Preparation of Fe2B boride coating on low-carbon steel surfaces and its evaluation of hardness and corrosion resistance

Fe₂B coating was prepared on low-carbon steel by surface alloying. A series of experiments were carried out to examine some surface properties of boride coating. The surface heat treatment of coated low-carbon steel was performed at 700 °C, 800 °C and 900 °C for 2 h, 4 h, 6 h and 8 h under hydrogen atmosphere. The boride coating was revealed by XRD analysis and the microstructure of the boride coating was analyzed by scanning electron microscopy (SEM). Depending on the temperature and time of the process, the hardness of the borided low-carbon steel ranged from 99 to 1100 HV. The hardness showed a maximum (about 1100 HV) at 900 °C for 8 h. The corrosion resistance of the borided samples was evaluated by the Tafel polarization and electrochemical impedance spectroscopy (EIS). Shift in the corrosion potential (E_corr) towards the noble direction was observed, together with decrease in the corrosion current density (I_corr), increase in the charge transfer resistance (R_ct) and decrease in the capacitance (C_c), which indicated an improvement in corrosion resistance with increasing temperature and time of the treatment.     

Evaluation of aged duplex stainless steel UNS S32750 susceptibility to intergranular corrosion by optimized double loop electrochemical potentiokinetic reactivation method

Optimized double loop electrochemical potentiokinetic reactivation (DL-EPR) in combination with microstructure observation were successfully applied to study the sensitization of super duplex stainless steel UNS S32750 aged at 900 °C for different duration to intergranular corrosion (IGC). The results indicated that the degree of sensitization increased gradually with the aging time increased from 10 min to 4 h due to the sigma phase precipitation, and I_r:I_a value reached the maximum value of 17.1%. However, further increasing aging time slightly decreased the sensitization due to the healing effect incurred by the diffusion of chromium from adjacent grains to chromium-depleted zone.     

Sulphuric acid corrosion of ultrafine-grained mild steel processed by equal-channel angular pressing

A bulk ultrafine-grained (UFG) mild steel with a ferrite grain size of approximately 200 nm and a dispersed distribution of iron carbide particles was fabricated by equal-channel angular pressing (ECAP) at 400 °C. The corrosion behaviour of the ECAP-processed mild steel and pure iron was investigated in a 0.5 mol/L H₂SO₄ solution. They exhibited a higher corrosion rate and better anodic passivity properties due to the presence of more crystalline defects. As a result of the refinement of the iron carbide particles, the forming ability of a continuous dense passive film was improved.     

Design and synthesis of a novel class of inhibitors for mild steel corrosion in acidic and carbon dioxide-saturated saline media

p-(9-(2-Methylisoxazolidin-5-yl)nonyloxy)benzaldehyde I, prepared using a cycloaddition protocol, was elaborated into its cinnamaldehyde derivative II which upon quarternization with propargyl chloride afforded III bearing an interesting blend of structural traits suitable for imparting inhibition of mild steel corrosion. Novel compounds I–III showed efficient inhibition against mild steel corrosion in CO₂–0.5 M NaCl (40 °C, 1 atm; 120 °C, 10 bar), 1, 4, 7.7 M HCl, and 0.5 M H₂SO₄ at 60 °C as determined by gravimetry and electrochemical methods. The presence of carbonaceous surface and nitrogen, as revealed by XPS study, indicated the formation of a film covering the metal surface, which imparted corrosion inhibition.     

High-temperature oxidation and hot corrosion of Cr2AlC

High-temperature oxidation and hot corrosion behaviors of Cr₂AlC were investigated at 800–1300 °C in air. Thermogravimetric–differential scanning calorimetric test revealed that the starting oxidation temperature for Cr₂AlC is about 800 °C, which is 400 °C higher than other ternary transition metal aluminum carbides. Thermogravimetric analyses demonstrated that Cr₂AlC displayed excellent high-temperature oxidation resistance with parabolic rate constants of 1.08 × 10⁻¹² and 2.96 × 10⁻⁹ kg² m⁻⁴ s⁻¹ at 800 and 1300 °C, respectively. Moreover, Cr₂AlC exhibited exceptionally good hot corrosion resistance against molten Na₂SO₄ salt. The mechanism of the excellent high-temperature corrosion resistance for Cr₂AlC can be attributed to the formation of a protective Al₂O₃-rich scale during both the high-temperature oxidation and hot corrosion processes.     

Effect of Er additions on ambient and high-temperature strength of precipitation-strengthened Al–Zr–Sc–Si alloys

The effect of substituting 0.01 at.% Er for Sc in an Al–0.06Zr–0.06Sc–0.04Si (at.%) alloy subjected to a two-stage aging treatment (4 h/300 °C and 8 h/425 °C) is assessed to determine the viability of dilute Al–Si–Zr–Sc–Er alloys for creep applications. Upon aging, coherent, 2–3 nm radius, L1₂-ordered, trialuminide precipitates are created, consisting of an Er- and Sc-enriched core and a Zr-enriched shell; Si partitions to the precipitates without preference for the core or the shell. The Er substitution significantly improves the resistance of the alloy to dislocation creep at 400 °C, increasing the threshold stress from 7 to 10 MPa. Upon further aging under an applied stress for 1045 h at 400 °C, the precipitates grow modestly to a radius of 5–10 nm, and the threshold stress increases further to 14 MPa. These chemical and size effects on the threshold stress are in qualitative agreement with the predictions of a recent model, which considers the attractive interaction force between mismatching, coherent precipitates and dislocations that climb over them. Micron-size, intra- and intergranular, blocky Al₃Er precipitates are also present, indicating that the solid solubility of Er in Al is exceeded, leading to a finer-grained microstructure, which results in diffusional creep at low stresses.     

Corrosion resistance of duplex stainless steel subjected to long-term annealing in the spinodal decomposition temperature range

The effect of thermal annealing up to 15,000 h between 300 °C and 500 °C on the corrosion resistance of the duplex stainless steel (DSS) 7MoPLUS has been investigated by using the DLEPR test. Spinodal decomposition in 7MoPLUS is unabated even after annealing for 15,000 h and no healing has been observed. The possible healing mechanisms in this temperature range (back diffusion of Cr atoms from the Cr-rich ferrite (α_Cr) and diffusion of Cr atoms from the austenite) and its absence in the present steel have been discussed.     

Mechanical and barrier properties of MOCVD processed alumina coatings on Ti6Al4V titanium alloy

This study focuses on the implementation of different aluminum oxide coatings processed by metal-organic chemical vapor deposition from aluminum tri-isopropoxide on commercial Ti6Al4V titanium alloy to improve its high temperature corrosion resistance. Films grown at 350 °C and at 480 °C are amorphous and correspond to formulas AlOOH, and Al₂O₃, respectively. Those deposited at 700 °C are composed of γ-Al₂O₃ nanocrystals dispersed in a matrix of amorphous alumina. Their mechanical properties and adhesion to the substrates were investigated by indentation, scratch and micro tensile tests. Hardness and rigidity of the films increase with increasing deposition temperature. The hardness of the coatings prepared at 350 °C and 480 °C is 5.8 ± 0.7 GPa and 10.8 ± 0.8 GPa respectively. Their Young's modulus is 92 ± 8 GPa (350 °C) and 155 ± 6 GPa (480 °C). Scratch tests cause adhesive failures of the films grown at 350 °C and 480 °C whereas cohesive failure is observed for the nanocrystalline one, grown at 700 °C. Micro tensile tests show a more progressive cracking of the latter films than on the amorphous ones. The films allow maintaining good mechanical properties after corrosion with NaCl deposit during 100 h at 450 °C. After corrosion test only the film deposited at 700 °C yields an elongation at break comparable to that of the as processed samples without corrosion. The as established processing–structure–properties relation paves the way to engineer MOCVD aluminum oxide complex coatings which meet the specifications of the high temperature corrosion protection of titanium alloys with regard to the targeted applications.     

Effect of annealing on microstructure and properties of Cu–30Ni alloy tube

The microstructure, mechanical properties and seawater corrosion resistance of annealed Cu–30Ni alloy tube were investigated using mechanical test, optical microscope, scanning electronic microscope and electrochemical measurement system, respectively. The recrystallizations gradually increased with the increase of annealing temperature and holding time. The hardness and tensile strength, which maintained invariability with annealing temperature at 680–720 °C, dramatically decreased with annealing temperature at 720–770 °C. As annealing temperature and holding time increase, corrosion potential (E_C) increased while corrosion rate (i_C) decreased at the beginning of seawater immersion. But after 15 days’ seawater immersion, as annealing temperature and holding time increase, E_C firstly increased and then decreased, on the contrary, i_C firstly decreased and then increased. The Ni-rich surface film and the Ni-rich sub-grains were responsible for the initial and extended immersion, respectively. It was found that the Cu–30Ni alloy tube annealed at 720 °C for 30 min exhibited favorable comprehensive mechanical properties and seawater corrosion resistance.     

Thermal aging of single-layer polymer light-emitting diodes composed of a carbazole and oxadiazole containing copolymer doped with singlet or triplet emitters

An electron-transporting monomer was synthesized that was structurally and energetically similar to the small molecule 2-biphenyl-4-yl-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (tBu-PBD). The monomer was copolymerized with 2-(9H-carbazol-9-yl)ethyl 2-methylacrylate and the resulting copolymer was utilized in organic light emitting devices which employed fluorescent coumarin 6 (C6) or phosphorescent tris(2-phenylpyridine)iridium(III) [Ir(ppy)₃] emitters. The copolymer devices exhibited a mean luminance of ca. 400 and 3552 cd/m² with the C6 and Ir(ppy)₃ emitters, respectively, that was stable with thermal aging at temperatures ranging from 23 °C to 130 °C. Comparable poly(9-vinyl-9H-carbazole)/tBu-PBD blend devices exhibited more pronounced variations in performance with thermal aging.     

Investigation of electrical properties of nanostructured carbon films derived from block copolymers

Electrical properties of nanostructured carbon (ns-C) films fabricated by pyrolysis of PAN–b–PBA copolymers were investigated. Films having cylindrical morphology and pyrolyzed at 400, 500 and 600 °C were investigated. Both carbide forming (Zr, Ti) and non-carbide forming (Cu, Pt) metals spanning a wide range of electron work functions (4.1–5.5 eV) formed ohmic contacts to the ns-C films in the as-deposited state. The conductivity of the ns-C films increased roughly three orders of magnitude for every 100 °C increase in the pyrolysis temperature. Hall-effect measurements showed that the films pyrolyzed at 600 °C were n-type with a majority carrier concentration and mobility of 5.8 × 10¹⁸ cm^?3 and 0.97 cm²/V s, respectively. Current–voltage measurements as a function of temperature (I–V–T) were performed on films pyrolyzed at 600 °C, whereas films pyrolyzed at 400 and 500 °C were too resistive for reliable resistivity–temperature and Hall-effect measurements. The resistivity as a function of temperature was analyzed by using the reduced activation energy method and was determined to follow variable-range hopping (VRH) mechanisms at and below room temperature. The data indicates a crossover from Efros–Shklovskii VRH [J. Phys. C 8, (1975) L49] to Mott VRH [J. Non-Cryst. Solids 1, (1968) 1] at temperatures above 100 K.     

Synthesis and hydrogen reduction of nano-sized copper tungstate powders produced by a hydrothermal method

The pure nano-sized copper tungstate (CuWO₄) powders were prepared by hydrothermal method and consequent annealing at 500 °C for 120 min. The thermogravimetric analysis was used to study dehydration processes, and the scanning electron microscopy (SEM) indicated that CuWO₄ particles were mostly spherical in the size range from 60 to 90 nm. Hydrogen reduction at 800 °C for 60 min converted the CuWO₄ to W–Cu composite powders. The hydrogen reduction results showed that nano-sized CuWO₄ particles calcining at 500 °C for 120 min indicated finer microstructure than the other calcination temperatures of 0 °C, 400 °C, 620 °C, 650 °C and 700 °C. W–Cu particles were observed finest and homogeneous in the size range from 90 to 150 nm by SEM images. Homogeneous distribution of W and Cu particles was clearly demonstrated by elemental mapping. Encapsulation of Cu phase by the W phase was observed by EDS and TEM. From FFT and HRTEM images, the orientation relationship of (01-1)_Cu ∥(01-1)_W and a semicoherent interface between W and Cu phases could be observed. A good correlation between the HRTEM image and the calculated lattice misfit (δ) was obtained.     

Determination of the corrosivity of Pb-containing salt mixtures

The composition of deposits from waste firing boilers is dominated by calcium-, alkali-chlorides and sulfates. Additionally, significant quantities of heavy metals such as Zn and Pb are usually present during waste combustion. In this high temperature corrosion study, two boiler materials, a low alloy ferritic steel (10CrMo9-10) and an austenitic stainless steel (AISI 347), were exposed to PbCl₂–ZnCl₂–KCl, PbCl₂–KCl and PbCl₂–K₂SO₄ salt mixtures as well as to the single salts of PbCl₂, K₂SO₄ and KCl. In the tests, the salt or the salt mixture was put on a pre-oxidized and cleaned steel sample and then heat treated at 400, 500 and 600 °C for 168 h (1 week) in a horizontal tube furnace. After the test, the metallographic cross-sections of the specimens were characterized with a SEM/EDX analyzer and the thickness of the corrosion layer was determined for each sample. The corrosion layer thickness measurements showed severe corrosion on 10CrMo9-10 already at 400 °C when 5 wt.% PbCl₂ was mixed with KCl or K₂SO₄ With all the PbCl₂ containing salt mixtures at temperatures around and above 500 °C a significant corrosion was observed on both materials. In order to estimate the relative corrosivity of PbCl₂ containing salt mixtures, the results from the laboratory experiments in this work were compared to an earlier study done with a corresponding ZnCl₂ containing salt mixture.     

Development of high DC-bias Mn–Zn ferrite working at frequency higher than 3 MHz

Modern high-frequency electronic technology demands Mn–Zn soft ferrite for high DC-bias and low power loss applications. In this study, DMR50B ferrite material with a very attractive DC-bias property and with a lower power loss at high frequency up to 3 MHz was developed employing a conventional ceramic powder processing technique based on our previous study of DMR50 material, indicating its magnetic properties can be further improved by microstructure homogeneity. The core loss is around 200 kW/m³ at 3 MHz, 10 mT and 100 °C, and only around 20 kW/m³ at 700 kHz, 30 mT and 100 °C; its cutoff frequency f_r is ～4 MHz and its incremental permeability μ_Δ remains constant until H_DC = 100 A/m. Furthermore, the electromagnetic characteristics and the microstructure of this new DMR50B material are also discussed.     

Thermal stability of diamond-like carbon films with added silicon

Diamond-like carbon (DLC) films with added silicon content from 0 to 19.2 at.% were deposited using r.f. PECVD (radio frequency plasma enhanced chemical vapor deposition). Fourier transform IR (FTIR) spectrometry, Raman spectrometry and X-ray photoelectron spectrometry (XPS) were used to determine the structural change of the annealed DLC films in ambient air. By increasing the annealing temperature the CH_n and Si–H groups in the FTIR spectra decrease because of hydrogen evolution, whereas the intensities of CO and Si–O peaks increase owing to oxidation. From Raman spectra, the integrated intensity ratio I_D/I_G of the pure DLC films and the silicon-doped films increases at 300 and 400 °C, respectively, whereas the observable shoulder of the D band occurs at 400 and 500 °C, respectively, which indicates that the addition of silicon improves the thermal stability of DLC films. Using XPS analysis, a surface reaction for the annealed films is investigated.     

Hot corrosion properties of composite coatings in the presence of NaCl at 700 and 900 °C

Cyclic hot corrosion tests have been carried out on three coatings (one NiCoCrAlY and two composite coatings) at 700 and 900 °C. The kinetic curves and evolution of microstructure show that the composite coating with a Cr-base interlayer performs best. The Cr₂O₃ scale is more effective to protect the coating at 700 °C than that at 900 °C. The corrosion process is accelerated by NaCl via forming volatile MCl_x and inducing the formation of molten voids in the coating or extra oxidation at the interface of fusant/oxide scale, determined by the temperature and the compositions of the coating.     

A transition from local equilibrium to paraequilibrium kinetics for ferrite growth in Fe–C–Mn: A possible role of interfacial segregation

Ferrite growth kinetics was measured under highly controlled decarburization conditions in an Fe–0.94%Mn–0.57%C (mass%) alloy. The observed growth kinetics closely followed the predictions of the local equilibrium non-partitioning (LE-NP) model over the temperature range 725–775 °C. A transition from LE-NP to paraequilibrium (PE) kinetics was observed, for the first time, as the temperature was increased from 775 °C to 825 °C. Long-lived parabolic kinetic states that are intermediate between LE-NP and PE were observed at these intermediate temperatures. Above 825 °C, ferrite growth appeared to follow the predictions of the PE model up to the T₀ temperature at which ferrite growth stopped. These novel results are attributed to the segregation of Mn to the moving interface. It is thought that this process has the effect of decreasing the Mn concentration on the austenite side of the interface and, consequently, extending the PE state.     

Low-temperature transformation kinetics of electron-beam deposited 5 wt.% yttria-stabilized zirconia

The transformation kinetics of ZrO₂ coatings stabilized with 5.6 mol% YO_1.5 (5YSZ), and deposited by electron-beam physical vapor deposition, were studied between room temperature and 600 °C using in situ Raman spectroscopy, and are described in the form of a transformation-time–temperature diagram. The coatings were found to be monoclinic (m) at temperatures below 375 °C, while above 400 °C they transformed to the tetragonal (t) phase. On cooling, the coatings transformed back to monoclinic below ∼375 °C. Between 375 and 400 °C, the transformation rates approached zero, indicating that the thermodynamic driving force for the transformation also approaches zero in this temperature range. This provides a direct measurement of the T₀(m–t) temperature for the 5YSZ composition.     

Corrosion and corrosion fatigue of AISI 420C (X46Cr13) at 60 °C in CO2-saturated artificial geothermal brine

The lifetime reduction of cyclically loaded AISI 420C (X46Cr13, 1.4034) constantly exposed to highly corrosive CO₂-saturated hot thermal water is demonstrated in in situ-laboratory experiments (60 °C, brine: Stuttgart Aquifer, flowing CO₂: 30 L/h). S–N plots, micrographic-, phase-, fractographic- and surface analysis were applied to obtain sustainable information on the corrosion and corrosion fatigue behavior. Maximum number of cycles (here 12.5 × 10⁶ cycles to failure) is reached at σ_a = 173 MPa. Hydroxide and siderite layers were found on pits and crack surfaces. No typical fatigue limit exists and pit corrosion prior to crack initiation may be identified as failure cause.