Plasma anodized ZE41 magnesium alloy sealed with hybrid epoxy-silane coating

Anodic coatings on magnesium ZE41 alloy were formed by DC plasma electrolytic oxidation (PEO) in spark regime in solution composed of NaOH, Na₂SiO₃ and KF. The positive effect of poly(ethylene oxide) addition into the anodizing electrolyte on PEO process, anodic film porosity and its protective performance was described. Anodic films were sealed with hybrid epoxy-silane formulation. The corrosion behavior of the coated ZE41 was studied through electrochemical impedance spectroscopy (EIS) in 0.6 M NaCl solution. Resulting duplex PEO/epoxy-silane coating provides good protective performance without significant signs of corrosion during 1 month of immersion test.     

Orientation dependence of the electrochemical corrosion properties of electrodeposited Cu foils

In this study, the electrochemical corrosion properties of electrodeposited Cu foils in a CuCl₂-containing acidic etching solution were investigated. The main passive product was CuCl and a trace amount of Cu₂O can also be detected. The (2 2 0)-oriented Cu foils exhibited higher corrosion potential and lower corrosion current density than those with (1 1 1) or (2 0 0) texture, suggesting a superior corrosion resistance against the etching solution. It is proposed that the preferred orientation and thus the differences in atomic stacking density on specific planes dominated the corrosion properties of the electrodeposited Cu foils instead of grain size or surface roughness.     

Comparison of electrochemical corrosion behaviour of MgO and ZrO2 coatings on AM50 magnesium alloy formed by plasma electrolytic oxidation

Two types of PEO coatings were produced on AM50 magnesium alloy using pulsed DC plasma electrolytic oxidation process in an alkaline phosphate and acidic fluozirconate electrolytes, respectively. The phase composition and microstructure of these PEO coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion behaviour of the coated samples was evaluated by open circuit potential (OCP) measurements, potentiodynamic polarization tests, and electrochemical impedance spectroscopy (EIS) in neutral 0.1 M NaCl solution. The results showed that PEO coating prepared from alkaline phosphate electrolyte consisted of only MgO and on the other hand the one formed in acidic fluozirconate solution was mainly composed of ZrO₂, MgF₂. Electrochemical corrosion tests indicated that the phase composition of PEO coating has a significant effect on the deterioration process of coated magnesium alloy in this corrosive environment. The PEO coating that was composed of only MgO suffered from localized corrosion in the 50 h exposure studies, whereas the PEO coating with ZrO₂ compounds showed a much superior stability during the corrosion tests and provided an efficient corrosion protection. The results showed that the preparation of PEO coating with higher chemical stability compounds offers an opportunity to produce layers that could provide better corrosion protection to magnesium alloys.     

Novel thermal protection coating based on Zr0.75Ce0.25O2/phosphate duplex system for polyimide matrix composites fabricated via a combined sol–gel/sealing treatment process

Thermal protection coating based on Zr_0.75Ce_0.25O₂/phosphate system was fabricated on polymer–matrix composites via a combined sol–gel/sealing treatment process. Phosphates sealed the cracks and enhanced the adhesion property via chemical bonding and binding. The Zr_0.75Ce_0.25O₂/phosphate duplex coating exhibited good thermal shock resistance and improved thermal oxidation resistance of the substrate. Due to the protection of the duplex coating, the weight loss of the specimen reduced from (4.83 ± 0.12)% to (0.98 ± 0.08)% and the mass ablation rate decreased from 0.088 ± 0.002 mg cm⁻² s⁻¹ to 0.018 ± 0.002 mg cm⁻² s⁻¹ when testing at 810 °C. Coating failure was attributed to the formation of cracks and delamination.     

Microstructure and corrosion resistance of Cr/Cr2N multilayer film deposited on the surface of depleted uranium

Depleted uranium is widely used in national defence and nuclear energy fields. However, the inferior corrosion resistance limits its application. A Cr/Cr₂N film was prepared by magnetron sputtering on the uranium to improve its corrosion resistance. The Cr/Cr₂N film exhibits modulation structure. The introduction of the Cr/Cr₂N increases the corrosion potential; the corresponding current density decreases about three orders of magnitude. After polarization corrosion, the surface morphology of the Cr/Cr₂N-coated on uranium keeps integrated. Only a thin layer of film (∼40 nm) is oxidized. The Cr/Cr₂N film shows great potential in improving oxidation and corrosion resistance of depleted uranium.     

A method for investigation of hot corrosion by gaseous Na2SO4

A novel in situ exposure method for investigating hot corrosion of aluminde coatings in the absence of an alkali sulphate melt is presented. The samples are exposed to Na₂SO₄(g) at a temperature above the dew point of the gas. Results from 100 and 500 h exposures at 900 °C of Ni-based In792 covered with SIF232 aluminide coating are presented. Already after 100 h of testing, attack characteristic for Type I hot corrosion is observed. The reactions taking place during the testing are discussed and the results are compared with those from the well-known ex situ salt hot corrosion test.     

An investigation of the corrosion mechanisms of WE43 Mg alloy in a modified simulated body fluid solution: The influence of immersion time

The corrosion mechanisms and kinetics of WE43 Mg alloy in a modified simulated body fluid (m-SBF) are investigated by electrochemical, hydrogen evolution and analytical techniques. The changes in the impedance response over time are related to four corrosion stages involving the formation of a partially protective corrosion layer and adsorption of Mg intermediates, formation of an inner passive MgO layer with increasing coverage over time, rupture of the corrosion layer and lateral growth of stable pits. ATR-FTIR, XRD and XPS results show the presence of an amorphous carbonated apatite/Mg(OH)₂ mixed corrosion layer.     

Ablation resistance of SiC–ZrC coating prepared by a simple two-step method on carbon fiber reinforced composites

SiC–ZrC ablation resistance coating was prepared on the surface of carbon fiber reinforced carbon (C/C) composites by simple pack cementation combined with low-cost slurry infiltration method. The results showed that SiC–ZrC coating could effectively protect C/C composites from ablation for 45 s at 3723 K under oxyacetylene torch. The mass and linear ablation rates (0.038 ± 0.01 mg/(s cm²) and 2.42 ± 0.15 μm/s) were largely reduced compared with that of uncoated C/C composites (0.530 ± 0.01 mg/(s cm²) and 1.75 ± 0.15 μm/s) after ablation for 20 s. The good ablation protective ability of SiC–ZrC coating is mainly attributed to the volatilization of SiO₂ and the formation of ZrO₂.     

Effect of heat flux on ablation behaviour and mechanism of C/C–ZrB2–SiC composite under oxyacetylene torch flame

The ablation property of C/C–ZrB₂–SiC composite under oxyacetylene torch flame with different heat flux was investigated. The ablation performance decreased with the increase of heat flux from 2400 to 4200 kW/m² and the ablation mechanism changed from chemical erosion to mechanical denudation. The good ablation resistance under 2400 kW/m² is attributed to the formation of SiO₂–ZrO₂ mixture. When heat flux is 3200 kW/m², the formation of ZrO₂ can partly prevent the diffusion of oxygen and provide ablation protection. But with heat flux increasing, the increase of mechanical denudation results in the reduction of ablation performance.     

Corrosion protection of AZ91 magnesium alloy by anodizing in niobium and zirconium-containing electrolytes

A new Nb + Zr-based anodized coating was designed for the corrosion protection of AZ91 magnesium alloy. Polarization curves and electrochemical impedance diagrams plotted in Na₂SO₄ electrolyte showed its high protective effect. Analysis of the chemical composition by X-ray photoelectron spectroscopy indicated that the coating mainly consisted of (i) magnesium metaborate and metaphosphate, (ii) MgF₂ and ZrF₄, and (iii) Nb₂O₅, ZrO₂ and MgO. A higher concentration of fluorine at both interfaces and an enrichment in Zr compared to Nb were revealed by SEM and EDS analyses. Thus, Zr-based compounds and MgF₂ play a key role in the anti-corrosion ability of the coating.     

Corrosion of ultra-high-purity Mg in 3.5% NaCl solution saturated with Mg(OH)2

Corrosion was evaluated for ultra-high-purity magnesium (Mg) immersed in 3.5% NaCl solution saturated with Mg(OH)₂. The intrinsic corrosion rate measured with weight loss, P_W = 0.25 ± 0.07 mm y⁻¹, was slightly smaller than that for high-purity Mg. Some specimens had somewhat higher corrosion rates attributed to localised corrosion. The average corrosion rate measured from hydrogen evolution, P_AH, was lower than that measured with weight loss, P_W, attributed to dissolution of some hydrogen in the Mg specimen. The amount of dissolution under electrochemical control was a small amount of the total dissolution. A new hydride dissolution mechanism is suggested.     

Glass coatings on stainless steels for high-temperature oxidation protection: Mechanisms

The oxidation behavior of a martensitic stainless steel with or without glass coating was investigated at 600–800 °C. The glass coating provided effective protection for the stainless steel against high-temperature oxidation. However, it follows different protection mechanisms depending on oxidation temperature. At 800 °C, glass coating acts as a barrier for oxygen diffusion, and oxidation of the glass coated steel follows linear law. At 700 or 600 °C, glass coating induces the formation of a (Cr, Fe)₂O₃/glass composite interlayer, through which the diffusion of Cr³⁺ or Fe³⁺ is dramatically limited. Oxidation follows parabolic law.     

Effect of Y2O3 content in the pack on microstructure and hot corrosion resistance of Y–Co-modified aluminide coating

Y–Co-modified aluminide coatings on nickel base superalloys were prepared by pack cementation method. Effect of Y₂O₃ content in the pack mixture on microstructure and hot corrosion resistance of the coatings was investigated. The results show that with the increase in Y₂O₃ content, the content of Co in the coatings increases. The mass gain of the coatings with Y₂O₃ addition of 1, 2 and 3 wt.% is 0.6, 0.55 and 0.42 mg/cm² after hot corrosion at 1173 K for 100 h, respectively. Y₂O₃ addition accelerates the diffusion of Co and thus increases the hot corrosion resistance of the coating.     

Synthesis of NiCrAl/MgO–ZrO2 cermet powders by chemical method for functionally graded coatings

Functionally graded materials (FGMs) are receiving great attention as they provide optimum thermal and mechanical properties without a discrete interface between two materials. In order to control the chemical composition and microstructure of FGMs, NiCrAl/MgO–ZrO₂ cermet powders were successfully developed in the present work. The NiCrAl/MgO–ZrO₂ powders were synthesized from a solution of NiCrAl, ZrO₂ and Mg hydroxide carbonate precursors using chemical synthesis. The powders were dried at 125 °C for 3 h and then pellet samples were sintered at 1381 °C for 30 min under N₂–5%H₂ atmosphere. The powders were characterized by scanning electron microscope, energy dispersive spectroscopy, X-ray mapping and X-ray diffraction. The obtained results showed that the optimum particle sizes of NiCrAl/MgO–ZrO₂ powders were between 45 and 90 μm. Microstructural studies have shown a uniform mixing in the cermet powders. It was also found that ZrO₂, MgO, MgZr₇O₁₄, Ni, Cr and Ni₅Al₃ phases were present in the cermet powder.     

An investigation on hot corrosion behavior of YSZ-Ta2O5 in Na2SO4 + V2O5 salt at 1100 °C

This study compares the hot corrosion performance of yttria stabilized zirconia (YSZ), and YSZ-Ta₂O₅ (TaYSZ) composite samples in the presence of molten mixture of Na₂SO₄ + V₂O₅ at 1100 °C. For YSZ, the reaction between NaVO₃ and Y₂O₃ produces YVO₄ and leads to the transformation of tetragonal ZrO₂ to monoclinic ZrO₂. For TaYSZ, minor amounts of NaTaO₃, TaVO₅ and Ta₉VO₂₅ are formed as the hot corrosion products with only traceable amounts of YVO₄. Due to the synergic effect of doping of zirconia with both Y₂O₃ and Ta₂O₅, the TaYSZ sample has a much better hot corrosion resistance than YSZ.     

Corrosion and hydrogen absorption of commercially pure zirconium in acid fluoride solutions

The corrosion and hydrogen absorption of commercially pure zirconium have been investigated in acidulated phosphate fluoride (APF) solutions. Upon immersion in 2.0% APF solution of pH 5.0 at 25 °C, a granular corrosion product (Na₃ZrF₇) deposits over the entire side surface of the specimen, thereby inhibiting further corrosion. In 0.2% APF solution, marked corrosion is observed from the early stage of immersion; no deposition of the corrosion product is observed by scanning electron microscopy. A substantial amount of hydrogen absorption is confirmed in both APF solutions by hydrogen thermal desorption analysis. The amount of absorbed hydrogen of the specimen immersed in the 2.0% APF solution is smaller than that in the 0.2% APF solution in the early stage of immersion. The hydrogen absorption behavior is not always consistent with the corrosion behavior. Hydrogen thermal desorption occurs in the temperature range of 300–700 °C for the specimen without the corrosion product. Under the same immersion conditions, the amount of absorbed hydrogen in commercially pure zirconium is smaller than that in commercially pure titanium as reported previously. The present results suggest that commercially pure zirconium, compared with commercially pure titanium, is highly resistant to hydrogen absorption, although corrosion occurs in fluoride solutions.     

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.     

Estimation of residual corrosion rates of steel under cathodic protection in soils via voltammetry

Steel coupons were buried in soil for 2 months under cathodic protection. Their residual corrosion rates were deduced from voltammetry and weight loss measurements. In aerated soils, the current density due to O₂ reduction, j_K,O2, was modelled with a mixed activation–diffusion controlled kinetic. The anodic part j_A of the current density j, computed as j_A = j − j_K,O2, obeyed Tafel law. Its extrapolation to the protection potential gave a corrosion rate (∼7 μm yr⁻¹) consistent with that obtained from weight loss measurements. With a deficient protection, corrosion rates remained at ∼80 μm yr⁻¹, a value given by both methods.     

Surface characterization of oxides grown on the Ti–13Nb–13Zr alloy and their corrosion protection

Oxide films were formed on the biocompatible alloy Ti–13Nb–13Zr in a phosphate buffer at open-circuit potential (E_oc), potentiodynamically up to 8 V, or by micro-arc oxidation (MAO) at 300 V. Their electrochemical properties were assessed in a phosphate buffer saline solution (PBS). EIS and SEM results showed that the E_oc and potentiodynamically formed oxide films were compact and behave as a monolayer, while the MAO oxide was a bilayered film (compact inner and porous outer layers). Open-circuit potential and EIS resistance values indicated that the MAO oxide provides the best corrosion protection for the alloy in PBS.     

Validated numerical modeling of galvanic corrosion of zinc and aluminum coatings

A time-dependent finite element model was developed to simulate the corrosion of zinc and aluminum coatings, galvanically coupled to a mild steel substrate in deaerated 0.01 M H₂SO₄ electrolyte. The simulations of galvanic corrosion for each of the coatings were compared to experimental measurements of open circuit potential, and changes in coating geometry measured via surface profilometry. Good agreement between the model predictions and corrosion tests were observed initially for both coatings. However, in the case of the zinc coating, divergence was observed between the simulation and the corrosion test after approximately 40 min, due to a decrease in the reactivity of the zinc surface.