Influence of salt spray corrosion on subsequent isothermal oxidation behaviours of AIP NiCoCrAlYSiB coatings

The influence of salt spray corrosion on subsequent oxidation behaviours of arc ion plated (AIP) NiCoCrAlYSiB coatings is investigated in this work. The results indicate impact of the salt spray corrosion on successive oxidation behaviour depends greatly on the degree of the previous corrosion attack. Accelerated oxidation kinetics and unsatisfactory oxide morphologies are observed on the as-deposited and as-annealed NiCoCrAlYSiB coatings that suffered serious salt spray attacks. Pre-oxidised coatings prove excellent performances during both salt spray tests and the subsequent oxidations. The mechanism of the corrosion attack influencing the isothermal oxidation behaviour is discussed.     

The effect of post-treatment time and temperature on cerium-based conversion coatings on Al 2024-T3

Corrosion performance, morphology, and electrochemical characteristics of cerium-based conversion coatings on Al 2024-T3 were examined as a function of phosphate post-treatment time and temperature. Corrosion resistance improved after post-treatment in 2.5 wt.% NH₄H₂PO₄ for times up to 10 min or temperatures up to 85 °C. Electrochemical impedance spectroscopy and polarization testing correlated to neutral salt spray corrosion performance. Hydrated cerium oxide and peroxide species present in the as-deposited coatings were transformed to CePO₄·H₂O for post-treatments at longer times and/or higher temperatures. Based on these results, processes active during post-treatment are kinetically dependent and strongly influenced by the post-treatment time and temperature.     

Preparation and hot corrosion behaviour of an Al-gradient NiCoCrAlYSiB coating on a Ni-base superalloy

A gradient NiCoCrAlYSiB coating was prepared on a Ni-base superalloy using arc ion plating (AIP) and subsequent gaseous phase aluminisation techniques. Hot corrosion of normal NiCoCrAlYSiB and the gradient coating in pure Na₂SO₄ and Na₂SO₄/NaCl (75:25, wt./wt.) salts was performed at 900 °C in static air. The corrosion results indicated an enhanced corrosion resistance to both salts for the gradient NiCoCrAlYSiB coating, which the improved performance of it should be attributed to the β aluminide ‘‘pool” at the surface layer. By partially sacrificing Al₂O₃ (i.e. Al), the gradient NiCoCrAlYSiB coating specimen behaved excellently in the two kinds of salts. The grain growth during the gaseous phase aluminisation and the corrosion mechanism, including the role NaCl played in the mixture salt corrosion, are discussed.     

Preparation and hot corrosion behaviour of a MCrAlY + AlSiY composite coating

A MCrAlY + AlSiY composite coating was prepared by arc ion plating. Hot corrosion behaviours of the composite coating and the reference NiCoCrAlYSiB coating were investigated. The results indicate that the composite coating consisted of β-(Ni,Co)Al with dispersed σ-NiCoCr and Cr₃Si in outer layer and Cr-rich phases in inner layer after annealing. As being corroded for 200 h, the composite coating proved better corrosion resistance than that of reference NiCoCrAlYSiB coating. The enhanced corrosion resistance of the composite coating was due to the gradient distribution of Al-enriched outer layer and Cr-enriched inner layer.     

Hot corrosion of YSZ/Al2O3 dispersed NiCrAlY plasma-sprayed coatings in Na2SO4-10 wt.% NaCl melt

This study examines the effect of ytrria stabilized zirconia (YSZ) dispersion on hot corrosion behaviour of NiCrAlY bond coat. Hot corrosion studies were conducted on NiCrAlY and NiCrAlY containing 25, 50 and 75 wt.% YSZ coatings obtained through the air plasma spray technique, in Na₂SO₄ + 10 wt.% NaCl environment at 800 °C. The results show that YSZ dispersion lowers the overall hot corrosion tendency of the NiCrAlY, though it enhances the inherent hot corrosion tendency of its metallic constituent (NiCrAlY). Furthermore, there exists a threshold oxide level beyond which it adversely affects the hot corrosion of the coating.     

Corrosion study of HK-40m alloy exposed to molten sulfate/vanadate mixtures using the electrochemical noise technique

Corrosion performance of HK-40m alloy obtained from electrochemical noise technique and polarization curves during 24 h of exposure in high sulfate (80 mol% Na₂SO₄-20 mol% V₂O₅) and high vanadate (80 mol% V₂O₅-20 mol% Na₂SO₄) molten salts at 700 °C are reported. Electrochemical noise signals were analyzed in the time and frequency domain. A statistical analysis obtaining the resistance noise, the current standard deviation and the localization index are presented as well as the determination of corrosion rates. Corrosion rates were supported by X-ray diffraction analysis of corrosion products and scanning electron microscopy analysis of corroded samples. Results from optical microscope examination of the corroded samples showed that HK-40m alloy suffered inter-granular corrosion when was exposed to the high vanadate salt, whereas exposed to the high sulfate salt, HK-40m corroded through a mixed corrosion process. A corrosion mechanism of HK-40m alloy was obtained together with the corrosion rate, showing the different behavior when exposing the alloy to a high vanadate and high sulfate molten salts.     

Corrosion resistance of HVOF-sprayed coatings for hard chrome replacement

HVOF-sprayed coatings (WC-17Co, WC-10Co-4Cr, Co-28Mo-17Cr-3Si) and electrolytic hard chrome (EHC) coatings corrosion resistances have been compared through electrochemical polarization tests (0.1 N HCl, 0.1 N HNO₃) and Corrodkote test. EHC coatings passivate in HNO₃, but undergo pitting corrosion in HCl and in Corrodkote test too. HVOF coatings do not passivate, but possess more noble corrosion potentials. Both in HNO₃ and HCl, they undergo more generalized corrosion, with similar i_corr; crevice corrosion along splat boundaries is sometimes detected after the HCl test. Their i_corr in 0.1 N HCl solution is lower than in several of EHC coatings. No visible damage in the HVOF coatings has occurred after the Corrodkote test.     

Erosion of oxide scales formed on Cr3C2-NiCr thermal spray coatings

Cr₃C₂-NiCr thermal spray coatings are extensively used to mitigate high temperature erosive wear in fluidised bed combustors and power generation/transport turbines. The aim of this work was to characterise the variation in oxide erosion response as a function of the Cr₃C₂-NiCr coating microstructure. Erosion was carried out at 700 °C and 800 °C with erodent impact velocities of 225-235 m/s. The erosion behaviour of the oxide scales formed on these coatings, was influenced by the coating microstructure and erosion temperature. Development of the carbide microstructure with extended heat treatment lead to variations in the erosion-corrosion response of the Cr₃C₂-NiCr coatings.     

Characterization and corrosion resistance of organically modified silicate-NiZn ferrite/polyaniline hybrid coatings on aluminum alloys

Hybrid coatings based on organically modified silicate (Ormosil)-NiZn ferrite/polyaniline (10-30 wt.%) were synthesized through a sol-gel technique. Tetraethylenepentamine, 3-glycidoxypropyltrimethoxysilane, tetraethoxysilane and Ni_0.5Zn_0.5Fe₂O₄/polyaniline were used as precursors for the hybrid coatings. These hybrid films were deposited via spin coating onto an aluminum alloy to improve the corrosion protection. The effects induced by the NiZn ferrite/polyaniline content on the chain dynamics, ferromagnetic behavior and corrosion performances of the coated samples were investigated. The rotating-frame spin-lattice relaxation times and scale of the spin-diffusion path length indicated that the configuration of the hybrid films was highly cross-linked, dense and adhered to the aluminum alloy substrates. The magnetic properties of the resulting hybrids showed super-paramagnetic behavior, such as zero coercive force (coercivity = 0 G) and a low blocking temperatures (45 K). Potentio-dynamic and salt-spray analysis revealed that the hybrid films provided an exceptional barrier and corrosion protection in comparison with untreated aluminum alloy substrates.     

The structure and high temperature corrosion performance of medium-thickness aluminide coatings on nickel-based superalloy GTD-111

Simple and Si-modified aluminide coatings having medium-thickness (40-60 μm) have been applied on the superalloy GTD-111 by a slurry technique. Hot corrosion and cyclic oxidation performance of the uncoated and the coated superalloy were investigated by exposing samples to a molten film of Na₂SO₄-40 %wt NaVO₃-10%wt NaCl at 780 °C and 1 h cyclic oxidation at 1100 °C in air, respectively. The presence of silicon in the aluminide structure increased the oxidation resistance by a factor of 1.7 times. In addition, a SiO₂-containing scale, which formed on the Si-containing coating surface, was stable during of the hot corrosion testing.     

Synergistic corrosion behavior of coated Ti60 alloys with NaCl deposit in moist air at elevated temperature

In the present paper, the corrosion behavior of Ti60 alloys with an aluminide, TiAlCr, and enamel coatings in moist air containing NaCl vapor at 700-800 °C were studied. The results showed that the TiAlCr and aluminide coatings failed to protect the substrate from corrosion due to the cyclic formation of volatile products during corrosion at 800 °C. However, an uneven continuous protective Al₂O₃ scale could form on the aluminide coating during corrosion at 700 °C. And the enamel coating could protect Ti60 from corrosion due to its high thermochemical stability and matched thermal expansion coefficient with substrates of Ti-base alloys during corrosion.     

Relation between corrosion behavior and microstructure of Mg-Zn-Y alloys prepared by rapid solidification at various cooling rates

The relation between corrosion resistance and microstructure of Mg-Zn-Y alloys with a long period stacking ordered (LPSO) phase has been investigated. In order to clarify the influence of microstructure evolution by rapid solidification on the occurrence of localized corrosion such as filiform corrosion, several Mg_97.25Zn_0.75Y₂ (at. %) alloys with different cooling rates were fabricated by the gravity casting, copper mould injection casting and melt-spinning techniques and their corrosion behavior and microstructures were examined by the salt immersion tests, electrochemical measurements, XRD and TEM. When the cooling rate was less than 3 × 10⁴ K s⁻¹, filiform corrosion propagated in the early stage of salt immersion test, due to formation of a massive block-shaped LPSO phase during casting. On the other hand, when the cooling rate was increased up to 3 × 10⁴ K s⁻¹, rapidly solidified (RS) alloys exhibited excellent corrosion resistance because of grain refinement and formation of a supersaturated single-phase solid solution. Large-sized Mg_97.25Zn_0.75Y₂ alloys fabricated by consolidation of the RS ribbons also exhibited excellent corrosion resistance with passivity. Enhancement of microstructural and electrochemical homogeneities in the Mg-Zn-Y alloys by rapid solidification techniques results in the passivity of substrate materials.     

Hot corrosion behaviour of low Al NiCoCrAlY cladded coatings reinforced by nano-particles on a Ni-base super alloy

Three NiCoCrAlY coatings with Al content lower than 5 wt.% reinforced by different kinds of nano-particles with the same addition and one without nano-particles were prepared on a Ni-base super alloy using laser cladding technique. Hot corrosion of the NiCoCrAlY coatings in Na₂SO₄/K₂SO₄ (75:25, wt./wt.) mixture was performed at 1050 °C in static air. Results indicate that the hot corrosion resistance of the coatings with nano-particles is better than that of the one without nano-particles, among which the one with nano-CeO₂ presented the best hot corrosion resistance. Effects of nano-particles on the hot corrosion behaviour were also discussed.     

Scratch-resistant anticorrosion sol-gel coating for the protection of AZ31 magnesium alloy via a low temperature sol-gel route

A novel hybrid sol-gel/polyaniline coating has been developed for application onto an AZ31 magnesium alloy for corrosion protection. Electrochemical Impedance Spectroscopy in 3.5 wt% NaCl and diluted Harrison’s solutions, along with salt spray tests showed that the coating possesses excellent corrosion resistance. The hybrid coating was modified by doping with silica nanoparticles (for scratch resistance) and cured at a low temperature of 75 °C. Whilst conventional sol-gel methods tend to limit the coating thickness values up to 10 μm, the new hybrid sol-gel/polyaniline system presented here allows thick coatings to be deposited, in this case, around 50-60 μm.     

The influence of CO2, AlCl3 · 6H2O, MgCl2 · 6H2O, Na2SO4 and NaCl on the atmospheric corrosion of aluminum

The influence of salt deposits on the atmospheric corrosion of high purity Al (99.999%) was studied in the laboratory. Four chloride and sulfate-containing salts, NaCl, Na₂SO₄, AlCl₃ · 6H₂O and MgCl₂ · 6H₂O were investigated. The samples were exposed to purified humid air with careful control of the relative humidity (95%), temperature (22.0 °C), and air flow. The concentration of CO₂ was 350 ppm or <1 ppm and the exposure time was four weeks. Under the experimental conditions all four salts formed aqueous solutions on the metal surface. Mass gain and metal loss results are reported. The corroded surfaces were studied by ESEM, OM, AES and FEG/SEM equipped with EDX. The corrosion products were analyzed by gravimetry, IC and grazing incidence XRD. In the absence of CO₂, the corrosivity of the chloride salts studied increases in the order MgCl₂ · 6H₂O < AlCl₃ · 6H₂O < NaCl. Sodium chloride is very corrosive in this environment because the sodium ion supports the development of high pH in the cathodic areas, resulting in alkaline dissolution of the alumina passive film and rapid general corrosion. The low corrosivity of MgCl₂ · 6H₂O is explained by the inability of Mg²⁺ to support high pH values in the cathodic areas. In the presence of carbon dioxide, the corrosion induced by the salts studied exhibit similar rates. Carbon dioxide strongly inhibits aluminum corrosion in the presence of AlCl₃ · 6H₂O and especially, NaCl, while it is slightly corrosive in the presence of MgCl₂ · 6H₂O. The corrosion effects of CO₂ are explained in terms of its acidic properties and by the precipitation of carbonates. In the absence of CO₂, Na₂SO₄ is less corrosive than NaCl. This is explained by the lower solubility of aluminum hydroxy sulfates in comparison to the chlorides. The average corrosion rate in the presence of CO₂ is the same for both salts. The main difference is that sulfate is less efficient than chloride in causing pitting of aluminum in neutral or acidic media.     

Study of the electrochemical behaviour of nanocrystalline zinc by statistical methods

Pulse electrodeposition was used to synthesize nanocrystalline (NC) zinc coatings from citric acid bath. The electrochemical behaviour of the NC zinc coatings was investigated by using potentiostatic and potentiodynamic polarization methods in 0.5 mol/L NaCl (pH = 12) solution and compared with that of cast zinc. Pitting corrosion behaviour was characterized by pitting potential, induction time and stable pit growth rate which were analyzed according to statistical method. The results showed that nanocrystallization increased the sensitivity of E_pit refer to potential sweep velocity, changed the type of the pit generation from B1 (parallel) to B2 (series), accelerated the pitting initiation process and inhibited the stable pit growth process of NC zinc.     

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.     

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.     

A newly synthesized glycine derivative to control uniform and pitting corrosion processes of Al induced by SCN anions - Chemical, electrochemical and morphological studies

A newly synthesized glycine derivative (termed GlyD), 2-(4-(dimethylamino)benzylamino)acetic acid hydrochloride, was used to inhibit uniform and pitting corrosion processes of Al in 0.50 M KSCN solutions (pH 6.8) at 25 °C. For uniform corrosion inhibition study, Tafel extrapolation, linear polarization resistance and impedance methods were used, complemented with SEM examinations. An independent method of chemical analysis, namely ICP-AES (inductively coupled plasma atomic emission spectrometry) was also used to test validity of corrosion rate measured by Tafel extrapolation method. GlyD inhibited uniform corrosion, even at low concentrations, reaching a value of inhibition efficiency up to 97% at a concentration of 5 × 10⁻³ M. Results obtained from the different corrosion evaluation techniques were in good agreement. This new synthesized glycine derivative was also used to control pit nucleation and growth on the pitted Al surface based on cyclic polarization, potentiostatic and galvanostatic measurements. The pitting potential (E_pit) and the repassivation potential (E_rp) increased by the addition of GlyD. Thus GlyD suppressed pit nucleation and propagation. Nucleation of pit was found to take place after an incubation time (t_i). The rate of pit nucleation and growth decreased with increase in inhibitor concentration. Morphology of pitting was also studied as a function of the applied anodic potential and solution temperature. Cross-sectional view of pitted surface revealed the formation of large distorted hemispherical and narrow deep pits. GlyD was much better than Gly in controlling uniform and pitting corrosion processes of Al in these solutions.     

Corrosion protection of magnesium alloy AZ31 sheets by spin coating process with poly(ether imide) [PEI]

In the present study, the potential of poly(ether imide) as corrosion protective coating for magnesium alloys was evaluated using the spin coating technique. The influence of different parameters on the coating properties was evaluated and the corrosion behaviour of the coatings was investigated using electrochemical impedance spectroscopy. The best corrosion protection was obtained preparing the coatings under N₂ atmosphere, using 15 wt.% solution in N′N′-dimethylacetamide (DMAc) which resulted in a coating of approximately 2 μm thickness, with an initial impedance of 10⁹ Ω cm² and of 10⁵ Ω cm² after 240 h of exposure to a 3.5% NaCl solution.