, SLM and conventional method. Regarding the friction behaviour, both methods exhibited similar COF in the order of 0.41–0.51. However, electrochemically, the potentiodynamic polarization curves presented some differences mainly in the potential range of the passive films and passive current density formed, with the passive current density being lower for the SLM method.

Long-Term Evolution of Corrosion Potential of Carbon Steel in Alkaline Radioactive Waste Environments

The long-term shifts of corrosion potential are important in predicting the likelihood of localized corrosion and stress corrosion cracking (SCC) of carbon steel used for storing radioactive wastes in underground storage tanks. Although considerable work has been done in understanding the passivity and corrosion potential of steel in various electrolytes, an important aspect of the current work is in assessing the effects of multiyear exposures of steel in waste simulants and their effects on corrosion potential. It is shown that SCC susceptibility of steel in nitrate increases at the long-term corrosion potential in solutions without organics (either by applying that potential or letting the corrosion potential increase over time). The long-term increase in corrosion potential results principally from a decrease in the passive current density with time of exposure. The present work shows that such a reduction in passive current density is accompanied by changes in the semi-conductive properties of the passive film, which itself may be a result of changes in stoichiometry of the film over time. Nitrite reduction is the most likely cathodic reaction with a small contribution from oxygen reduction. However, the presence of organic species in the environment can result in additional anodic reactions that may decrease the corrosion potential.

CORROSION AND WEAR PROPERTIES OF ELECTRODEPOSITED TERTIARY NANOCOMPOSITE Zn–Ni (ALUMINA–YITTRIA–GERAPHENE) COATING

Nanocomposite Ni–Zn coatings containing 80 wt.% Al2O3, 5 wt.% Y2O3 and 15 wt.% graphene were fabricated by pulsed electrodeposition method in nickel–zinc sulphate-based electrolyte and effects of pulse current parameters on nickel and other element contents, microstructure, resistance to corrosion and tribological properties of the coatings were investigated. The pulsed current with duty cycle from 10% to 50% was applied to different samples and frequency changed gradually from 500 to 4000[Formula: see text]Hz in five steps during coating process. Increasing the duty cycle led to decrease of absorbed nanoparticles in the surface of the coatings from 4.4 vol% to 3.58 vol% The sample coated with 10% duty cycle had utmost alumina content in the coating surface, 3.5 vol% in first layer up to 4.4 vol% in fifth layer. The sample coated with 30% duty cycle had higher corrosion resistance with passive current density of 2.5[Formula: see text]mA/cm2. Furthermore, the results showed that by increasing the duty cycle, wear rate had been increased up to 1.3[Formula: see text][Formula: see text][Formula: see text]10[Formula: see text][Formula: see text]mm2/N[Formula: see text]m.

Influence of Cu on the Passivation Behavior of Fe-20Cr-xCu (x = 0, 2, 4 wt%) Alloys in Sulfuric Acid Solution

Effects of Cu on the polarization behavior of Fe-20Cr-xCu (x = 0, 2, 4 wt%) were examined in deaerated 0.1 M sulfuric acid (H2SO4) solution. In the active region, corrosion resistance of the alloys is improved with Cu content as a result of the enrichment of Cu on the surface of the alloys that occurs by the preferential dissolution of Fe. In the passive region, the effects of Cu on the passivation behavior of the alloys was dependent on the applied potential. At potentials active to −100 mV vs. saturated calomel electrode (SCE), the passivity of the alloys is improved with Cu content, which is confirmed by the increase in passivation rate with a decrease in oxidation tendency. In contrast, above −100 mVSCE, where the oxidation reaction of Cu+ to Cu2+ occurs, the passivity of the alloys was degraded significantly with Cu content, as confirmed by the increase in passive current density as well as the significant decrease in passivation rate. The deleterious effects of Cu on the passivation behavior of Fe-20Cr-xCu (x = 0, 2, 4 wt%) decreased at potentials noble to 500 mVSCE, primarily as a result of the significant decrease in Cu content in passive film and hence of the decrease in the oxidation reaction of Cu+ to Cu2+.

Role of Alloying Elements on the Cytotoxic Behavior and Corrosion of Austenitic Stainless Steels

Super stainless steel has been used to solve corrosion problems of biomaterials because it shows the excellent corrosion resistance as like Ti and Ti alloys and has better mechanical properties than Ti and Ti alloys. We designed high Mo and Ni bearing super austenitic stainless steel. To obtain desirable microstructure, Cr, Ni, Mo, N contents were controlled. This work focused on the role of alloying elements on cytotoxic behavior and corrosion of stainless steel. In acidic chloride solution, when the alloys had high PRE values, the alloys showed high pitting resistance and low critical current density. However, in Hanks’ solution, the higher PRE’s alloys showed high critical passive current density. Namely, it was revealed that EDTA among Hanks’ solution played an important role to increase the critical passive current density of high Mo and Ni bearing super stainless steels, regardless of PRE’s value of the alloys. Therefore, even if the PRE values of the alloys were higher, high Ni and Mo bearing alloys would reveal more cytotoxic and high metal ion release rate than 316L stainless steel.

Methodologies for Predicting the Performance of Ni-Cr-Mo Alloys Proposed for High Level Nuclear Waste Containers

For the geologic disposal of the high level nuclear waste (HLW), aqueous corrosion is considered to be the most important factor in the long-term performance of containers, which are the main components of the engineered barrier subsystem. Container life, in turn, is important to the overall performance of the repository system. The proposed container designs and materials have evolved to include multiple barriers and highly corrosion resistant Ni-Cr-Mo alloys, such as Alloys 625 and C-22. Calculations of container life require knowledge of the initiation time and growth rate of localized corrosion. In the absence of localized corrosion, the rate of general or uniform dissolution, given by the passive current density of these materials, is needed. The onset of localized corrosion may be predicted by using the repassivation and corrosion potentials of the candidate container materials in the range of expected repository environments. In initial corrosion tests, chloride was identified as the most detrimental anionic species to the performance of Ni-Cr-Mo alloys. Repassivation potential measurements for Alloys 825, 625, and C-22, conducted over a wide range of chloride concentrations and temperatures, are reported. In addition, steady state passive current density, which will determine the container lifetime in the absence of localized corrosion, was measured for Alloy C-22 under various environmental conditions.

Localized Corrosion of Amorphous Fe-Ni-Cr-P-B Alloys

Experiments were performed with a series of amorphous Fe-Ni-Cr-P-B alloys of varying Cr content to characterize their susceptibility to crevice corrosion. Potentiodynamic and potentiostatic polarization of freely exposed specimens was performed in simulated crevice solutions (1M NaCI at pH 1–7, and 6% FeCI3 at PH 1.4). Crevice corrosion experiments were conducted in an artificial crevice cell. Polarization in the simulated crevice electrolytes indicated that alloys containing 2 to 16 At.% Cr can passivate in even 1M NaCI, pH 1, and they do not experience significant pitting at potentials active to that at which oxygen evolution occurs. In agreement with previously published results,1 only a few At.% Cr were needed to confer passivity; even the alloy containing 2% Cr achieved a passive current density of only 10−1 A/2 in 1 M NaCI at pH 1, and it did not pit below about 1.20 V(SCE). Results indicated, however, that the alloys meet certain requirements for susceptibility to crevice corrosion and occluded cell corrosion in general; that is, they passivate at sufficiently anodic potentials, and the intensity of active dissolution increases with decreasing pH. The amorphous alloys were considerably more inert to corrosion than T304 and Incoloy 800 steels, suggesting a lesser susceptibility to crevice attack. Experiments with the artificial crevice cell confirmed that the amorphous alloys can indeed be forced to crevice corrode, but only at very anodic (transpassive) potentials. They are considerably more resistant to crevice attack than the crystalline Cr-containing alloys.

Contribution to the Interpretation of Current Maxima in the Passivity Range of Austenitic Stainless Steels

Passive current density maxima have been observed by various workers during the anodic polarization of different stainless steels in sulfuric acid solutions and have been explained by many theories. This paper describes the precorrosion time and some other pretreatments of the sample surface which change drastically the anodic polarization curve of AISI Type 304 stainless steel in 2N H2SO4, lowering the critical current density and causing two passive maxima. Our results are used in a critical analysis of the conclusions of other authors. The passive current density maxima are attributed to the oxidation of hydrogen formed during precorrosion and absorbed into the alloy and of ions present in the solution and coming from the alloy dissolution.