Search

The band structures and band gap energies, E (g), of passive films formed on titanium (Ti) in simulated bioliquids, Hanks' solution (Hanks) and saline, were evaluated. Ti was polarized at 0, −0.1, and −0.2 V-Ag/AgCl, E (f), for 1 h. After polarization, the surfaces were characterized using X-ray photoelectron spectroscopy, and the photoelectrochemical responses were evaluated. The current change during photoirradiation was recorded as a photocurrent transient at each measuring potential, E (m), and by changing the wavelength of the incident light. Passive films consisted of a very thin TiO2 layer containing small amounts of Ti2O3 and TiO, hydroxyl groups, and water. During polarization in Hanks, calcium and phosphate ions were incorporated or formed calcium phosphate but not in saline. Calcium phosphate and hydroxyl groups influenced the band structure. E (g) was graded in Hanks but constant in saline, independent of E (f) and E (m). The passive film on Ti behaved as an n-type semiconductor containing two layers: an inner oxide layer with a large E (g) and an outer hydroxide layer with a small E (g). In Hanks, E (g) was 3.3–3.4 eV in the inner oxide layer and 2.9 eV in the outer hydroxide layer. In saline, E (g) was 3.3 eV in the inner layer and 2.7 eV in the outer layer. Calcium phosphate and hydroxyl groups influenced the band structure of the passive film. The E (g) of the outermost surface was smaller than that of TiO2 ceramics, which is probably one of the principles of the excellent biocompatibility of Ti among metals.

Titanium (Ti) is always covered by thin passive films. Thus, valence band (VB) spectra, obtained using X-ray photoelectron spectroscopy (XPS), are superpositions of the VB spectra of passive films and that of the metallic Ti substrate. In this study, to obtain the VB spectra only of passive films, angular resolution (for eliminating the substrate Ti contribution) and argon ion sputtering (for removing passive films) were used along with XPS. The passive film on Ti was determined to consist of a very thin TiO2layer with small amounts of Ti2O3, TiO, hydroxyl groups, and water with a thickness of 5.9 nm. The VB spectra of Ti were deconvoluted into four peak components: a peak at ~1 eV, attributed to the Ti metal substrate; a broad peak in the 3–10 eV range, mainly attributed to O 2p (~6 eV) and O 2p-Ti 3d hybridized states (~8 eV), owing to the π (non-bonding) and σ (bonding) orbitals in the passive oxide film; and a peak at ~13 eV, attributed to the 3σ orbital of O 2p as OH−or H2O. The VB region spectrum between approximately 3 and 14 eV from Ti is originating from the passive film on Ti. In particular, characterization of VB spectrum obtained with a takeoff angle of less than 24° is effective to obtain VB spectrum only from the passive film on Ti. The property as n-type semiconductor of the passive film on Ti is probably higher than that of rutile TiO2ceramics.

The effect of temperature (from 288 to 308 K) and concentration of sulfide ions (up to 40 ppm) on the corrosion behavior of AISI 304L and AISI 316L stainless steels in seawater was studied with measurements of open-circuit potential, linear and potentiodynamic polarization, and electrochemical impedance spectroscopy. An increase in temperature and pollutant concentration negatively affects the corrosion stability of stainless steels at the open circuit (the resistance, compactness, and thickness of the surface layer decrease and the corrosion current increases), in the passive region (the passivation current increases, the depassivation potential decreases, and the passive potential region narrows), and in the transpassive potential region (the rate of metal dissolution increases). The occurrence of pitting corrosion on the surface of the samples was confirmed with optical microscopy and a non-contact 3D profilometer. A few large pits (depth 80–100 μm and width 100 μm) were formed on the surface of AISI 304L steel, while several smaller pits (depth 40–50 μm and width 50 μm) were formed on the surface of AISI 316L steel. With increasing temperature and sulfide ion concentration, the width, depth, and density of the pits increased on both steel samples. In the studied temperature and concentration range of sulfide ions, the AISI 316L steels exhibited higher corrosion resistance. Overall, the influence of sulfide ions on steel corrosion was more pronounced than the influence of temperature.

There is limited research on metastable pitting corrosion in an acidic environment, and acid is a major challenge for material corrosion. Therefore, this work investigated the metastable pitting corrosion of laser powder bed fusion (LPBF)-produced Ti–6Al–4V, in Hank’s solution, at different pH values (pH = 3, 5, and 7). This work investigated the effect of acid on the characteristics of passive films, as well as the change in metastable pitting behavior. Based on the results of electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS), the passive film will be inhibited and dissolved under the influence of H+. The higher the concentration of H+, the thinner the passive film. Potentiodynamic polarization tests reveal that LPBFed Ti–6Al–4V in Hank’s solution, at pH 3, has more obvious metastable pitting corrosion. This is because the higher the H+ concentration, the more Cl- is adsorbed on the surface of the passive film, which is prone to generate soluble chlorides by competitive adsorption with oxygen atoms and thus develop into metastable pitting corrosion.

The development of a safe permanent disposal plan is essential for the long-term disposal of used fuel bundles. Nuclear Waste Management Organization (NWMO) has been investigating the deep geologic disposal of nuclear waste which offers the optimum passive safety system with a negligible probability of release of radionuclides into the environment. The proposed used fuel containers (UFC) for the permanent disposal of high-level nuclear waste in Canada is comprised of a carbon steel vessel coated with a 3 mm corrosion-resistant copper layer deposited using a combination of electrodeposition and cold spray deposition. Although copper is often considered to be thermodynamically stable in anoxic conditions, different corrosion processes might occur upon exposure to aggressive anions, such as chloride, sulfate, and sulfide. The presence of oxygen could increase the possibility of passive film formation on Cu which might result in localized corrosion (specifically pitting) and unexpected failure of UFC; so, the pitting corrosion probability of Cu in these conditions should be evaluated carefully. This research has focused on the development of a new technique to investigate the pitting probability of copper (Cu) in Deep Geological Repository (DGR) conditions using multielectrode arrays and statistical models. Investigation of pitting probability of Cu required the combination of different types of electrochemical experiments including potentiodynamic and potentiostatic polarizations since potentiodynamic polarization test gives us information about the Eb and Erp, however, potentiosatic polarizations simulates the natural condition in which a constant potential is applied to the Cu electrode. As a result, two different applied potentials were chosen based on histograms of Ecorr and Eb which had been collected using potentiodynamic polarization experiments to simulate the natural conditions. Ecorr+ 20 mV and Eb-20 mV vs SCE were selected as the highest and lowest applied potentials, respectively. Also, the morphology and composition of various types of passive films were investigated using SEM and XPS techniques. These information gave us useful information to evaluate the pitting possibility of Cu in the presence of different types of passive films such as Cu2O, CuO, and Cu(OH)2 as well as which film was more resistant to pitting corrosion.

Aqueous zinc ions lower the corrosion rate of Alloy 600, which helps lower the radiation dose rate in pressurized water reactors (PWRs). The influence of zinc on the electrochemical behavior of Alloy 600 in PWR primary water (PW) at 320 °C was investigated using a combination of electron microscopy and electrochemical impedance spectroscopy (EIS). Secondary electron microscopy (SEM) and scanning transmission electron microscopy (STEM)/energy-dispersive X-ray spectroscopy (EDS) indicated duplex surface films were formed on the Alloy 600 in PWR PW with and without 100 ppb of zinc. There was no effect of zinc on the chromium-rich inner layer (IL) (of Cr2O3 and/or CrOOH). Zinc had a significant effect on the outer layer (OL). In the absence of zinc, a highly porous OL formed that was mostly composed of nickel oxide whiskers. In the presence of zinc, a zinc-containing, denser OL of oxide was formed. The EIS data were acquired in laboratory simulated PWR PW at 320 °C with and without 100 ppb zinc. The spectra were measured at nine different values of potential that spanned a 500 mV-wide range. The EIS indicated there was no effect of zinc on the oxidation rate of metals at the alloy/IL interface nor on the transport of ions through the IL. Zinc lowered the corrosion rate because the dense OL inhibited the release of nickel ions from the IL into the solution.

Restaurant wastewater contains a high concentration of O&G, up to 3434 mg/L. This study aims to (a) assess the efficiency of EC combined with US methods for O&G removal in restaurant wastewater, (b) identify the optimum condition for COD degradation using EC treatment via response surface methodology (RSM), and (c) determine the morphological surface of the aluminium (Al) electrode before and after EC treatment. The wastewater samples were collected from the Lembaran cafeteria at the Universiti Sains Malaysia (USM). The efficiency of EC, US, and US-EC, combined methods for O&G removal, was investigated using a batch reactor (pH 7). The interelectrode distance (ID, 2–6 cm), electrolysis time (T, 15–35 min), and current density (CD, 40–80 A/m2) were analysed, followed by RSM. The response variables were O&G (1000 mg/L) and chemical oxygen demand (COD low range, 1000 mg/L). The central composite design (CCD) with a quadratic model was used to appraise the effects and interactions of these parameters. The morphological surface of the electrode used was observed via scanning electron microscopy (SEM). The optimum removal efficiencies obtained were 95.4% (O&G) and 75.9% (COD) (ID: 2.4 cm, T: 30.5 min, and CD: 53.2 A/m2). The regression line fitted the data (R2 O&G: 0.9838, and R2 COD: 0.9558). The SEM images revealed that the use of US was useful in minimising cavitation on the electrode surface, which could lower the EC treatment efficacy. The US-EC combined technique is highly recommended for O&G removal from the food industry’s wastewater.

Hydrogen production could be enhanced by increasing the potassium hydroxide (KOH) concentration, but higher KOH concentrations result in higher corrosion rates. Therefore, a deep investigation of the electrochemical behavior of stainless steel (SS 316L) in the KOH solution is needed. This study investigates the influence of KOH concentrations on the electrochemical behavior, surface morphology, structure, and sample phases of SS 316L. The investigations were conducted by some electrochemical techniques, UV-vis, scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD). The corrosion rate was found to increase, and solution resistance to decrease with increasing KOH concentration. Samples tested in 5, 30, and 50 g l-1 of KOH showed corrosion rates of 0.457, 2.362, and 5.613 µm year-1, respectively. A wide passive region and the noblest pitting potential were noticed for the sample with 5 g l-1 of KOH. Moreover, Mott-Schottky plots and characteristic wavelengths of UV-Vis suggest the formation of iron and chromium oxides by the passivation of samples. The SEM analysis showed a dynamic change of surface morphology from the lowest to the highest concentration with the intergranular corrosion found at the grain boundaries area. In conclusion, concentrations < 50 g l-1 KOH could be recommended since they would support the optimum remaining life of SS 316 L plates in HHO generators.

The effects of secondary passivation on the electrochemical behavior of passive films on the surface of 2205 duplex stainless steel were studied by potentiodynamic polarization, EIS and Mott-Schottky analysis. The composition and surface morphology of the passive film were analyzed by ICP-MS, ARXPS and SEM. These results manifested that the passive film formed by secondary passivation has good pitting resistance, due to the increase of the content of oxidized Fe3+, Cr3+ and Mo4+ in the passive film, which improves and increases the dissolution and migration rate of metal cations in the passive film, thus improving the compactness and stability of the passive film. The Mott-Schottky plots demonstrated that the five kinds of passive films have different semiconductor properties. The estimated doping density shows that the donor density of the passive film was significantly reduced by secondary passivation. In addition, Cl- and SO42- are permeated in the passivation layer and their adsorption on the surface of the passive film can be inhibited by the secondary passivation.

In this work, the synergistic inhibitions of tungstate (WO42−) and molybdate (MoO42−) anions, including role and mechanism, on the initiation of pitting corrosion (PC) for Q235 carbon steel in chloride (Cl−) solution were investigated with electrochemical and surface techniques. The pitting potential (Ep) of the Q235 carbon steel in WO42− + MoO42- + Cl− solution was more positive than that in WO42− + Cl− or MoO42− + Cl− solution; at each Ep, both peak potential and affected region of active pitting sites in WO42− + MoO42− + Cl− solution were smaller than those in WO42− + Cl− or MoO42− + Cl− solution. WO42− and MoO42− showed a synergistic role to inhibit the PC initiation of the Q235 carbon steel in Cl− solution, whose mechanism was mainly attributed to the influences of two anions on passive film. Besides iron oxides and iron hydroxides, the passive film of the Q235 carbon steel formed in WO42− + Cl−, MoO42− + Cl−, or WO42− + MoO42− + Cl− solution was also composed of FeWO4 plus Fe2(WO4)3, Fe2(MoO4)3, or Fe2(WO4)3 plus Fe2(MoO4)3, respectively. The film resistance and the defect quantity for Fe2(WO4)3 plus Fe2(MoO4)3 film were larger and smaller than those for FeWO4 plus Fe2(WO4)3 film and Fe2(MoO4)3 film, respectively; for the inhibition of PC initiation, Fe2(WO4)3 plus Fe2(MoO4)3 film provided better corrosion resistance to Q235 carbon steel than FeWO4 plus Fe2(WO4)3 film and Fe2(MoO4)3 film did.

Chloride-induced corrosion of steel rebars is one of the major causes of the premature failures of reinforced concrete structures served in different environments. This paper investigates the semiconducting behaviour and corrosion resistance of the passive film formed on the corrosion-resistant rebars exposed to simulated concrete pore solutions with different pH values and chloride concentrations. The electronic properties of the passive film were studied using potentiodynamic measurements and capacitance measurements (Mott–Schottky analysis). The results indicate that, firstly, the passive film of corrosion-resistant steel rebar shows n-type semiconducting behaviour with shallow and deep donor states in the band gap during passivation, and the deep donor energy level of corrosion-resistant steel rebar passive film is not sensitive to the decreasing pH value; secondly, under the same conditions, the passive film of corrosion-resistant rebars has a larger negative flat-band potential and thicker space charge layer than hot-ribbed rebars; thirdly, n-type semi-conductivity at a higher potential disappears once the chloride concentration at the rebar surface attains the chloride threshold value; and finally, a reverse charge layer forms on the surface of CR rebar at 0.50 V potential.

The passive film stability on stainless steel can be affected by hydrogen absorption and lead to microstructure embrittlement. This work shows that the absorption of hydrogen results in surface degradation due to oxide reduction and ionic defect generation within the passive film, which decomposes and eventually vanishes. The passive film provides a barrier to entering hydrogen, but when hydrogen is formed, atomic hydrogen infuses into the lattices of the austenite and ferrite phases, causing strain evolution, as shown by synchrotron x-ray diffraction data. The vacancy concentration and hence the strains increase with increasing electrochemical cathodic polarization. Under cathodic polarization, the surface oxides are thermodynamically unstable, but the complete reduction is kinetically restrained. As a result, surface oxides remain present under excessive cathodic polarization, contesting the classical assumption that oxides are easily removed. Density-functional theory calculations have shown that the degradation of the passive film is a reduction sequence of iron and chromium oxide, which causes thinning and change of the semiconductor properties of the passive film from n-type to p-type. As a result, the surface loses its passivity after long cathodic polarization and becomes only a weak barrier to hydrogen absorption and hence hydrogen embrittlement.

Among metals, Ti and majority of its alloys exhibit excellent biocompatibility or tissue compatibility. Although their high corrosion resistance is a factor in the biocompatibility of Ti and Ti alloys, it is clear that other factors exist. In this review, the corrosion resistance and passive film of Ti are compared to those of other metallic biomaterials, and their band gap energies, Egs, are compared to discuss the role of Eg in the reactivity with living tissues. From the perspective of the material's surface, it is possible to explain the excellent biocompatibility of Ti by considering the following factors: Ti ions are immediately stabilized not to show toxicity if it is released to body fluids; good balance of positive and negative charges by the dissociation of surface hydroxyl groups on the passive film; low electrostatic force of the passive film inducing a natural adsorption of proteins maintaining their natural conformation; strong property as n-type semiconductor; lower band gap energy of the passive film on Ti generating optimal reactivity; and calcium phosphate formation is caused by this reactivity. The results suggest that due to the passive oxide film, the optimal balance between high corrosion resistance and appropriate reactivity of Ti is the predominate solution for the excellent biocompatibility of Ti.

The adsorption characteristics of corrosive anions (Cl−, HS−, S2−, HCO3− and CO32−) on TiO2 of TC4 titanium alloy in a NaCl solution containing H2S and CO2 were studied by density functional theory (DFT). The stable adsorption configuration of each corrosive species on the TiO2 (110) surface was obtained by geometric optimization, and the electronic structure and interface binding energy were calculated and analyzed. The results showed that the optimal adsorption positions of Cl−, HS−, S2−, HCO3− and CO32− on TiO2 (110) were all bridge positions. There was a strong charge interaction between the negatively charged Cl, S and O atoms in Cl−, HS−, S2−, HCO3− and CO32− and the positively charged Ti atoms of TiO2. The interface bonding was mainly caused by charge movement from around Ti atoms to around Cl, O, S atoms. The energy levels were mainly caused by the electron orbital hybridization of Cl-3p5, S-3p4, O-2p4 and Ti-3d2. All adsorption configurations were chemical adsorption. The order of influence of the five ions on the stability of TiO2 was S2− > CO32− > Cl− > HS− > HCO3−. Finally, a novel corrosion mechanism was proposed to illustrate the dynamic evolution processes of pits.

This study presents microstructural, electrochemical, and mechanical characterizations on pure copper fabricated using constrained groove pressing (CGP) in quiescent phosphate buffer solution (pH = 11.73). We tested the effect of surface modifications on electrochemical features using electrochemical impedance spectroscopy (EIS) and Mott–Schottky analysis. A three-dimensional simulation via finite element modeling evaluates the CGP effects on the maximum principal stress and the plastic strain distribution. The transmission electron microscopy (TEM) micrographs confirm that the increasing CGP cycles result in a nano/ultrafine grain microstructure. Our electrochemical study showed that the CGP process can positively impact the passive behavior of pure copper. Mott–Schottky analyses illustrated that the point defect densities decline in the p-type semiconducting passive layer upon receiving CGP. Besides, increasing the CGP cycles lowers the corrosion current density and produces less defective passive films.

In the present work, the electronic properties of the passive films formed on equiatomic CoCrFeNi and CoCrFeNiAl high-entropy alloys were investigated in sodium chloride solution. The point defect model (PDM) was employed to study the effect of aluminum addition on the transport of p-type or n-type dopants through the passive films. The crystalline structure of the alloys was determined by X-ray diffraction. CoCrFeNi alloy was comprised of one single face-centered cubic crystalline phase. Aluminum addition promoted the formation of a mixed B2 and BCC structure. X-ray photoelectron spectroscopy (XPS) analysis allowed the examination of the correlation between passive film composition and their electronic properties. The major species in the passive film of the CoCrFeNi alloy were Cr2O3 and Cr(OH)3 while Al2O3 and Al(OH)3 were the predominant species in that of the Al-containing alloy. Electrochemical impedance spectroscopy (EIS) was employed to evaluate the corrosion resistance of the alloys. Aluminum addition promoted the formation of a thicker and more defective passive film. The pitting corrosion susceptibility of the HEAs was dependent on the passive film composition and increased with Al addition. The predominance of n-type defects in the passive film of the CoCrFeNiAl alloy was enhanced upon Al addition. Interaction between passive film composition and the electronic properties of the film played a central role in the corrosion resistance of the high entropy alloys.

A partial pre-rusted wire beam electrode (WBE) was designed to study the influence of the rust layer on rebar corrosion in the carbonated simulated concrete pore solution (SCPS). The results show that the passive film generated on the pre-rusted steel area is more fragile than that formed on the fine polished steel area in carbonaceous media. Nevertheless, the pitting corrosion resulting from the presence of chloride ions still tends to occur on the fine polished steel surface due to the local acidification process being hindered by the rust layer. The rust layer could play a more important role than the passive film in inhibiting the initiation of chloride-induced corrosion on rebar. The expansion path of the corrosion product would be blocked by the rust layer, leading to the pit propagating in the fine polished region. Furthermore, the growth of pitting corrosion is greatly accelerated due to the catalytic cathodic reaction of the rust layer.

In the present work, a low-pressure chemical vapor deposition (LPCVD) Ti0.17Al0.83N and state-of-the-art arc ion plating PVD-Ti1−xAlxN (x = 0.25, 0.55, 0.60, 0.67) coatings were deposited on cemented carbide substrate. The morphological, structural, and electrochemical properties of LPCVD-Ti0.17Al0.83N and PVD-Ti1−xAlxN coatings were compared. The X-ray diffraction (XRD) results and scanning electron microscopy (SEM) images revealed that the LPCVD-Ti0.17Al0.83N coating had a face-centered cubic (fcc) structure, while presenting a crack-free surface morphology and a compressive residual stress of −131.9 MPa. The PVD coatings with a composition of x ≤ 0.60 had an fcc structure, while the PVD-Ti0.33Al0.67N coating consisted of fcc and w-AlN phases. The results of the electrochemical corrosion test showed that the LPCVD-Ti0.17Al0.83N coating had the lowest corrosion current density in a 3.5 wt.% NaCl solution. After a 20-day immersion corrosion test in a 5 mol/L HCl solution, the LPCVD-Ti0.17Al0.83N coating displayed higher stability than the PVD-Ti1−xAlxN coating. The results of electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) analysis revealed that more uniform and denser passivation film, as well as higher Al2O3 proportion in the Al2O3/TiO2 composite passive layer, led to the outstanding corrosion resistance of the LPCVD-Ti0.17Al0.83N coating.

The composition and characteristics of the passive film formed in the simulated carbonation solution with the concentration of 1.0% and 2.0% rust inhibitor were studied by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The formation mechanism of steel passive film under the action of rust inhibitor was expounded. The results showed that the anticorrosion performance was the best and stable passive film was formed on the steel surface when the content of rust inhibitor was 2.0%; under the action of rust inhibitor, the main compositions of passive film on the steel surface were Fe2O3 and FeOOH. Among anodic rust inhibitors, Na2MoO4 was more effective than Na2CrO4. Between cathodic rust inhibitors, the anticorrosion performance of benzotriazole (BTA) was better than N-N dimethyl ethanolamine (DMEA). Among the composite rust inhibitors, the corrosion resistance of Na2MoO4 + BTA group was superior, and the corrosion resistance of Na2MoO4 + DMEA group was the worst. The composite rust inhibitor was better than the single rust inhibitor, and the inhibition efficiency was the highest when Na2MoO4 and BTA were mixed together.

To compare the corrosion inhibition behaviors of rust inhibitors with different mechanisms on steel bars, the rust resistance effect of sodium molybdate (Na2MoO4), sodium chromate (Na2CrO4), benzotriazole (BTA), N-N dimethyl ethanolamine, sodium molybdate (Na2MoO4) + benzotriazole (BTA), and sodium chromate (Na2CrO4) + benzotriazole (BTA) on steel bars in a simulated chloride concrete pore solution was studied. The rust resistance effects of different types of rust inhibitors were assessed by electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The effects of different types of rust inhibitors on the film formation characteristics of a passive film on a steel bar surface were expounded. The results show that: When sodium molybdate (Na2MoO4) and benzotriazole (BTA) acted together, the impedance value and the capacitive reactance arc radius were the largest, and the density of the passive film and the inhibition efficiency were the highest. The composition of the passive film was primarily composed of iron compounds, and it also contained oxide and adsorption films that were formed on the steel bar surface by the rust inhibitors. The rust resistance effect was proportional to the compactness of the passive film.

The electrochemical behavior and passive film properties of Hastelloy C22 alloy, laser-cladding C22 coating, and Ti–6Al–4V alloy in sulfuric acid dew-point corrosion environment were investigated through a combination of electrochemical measurements and surface analyses. The C22 alloy and laser-cladding C22 coating exhibited similar passivation and repassivation behavior without pitting corrosion, resulting from a similar passive film with a bilayer structure consisting of a Cr2O3-dominated compact inner layer and a porous outer layer containing oxides of Mo and hydroxides of Ni and Cr. The slightly poorer corrosion resistance and higher sensitivity to localized corrosion exhibited by the C22 coating were attributed to the microscale heterogeneity of the passive film resulting from the element segregation in the microstructure introduced by the laser-cladding process. The corrosion of the TC4 alloy performed as the preference dissolution of the β phase. Compared to the C22 alloy and C22 coating, the TC4 alloy exhibited more stable passivation behavior but poorer corrosion resistance, which is attributed to a compact but less protective single-layer passive film consisting of oxides of Ti and Al. An increase in temperature degrades passive film stability and accelerates the charge transfer process.

Using Cr-bearing low-alloy steel is an effective preventive measure for marine structures, as it offers superior corrosion resistance when compared to plain carbon steel. However, it remains unclear how quenching and tempering heat treatment, which is commonly applied to steel reinforcement in some specific environments to improve its mechanical properties, affects its corrosion resistance. In the present work, the impact of heat treatment on the passive film and depassivation behavior of the 0.2C-1.4Mn-0.6Si-5Cr steel are studied. The results reveal that quenching and tempering result in grain refinement of the Cr-bearing steel, which increases its hardness. However, this refinement causes significant degradation in its corrosion resistance. The critical [Cl−]/[OH−] ratio after quenching and tempering is determined to be approximately 6.6 times lower than that after normalization, and the corrosion rate is 1.6 times higher. After quenching and tempering, the passive film predominantly comprises iron oxides and hydroxides, with relatively high water content and defect density. Additionally, the FeII/FeIII ratio and film resistance are relatively low. In comparison, after normalization, the steel exhibits high corrosion resistance, with the passive film formed offering the highest level of protection.

Metallic elemental powder mixture and pre-alloyed metallic powder are both frequently used powder feedstock in the additive manufacturing process. However, little research has been conducted to compare the corrosion behavior of selective laser melting (SLM) alloys, fabricated by pre-alloyed metallic powder and mixed metallic powder. Hence, it is important to investigate the corrosion behavior of SLMed alloys, as well as the corresponding cast ingot, with the aim to better understand the feasibility of designing new materials. In this work, the SLM-produced Ti6Al4V3Cu alloys were manufactured using a metallic elemental powder mixture and pre-alloyed metallic powder, respectively. The corrosion behavior of the different Ti6Al4V3Cu alloys was investigated in following electrochemical tests and ion release measurements. The results showed that the Ti6Al4V3Cu alloy prepared by pre-alloyed metallic powder showed better corrosion resistance than that produced from mixed metallic powder. Moreover, the SLM-produced Ti6Al4V3Cu alloys performed significantly better in corrosion resistance than the cast Ti6Al4V3Cu. The results are expected to achieve a better understanding of the feasibility of designing new materials using mixed powders, contributing to reducing development costs and cycles.

Ni-Cr-Mo alloy coatings with different content of Cr element were produced on carbon structural steel by laser cladding. Coatings exhibited similar phase composition containing γ-Ni solid solution and similar microstructure composed of primary grains and eutectics with segregated Mo. Corrosion behavior of coatings in a sulfuric acid dew point corrosion environment were investigated through the combination of immersion test, electrochemical measurements, and surface analysis. Coatings exhibited similar passive behavior and passive film with a bilayer structure consisting of a compact inner layer and a porous outer layer. An increase in Cr resulted in a more protective passive film with thicker and more compacting inner layer. However, the corrosion resistance was firstly degraded as the content of Cr increased from 18 wt.% to 22 wt.%, and then improved as the content of Cr increased from 22 wt.% to 26 wt.%. This phenomenon was attributed to the competition effect between the increase in Cr and relative decrease in Ni.

The role of nitrogen, introduced by deep cryogenic treatment (DCT), has been investigated and unraveled in relation to induced surface chemistry changes and improved corrosion resistance of high-alloyed ferrous alloy AISI M35. The assumptions and observations of the role of nitrogen were investigated and confirmed by using a multitude of complementary investigation techniques with a strong emphasis on ToF-SIMS. DCT samples display modified thickness, composition and layering structure of the corrosion products and passive film compared to a conventionally heat-treated sample under the same environmental conditions. The changes in the passive film composition of a DCT sample is correlated to the presence of the so-called ghost layer, which has higher concentration of nitrogen. This layer acts as a precursor for the formation of green rust on which magnetite is formed. This specific layer combination acts as an effective protective barrier against material degradation. The dynamics of oxide layer build-up is also changed by DCT, which is elucidated by the detection of different metallic ions and their modified distribution over surface thickness compared to its CHT counterpart. Newly observed passive film induced by DCT successfully overcomes the testing conditions in more extreme environments such as high temperature and vibrations, which additionally confirms the improved corrosion resistance of DCT treated high-alloyed ferrous alloys.

Journal of alloys and compounds 895, 162657 (2022). doi:10.1016/j.jallcom.2021.162657, The surface chemistry and thickness of the native oxide, hydroxide, and modified sub-surface layer of three Ni superalloys (alloy 59, 625, and 718) were determined by synchrotron X-ray Photoelectron Spectroscopy (XPS) and X-ray Reflectivity (XRR). Taking advantage of the synchrotron radiation techniques, a procedure for normalizing the photoelectron intensity was employed, which allowed for accurate quantitative analysis revealing a total oxide thickness for all samples of 12���13 ��, a hydroxide layer of 2���3 ��, and a thickness of the sub-surface alloy layer of 20���35 ��. The thickness results were compared to structural atomic models suggesting that the oxide thickness corresponds to four planes of metal cations in the oxide matrix. The XPS data revealed that the native oxides were enriched in Cr$^{3+}$, Mo$^{(4,5,6)+}$, and Nb$^{5+}$, while no Ni oxide was detected. The hydroxide layer mainly contained Ni$^{2+}$ and Cr$^{3+}$ hydroxide. The sub-surface layer was enriched in Ni and depleted in Cr, Fe, Mo, and Nb. The obtained oxide composition can be explained using thermodynamics, and it was found that the oxide composition correlates with the enthalpy of oxide formation for the metal elements in the alloys. Finally, the advantages of synchrotron radiation for composition and thickness determination are discussed., Published by Elsevier, Lausanne

Cathodic protection (CP) is an electrochemical technique used to reduce corrosion rate of carbon steel to negligible values. Despite the worldwide use of this technique for protecting underground structures for almost 100 years, the working mechanism of CP is still under debate. The aim of this work is to verify with in-situ spectroscopy measurements of electromagnetic radiation, the formation of a passive film on carbon steel during CP. In this study, active reflectance spectroscopy (ARS) and electrochemical impedance spectroscopy (EIS) measurements were used to verify the passive film formation and modifications in the carbon steel-electrolyte interface in CP condition. The increase in absorbance in the UV region and the increase in the measured impedance suggest the modification in the electrolyte chemistry and the formation of passive film on the steel surface under cathodic polarization.

Austenitic stainless steels that exhibit good corrosion resistance have recently found increasing applications in industry and transportation. This article addresses the influence of cold rolling and deformation on the pitting corrosion resistance of AISI 301LN and 316L stainless steels. The results indicate that the content of martensite increases as the cold rolling reduction also increases. The current work combined different techniques such as optical microscopy and scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) analyses. Corrosion tests were carried out, in accordance with the ASTM standards. The results confirm that the 316L steel performed better than the 301LN, regarding pitting corrosion, even when deformed. This is due to the high molybdenum (Mo) content, which guarantees greater corrosion resistance. The conducted corrosion tests showed that the increase of cold deformation reduces the resistance to pitting and overall corrosion in both steels. It was found that the 301LN stainless steel has higher susceptibility to deformation-induced martensite and, despite the addition of nitrogen, it still has a lower performance relative to the 316L steel. The current work focused on evaluating the formation of pits and the dynamics of the microstructures of the AISI 301LN and 316L steels with their mechanical properties and corrosion resistance in a saline environment including chlorides.

The corrosion resistance of stainless steel is attributed to the extraordinary protectiveness of the ultrathin native passive film (~3 nanometers) on alloy surface. This protectiveness, independent of alloying, can possibly be further increased by modifying the native film to resist corrosion in harsh conditions. However, the modification based on the film itself is extremely difficult due to its rapid, self-limiting growth. Here we present a strategy by using low-temperature plasma processing so as to follow the growth kinetics of the native film. The native oxide film is restarted and can uniformly grow up to ~15 nanometers in a self-limiting manner. High-resolution TEM found that the film exhibited a well-defined, chemical-ordering layered structure. The following corrosion tests revealed that the anodic current density of the alloy decreased by two orders of magnitude in 0.6 M NaCl solution with a remarkable increase of pitting potential. This enhancement is also observed in Fe-Cr alloys with Cr contents above ~10.5 wt.%. The superior protectiveness of the alloy is thus attributed to the continuous and thickened high-quality ultrathin Cr2O3 layer in the restarted film.

Concrete, especially mass concrete, releases a large amount of heat during the hydration process, resulting in the passivation of reinforcement at high temperatures. However, the passivation study of reinforced concrete is mostly conducted at room temperature, and the influence of temperature on passive film behavior is not clear at present. The passivation film of reinforcing steel directly determines the corrosion resistance of reinforcing steel and affects the service life of reinforced concrete. Herein, the passivation of Q235 steel soaking in simulated concrete pore (SCP) solution at 20 °C, 40 °C, and 60 °C is explored. It is found that the passivation process is divided into two stages, with 24 h as the boundary; within 24 h the passivation was carried out rapidly, and the passive film is in a relatively stable state after 24 h. In addition, the higher the temperature, the faster the passivation. Moreover, under the condition of higher temperatures, more Fe3+ compounds are produced, and the semiconductor properties of passivated films are more stable. Based on experiments, the passivation mechanism affected by temperature was analyzed in detail.

Newly developed Ti&ndash, 10Mo&ndash, 6Zr&ndash, 4Sn&ndash, 3Nb has fascinating mechanical properties to be used as a biomedical material. However, there is still a lack of investigation focusing on the corrosion behavior of Ti&ndash, 3Nb. In this work, the microstructure and corrosion behavior of as-cast Ti&ndash, 3Nb was investigated by optical microscopy, X-ray diffraction, and electrochemical measurements. Hank&rsquo, s solution was used as the electrolyte. A classical as-cast Ti&ndash, 6Al&ndash, 4V was used as reference. The results showed that Ti&ndash, 3Nb has a higher corrosion potential and a lower corrosion current density compared with Ti&ndash, 4V, indicating better corrosion resistance. However, after applying anodic potentials, Ti&ndash, 3Nb shows larger passivation current density in both potentiodynamic polarization and potentiostatic polarization tests. This is because more alloying elements contained in Ti&ndash, 3Nb trigger the production of a larger number of oxygen vacancies, resulting in a higher flux of oxygen vacancy. This finding illustrates that the passive film on Ti&ndash, 3Nb is less protective compared with that on Ti&ndash, 4V when applying an anodic potential in their passivation range.

This paper describes the corrosion process of reinforcing steel under both chloride attack and mechanical loading. Molecular dynamics simulation is adopted to investigate the evolution process of passive film and oxidation process of reinforcement. The mean square displacement and self-diffusion coefficient of O2 under different conditions are also obtained based on molecular dynamics simulation. It is shown that the present of chlorides can increase the energy of Fe atoms on the surface of Fe3O4 substrate, leading Fe atoms to separate from Fe3O4 easily and to get more chance to react with oxygen atoms. The interaction of chloride attack and mechanical loading can aggravate the damage of passive film on the surface of reinforcement and result in an increase of oxidation depth of Fe matrix. The coupled effects of chloride and static loading can accelerate the diffusion of O2 to the iron matrix, thus increasing the oxidation reaction between the iron matrix and O2.

[EN] In this paper, the degradation mechanisms of the ceramic and the metal in Titanium/Zirconia pairs for biomedical applications were analyzed. To do that, an experimental set-up with well-controlled mechanical and chemical conditions was used based on a unidirectional ball-on-disk tribometer coupled to a potentiostat. Tribocorrosion tests were carried out in artificial saliva at different applied potentials, this is, different chemical conditions of the surface. Wear damage of the titanium/zirconia pair was influenced by the properties and the behavior of wear debris in the contact. Under passive conditions metallic and oxidized titanium particles (formed by the cyclic removal of the passive film and subsequent repassivation) were smeared and mechanically mixed within the contact forming compacted wear debris through which the loading was carried out. Properties and amount of oxidized titanium lead to low wear at low passive conditions (OCP) and higher wear at high passive conditions. Zirconia did not suffer any damage under all the studied conditions and oxidized titanium was transferred to the ball at anodic applied potentials., Authors would like to acknowledge the Generalitat Valenciana for the financial support under the PROMETEO/2016/040 and GV/2017/042 projects. A. Dalmau acknowledges the Generalitat Valenciana for her contract (APOSTD/2017/051).

In this work, the corrosion performance of a new developed high-entropy alloy (HEA) of the composition AlCrFeMnTi (at. %) was studied. The corrosion testing of the AlCrFeMnTi HEA was carried out in 2.5 M NaOH, 0.6 M NaCl, and in 0.5 M HSO. The correlation between the microstructure, elemental composition, density, hardness, and corrosion resistance of the new developed AlCrFeMnTi HEA was investigated. The impedance response showed protective corrosion behavior for the neutral and acidic pH, while the alkaline environment led to a passivity breakdown due to dissolution of Al forming Al(OH). The outcome of the presented study suggests that new developed HEA is suitable to be used in industrial environments with a neutral and acidic pH., This research was funded by Firestone Research, grant number 639430, and the University of Akron Fellowship, FRC-207160 and FRC-207865. P.A and P.P. acknowledge financial support of the Spanish Ministry of Science and Innovation under project PID2019-104382RBI00. The assistance of A. Stark during HEXRD measurements in P07 beamline of PETRA III, at the Deutsches-Elektronen-Synchrotron (DESY), is acknowledged in the framework of proposal I20210087EC.

Three corrosion potentials and three corrosion current densities are clearly identified before the passivation for both dynamic polarization curves of equimolar CoCrFeNi high-entropy alloy (HEA) and 304 stainless steel (304SS) in 0.5 M H2SO4 aerated aqueous solution, by decomposing anodic and cathodic polarization curves. The passivated current density of the former is greater than the latter, compliant with not only the constant of solubility product (ksp) and redox equilibrium potential (Eeq) of each metal hydroxide but also the sequence of bond energy (Eb) for monolayer hydroxide on their facets derived from the first principle founded on density function theory. However, the total amount of ion releasing from HEA is less than 304SS, since the hydroxide/oxide film formed in the air of the latter containing greater amounts of Fe(Ⅱ) and Mn(Ⅱ) is less stable around corrosion potentials while they are further oxidized into more stable Fe(Ⅲ) and Mn(ⅢorⅣ) with much lower ksp, leading to the much less increasing ratios of ion releases from 0.25 to 0.6 V.

The effect of high temperature on corrosion behavior and passive film composition of Ni-based alloy 825 in H2S-containing environment was investigated by Confocal Laser Microscope (CLM), Scanning Electron Microscope (SEM), Energy Dispersive Spectrometer (EDS), and X-ray Photoelectron Spectroscopy (XPS). The experiment was carried out at 150 and 230°C in NaCl solution. The partial pressure of H2S was set to 1.2 MPa and CO2 was set to 3.2 MPa. The results showed that Ni-based alloy 825 presented good general corrosion resistance. Pitting corrosion was likely to occur at 230°C because of Cr depleted in the passive film. NiS appeared at high temperature and is damaging to protectiveness of passive film.

In the present study, 0.115 M L-arginine (LA) has been used as an eco-friendly inhibitor in simulated concrete pore solutions (SP-0) in order to form passive films on a steel rebar–solution interface until 144 h. Hence, 0.51 (SP-1) and 0.85 M NaCl (SP-2) were added in LA containing SP-0 solution to breakdown the passive film and to initiate corrosion reactions. The electrochemical results show that the charge transfer resistance (Rct) of steel rebar exposed to SP-1 and SP-2 solutions increased with respect to immersion periods. The sample exposed to the SP-2 solution initiated the corrosion reaction at the steel rebar–solution interface after 24 h of NaCl addition and formed pits, on the other hand, the sample without NaCl added, i.e., SP-0, showed agglomeration and dense morphology of corrosion products.

Application of low-nickel stainless steel anchor was one of the economical and effective methods to solve the durability problem of slope engineering. At present, there are scarce reports about corrosion characteristics of low-nickel stainless steel in various underground waters. For investigating the corrosion initiation of stainless steel anchor bolt in corrosive underground water, the effect of SO42−, HCO3− and pH value on the corrosion behavior of 201 low-nickel stainless steel was studied via electrochemical methods. As the SO42− concentration, HCO3− concentration or pH value increase, the open circuit potential and polarization resistance increase, while the double-layer capacitance, donor density and passive current density decrease. The results indicate that corrosion is inhibited by SO42−, HCO3− and OH− in underground water. In addition, the inhibitive efficiency of SO42− and HCO3− increases with the SO42− and HCO3− concentration.

The ion transport mechanisms in passive films pre-formed on CoCrFeMnNi high entropy alloy surfaces in acid solution were studied in-situ by time-of-flight secondary ion mass spectrometry upon oxidation in 18O2 gas. The bilayer structure of the passive film, with outer Fe, Co-rich and inner Cr, Mn-rich layers, is thermally stable, but with reduction of Fe and Co oxides and formation of Mn oxide in the outer layer above 150°C. At 170°C, the transport mechanism is mainly outward cation diffusion with slower inward oxygen diffusion. The re-oxidation growth rate follows a parabolic law and the value of parabolic constant was determined.

In this study, the resistance to pitting corrosion of Fe-18Cr-9Mn-5.5Ni-0.3(C + N) austenitic stainless steel γ-SSs (in wt%) with different C/(C + N) ratios (0.02, 0.29, and 0.60) was evaluated. It was found to be difficult to form a γ-matrix without precipitation, because the Cr23C6 precipitation rate in the γ-SSs with the C/(C + N) value of 0.60 was too fast. Thus, it was recommended to maintain the C/(C + N) ratio below 0.6 in Fe-18Cr-9Mn-5.5Ni-0.3(C + N) γ-SSs. As a result of the potentiodynamic polarization tests, the γ-SS with a C/(C + N) ratio of 0.29 showed the highest resistance to pitting corrosion, and the resistance level of this alloy was superior to that of the AISI 304 γ-SS. Analysis of the passive film and matrix dissolution rates revealed that a higher C/(C + N) ratio of γ-SS increased the protective ability of the passive film and decreased the growth rate of the pits. Therefore, it could be concluded that partial substitution of C for N was advantageous for improving the pitting corrosion resistance of Fe-18Cr-9Mn-5.5Ni-0.3(C + N) γ-SSs, as long as C and N existed in a solid solution state.

Localized plastic deformation has been observed to render nuclear reactor components more susceptible to stress corrosion cracking (SCC). However, it is not fully clear how localized strain impacts corrosion (oxidation) behavior. Herein, the surface reactivity and corrosion behavior of 304 L stainless steel specimens, deformed to different strain levels, was analyzed using advanced multimodal and multiscale methods. For the first time, we observed that localized deformation regions, e.g., deformation bands and α’-martensite featured smaller Volta potentials than the parent austenite matrix. This resulted in the establishment of localized corrosion potential gradients and the emergence of accelerated microscale galvanic corrosion. Particularly, regions that featured higher dislocation concentrations were more reactive on account of the reduction in the activation energy of corrosion due to the stored energy. The superposition of surface reactivity and strain distributions reveals that, SCC cracking is expected to initiate in regions of strain localization wherein micro-galvanic corrosion is favored.

In this work, the influence of temperature on the corrosion behavior of AlCoCrFeNi2.1 eutectic high-entropy alloy in a chloride-containing sulfuric acid solution was investigated using electrochemical measurement, X-ray photoelectron spectroscopy, and scanning electron microscopy. Results show that the passive film of AlCoCrFeNi2.1 is stable in chloride-containing sulfuric acid solutions at low temperatures, while an unstable film forms on the alloy at high temperatures. Furthermore, temperature changes the proportion of hydroxide and oxide in Fe and Cr, but it has no noticeable effect on Al and Ni, which is a significant factor on the passive behavior. L12 phase exhibits good corrosion resistance at different temperatures. Pitting occurred on B2 phase in the chloride-containing sulfuric acid solution at a low temperature of 5 °C, while pitting and dissolution take place on AlCoCrFeNi2.1 in the acid solution at room temperature and above.

The effects of solution concentration and temperature on the electrochemical behavior of the Fe-based amorphous (AMCs) coatings in NaCl solution were studied by using conventional electrochemical measurement and XPS analysis. Results indicated that as solution concentration and temperature increased, the current density for the Fe-based AMCs increased and the pitting potential decreased. This reduced corrosion resistance was derived from the decreased passivation index and polarization resistance, which contributed to the decrease of the induction time (tm) for the initiation of corrosion pit. The passive films formed on Fe-based AMCs exhibit two types’ semiconductors higher and lower the flat band potential. The decrease of protectiveness of the passive film was mainly correlated with the reduction of Cr2O3 content, the increase of carrier density and the decrease of the work function (WF).

Typical engineering materials may experience high corrosion rates when exposed to aggressive service conditions. Such conditions include high concentrations of aggressive anions and oxidants, high solution acidity, and/or high temperatures. Alloys with high corrosion resistance may be selected to avoid material failure caused by corrosion, for example, Ni-Cr-Mo alloys. These alloys exhibit excellent corrosion resistance due to the formation of a passive oxide film, primarily containing Cr and Mo. However, film breakdown can result in localized corrosion, e.g., crevice corrosion. Localized corrosion processes can penetrate deep into the bulk alloy, threatening its integrity. While the oxide film and localized corrosion processes have been extensively studied, many mechanistic features remain unresolved. Here, the corrosion behaviour of commercially available Ni-Cr-Mo alloys has been studied using electrochemical, spectroscopic, and microscopy techniques. Electrochemical measurements (Chapter 3) showed that increases in Cr content improve passive film properties, while increases in Mo content improve film stability in acidic solutions. Atomic emission spectroelectrochemistry (AESEC) measurements (Chapters 4 and 5) revealed a dynamic role for Mo deposition during film breakdown. Transpassive dissolution occurring in neutral solutions led to the deposition of Mo-rich species, while repassivation resulted in their release to solution. Surface activation followed by repassivation in acidic solution was found to cause a similar process. The mechanism of crevice corrosion (Chapter 6) was investigated using a galvanostatic technique combined with weight loss measurements. Internal cathodic reactions were found to be an important feature, intensifying damage by as much as 76 %, depending on the Mo content of the alloy. The effect of fluoride (Chapter 7) on the corrosion behaviour of Ni- and Fe-based alloys was investigated. Film stability was found to decrease in the presence of trace quantities, especially as the applied potential was increased. The findings reported throughout this thesis suggest a delicate balance between Cr and Mo is required for optimal corrosion performance, however, an optimal alloy composition has yet to be determined. The results presented in this thesis provide new mechanistic information necessary for understanding the corrosion of these industrially important alloys.

In this work, the characteristics of passive films formed on as-cast Ti-6Al-4V before and after transpassivation by electrochemical methods will be studied. A simulated body fluid of Hank’s solution was used as the electrolyte in this work. According to the potentiodynamic polarization test, the passivation range, transpassive range, and repassivation range of as-cast Ti-6Al-4V were obtained. Afterward, the potentiostatic polarization was employed to passivate the Ti-6Al-4V in both passivation and repassivation ranges. Electrochemical impedance spectroscopy (EIS) was used to analyze the characteristics of formed passive films. Different electrochemical behavior of as-cast Ti–6Al-4V is found in passivation and repassivation ranges. The passivation current density of the sample in the repassivation range is significantly larger than that in the passivation range. Meanwhile, the growth rate of passive film in the repassivation range is also greater than that in the passivation range. Although the sample shows a higher charge transfer impedance in the repassivation range, metastable pitting corrosion is also observed, indicating the formation of the unstable passive film. Such results advance the understanding of as-cast Ti-6Al-4V polarized under different potentials for potential biomedical applications.

Ni-based alloys are widely used in various engineering applications, such as oil and gas, nuclear power plant and chemical process, due to their excellent corrosion resistance and good mechanical properties at elevated temperature. To extend the sustainability and the versatility of Ni-based alloys, development of new grades or improvement of the existing ones is still ongoing. To carry out the alloy design, the study on the role of alloying elements on nickel-based alloys properties and corrosion resistance remains the best scientific approach. Within this framework, the main objective of this thesis is to develop a methodology based on electrochemical characterization and analysis for understanding the role of the alloying elements on the corrosion behavior and on the passive film properties of Ni-based alloys. The proposed methodology relies on successive electrochemical impedance spectroscopy measurements and advanced analyses of the impedance data, to follow the evolution of the passive film properties grown on different binary (Ni-Cr) and ternary (Ni-Cr-Fe) alloys at different potentials. The obtained impedance diagrams were interpreted with enhanced graphical method and fitted with Power-Law Model for giving physical meanings to passive film. This experimental procedure provides supplementary data from conventional polarization curves, such as evolution of the passive film thickness and resistivity profiles through the film. In this work, the role of alloying elements, especially chromium and iron, on the passive film properties and reactivity is studied. Understanding how alloying elements act on the passive film nature, structure and properties will provide some guidelines to engineers and scientists for designing or choosing the more adapted alloys. Finally, the proposed experimental method is also applied on certain commercial alloys. Their responses are compared with that of the closest binary and ternary alloys to evidence the discriminating factors that control the passive behavior., Les alliages base nickel sont largement utilisés dans divers domaines d’application, tels que le pétrole et le gaz, le nucléaire et la chimie, grâce à leur excellente résistance à la corrosion et leurs bonnes propriétés mécaniques à haute température. Dans le but de prolonger la durée de vie de certains composants stratégiques, de nombreuses études sur l’influence des éléments d'alliage sur les propriétés et la résistance à la corrosion des alliages base nickel ont été réalisées ou sont toujours en cours. Dans ce cadre, l'objectif principal de cette thèse est de développer une méthodologie basée sur la caractérisation et l'analyse électrochimique permettant de comprendre l’effet des éléments d'alliage sur les propriétés du film passif des alliages base nickel. La méthodologie proposée se base sur des mesures successives de spectroscopie d'impédance électrochimique et sur une analyse approfondie des diagrammes d’impédance, permettant de suivre l'évolution des propriétés du film passif formé sur différents alliages binaires (Ni-Cr) et ternaires (Ni-Cr-Fe) à différents potentiels. Les diagrammes d’impédance obtenus ont été interprétés avec une méthode graphique avancée et ajustés par le modèle en loi de puissance fournissant ainsi des informations caractéristiques du film passif. Cette méthodologie permet ainsi de déterminer et d’obtenir des informations complémentaires aux traditionnelles courbes de polarisation, telles que l’évolution de l’épaisseur du film et des profils de résistivité au travers de son épaisseur. Cette méthodologie a aussi été appliquée à des alliages commerciaux. Leurs réponses ont été comparées à celles obtenues pour les alliages binaires ou tertiaires dont les compositions sont les plus proches afin de mettre en évidence quels sont les paramètres microstructuraux ou chimiques les plus discriminants. Dans ce travail, le rôle des éléments d'alliage, en particulier le chrome et le fer, sur les propriétés et la réactivité du film passif a été étudié. Comprendre comment les éléments d'alliage agissent sur la nature, la structure et les propriétés du film passif fournira quelques lignes directrices aux ingénieurs et scientifiques pour la conception ou le choix des alliages les plus adaptés.

Materials 14(4), 929 (2021). doi:10.3390/ma14040929, This study investigated the structural heterogeneity, mechanical property, electrochemical behavior, and passive film characteristics of Fe���Cr���Mo���W���C���B���Y metallic glasses (MGs), which were modified through annealing at different temperatures. Results showed that annealing MGs below the glass transition temperature enhanced corrosion resistance in HCl solution owing to a highly protective passive film formed, originating from the decreased free volume and the shrinkage of the first coordination shell, which was found by pair distribution function analysis. In contrast, the enlarged first coordination shell and nanoscale crystal-like clusters were identified for MGs annealed in the supercooled liquid region, which led to a destabilized passive film and thereby deteriorated corrosion resistance. This finding reveals the crucial role of structural heterogeneity in tuning the corrosion performance of MGs., Published by MDPI, Basel

Several factors during steel manufacture and fabrication can alter the passivity and corrosion behaviour of stainless steels. These include alloying, deformation, welding and heat treatments. In this work a combination of local techniques, such as Scanning Vibrating Electrode Technique (SVET) and Scanning Kelvin Probe Force Microscopy (SKPFM), and surface and microstructural analytical techniques, such as Electron Backscatter Diffraction (EBSD), Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS) is used to elucidate the influence of process variables on the corrosion resistance of stainless steels. Different manufacturing processes, such as casting, rolling or additive manufacturing (AM) (Paper V), fabrication processes, such as welding (Paper III) and deformation (Paper I and IV), and post-processing, such as heat and high pressure treatments (Paper II and V) and post-weld cleaning (Paper II) are studied. The results showed that deformation can have a major impact on microstructure but a smaller impact on corrosion. Plastic deformation decreased irreversibly the Volta potential, whereas elastic deformation did not have any permanent effect. The potential was dependent on composition, passive film thickness and deformation but not on the crystallographic orientation. Thermal oxides formed after welding were detrimental to corrosion resistance. SVET showed anodic activity on the weld areas with oxides where pitting initiated, explained by the interplay between the composition and the thickness of the oxides. Brushing combined with pickling was recommended for restoring the corrosion resistance. Long-term ageing of a Cu-alloyed 2507 caused precipitation of Cu-rich ε particles which showed the lowest potential and were most severely pitted, plus nitrides which were the most noble. Intermetallic sigma phase observed in the AM 2507 after HIP heat treatment with slow cooling resulted in a decreased corrosion resistance. However, it was concluded that AM 2507 exhibits similar or even higher corrosion resistance than conventional 2507 if appropriate post-processing parameters are applied. This thesis provides scientific insights to facilitate the correct manufacturing, fabrication and use of stainless steels. Flera faktorer under ståltillverkning och fabrikation kan förändra passivitet och korrosionsbeständighet hos rostfritt stål. Dessa inkluderar legering, deformation, svetsning och värmebehandlingar. I detta arbete har en kombination av lokala tekniker, såsom Scanning Vibrating Electrode Technique (SVET) och Scanning Kelvin Probe Force Microscopy (SKPFM), och yt- och mikrostruktur analytiska tekniker, såsom Electron Backscatter Diffraction (EBSD), Auger Electron Spectroscopy (AES) och X-ray Photoelectron Spectroscopy (XPS) använts för att studera processvariablernas påverkan på korrosionsbeständighet hos rostfritt stål. Olika tillverkningsprocesser, såsom gjutning, valsning eller additiv tillverkning (AM) (Papper V), produktionsprocesser, såsom svetsning (Papper III) och kall deformation (Papper I och IV), och efterbehandling, såsom värme- och tryckbehandlingar (HPHT) (Papper II och V) och svetsrengöring (Papper II) har undersökts. Resultaten visade att deformation kan ha en stor inverkan på mikrostruktur men en mindre påverkan på korrosionsegenskaper. Plastisk deformation minskade Volta potentialen irreversibelt, medan elastisk deformation inte hade någon permanent effekt. Potentialen var beroende av sammansättning, passiv filmtjocklek och deformation men inte på kristallografisk riktning. Termiska oxider som bildades efter svetsning minskade korrosionsbeständigheten. SVET visade anodisk aktivitet på svetsområdena med oxider där punktangrepp initierades; detta kan förklaras genom samverkan mellan sammansättningen och oxidtjockleken. Borstning i kombination med betning rekommenderades för att återställa korrosionsbeständigheten. Långtidsvärmebehandling av ett Cu-legerat 2507 orsakade utskiljning av Cu-rika ε partiklar som visade lägst potential och korroderade snabbast, plus nitrider som var ädlaste. Intermetalliska sigmafas i AM 2507 efter HPHT med långsam kylning resulterade i minskad korrosionsbeständighet. Slutsatsen var att AM 2507 uppvisar liknande eller bättre korrosionsbeständighet än konventionella 2507 om rätt efterbearbetningsparametrar tillämpas. Denna avhandling bidrar med vetenskapliga insikter som kan främja korrekt tillverkning, fabrikation och användning av rostfritt stål. QC 2021-09-23

Ni-Co alloys are used in shielding of leading edge of helicopter blades, due to their good corrosion and wear resistance, mechanical strength and magnetic properties. These binary alloys form a solid solution with a face-centered cubic structure when the cobalt content is less than about 70 %. The study materials Ni, Ni-20Co and Ni-50Co were produced by fusion, before undergoing cold rolling deformation, and annealing at 850 °C. They are characterized by a microstructure with a grain size between 5 and 14 µm, a high fraction of Σ3 boundaries, a low stress state and an absence of texture. The observation of the microstructural evolution during annealing made it possible to determine the kinetics of recrystallization and to identify the Cube texture as a limiting factor. The electroformed, nanocrystalline industrial alloy Ni-20Co was also examined, and its grain size was increased to resemble the model material. Corrosion properties were studied in two electrolytes (30 g/L NaCl and 0.5 M H2SO4) to determine the impact of composition and microstructural parameters on corrosion resistance and passivation mechanisms. Potentiodynamic polarization curves showed the passive character of nickel rich samples, and revealed the detrimental influence of cobalt. However, a composition similar to that of the industrial alloy does not significantly degrade the corrosion performance. A high density of grain boundaries, a large fraction of twin boundaries, or a (111) orientation, are identified as beneficial microstructural parameters for corrosion resistance. A higher susceptibility to pitting, due to Cl- ions, is observed for the industrial alloy, certainly related to the electroforming process. Finally, the model materials exhibit an active-passive behaviour, typical of nickel alloys in an acidic medium. Electrochemical impedance spectroscopy and Mott-Schottky tests enable to characterize the electrochemical and semiconductor properties of the passive layers. The different results seem to converge towards a less efficient passivity linked to an increasing fraction of cobalt in solid solution., Les alliages Ni-Co sont utilisés dans l’industrie aéronautique comme revêtement de bord d’attaque de pales d’hélicoptères, en raison de leur résistance à la corrosion et à l’usure, leurs propriétés mécanique et magnétiques. Ces alliages binaires forment une solution solide de structure cubique à faces centrées lorsque le taux de cobalt est inférieur à environ 70 %. Les matériaux d’étude Ni, Ni-20Co et Ni-50Co ont été élaborés par fusion avant de subir une déformation par laminage à froid, et un recuit à 850 °C. Ils se caractérisent par une microstructure équiaxe dont la taille de grains est entre 5 et 14 µm, une forte proportion de joints Σ3, un faible état de contraintes et une absence de texture. Le suivi de l’évolution microstructurale au cours du recuit a permis de déterminer les cinétiques de recristallisation et d’identifier la texture Cube comme un facteur limitant. En parallèle, l’alliage industriel électroformé et nanocristallin Ni-20Co a été examiné, et sa taille de grains a été augmentée artificiellement pour correspondre à celle du matériau modèle. Les propriétés en corrosion ont été étudiées dans deux électrolytes (NaCl 30 g/L et H2SO4 0.5 M) pour déterminer l’impact de la composition et des paramètres microstructuraux sur la tenue en corrosion et les mécanismes de passivation. Les courbes de polarisation potentiodynamique montrent le caractère passif des échantillons riches nickel, et révèlent l’influence néfaste du cobalt. Toutefois, une composition analogue à celle de l’alliage industriel ne dégrade pas significativement les performances en corrosion. Une forte densité de joints de grains, une fraction importante de joints de macle, ou bien une texture (111), sont identifiés comme des paramètres microstructuraux bénéfiques à la résistance en corrosion. La susceptibilité à la piqûration en NaCl est plus marquée pour l’alliage industriel, certainement en lien avec le processus d’électroformage. Les matériaux modèles présentent enfin un comportement actif-passif, typique des alliages de nickel en milieu acide. Des essais de spectroscopie d’impédance électrochimique et de Mott-Schottky ont permis de caractériser les propriétés électrochimiques et semi-conductrices des couche passives. Les résultats semblent converger vers une passivité moins efficace liée à une fraction croissante de cobalt en solution solide.