Sensing electrochemical activity in polymer-coated metals during the early stages of coating degradation by means of the scanning vibrating electrode technique

Application of scanning vibrating electrode technique (SVET) for corrosion studies of organic coatings on reactive metals is presented. SVET was used to monitor the electrochemical processes at painted steel immersed in either 10 mM Na₂SO₄ or 10 mM NaCl aqueous solutions. The coated samples were investigated after a scratch was operated through the polymer matrix down to the metal–substrate surface in order to simulate a defect across the coating. SVET imaging probes that the electrochemical behaviour of the system is different depending on the electrolyte employed. Enhanced coating delamination originating from the defect is observed when chloride ions are present in the environment.     

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.     

Enhanced corrosion resistance of magnesium alloy AM60 by cerium(III) in chloride solution

Cerium(III) was utilised to enhance the corrosion resistance of AM60 in NaCl solution. Ce³⁺ can suppress corrosion deterioration up to 1.0 mM. Beyond that level corrosion rate increases till a steady value. Surface film resistance increases with time evolution until 24 h, then decreases and stabilizes. The converted film after 240 h immersion exhibits self-healing and thickening when re-exposed to plain chloride solution. SEM and EDX confirmed that when Ce is present as additive in solution, more compact coating is formed better than its presence as a post coating on the alloy surface before being immersed in the corrosive environment.     

Effect of electrolyte and monomer concentration on anticorrosive properties of poly(N-methylaniline) and poly(N-ethylaniline) coated mild steel

The electrosynthesis of poly(N-methylaniline) (PNMA) and poly(N-ethylaniline) (PNEA) coatings on mild steel in aqueous oxalic acid solutions was carried out by potentiodynamic synthesis technique. The effects of monomer and electrolyte concentrations on electrochemical growth of PNMA and PNEA coatings on mild steel substrates were investigated. Repassivation peak did not appear during electrosynthesis of PNMA and PNEA coatings from solutions containing 0.1 M monomer and 0.1 M electrolyte. The tests for corrosion protection of the polymer coated and uncoated mild steel substrates were done in 3% NaCl solutions by dc polarization and electrochemical impedance spectroscopy (EIS) techniques. Corrosion tests revealed that PNMA and PNEA coatings exhibited effective anti-corrosive properties. The acidity of the polymerization solution was found to influence the anticorrosive behavior of the polymer coating.     

Microstructure and mechanical properties of hot pressed Mo–Cr–Si–Ti in-situ composite,and oxidation behavior with silicide coatings

The present study deals with the synthesis of Mo–16Cr–4Si–0.5Ti (wt.%) alloy by means of the reactive hot pressing method. The microstructure of the synthesized alloy consisted of (Mo, Cr, Ti)₃Si, and the discontinuous α-(Mo, Cr, Ti)_SS phases. The isothermal oxidation behavior of the alloy was investigated in air at 1273 K for 50 h. The alloy exhibited superior oxidation behavior in comparison with single phase molybdenum alloys, because of the formation of SiO₂ and Cr₂O₃ over the alloy surface. The flexural strength determined from three-point bend testing of single edge notch bend specimens was 615 ± 15 MPa. The dominant mechanism of fracture was identified as transgranular mode of crack propagation. To extend the life of the alloy under oxidizing atmosphere, silicide based oxidation resistant coatings were developed, using halide activated pack cementation process. The kinetic behavior of growth of the coating was established and the activation energy of the coating process was determined to be 52.5 kJ/mol. Isothermal oxidation tests of the coated alloy at 1273 K for 50 h, revealed a small weight gain at the initial stages of oxidation followed by no change of weight, indicating the protective nature of the coating.     

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.     

Improvement in oxidation resistance of a Ni3Al-based superalloy IC6 by rhenium-based diffusion barrier coatings

The oxidation behavior of a Ni₃Al-based superalloy IC6 coated with a duplex Re–Cr–Ni–Mo diffusion barrier layer and an Al reservoir layer was investigated in air at 1423 K for up to 1080 ks. The diffusion barrier layer was formed by electroplating Re(Ni) and Ni films on the alloy, followed by Cr pack cementation at 1573 K, and as a result, forms a continuous inner Re–Cr–Ni–Mo diffusion barrier layer and an outer Ni(Cr,Mo,Al) layer. Then a Ni film was electroplated on the Ni(Cr,Mo,Al) layer, followed by Al-pack cementation at 1273 K for 18 ks, to form an Al reservoir layer with a duplex Ni₂Al₃ and γ-Ni(Cr,Mo,Al) layers. After oxidation at 1423 K in air for 1080 ks, the Al reservoir layer changed to a γ-Ni–4Cr–5Mo–12Al (all in at%) layer, on which a protective α-Al₂O₃ scale formed. The Re–Cr(Mo)–Ni layer was stable and effectively retarded the interdiffusion between the Al reservoir layer and the alloy, as a result, the depth of the microstructural change zone of the alloy was less than 15 μm. In contrast, the bare and the coated IC6 superalloy only with an Al reservoir layer were significantly oxidized, accompanied by serious spallation of oxide scales. After oxidation at 1423 K for 1080 ks, the depth of the microstructural change zone of the alloy was about 200 μm for the bare and coated alloy only with an Al reservoir layer. These results indicate that the oxidation resistance of IC6 superalloy can be effectively improved by coating with a Re–Cr–Ni–Mo diffusion barrier layer and an Al reservoir layer.     

Electrochemical synthesis and corrosion behavior of polyaniline-benzoate coating on copper

Electrochemical polymerization of polyaniline (PANI) coating on copper electrode was performed galvanostatically in the current density range between 0.50 and 1.25 mA cm^?2, from aqueous solution of 0.3 mol dm^?3 sodium benzoate and 0.2 mol dm^?3 aniline. The corrosion behavior of PANI coated copper and copper electrode exposed to 0.5 mol dm^?3 sodium chloride solution was investigated by potentiodynamic and electrochemical impedance spectroscopy techniques. It was observed that thin PANI (5 μm) coating had provided efficient protection (～96%) to copper in 0.5 mol dm^?3 sodium chloride solution. Unusual initial impedance behavior to that normally observed with conventional organic coatings was attributed to dedoping of benzoate anions from the polymer coating.     

High temperature EMAT design for scanning or fixed point operation on magnetite coated steel

Bulk thickness measurements were performed at elevated temperatures on magnetite coated low carbon steel pipe and aluminium samples, using a permanent magnet electromagnetic acoustic transducer (EMAT). The design presented here exploits the non-contact nature of EMATs to allow continuous operation at elevated temperatures without physical coupling, sample preparation (in the form of oxide scale removal), or active cooling of the EMAT. A non-linear change in signal amplitude was recorded as the magnetite coated sample was heated in a furnace, whereas a steady decrease in amplitude was observed in aluminium. For a magnetite coated pipe sample, after a dwell time of 3 h, a SNR of 33.4 dB was measured at 450 °C, whilst a SNR of 33.0 dB was found at 25 °C. No significant EMAT performance loss was observed after one month of continuous exposure to 450 °C. EMAT-sample lift-off performance was investigated at elevated temperature on magnetite coated steel; a single-shot SNR of 31 dB for 3.0 mm lift-off was recorded at 450 °C, highlighting the suitability of this design for scanning or continuous fixed point inspection at high temperature.     

Glass–ceramic coatings on titanium alloys for high temperature oxidation protection: Oxidation kinetics and microstructure

A glass–ceramic coating is applied on Ti–6Al–4V alloy for oxidation protection at 800 °C. Its dynamic oxidation and microstructure evolution are investigated. The titanium alloy substrate is effectively protected by the glass–ceramic coating, of which the oxidation develops at constant rate. The linear relationship of oxidation is deduced dm/dt = Dρ(C₁ − C₂)/(bC′), and the diffusion coefficient of oxygen at 800 °C in glass is obtained. Oxygen diffusion through glass coating is the controlling step. After the initial firing, silicide interlayer forms between the glass coating and titanium alloy substrate, where the ratio of Ti/Si decreases after oxidation due to Si diffusion and Ti consumption.     

The effect of an artificially synthesized simonkolleite layer on the corrosion of electrogalvanized steel

Electrochemical behavior of low alloy steel covered by a synthetic layer of simonkolleite was studied alone and in galvanic couple with zinc. Simonkolleite on steel inhibits cathodic oxygen reduction in 5 wt.% NaCl at pH = 7 and 9 but it is unstable at pH = 11. In NaCl solution the galvanic current density between Zn and steel is reduced by more than twice if the synthetic simonkolleite layer is deposited on steel. The polarity between Zn and steel is inverted after several hours in NaHCO₃ solutions. X-ray diffraction confirmed the instability of simonkolleite in this conditions and its transformation into carbonate containing salts.     

Electroless Ni-P plating on Mg-10Gd-4.8Y-0.6Zr magnesium alloy with a new pretreatment process

A new pretreatment process for electroless Ni-P plating on Mg-10Gd-4.8Y-0.6Zr was investigated in this paper. The morphology, component, chemical composition and structure of the pretreatment layers and Ni-P coating were analyzed by scanning electronic microscopy, energy dispersive spectroscopy and X-ray diffraction spectroscopy. The structure of Ni-P coating was also detected by transmission electron microscopy (TEM). Potentiodynamic polarization analysis and salt spray test were used to test the corrosion resistance of the Ni-P coating. Experimental results indicate that: metal Cr generated in the new pretreatment process, which provided active points for later zinc immersion process, was beneficial to nickel deposition. The subsequent Ni-P coating was amorphous, in which the content of P was 9.43 wt.%. It was uniform and its thickness reached about 50 μm at 2 h deposition. Compared to Mg-10Gd-4.8Y-0.6Zr alloy substrate, the corrosion potential of the coated alloy shifted by 1090 mV positively and the corrosion current density decreased one order of magnitude in 3.5 wt.% NaCl solution. The salt spray test time of Ni-P coating was 210 h. All of these results suggest that the electroless nickel plating procedure researched in this paper is suitable for Mg-10Gd-4.8Y-0.6Zr alloy.     

The influence of SDS and CTAB surfactants on the surface morphology and surface topography of electroless Ni–P deposits

The effect of surfactants sodium dodecyl sulphate (SDS) and cetyltrimethyl ammonium bromide (CTAB) on the surface roughness, surface morphology and surface topography of electroless nickel (EN)–phosphorus surface protective coating obtained from an alkaline bath is presented in this paper. In this study the influence of surfactant concentrations on the surface roughness of coated samples were investigated. It was observed that the surface roughness, surface morphology and surface topography of Ni–P coating were clearly influenced by the addition of surfactants SDS and CTAB. EN deposits with addition of surfactant SDS and CTAB at a concentration of 0.6 g/l produce a smooth surface and the average roughness (R_a) value is 1.715 μm for SDS and 1.607 μm for CTAB which is less than the R_a value of EN deposit without surfactant addition (1.885 μm). The mean average roughness (R_a) value with addition of surfactant is 1.796 μm.EN deposit with addition of surfactants consists of a significant fraction of particles of nickel. In the presence of SDS, fine nickel particles have dispersed uniformly on the substrate surface resulting in smoother surface finish of the deposited layers. In the presence of CTAB, at lower concentrations (upto 0.6 g/l) coalescence of nickel particles have been deposited on the substrate surface and at the higher concentration (above 0.6 g/l) uniformly improved surface finish of the deposited layer is resulted. The complete experimental details, results obtained and their analysis are presented in this paper.     

Microstructure and oxidation resistance of Si–Mo–B coating for C/SiC coated carbon/carbon composites

A self-sealing Si–Mo–B oxidation resistance coating was prepared on C/SiC coated carbon/carbon (C/C) composites by slurry and high temperature treatment method. The oxidation resistance of the coating increases at 1173 K and first increases then decreases at 1873 K with the increase of B content from 0 to 20 wt.%. The C/SiC/gradient Si–Mo–B multilayer coating can protect C/C composite from oxidation for 100 h at 1173 K and 125 h at 1873 K. The good oxidation resistance of the coating in broad temperature range could be attributed to its good self-sealing property.     

A comparative study of two kinds of Y and Al modified silicide coatings on an Nb–Ti–Si based alloy prepared by pack cementation technique

Two kinds of Y and Al modified silicide coatings on an Nb–Ti–Si based alloy were prepared by pack cementation technique. The microstructure and oxidation behavior of both coatings were studied. Both coatings had a multiple layer structure, but the outer layers were composed of either Y- and Al-doped (Nb,X)Si₂ or Y-doped (Nb,X)₃Si₅Al₂ + (Nb,X)Si₂ phases, respectively. The former coating could protect the substrate alloy from oxidation at 1250 °C for 100 h, but the latter coating could only endure for less than 20 h. The scale formation mechanisms and microstructural changes of both coatings upon oxidation have been illustrated.     

Erosion performance and characterization of nanolayer (Ti,Cr)N hard coatings for gas turbine engine compressor blade applications

(Ti,Cr)N nanolayer coatings were deposited on Ti–6Al–4V, 17-4PH and Inconel 718 substrates using cathodic arc physical vapor deposition for improved erosion and corrosion resistance. Coating corrosion performance was highly dependent on the coating thickness and packing factors and correlated with increased chromium content within the (Ti,Cr)N nanolayer coatings. The change in cathode current predominantly affected coating thickness and the bias affected the packing factor. Erosion tests of the coated and uncoated substrates at both 30° and 90° erodent impingement angles were conducted using angular aluminum oxide media at particle velocities up to 145 m/s. Chromium evaporator current and substrate bias were varied to change film stoichiometry and microstructure for erosion performance evaluation. When chromium evaporator current was varied, the increase in chromium content led to an increase in binary CrN phase volume and a decrease in TiN phase volume. The increase in CrN phase volume decreased both hardness and erosion performance at both impingement angles. Lower bias values resulted in better erosion performance. At 30° erodent impingement, all coated samples outperformed the uncoated substrate; whereas, for 90° impingement, only coatings deposited at low bias values (? 25 V, ? 50 V, and ? 100 V) and high Ti:Cr ratios (> 2.4) outperformed the uncoated substrate. The primary coating failure mechanism was microchipping.     

Oxidation protection of SiC-coated C/C composites by SiC nanowire-toughened CrSi2–SiC–Si coating

Oxidation protective SiC nanowire-toughened CrSi₂–SiC–Si coating was prepared on SiC-coated carbon/carbon composites by chemical vapor deposition and pack cementation. SiC nanowires in the coating suppressed the cracking of the coating via various toughening mechanisms including nanowire pullout, microcrack bridging by nanowire and microcrack deflection, resulting in a good oxidation inhibition for the coated samples. The results showed that the maximal weight loss of the coated samples was only 2.55% in thermogravimetric analysis from room temperature to 1500 °C, and the weight loss of the coated samples was only 1.24% after isothermal oxidation at 1500 °C for 316 h.     

Cracking investigation of W and W(C) films deposited by physical vapor deposition on steel substrates

This paper presents the investigation of the cracking of coatings deposited on steel substrates. The coating on substrate systems consisted on pure tungsten films (W) and films of solid solutions of carbon in tungsten [W(C)], which were deposited by direct current reactive magnetron sputtering on stainless steel substrates. The systems were strained uniaxially with a microtensile device adapted to a scanning electron microscope. The mechanical response was analyzed from the experimental results: the straining of the samples showed an evolution of the density of cracks in the coating, which was described trough an empirical equation based on the Weibull distribution function. The density of cracks, which corresponds to the crack saturation of the coating, appeared to vary inversely with coating thickness. Critical parameters relative to their mechanical stability were also determined from the experimental results: the strain energy release rate for crack extension through the film, G^f_c, and the fracture toughness, K^f_Ic, of the coatings. These values are included between 0.2 and 14 J m⁻², and between 0.1 and 2.5 MPa m^−1/2. The fracture resistance of W and W(C) coatings was found to be correlated to their thickness and microstructure.     

Effect of hot-dip aluminizing on the oxidation resistance of Ti–6Al–4V alloy at high temperatures

Hot-dip aluminizing and interdiffusion treatment were used to develop a TiAl₃-rich coating on Ti–6Al–4V alloy. Interrupted oxidation at temperatures from 600 to 900 °C and isothermal oxidation at 700 and 800 °C of the coating were conducted. The coating markedly decreases the oxidation rate in comparison with the alloy at temperatures below 800 °C during the interrupted oxidation. The oxidation kinetics follows parabolic relations at 700 and 800 °C during the isothermal oxidation. A layered structure of Al₂O₃/TiAl₃/TiAl₂/TiAl/alloy from the outside to the inside forms after oxidation at 700 °C without changing the main body of the coating.     

Low-temperature processing of Fe–Al intermetallic coatings assisted by ball milling

A novel technique has been developed to produce Fe–Al intermetallic coatings on steel. This technique applies mechanical vibration to a retort, which is loaded with Al powder, alumina filler, ammonium chloride activator and FeCrAl alloy balls. The operation temperature was from 440 to 600 °C. This technique produced coatings with thickness of 17 μm for 15 min and 90 μm for 120 min treatment at 560 °C. The coatings appear to be homogeneous, with a high density and free of porosity, and have excellent adherence to the substrate. The coatings consisted mainly of η-Fe₂Al₅ with small amounts of θ-FeAl₃ and β-FeAl, and exhibited a nano-structure. Microstructure studies suggested that the formation of the intermetallic phases at a low temperature has a complex mechanism, including the formation of a thin Al layer on the substrate by ball milling; Al-rich phases nucleation, growth and formation of an initial alloy layer; severe plastic deformation which increases the local temperature and produces a nano-structure; and fast outward diffusion of Fe and formation of Fe–Al intermetallics. This technique reduced the treatment temperature and duration significantly compared with the conventional Al pack cementation processes, providing a new approach to industrial diffusion coatings with great energy and time savings.