Study of structure and corrosion resistance of Fe-based amorphous coatings prepared by HVAF and HVOF

Fe-based amorphous coatings with a composition of Fe_49.7Cr₁₈Mn_1.9Mo_7.4W_1.6B_15.2C_3.8Si_2.4 have been prepared on a mild steel substrate by High velocity air fuel (HVAF) and High velocity oxygen fuel (HVOF) processes. The microstructure and corrosion resistance in 3.5 wt.% NaCl solution of the coatings prepared by the two processes were comparatively studied. It was found that the two coatings exhibit dense structure with the porosity of 0.4% and compact bonding with the substrate. However, HVOF coating contains higher oxygen content than HVAF coating, resulting from the formation of significant oxide contours between the partially melted particles in HVOF process. Electrochemical polarization tests and electrochemical impedance spectroscopy (EIS) analysis indicate that the HVAF coating has better corrosion resistance than the HVOF coating. The preferential corrosion along the oxide contours thus providing efficient diffusion channels for electrolyte accounts for the poor corrosion resistance in HVOF coating. The present results demonstrate that HVAF with less cost can be a promising spray process to fabricate the Fe-based amorphous coating for industrial applications.     

Corrosion characteristics of polyaniline-coated 316L stainless steel in sulphuric acid containing fluoride

The corrosion resistance of conducting polyaniline (PANi) coatings deposited on 316L stainless steel (316L SS) at various cycle numbers of cyclic voltammetry (2-, 3- and 4-cycles) by electro-polymerization in sulphuric acid solution containing fluoride was investigated by electrochemical techniques. The corrosion resistance of the 316L SS substrate was considerably improved by the PANi coating. The increase of the cycle number of cyclic voltammetry increased the thickness and enhanced the performance of the PANi coating due to low porosity.     

A study on mechanism of corrosion protection of polyaniline coating and its failure

Polyaniline (PANI) coatings were electrochemically deposited on substrates of stainless steel and platinum in solutions of 0.2 M H₂SO₄ and 0.1 M aniline by cyclic voltammetry. The corrosion protection of the PANI coatings and their failure were investigated in 0.2 M H₂SO₄ solution. It was observed that the corrosion protection ability of the coating to steel substrate was increased with the increase of the coating thickness. The corrosion protection ability was mainly attributed to the passivating effect of PANI due to its oxidizing ability in its emeraldine state. During its operation, the PANI coating in emeraldine state tended to gradually lose its corrosion protection ability. This gradual failure of the PANI coating, but faster than expected, was confirmed to be related to a gradual reduction of the emeraldine PANI and a gradually increased resistance between the PANI coating and the stainless steel substrate. These findings lead to a new mechanism for the corrosion protection of PANI coating and its failure.     

Molten salt synthesis of perovskite conversion coatings: A novel approach for corrosion protection of stainless steels in molten carbonate fuel cells

A novel conversion coating process has been developed to meet the stability requirements of stainless steel hardware in the demanding MCFC fuel cell environments. The process applies a perovskite-based coating by exploiting spontaneous oxidizing reactions of the metallic surface with La₂O₃ in eutectic alkali carbonate mixtures. By using well controlled synthesis procedures, conversion coating layers covering the entire metallic surface with a uniform and compact structure could be obtained. The as-formed coatings with a surface morphology of agglomerated crystallite particles consisted of a thin (<5 μm) LaFeO₃ perovskite layer grown over a thicker (>5 μm) LiFeO₂-rich layer. Test coupons of 316L stainless steel with the perovskite conversion coating were analyzed for corrosion protection and interfacial resistivity properties. It was found that the conversion coating is highly conductive while showing excellent long-term corrosion stability in simulated MCFC environments. These results suggested that perovskite coatings formed by molten salt conversion reactions could be particularly attractive to confer optimal protection and electrical continuity to MCFC current collectors.     

Electrodeposition of chemically and mechanically protective Al-coatings on AZ91D Mg alloy

Electrodeposition of aluminium coatings upon AZ91D in aluminum chloride/1-ethyl-3-methylimidazolium chloride ionic liquid was achieved. Post-plating heat treatment processes compatible with AZ91D solution treatment (420 °C) and aging treatment (200 °C) were explored to improve coatings adhesion and hardness, and to maintain corrosion resistance. 420 °C treatment produced a β-phase (Mg₁₇Al₁₂) enriched two-phase coating; whilst treatment at 200 °C leads to a tri-layer structure, rich in γ-phase (Mg₂Al₃). The 200 °C treatment was shown to be most effective for corrosion resistance, eradicating water reduction as the principal cathodic reaction and increasing surface hardness.     

The effect of heat treatment temperature on the structure and barrier performance of a zirconia coating electrodeposited by pulse current

Zirconia coatings were prepared by heat-treating the electrodeposited zirconium hydroxide produced by pulse current on 316L stainless steel. The results showed that a coating with amorphous structure obtained after heat treating at 200 °C, reveals no corrosion barrier performance. Heat treating at 400 °C resulted in zirconia coating with nanocrystalline/amorphous structure, which reveals a strong corrosion barrier performance. This coating shows a high value of pore resistance and Warburg behaviour in electrochemical impedance spectroscopy. As the temperature was raised to 600 °C, an entirely nanocrystalline structure was found. However, this coating revealed muddy shape cracks, with inferior corrosion barrier performance.     

Hot corrosion of a novel (Ni,Co)O/(Ni,Co)Fe2O4 composite coating thermally converted from an electrodeposited Ni–Co–Fe2O3 composite coating

Ni–Co–Fe₂O₃ composite coatings were successfully developed by sediment co-deposition. In order to improve their hot corrosion resistance, a pre-oxidation treatment was conducted at 1000 °C for 6 h. The corrosion behaviour of the oxidised composite coating was investigated at 960 °C in an atmosphere consisting of a mixture of Na₃AlF₆–AlF₃–CaF molten salts and air. They exhibited good hot corrosion resistance due to not only the pre-formed oxide scale with (Ni,Co)O and (Ni,Co)Fe₂O₄ phases after pre-oxidation, but also the formation of (Ni,Co,Fe)Al₂O₄ phases in the outer layer and a well-distributed NiFe₂O₄-enriched phase along the grain boundaries in the subscale area during the corrosion process.     

Stability of electroplated titanium diboride coatings in high-temperature corrosive media

The corrosion behaviour of molybdenum and steel materials, protected by electrochemically plated TiB₂ coatings, in contact with liquid aluminium alloys and liquid glass in air has been studied. The corrosion performance tests followed by characterization of the treated samples by optical microscopy, SEM, elemental EDX analysis, have shown high corrosion resistance of the coatings to the liquid metal. However, the coating was not stable in contact with molten glass in an oxidative atmosphere at temperatures higher as 750-800 °C. Dissolution of the corrosion products in the melt facilitates the destruction of the coating.     

Development of anodic coatings on aluminium under sparking conditions in silicate electrolyte

Spark anodizing of aluminium at 5 A dm⁻² in sodium metasilicate/potassium hydroxide electrolytes is studied, with particular emphasis on the mechanism of coating growth, using transmission electron microscopy and surface analytical techniques, with coatings typically 10 μm, or more, thick. Two-layered coatings develop by deposition of an outer layer based on amorphous silica, associated with low levels of alkali-metal species, at the coating surface and growth of an inner, mainly alumina-based, layer, with an amorphous region next to the metal/coating interface. Formation of crystalline phases in the inner layer, mainly γ-Al₂O₃, with some α-Al₂O₃ and occasional δ-Al₂O, is assisted by local heating, and possibly also by ionic migration processes, arising from the rapid coating growth at sites of breakdown. Due to local access of electrolyte species in channels created by breakdown events, the silicon content in the inner coating regions varies widely, ranging from negligible levels to about 10 at.%. Silica deposition at the coating surface and formation of Al₂SiO₅ and Al₆Si₂O₁₃ phases is promoted by increased time of anodizing and concentration of metasilicate in the electrolyte. However, at sufficiently high concentration of metasilicate and pH, when more extreme voltage fluctuations accompany breakdown, the two-layered nature of coatings is replaced by a mixture of aluminium-rich and silicon-rich regions throughout the coating thickness.     

A comparative H2S corrosion study of 304L and 316L stainless steels in acidic media

H₂S corrosion of 304L and 316L in oxygen-free Na₂SO₄ + Na₂S solution at pH 3 and temperature of 60 °C were investigated by EIS, potentiodynamic polarisation, multi-component Pourbaix diagrams and microstructure characterization. At similar conditions, lower corrosion rate was observed on 316L, attributed to its denser (1.5 times) and smoother (6%) surface layer and confirmed by SEM micrograph. During polarisation, H₂S increases significantly the critical current density on 304L and passivation current density, i_p, on 316L. Higher i_p on 316L was associated to simultaneous FeS₂–MoS₂ preservation, confirmed by XRD examination. H₂S could have an inhibiting effect on 304L in passivity region.     

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.     

Corrosion of zinc–magnesium coatings: Mechanism of paint delamination

Due to their promising corrosion properties, metallic coatings containing magnesium are currently widely investigated for use as protective coatings for steel sheet. Particularly, alloying zinc coatings with magnesium results in a remarkable improvement of the corrosion resistance of the painted system. While some aspects of this improvement have been understood, the progress of the corrosive degradation of the alloy coating/paint interface has not been reported in detail. In this paper, the delamination of a model polymer from the intermetallic MgZn₂ is described and a degradation mechanism proposed. Aspects for the design of stable interfaces are discussed.     

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.     

Corrosion behaviour of thermal sprayed nitinol coatings

NiTi alloy is here investigated as an alternative coating to stainless steel since it is considered to possess good corrosion properties. Three different thermal spray techniques (high velocity oxy-fuel -HVOF-, vacuum plasma spray -VPS- and atmospheric plasma spray quenching -APS+Q-) have been used for building the coatings, and electrochemical tests have been carried out for corrosion evaluation. Open-circuit tests have revealed that the VPS-coating shows fairly good corrosion resistance, both in the as-sprayed and polished forms. The HVOF coatings however, showed a strong dependence on the surface conditions and APS+Q is dominated by electrolyte penetration through coating cracks, thus exhibiting a higher i_corr.     

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.     

电解液成分对等离子体电解氧化TC4合金腐蚀行为和摩擦学性能的影响（英文）

在TC4合金表面制备4种典型等离子体电解氧化(PEO)涂层,研究电解质组成对PEO涂层腐蚀行为和摩擦学性能的影响。结果表明,PEO涂层的腐蚀行为和摩擦学性能与电解质成分密切相关。在含NaH₂PO₂的电解液中制备的PEO涂层由于内氧化膜较致密而具有最好的耐蚀性能,而在含NaAlO₂的电解液中制备的PEO涂层由于含有Al₂O₃而具有最好的摩擦学性能。为制备具有良好耐蚀性和耐磨性的PEO涂层,以NaH₂PO₂和NaAlO₂为电解液主要成分制备了复合PEO涂层。     

Hot corrosion behaviour of plasma sprayed YSZ/LaMgAl11O19 composite coatings in molten sulfate–vanadate salt

Hot corrosion studies of thermal barrier coatings (TBCs) with different YSZ/LaMgAl₁₁O₁₉ (LaMA) composite coating top coats were conducted in 50 wt.% Na₂SO₄ + 50 wt.% V₂O₅ molten salt at 950 °C for 60 h. Results indicate that TBCs with composite coating top coats exhibit superior oxidation and hot corrosion resistances to the TBC with the traditional YSZ top coat, especially for which has a LaMA overlay. The presence of LaMA can effectively restrain the destabilization of YSZ at the expense of its own partial degradation. The hot corrosion mechanism of LaMA coating and the composite coatings have been explored.     

Sintering atmosphere and temperature effects on hydroxyapatite coated type 316L stainless steel

Electrophoretically deposited hydroxyapatite (HAP) coatings on type 316L SS was developed at the optimum coating parameters of 60 V and 3 min. Sintering of the coating enhances the metal-ceramic bond strength, but HAP structure is sensitive to temperature as it decomposes to other calcium phosphate phases. Sintering of HAP coatings in air at 900 °C for 1 h indicate the formation of a composite surface containing oxides of the alloy and decomposition products of HAP, mainly tricalcium phosphate. Open circuit potential-time measurements, potentiodynamic cyclic polarisation and electrochemical impedance experiments performed in Ringer’s solution indicate that the corrosion performance of HAP coatings were severely affected by the sintering atmosphere and temperature. Higher capacitance and low polarisation resistance values obtained from electrochemical impedance spectroscopic studies further indicate that the coatings are more prone to dissolution on comparison with the pristine type 316L SS. The sintering of the coatings in vacuum at 600, 800 and 900 °C for 1 h did not alter the phase purity of the coatings, and shifted the electrochemical parameters towards noble direction. Sintering of the coatings in vacuum lead to the formation of an adherent, stoichiometric HAP coating with enhanced corrosion resistance.     

Electrochemical corrosion behaviour of microcrystalline aluminium in acidic solutions

The electrochemical corrosion behaviour of microcrystalline pure aluminium coating, fabricated by a magnetron sputtering technique, has been investigated in both 0.5 mol/l NaCl and 0.5 mol/l Na₂SO₄ acidic (pH = 2) aqueous solutions. The corrosion resistance of the microcrystalline Al coating has deteriorated more compared with that of the cast pure Al in Na₂SO₄ acidic solution. However, its oxide film has a higher pitting resistance in the NaCl acidic solution. Chloride ions play a big role in the formation of the oxide film on the microcrystalline Al coating. The higher pitting resistance was attributed to the more acidic isoelectric point which the oxide film achieved.     

Corrosion behaviour of AIP NiCoCrAlYSiB coating in salt spray tests

NiCoCrAlYSiB coatings were deposited by arc ion plating (AIP) and annealed/pre-oxidised under various conditions. The corrosion behaviour of as-deposited and annealed/pre-oxidised coatings was studied by salt spray testing in a neutral mist of 5 wt% NaCl at 35 °C for 200 h. The results showed that the as-deposited NiCoCrAlYSiB coating behaved poorly while the annealed and pre-oxidised ones performed much better in salt spray tests. The dense microstructure in annealed coatings and formation of α-Al₂O₃ scales on the surface during pre-oxidation improved the corrosion resistance in salt spray test. The corrosion process was investigated from the aspects of corrosion products, and its electrochemical mechanism was proposed as well.