Development of polyorganosilazane–silicone marine coatings

The objective of the present work is the development and characterization of marine coatings based on polyorganosilazanes (PSZ). Two types of coatings containing silicone oils and biocidal compounds were investigated as anticorrosive and antifouling coatings. The flexibility, hydrophobicity and adhesion properties of the PSZ-based coatings on aluminum substrates were studied. Static immersions in natural seawater were investigated to evaluate the antifouling performances of these coatings. The corrosion properties were studied by salt spray tests. Results demonstrated that coatings based on silicone oils appeared to be the most efficient coatings in terms of antifouling and anticorrosive properties. Ten-month antifouling efficiency was revealed for biocide-free polydimethylsiloxane-based PSZ coatings in natural seawater static immersion. The adjunction of dicopper oxide as biocidal pigments was shown to decrease the stability in cans of the corresponding paints and therefore decreasing the flexibility of coatings. In addition, this pigment affected badly the anticorrosive properties of the coatings together with a short antifouling efficiency time. Thus, the silicone oil-based PSZ displayed remarkable advantages in addition to their dual antifouling and anticorrosive properties which are the absence of biocidal compounds released in marine environment and the absence of volatile solvent.     

Electrodeposition of Zn–SiC nanocomposite coatings

Zn–SiC composite coatings were obtained on mild steel substrate by electrodeposition technique with high-current efficiency. A slightly acidic chloride bath, containing SiC nanoparticles and gelatine as additive, was used. The electrodeposition was carried out under galvanostatic control with pulsed direct current; the effect of experimental parameters (temperature, average current density and particles concentration) on composition, morphology and structure of the deposit was studied. Coatings were characterized by means of scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffractometry and Vickers microhardness measurements. Zn–SiC electrodeposits with the best characteristics were obtained by performing electrodepositions at 45 °C, with 20 g L^?1 SiC in the bath and with average current density in the range 100–150 mA cm^?2. Under these experimental conditions, homogeneous and compact coatings, with low-grain size and SiC content ranging from 1.7 to 2.1 wt%, were found to be electrodeposited. Microhardness measurements showed for these deposits an increase of about 50 % with respect to those without nanoparticles obtained in the same experimental conditions.     

Properties of electrodeposited silver–cobalt coatings

Silver–cobalt coatings were elecrodeposited from cyanide–pyrophosphate electrolytes with and without additives (diammonium oxalate monohydrate and 2-Butyne-1,4-diol). The deposition rate, the alloy composition, as well as some physicomechanical (internal stress, microhardness, plug-in forces, abrasion resistance) and electrical properties (contact resistance, magnetoresistance) of the obtained coatings, depending upon the applied current density or the cobalt content, respectively, were investigated. The increase in current density led to the increase in the Co content of the coatings and to a decrease in the grain size of both silver and cobalt phases. Granular structural type of deposits with a magnetoresistance of about 4% ratio was shown. The tensile stress observed in pure Ag deposits increased in the presence of Co and with an increase in its content in the alloy. The increase in the microhardness, abrasion resistance and electrical resistance of the Ag–Co coatings depended almost linearly on the Co content. The performed high speed electroplating showed significant increase in the deposition rate and smoothness of the coatings.     

Corrosion resistant behaviour of PANI–metal bilayer coatings

The present work discusses on the corrosion resistant behaviour of polymer metal bilayer coatings, viz. polyaniline (PANI), polyaniline–nickel (PANI–Ni), nickel–polyaniline (Ni–PANI), polyaniline–zinc (PANI–Zn) and zinc–polyaniline (Zn–PANI). The coatings were synthesized by means of cyclic voltametric method. The coatings thus obtained were uniform in nature and highly adherent to the mild steel substrate. The effectiveness of the coatings in preventing corrosion was tested by electrochemical impedance studies (EIS) using Nyquist and Bode plots and potentiodynamic polarization studies as well. Among the various coatings synthesized, the PANI–Zn coating was found to offer the maximum protection, followed by PANI–Ni coatings. Metal–PANI coatings were found to offer the least resistance to corrosion. The coatings thus obtained were characterized by scanning electron microscopic (SEM) analysis and the results are discussed.     

Structure and properties of electrodeposited Ni–Co–YZA composite coatings

The aim is to develop an economical composite coating with high thermal stability. Ni–Co alloys are found to possess better thermal, physical and mechanical properties compared to Ni. Also, oxide particles as distributed phase can impart better thermal stability. Hence, particulates of composite Yttria stabilised zirconia, a commonly used high temperature material and alumina (YZA) were reinforced in various Ni–Co alloy matrices through electrodeposition. The influence of YZA on the microhardness, tribology and corrosion behaviour of Ni–Co alloys with Co contents of 0 wt.%, 17 wt.%, 38 wt.% and 85 wt.% was evaluated. Optical and Scanning Electron Microscopy (SEM) confirmed the presence of YZA particles and Energy Dispersive X-ray Analysis (EDX) revealed the composition. Tribology testing showed that composite containing 38 wt.% Co displayed better wear resistance. It was found from the immersion corrosion studies that Ni–17Co–YZA coating displayed improved corrosion resistance. Thermal stability studies showed that Ni–85Co–YZA coating retained its microhardness at temperatures of 600 °C. Thus, these coatings can be tailored for various applications by varying the cobalt content.     

Structural characterization of plasma sprayed basalt–SiC glass–ceramic coatings

In the present study, the effect of SiC addition on properties of basalt base glass–ceramic coating was investigated. SiC reinforced glass–ceramic coating was realized by atmospheric air plasma spray coating technique on AISI 1040 steel pre-coated with Ni + 5 wt.%Al bond coat. Composite powder mixture consisted of 10%, 20% and 30% SiC by weight were used for coating treatment. Controlled heat treatment for crystallization was realized on pre-coated samples in argon atmosphere at 800 °C, 900 °C and 1000 °C which determined by differential thermal analysis for 1–4 h in order to obtain to the glass–ceramic structure. Microstructural examination showed that the coating performed by plasma spray coating treatment and crystallized was crack free, homogeneous in macro-scale and good bonded. The hardness of the coated samples changed between 666 ± 27 and 873 ± 32 HV_0.01 depending on SiC addition and crystallization temperature. The more the SiC addition and the higher the treatment temperature, the harder the basalt base SiC reinforced glass–ceramic coating became. X-ray diffraction analysis showed that the coatings include augeite [(CaFeMg)–SiO₃], diopside [Ca(Mg_0.15Fe_0.85)(SiO₃)₂], albite [(Na,Ca)Al(Si,Al)₃O₈], andesine [Na_0.499Ca_0.492(Al_1.488Si_2.506O₈] and moissanite (SiC) phases. EDX analyses support the X-ray diffraction analysis.     

Effects of Mo addition on tribological performance of plasma-sprayed Ti–Si–C coatings

Ti–Si–C–Mo composite coatings were fabricated by plasma spraying using Ti, Si, graphite and Mo powders. The effect of Mo on microstructure and tribological performance of the Ti–Si–C coatings were investigated. The results showed that the Ti–Si–C coating consisted of TiC, Ti₃SiC₂, Ti₅Si₃, and residual graphite. The Ti–Si–C–Mo coatings consisted of TiC, Ti₃SiC₂, Ti₅Si₃, residual graphite, Mo and Mo₅Si₃ phases. With increasing Mo contents, the fractions of Mo and Mo₅Si₃ phases increased, and the fractions of Ti₃SiC₂ and Ti₅Si₃ phases decreased. All the coatings existed a typical lamellar structure. The addition of Mo enhanced the hardness and fracture toughness of Ti–Si–C coating by 16% and 52%, respectively. The coating porosity decreased by 57.6%. The wear resistance of the Ti–Si–C coating was also improved and the mass loss decreased by 83%. The wear mechanism of the Ti–Si–C–Mo coatings was the combination of abrasive wear, adhesive wear, and tribo-oxidation wear.     

Effects of saccharin on the electrodeposition of Ni–Co nanocrystalline coatings

Nickel–Co nanocrystalline coatings were electrodeposited onto a carbon steel substrate with and without saccharin addition. In the absence of saccharin, current density and adsorption of hydrogen complexes and/or intermediate components were distinguished as two effective parameters causing nanocrystalline electrodeposits. In the latter case, the growth active sites can be blocked easily at low current densities. By increasing the current density, a lower degree of adsorption was associated by a significant increase in surface diffusion of adions resulting in grain growth. Although, the nucleation rate is expected to increase with current density, it seems that the Ni–Co grain size is not reduced by the nucleation rate. Adsorption of saccharin molecules and/or decomposed sulfide species occurred in the saccharin contained bath, resulting in slow surface diffusion of adions. Therefore, finer grains were obtained which produced a smooth morphology instead of the pyramidal forms obtained in the absence of saccharin.     

Influence of additives on microstructure and property of microarc oxidized Mg–Si–O coatings

Ceramic coatings were fabricated on ZK60 magnesium alloy substrate by microarc oxidation (MAO) in Na₂SiO₃–KOH base electrolyte with four kinds of additives (i.e. KF, NH₄HF₂, C₃H₈O₃ and H₂O₂). The effects of these additives on microstructure and property of coatings were investigated. The surface morphology, phase composition and corrosion resistance of the ceramic coatings were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and simulation body fluid (SBF) immersion test respectively. It is found that different additives can change the spark discharge phenomenon during microarc oxidation. It is proved that both potassium fluoride (KF) and ammonium bifluoride (NH₄HF₂) promote discharge and accelerate reaction while the introduce of glycerol (C₃H₈O₃) leads to the refining of sparks and reduction of thermal effects. Results also demonstrate that the introduce of hydrogen peroxide (H₂O₂) contributes to the increase of coating surface roughness and enlargement of surface micropore size. XRD results indicate that the ceramic coatings are mainly composed of Mg₂SiO₄, MgSiO₃ and SiO₂. The introduce of H₂O₂ hinders the reaction between SiO₂ and MgO and creates favorable conditions for the formation of the MgO phase. The ceramic coatings formed in base electrolyte containing 7 g/L NH₄HF₂ and 5 mL/L C₃H₈O₃ exhibit the highest corrosion resistance.     

Study of the degradation of hybrid sol–gel coatings in aqueous medium

The design and development of suitable multilayered functional coatings for delaying corrosion advance in metals and become controlled-release vehicles requires that the properties of the coatings are known. Coatings prepared by the sol–gel method provide a good approach as protective layers on metallic surfaces. This kind of coating can be prepared from pure chemical reagents at room temperature and atmospheric pressure, with compositions in a very wide range of environmentally non-aggressive precursors. Sol–gel coatings based on siloxane bonded units were prepared, starting with an organic–inorganic hybrid system. Synthesis procedures included acid-catalysed hydrolysis, sol preparation, and the subsequent gelation and drying. The alkoxide precursors used were methyl-triethoxysilane (MTMOS) and tetraethyl-orthosilicate (TEOS) in molar ratios of 10:0, 9:1, 8:2 and 7:3. After determination of the optimal synthesis parameters, the materials were characterised by solid ²⁹Si nuclear magnetic resonance (²⁹Si NMR), Fourier transform infrared spectroscopy (FTIR), contact angle measurement and electrochemical impedance spectroscopy (EIS) test. Finally, the materials were assayed by controlling their weight in contact with water, to determine their ability to degrade by hydrolysis. Electrochemical analysis reveals the formation of pores and water uptake during the degradation. The quantity of TEOS is one of the principal parameters that determine the kinetics of degradation. There is a correlation between the degradation process obtained for long periods and the electrochemical parameters obtained by EIS in short times. The study tries to incorporate knowledge that can be used for designing the degradation process of the functional coatings and to control their properties in short times.     

Structure–property correlation study of hyperbranched polyurethane–urea (HBPU) coatings

This paper deals with the structure–property relation of different HBPU coatings based on the variation of parameters like, NCO/OH ratio, generation number and type of diisocyanates used. For this, the NCO terminated HBPU prepolymers were synthesized first by reacting the different generation hyperbranched polyesters (HBPs) with excess diisocyanates. In the next step, these HBPU prepolymer coated films were completely moisture cured to get the desired HBPU coatings. The synthesized polymers were confirmed by ¹H, ¹³C NMR and FT-IR spectroscopy methods whereas structure–property relation was drawn from the FT-IR peak deconvolution technique. The degree of branching (DB) and percent composition of different structural units present in the HBPs were calculated from the ¹H and ¹³C NMR data by using Fretch equation. The melt viscosity study of different HBP samples suggests that most polyester sample showed Newtonian behavior. The coating film properties were studied by DMTA, TGA, UTM, and contact angle measurement instruments. DMTA and TGA data shows that the increase of NCO/OH ratio and generation number had a favorable impact on storage modulus (E′), glass transition temperature (T_g), onset degradation temperature (T_1ON) and char residue values of the coatings. The contact angle and UTM data suggest that the hydrophobicity and tensile strength increases but flexibility decreases with increasing the NCO/OH ratio.     

Bond strength of hybrid sol–gel coatings with different additives

The hybrid sol–gel coating on Al 2024-T3 was modified by adding polyaniline, TiO₂, or γ-Al₂O₃ nanoparticles in the formulation separately. The coating was then used as an adhesive to bond Al 2024-T3 alloys, forming a single lap joint. The bond strength of the sol–gel coating was investigated using a universal tensile test machine. The lap shear strength of the original sol–gel coating was about 1.38 MPa and it was increased up to 2.26 MPa after the modification by adding 0.05 wt% PANI microparticles in the sol–gel coating. The small increase in strength was attributed to an improvement in its adhesive flexibility because of incorporation of the long-chain organic polymer in its structure. Furthermore, the addition of different amounts of TiO₂ nanoparticles in the unmodified sol–gel coating also led to an increase in shear strength compared to the undoped sol–gel coating. Typically, a sol–gel coating containing 2.0 wt% of TiO₂ recorded the highest adhesive strength of about 4.0 MPa. A similar increase in strength was observed when doping γ-Al₂O₃ nanoparticles into the original hybrid sol–gel coating. Adding 0.5 wt% of γ-Al₂O₃ in the sol–gel coating increased the adhesive bonding strength up to 4.48 MPa. The fracture surface of the specimen was separately observed by SEM and Optical Microscopy in order to examine potential evidences of mechanism and nature of failure. The reason why the adhesive strength increased after the modification of the sol–gel coating is discussed in this article.     

Electrochemical synthesis of polythiophene–polyacrylamide composite coatings used for pseudocapacitors

Polythiophene films have been electrochemically deposited onto uncoated and coated with polyacrylamide steel mesh. It has been found that the specific capacitance of the polythiophene film deposited on a sublayer of polyacrylamide was 30% higher than of the film of polythiophene deposited on the steel mesh without the underlayer.     

Adhesion and residual stress of plasma sprayed Alumina–titania coatings

Plasma-sprayed coatings are formed by the impacting of particles onto a fixed substrate layer-by-layer. Residual stresses inside the coatings are essential for their influencing on the coatings’ performance and durability during service life. In the present work, heat transfer and elastic–plastic residual stresses generation during plasma spraying in Al₂O₃–13wt.%TiO₂/NiCrAl (AT13) coating system were analyzed by finite element analysis (FEA). The sophisticated spraying process was simulated and the laminated structure of the coating was modeled under three-dimension. In this simulation, radial and axial compressive stresses were concentrated at the interfaces and inside the bond layer. Besides, at the specimen corner of the free edge, there were high tensile radial and axial stress concentrations. Such remarkable stresses, no matter tensile or compressive, may lead to the delamination and failure of coatings. Comparing with the numerical results, X-ray diffraction measurement was conducted on the AT13 coatings. As a result, the tested values matched well with the FEA simulated results.     

Influence of cobalt on manganese incorporation in Ni–Co coatings

The effect of alloying <1 wt% Mn with plain Ni, Ni–Co alloys and plain Co coatings in terms of the structure and properties has been studied. The alloys were electrodeposited from an additive free sulphamate electrolyte. The Mn concentration in the electrolyte was maintained at 5 g L⁻¹ so as to obtain <1 wt% Mn content in the alloy coatings. The Energy Dispersive X-ray analysis (EDX) showed that the Mn content reduced from 0.97 to 0.05 wt% with increase in Co content from 0 to 98 wt% in the alloy coating. An increase in microhardness was obtained on the addition of Mn to Ni/Ni–Co alloys. The X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) studies revealed a change in crystal structure and morphology. Pin-on-disc tribology test revealed better wear performance of Ni–18 wt%Co–Mn alloy coating compared to the other Ni–Mn/Ni–Co–Mn alloy coatings.     

Synthesis of silane monomer-modified styrene–acrylate microemulsion coatings by photopolymerization

A relatively concentrated silane monomer-modified styrene–acrylate microemulsion coating with high monomer to surfactant ratio of 7.5:1 has been prepared by microemulsion photopolymerization. The properties and the structure of the microemulsion coatings were investigated using TEM, FTIR and UV–vis measurements. The microemulsions are transparent with high transmittance in the visible range. The particle sizes of the produced latexes are in the range of 34–52 nm with the number average diameter of 40.9 nm and D_w/D_n of 1.16. FTIR spectrum indicates the possible structure of the silane monomer-modified styrene–acrylate copolymer, and confirms the hydrolysis and condensation resulting in siloxane bonds between polymer particles. The microemulsion coatings show enhanced acid, base and water resistance with decrease of surfactant content and increase of silane coupling agent.     

Zinc–magnesium-pigment rich coatings for corrosion protection of aluminum alloys

Chromate (Cr(VI))-based pigments have been widely used for corrosion protective coatings because of their outstanding protection efficiency especially for aluminum alloy products. However, due to environmental issues associated with Cr VI, more and more requests are being made for alternative solutions. In the presented work zinc was modified by alloying with magnesium to achieve a combination of properties – cathodic protection and less reactivity during production, storage and application of the pigments. The magnesium content leads to a lowering of the electrochemical potential which allows the cathodic protection of aluminum alloys. zinc–magnesium pigments were prepared in different compositions with special attention to the intermetallic phases MgZn, Mg₂Zn₃, and MgZn₂. Pigments were produced and a zinc–magnesium rich coating was formulated and compounded. Pickled samples of AA 2024 unclad were coated and the corrosion behavior investigated. A durability of more than 10,000 h in salt spray test could be achieved.     

Soy-based UV-curable thiol–ene coatings

Novel soy-based thiols and enes were synthesized and characterized. Then, soy-based thiol–ene UV-curable coatings were formulated and their coating physiochemical properties were investigated in detail. The use of biorenewable resources, combined with environmentally friendly UV-curable technology, provides a “green + green” solution to the stricter regulations in the coatings industry. Novel soy-based thiols and enes were synthesized through the Lewis acid-catalyzed ring opening reaction of epoxidized soybean oil with multifunctional thiols or hydroxyl functional allyl compounds. FTIR and NMR confirmed the formation of the target compounds. The soy-based thiols and enes were formulated with petrochemical-based enes and thiols, respectively, to make thiol–ene UV-curable coatings. Typical coating film properties, thermal properties, and photopolymerization kinetics of these coatings were studied. Soy-based thiol–ene coatings having lower functionality thiols and enes have poor UV curability and coating properties, which was attributed to the lower crosslink density. Soy-based thiols and enes with higher functionality can be UV-cured in combination with petrochemical-based enes or thiols even without the presence of free radical photoinitiators. Better coating film properties were obtained from these higher functionality thiol–ene systems that were toughened by commercial hyperbranched acrylates.     

Electrochemical behavior of hydrophobic silane–zeolite coatings for corrosion protection of aluminum substrate

The anticorrosive properties of a silane–zeolite composite coating applied on a 6061 aluminum substrate was investigated. The composite film, deposited by dip-coating technique, was prepared with different contents of SAPO34 powder (60–90 wt%) with the purpose of evaluating the protective action offered by the zeolite-filled silane matrix. Corrosion protection performance, during immersion in 3.5% NaCl solution, was evaluated by means of a potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS). The coating evidenced good barrier properties and high hydrophobicity. The addition of zeolite in the silane matrix induced, as expected, a reduction of cathodic and anodic current. The zeolite improved the barrier properties of the hybrid sol–gel films, enhancing the resistance to localized corrosion attacks. Better results were observed for the sample with 80 wt% of zeolite filler that evidenced still acceptable protective action after 3 days of immersion in the sodium chloride solution.