Anticorrosive painting for a wide spectrum of marine atmospheres: Environmental-friendly versus traditional paint systems

This work presents some of the main results obtained in different marine atmospheres by Working Groups 1 and 2 (dedicated to anticorrosive protection of steel by paint coatings) of the Ibero-American PATINA network, developed in the context of the CYTED Programme. As marine atmospheres it includes natural atmospheres with a salinity level above S1 classification of ISO standard 9223 (>3 mg Cl⁻ m⁻² day⁻¹), and sulphur dioxide contamination only up to classification P1 of the same standard (maximum of 35 mg SO₂ m⁻² day⁻¹). Consideration is also made of accelerated tests traditionally used to assess the anticorrosive behaviour of the substrate/paint coatings contemplated in the study, namely salt spray, artificial weathering and different cycles involving ultraviolet radiation, humidity, temperature and different contamination conditions. The substrates were steel, hot-dip galvanised steel and electrogalvanised steel (Zincorr^® sheet). The paint systems applied on these substrates, with or without pretreatments, were solventborne, waterborne, high solid and powder paint systems. As a result it has been possible to conclude which of the studied anticorrosive coating types were the most suitable for each of the different types of marine atmospheres considered in the study.     

The corrosion behavior of Fe-10Cr nanocrystalline coating

Fe-10Cr nanocrystalline (nc) coatings with a grain size of 20-30 nm were synthesized on glass substrates by magnetron sputtering. The corrosion behavior was investigated in 0.05 mol/L H₂SO₄ + 0.25 mol/L Na₂SO₄ and 0.05 mol/L H₂SO₄ + 0.5 mol/L NaCl solution by polarization curves, EIS and Mott-Schottky analysis. The results showed that compared to Fe-10Cr cast alloy, the active dissolution of the coating was accelerated; the passive film contained more Cr and therefore the coating was easier to passivate. The passive films formed on Fe-10Cr nc and cast alloy exhibited n-type semiconducting behavior in acidic solutions without Cl⁻ and p-type semiconducting behavior in acidic solutions with Cl⁻. The lower breakdown potential for both materials in the solution with Cl⁻ is related to the p-type passive film formed on them. For Fe-10Cr nc, lower donor density and increased Cr content were responsible for the chemical stability of the passive film.     

Thermal conductivity in mullite/ZrO2 composite coatings

Mullite-based multilayered structures have been suggested as promising environmental barrier coatings for Si₃N₄ and SiC ceramics. Mullite has been used as bottom layer because its thermal expansion coefficient closely matches those of the Si-based substrates, whereas Y–ZrO₂ has been tried as top layer due to its stability in combustion environments. In addition, mullite/ZrO₂ compositions may work as middle layers to reduce the thermal expansion coefficient mismatch between the ZrO₂ and mullite layers. Present work studies the thermal behaviour of a flame sprayed mullite/ZrO₂ (75/25, v/v) composite coating. The changes in crystallinity, microstructure and thermal conductivity of free-standing coatings heat treated at two different temperatures (1000 and 1300 °C) are comparatively discussed. The as-sprayed and 1000 °C treated coatings showed an almost constant thermal conductivity (K) of 1.5 W m⁻¹ K⁻¹. The K of the 1300 °C treated specimen increased up to twice due to the extensive mullite crystallization without any cracking.     

Lanthanum-titanium-aluminum oxide: A novel thermal barrier coating material for applications at 1300 °C

Considerable efforts are being invested to explore new thermal barrier coating (TBC) materials with higher temperature capability to meet the demand of advanced turbine engines. In this work, LaTi₂Al₉O₁₉ (LTA) is proposed and investigated as a novel TBC material for application at 1300 °C. LTA showed excellent phase stability up to 1600 °C. The thermal conductivities for LTA coating are in a range of 1.0-1.3 W m⁻¹ K⁻¹ (300-1500 °C) and the values of thermal expansion coefficients increase from 8.0 to 11.2 × 10⁻⁶ K⁻¹ (200-1400 °C), which are comparable to those of yttria stabilized zirconia (YSZ). The microhardness of LTA and YSZ coatings were in the similar level of ∼7 GPa, however, the fracture toughness value was relatively lower than that of YSZ. The lower fracture toughness was compensated by the double-ceramic LTA/YSZ layer design, and the LTA/YSZ TBC exhibited desirable thermal cycling life of nearly 700 h at 1300 °C.     

Electrodeposited Pd-Co catalyst for direct methanol fuel cell electrodes: Preparation and characterization

Pd-Co alloy has been recently proposed as a catalyst for the cathode of direct methanol fuel cells with both excellent oxygen reduction activity and methanol tolerance, hence electrodeposition of this alloy is an attractive approach for synthesizing porous metal electrodes with high methanol tolerance in direct methanol fuel cells. In this study, we electrodeposited two types of Pd-Co films onto Au substrates by applying different current density (−10 or −200 mA cm⁻²); and then characterized them in terms of morphology, composition, crystal structure, and catalytic activity. Pd-Co deposited at −10 mA cm⁻² was smooth and possessed smaller particles (ca. 10 nm), while that at −200 mA cm⁻² was dendritic (or rough) and possessed larger particles (ca. 50 nm). Both the Pd-Co alloys were found to be almost the same structure, i.e. a solid solution of ca. Pd₇Co₃ with Pd-skin, and also confirmed to possess comparable activity in oxygen reduction to Pt (potential difference at 1.0 μA cm⁻² was 0.05 V). As for methanol tolerance, cell-voltage was not influenced by addition of 1 mol dm⁻³ methanol to the oxidant solution. Our approach provides fundamental technique for synthesizing Pd-Co porous metal electrodes by electrodeposition.     

Characterisation of thermal barrier coatings and ultra high temperature composites deposited in a low pressure plasma reactor

A low pressure plasma process working at 600-800 Pa was used to deposit from aqueous solution ZrO₂-4 mol% Y₂O₃ (Yttria partially stabilized Zirconia-YpSZ) layers and stacks of Ta₂O₅/YpSZ layers for use as thermal barrier coatings (TBC). The observation of the cross section revealed a high porosity. The thermal diffusivity of the layers (1 × 10⁻⁷ m² s⁻¹) was measured by a laser flash technique and compared with values obtained on air plasma sprayed material (3 × 10⁻⁷ m² s⁻¹). The plasma reactor were also used to deposit ZrB₂-ZrO₂-SiC layers used as Ultra High Temperature Composite (UHTC) from aqueous solutions of zirconyl and Boron nitrates containing suspensions of SiC. Layers up to 100 μm thick were obtained on SiC substrates. XRD was used to study the crystallinity of the layer. The presence of ZrB₂ and SiC phases was confirmed after the deposition. XRD analysis showed that heat treatment at 1073 K under oxidizing conditions led to the loss of ZrB₂ and the appearance of ZrO₂ phases. To understand the behaviour of the layers to interaction with atomic oxygen (combustion for TBC and spacecraft re-entry phase for UHTC), we have measured the atomic oxygen recombination coefficient to determine the number of adsorption sites on the surface of the coatings. This was accomplished by using a low pressure plasma reactor coupled with optical spectroscopic measurements as a diagnostic technique.     

Oxygen reduction reaction on electrodeposited zinc oxide electrodes in KCl solution at 70 °C

The reduction of oxygen was studied in 0.1 M KCl at 70 °C using the rotating disk electrode (RDE) technique on platinum and electrodeposited ZnO thin film electrodes deposited on platinum substrates. In the absence of Zn²⁺ ions in solution, a Tafel slope of 139 mV dec⁻¹ was obtained, a value close to that measured on bare platinum electrode (133 mV dec⁻¹) and ascribed to the limitation of the reaction rate by the first electron transfer. The main difference between the noble metal and the oxide electrode was a shift of the curves towards more negative potentials. In the presence of Zn²⁺ ions, the current density decreased significantly and the Tafel slope was measured at 282 mV dec⁻¹ showing that the electrode was partially blocked by zinc oxide formation reaction intermediates.     

Efficient diffusion barrier layers for the catalytic growth of carbon nanotubes on copper substrates

We have investigated the growth of carbon nanotube (CNT) films on copper substrates by the catalytic chemical vapour deposition route. Ferrocene was used as the catalyst precursor and toluene was the carbon feedstock. The copper substrates were coated with nitride and oxide amorphous ceramic barrier coatings in order to prevent diffusion of the iron catalyst during growth. It was found that virtually no CNT grew on pure copper, but long and densely packed mats of CNTs could be grown on TiN-coated copper. Copper substrates coated with SiN_x and In₂O₃:Sn (ITO) also showed better results than pure copper, although the CNT density was much lower than that obtained from TiN/Cu. Auger electron spectroscopy (AES) showed that Fe diffusion occurred into SiN_x/Cu and ITO/Cu substrates, which partially inhibited its catalyst activity. In contrast, AES did not detect the presence of diffused Fe into the TiN coating. The estimation of the diffusion coefficient by AES depth profiles for Fe in SiN_x, was 3 · 10⁻³ nm² s⁻¹. This value establishes an upper limit for Fe diffusion on substrates for proper nanotube nucleation and growth. Secondary ion mass spectrometry provided complementary information on the composition profiles with depth.     

Investigation of anti-corrosion behavior of waterborne organosilane–polyester coatings for AA6011 aluminum alloy

Thermal-cured, sol–gel derived, waterborne organosilane–polyester coatings (SiE) have been developed using methyltrimethoxysilane, 3-glycidoxytrimethoxysilane and polyester resin for corrosion protection of aluminum AA6011. The structural and morphological features of the coatings were analyzed by Fourier transform infrared spectroscopy (FT-IR) and atomic force microscopy (AFM). Results show that the coatings on aluminum were smooth, continuous and defect-free. Performance of the SiE coatings were investigated and compared with pure organosilane coating and polyester coating using potentiodynamic polarization studies, contact angle measurement and pencil hardness test. Results from polarization studies have shown that the SiE coated substrate (4.6–13.1 × 10⁻⁷ A/cm²) provided a better corrosion protection than the polyester coated substrate (7.8 × 10⁻⁶ A/cm²) due formation of aluminum–oxygen–silicon covalent bond at aluminum-coating interface. Furthermore, SiE coatings provided better hydrophobicity and hardness than the polyester coating.     

Non-catalyzed cathodic oxygen reduction at graphite granules in microbial fuel cells

Oxygen is the most sustainable electron acceptor currently available for microbial fuel cell (MFC) cathodes. However, its high overpotential for reduction to water limits the current that can be produced. Several materials and catalysts have previously been investigated in order to facilitate oxygen reduction at the cathode surface. This study shows that significant stable currents can be delivered by using a non-catalyzed cathode made of granular graphite. Power outputs up to 21 W m⁻³ (cathode total volume) or 50 W m⁻³ (cathode liquid volume) were attained in a continuous MFC fed with acetate. These values are higher than those obtained in several other studies using catalyzed graphite in various forms. The presence of nanoscale pores on granular graphite provides a high surface area for oxygen reduction. The current generated with this cathode can sustain an anodic volume specific COD removal rate of 1.46 kg_COD m⁻³ d⁻¹, which is higher than that of a conventional aerobic process. This study demonstrates that microbial fuel cells can be operated efficiently using high surface graphite as cathode material. This implies that research on microbial fuel cell cathodes should not only focus on catalysts, but also on high surface area materials.     

The influence of a new fabrication procedure on the catalytic activity of ruthenium-selenium catalysts

A new procedure has been introduced to enhance catalytic activity of ruthenium-selenium electro-catalysts for oxygen reduction, in which materials are treated under hydrogen atmosphere at elevated temperatures. The characterisation using scanning electron microscopy, energy dispersive spectroscopy or energy dispersive X-ray spectroscopy exhibited that the treatment at 400 °C made catalysts denser while their porous nature remained, led to a good degree of crystallinity and an optimum Se:Ru ratio. The half cell test confirms feasibility of the new procedure; the catalyst treated at 400 °C gave the highest reduction current (55.9 mA cm⁻² at −0.4 V) and a low methanol oxidation effect coefficient (3.8%). The direct methanol fuel cell with the RuSe 400 °C cathode catalyst (2 mg RuSe cm⁻²) generated a power density of 33.8 mW cm⁻² using 2 M methanol and 2 bar oxygen at 90 °C. The new procedure produced the catalysts with low decay rates. The best sample was compared to the Pt and to the reported ruthenium-selenium catalyst. Possible reasons for the observations are discussed.     

ZrC ablation protective coating for carbon/carbon composites

A zirconium carbide (ZrC) protective coating was deposited on carbon/carbon (C/C) composites by atmospheric pressure chemical vapor deposition. The phase compositions, surface and cross-section microstructures, and anti-ablative properties of the coatings were investigated. Results show that the method is an effective route to prepare a dense and thick ZrC coating on C/C composites. The coating can effectively protect C/C composites from ablation for 240 s in an oxy-acetylene torch system with a mass ablation rate of 1.1 × 10⁻⁴ g/cm² s and a linear ablation rate of 0.3 × 10⁻³ mm/s.     

Oxygen reduction on platinum electrodes in base: Theoretical study

Electrode-potential-dependent activation energies for electron transfer have been calculated using a local reaction center model and constrained variation theory for the oxygen reduction reaction on platinum in base. Results for four one-electron transfer steps are presented. For the first, O₂(ads) is predicted to be reduced to adsorbed superoxide, O₂⁻(ads), which dissociates with a low activation barrier to O(ads) + O⁻(ads). Then a proton transfer form H₂O(ads) to O⁻(ads) takes place, forming OH(ads) + OH⁻(aq). The second electron transfer reacts O(ads) with H₂O(aq) to form a second OH(ads) + OH⁻(aq). The third and fourth electron transfers react the two OH(ads) with two H₂O(aq) to form two H₂O(ads) + two OH⁻(aq). All three different surface reduction reactions are predicted to have reversible potentials in the −0.24 V(SHE) to −0.29 V(SHE) range for 0.1 M base and activation energies for the superoxide formation step are close to the experimentally observed range in 0.1 M base for the overall four-electron to water over the three low index (1 1 0) (1 0 0) and (1 1 1) surfaces: 0.38-0.49 eV at 0.35 eV respectively at 0.88 V(RHE). Predicted reversible potentials for forming O₂⁻(ads) are compared with estimates from the experimental literature. The difference between the acid mechanism, where the peroxyl radical, OOH(ads) is the first reduction intermediate, and the base mechanism, where superoxide, O₂⁻(ads) is the first reduction intermediate, is discussed.     

Kinetics and mechanism of oxygen reduction at hydrous oxide film-covered platinum electrode in alkaline solution

This study uses rotating ring-disk electrode (RRDE) and linear sweep voltammetry (LSV) to characterize oxygen reduction kinetics in alkaline solution on platinum electrodes with various thickness of hydrous oxide (oxyhydroxy) film. Oxyhydroxy films are created on Pt electrodes by pretreatment in 1.0 mol dm⁻³ KOH at a constant voltage. The pretreatment voltage ranges from −1.2 to 1.0 V and is increased stepwise before each new experimental run to produce seven discreet films. LSV plots show oxyhydroxy film thickness strongly inhibits oxygen reduction and is inversely proportional to RRDE oxygen reduction current I_D for LSV voltages E_D from −0.1 to −0.46 V, but this trend reverses at E_D more negative than −0.46 V so that the worst-performing electrode becomes the best. However, this improvement disappears at around −0.8 V, suggesting this change involves a negatively charged ion, possibly embedded into the metal in the top few atomic layers either interstitially or substitutionally. The 1.0 V-pretreated electrode in the E_D range from −0.46 to −0.9 V of highest oxygen reduction current also exhibits the lowest hydrogen peroxide production, with zero H₂O₂ produced at −0.6 V, indicating the brief presence of the oxyhydroxy film on the Pt surface has strong lingering effects. The post-oxyhydroxy Pt surface is very different than the native Pt for oxygen reduction pathway and efficiency. Reaction order with respect to oxygen is close to 1. The rate constants of the direct O₂ to H₂O electroreduction reaction are increased with decreasing the potential from −0.2 to −0.6 V, but the O₂ to H₂O₂ electroreduction is contrary to this expectation. The rate constants of H₂O₂ decomposition on the oxyhydroxy film-covered Pt electrode are near constant around 1 × 10⁻⁴ cm s⁻¹ at E_D > −0.5 V.     

Electrochemical formation of nanometric layers of tin selenides on Ti surface

Photoelectrically active tin selenide coatings of nanometric thickness were manufactured by electrodeposition from separate solutions of Sn and Se precursors. Sn was deposited from acidic SnSO₄ electrolytes and Se was deposited from H₂SeO₃ solutions. Fine-grained Sn coatings were deposited at potential φ = −0.3 V with 100% current efficiency. Se coatings were formed at two potentials: φ = −0.5 V, forming Se⁰, and φ = −0.85 V, forming Se²⁻ ions. After the Sn coating was immersed into H₂SeO₃ solution, small quantities (∼2 at.%) of SnSe were formed and SeO₃²⁻ was adsorbed on the surface. A short-time deposition of Se at φ = −0.5 V passivated the surface, so no Sn dissolution is observed upon anodic polarization. XPS and Auger data indicated that under those conditions 20 at.% of Se⁰ and only 2 at.% of SnSe were formed. Thickening of Sn and Se layers led to formation of larger quantities of Se⁰ (75 at.%) and SnSe (4-5 at.%) on the surface, whereas deeper layers contained up to 10 times more of SnSe phase. Upon deposition of Se at φ = −0.85 V, new SnSe₂ phase was formed and the quantity of SnSe phase is increased and that of Se⁰ was reduced. All coatings formed exhibited photoelectric properties.     

Microstructure and thermo-physical properties of yttria stabilized zirconia coatings with CMAS deposits

Yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) are used to protect hot-components in aero-engines from hot gases. In this paper, the microstructure and thermo-physical and mechanical properties of plasma sprayed YSZ coatings under the condition of calcium-magnesium-alumina-silicate (CMAS) deposits were investigated. Si and Ca in the CMAS rapidly penetrated the coating at 1250 °C and accelerated sintering of the coating. At the interface between the CMAS and YSZ coating, the YSZ coating was partially dissolved in the CMAS, inducing the phase transformation from tetragonal phase to monoclinic phase. Also, the porosity of the coating was reduced from ∼25% to 5%. As a result, the thermal diffusivity at 1200 °C increased from 0.3 mm²/s to 0.7 mm²/s, suggesting a significant degradation in the thermal barrier effect. Also, the coating showed a ∼40% increase in the microhardness. The degradation mechanism of TBC induced by CMAS was discussed.     

Light-colored compound conductive coatings based on CuI: Effect of volume fraction of CuI on morphology and electrical conductivity

A novel family of compound conductive coating has been designed and developed using Cu(I)I (cuprous iodide) as electrical conducting material. A carefully designed polyurethane matrix material allowed incorporation of large volume fraction of CuI, much above its critical volume concentration, while maintaining good film integrity. Electrical conductivity of a series of such coatings has been studied using dielectric spectroscopy, as a function of volume fraction of CuI and temperature. The light-colored (white–beige colored) coatings having electrical conductivity of the order of up to 10⁻³–10⁻² S cm⁻¹ have been demonstrated. Scanning electron microscopy (SEM) has been used to characterize bulk coating microstructures. The result of this study provides useful insight for fabrication of compound conductive coatings based on CuI.     

Modification of copper with self-assembled organic coatings

Self-assembled monolayers of n-alkanethiols (CH₃(CH₂)_n−1SH; n = 12 and 18) were prepared on a copper surface, and the quality and protection efficiency of resulting coatings against aqueous corrosion of copper were investigated. A combination of physical (scanning electron microscopy, energy dispersive X-ray spectroscopy and contact angle measurements) and electrochemical (cyclic voltammetry and impedance spectrometry) methods was used to correlate the structure of the coatings with their barrier properties during exposures in an aqueous solution. Impedance results reveal that the coatings behave almost like an ideal dielectric, with a resistance of several MΩ cm² and a coating capacitance that agree well with the value calculated according to the theory of dielectrics. Protection efficiency of chemisorbed alkanethiol coatings determined from dc and ac measurements were above 99%.     

The influences of oxygen ion implantation on the structural and electrical properties of B-doped diamond films

The oxygen ion with a dose of 10¹⁴ (called CVDBO14) and 10¹⁵ cm^− 2 (called CVDBO15) was implanted into boron doped diamond films synthesized in chemical vapor deposition. The structural and electrical properties of different samples were characterized by XPS, Raman spectroscopy and 4-probe resistivity measurements. The results show that oxygen ion exists both in the diamond surface and the subsurface of the films. The FWHM values of CVDBO15 samples are higher than those of CVDBO14 samples, indicating that more damages existed in CVDBO15 samples. The resistivity of CVDBO15 sample series is smaller than those of CVDBO14 sample series, and the film with a larger FWHM value exhibits low resistivity. In the 1150 °C annealed sample, the activation energy decreases from 0.50 eV to 0.39 eV with the oxygen ion dose increasing from 10¹⁴ to 10¹⁵ cm^− 2. It is indicated that oxygen ion and the defects produced by ion implantation give contributions to the conductivity in diamond films. Some surface hydrogen is removed and pi-bonded carbon as well as C-H vibration is formed after annealing, which is also relative to the lower resistivity in the samples.     

Improvement of the electrochemical behavior of steel surfaces using a TiN[BCN/BN]n/c-BN multilayer system

The aim of this work is to improve the electrochemical behavior of AISI 4140 steel substrates by using a TiN[BCN/BN]_n/c-BN multilayer system as a protective coating. We grew TiN[BCN/BN]_n/c-BN multilayers via reactive r.f. magnetron sputtering technique, systematically varying the length period (Λ) and the bilayer number (n), maintaining constant the total thickness of the coating and all other growth parameters. The coatings were characterized by FTIR spectroscopy that showed bands associated to h-BN bonds, and c-BN stretching vibrations centered at 1385 cm^− 1 and 1005 cm^− 1, respectively. Film composition was studied via X-ray photoelectron spectroscopy where typical signals for C1s, N1s and B1s are shown. The electrochemical properties were studied by electrochemical impedance spectroscopy and Tafel curves. In this work, the maximum corrosion resistance for the coating with (Λ) equal to 80 nm was obtained, corresponding to n = 25 bilayers. The polarization resistance and corrosion rate were around 10.1 kOhm cm² and 0.22 mm/year; these values were 83 and 15 times higher, respectively, than uncoated AISI 4140 steel substrate (0.66 kOhm cm² and 18.51 mm/year). Optical microscopy was used for surface analysis after corrosive attack. The improvement of the electrochemical behavior of the AISI 4140 coated with this TiN[BCN/BN]_n/c-BN multilayer system can be attributed to the presence of several interfaces that offer resistance to diffusion of Cl⁻ of the electrolyte toward the steel surface.