Effect of cerium concentration on corrosion resistance and polymerization of hybrid sol–gel coating on martensitic stainless steel

Stainless steels are increasingly used in the aeronautics field for the manufacture of structural parts. One of them, the X13VD martensitic stainless steel (X12CrNiMoV12-3), known for its good mechanical properties, has a poor corrosion resistance in confined or severe environments. In the past years, Cr(VI) based pre-treatments have been currently used for corrosion protection of different metals, however, they are toxic and due to environmental regulations, they will be definitely banned in a near future. Alternatives to replace Cr(VI) show advantages and drawbacks considering key properties such as: corrosion resistance, adhesion of coatings, fatigue resistance, durability and reliability. However, some of their possible alternatives show high potential. In this paper, a process was developed to improve the corrosion resistance of the martensitic stainless steel. Organic–inorganic hybrid coatings with different cerium concentrations were deposited onto stainless steel by sol–gel process. Corrosion resistance of the coatings was evaluated by electrochemical impedance measurements and it has been proved that cerium concentration of 0.01 M into hybrid coating was an optimal content. Adhesion tests were also carried out by "nanoscratchtest" to characterize the coatings mechanical properties as a function of cerium concentration but results do not clearly show the influence of cerium for the coating adhesion toward the substrate. To try to correlate with the electrochemical properties, liquid 29Si NMR spectroscopy was then performed to investigate hydrolysis and condensation reactions of sol–gel process, and by this method, it was demonstrated that for higher cerium concentration (>0.01 M) there is a modification of the chemical structure of the sol–gel network

Structure, morphology and mechanical properties of electrodeposited composite coatings Ni–P/SiC

Physical properties of the NiP/SiC deposits are discussed according to the electroplating parameters and heat treatments. The insertion of silicon carbide in the coatings does not modify their rigidity (E = 230 GPa), increases their hardness slightly (50 Vickers) and decreases the residual stresses in the coatings. The phosphorus content has a major effect on the structure and the physical properties. Indeed, the insertion of phosphorus into the deposits generates a grain size reduction or even an amorphisation which results in morphological modifications observed by AFM. In parallel a very important hardening is associated with incorporation of phosphorus: hardness is multiplied by 3 to reach 600 HV0.1. The deposits tend to become crystalline following the heat treatments. With 420 °C, the precipitation of a Ni3P phase which distorts the crystal lattice is observed, increasing the hardness of the deposits

Development of nickel phosphorus coatings containing micro particles of talc phyllosilicates

The present work aims to characterized nickel phosphorus coatings co-deposited with talc particles on steel. The NiP-talc composite deposits were developed to serve as hard coatings with a lubricating effect at 600 C. This process, which is free of hexavalent chromium, could provide a reliable substitute for the electrodeposition of hard chromium coating used in industrial applications. Local responses to static and dynamic mechanical loading have been obtained by nano- and microhardness, microtensile and nanoscratch testing. The hardness and stiffness values slightly decrease when the amount of talc increases for untreated coatings. In contrast, a 420 C heat treatment leads to high hardness and Young’s modulus values due to crystallization. Moreover, a 600 C heat treatment lowers these values through overageing. A 420 C heat treatment greatly improves the adherence and the cohesion of the coatings containing talc

Effect of reinforcing submicron SiC particles on the wear of electrolytic NiP coatings Part 1. Uni-directional sliding

As-plated and annealed NiP coatings and composite NiP-SiC coatings were investigated in uni-directional ball-on-disc sliding tests. Abrasive wear was noticed in the case of composite NiP coatings containing submicron SiC particles, whereas in NiP coatings oxidational wear was active. The addition of submicron SiC particles not only increases the hardness of these electrolytic coatings but also hinders the formation of an oxide film in the sliding wear track. As a consequence, the wear loss on as-plated NiP coatings is not markedly reduced by the addition of SiC particles. On the contrary, a heat treatment at 420 °C for 1 h decreases the wear loss on both pure NiP and composite NiP-SiC coatings. During that heat treatment, Ni3P precipitates are formed in the NiP matrix and owing to this fact, the hardness of both pure NiP and composite NiP-SiC coatings increases. However, the heat treatment of composite NiP-SiC coatings induces the sensitivity for crack formation in the NiP matrix around these SiC particles. As a result, the pull out of SiC particles in the wear track occurs easily during sliding, and the wear loss of composite NiP-SiC coatings remains above the wear loss on NiP coatings

Effect of reinforcing submicron SiC particles on the wear of electrolytic NiP coatings Part 2: Bi-directional sliding

As-plated and heat-treated electrodeposited NiP and composite NiP-SiC coatings were investigated in bi-directional ball-on-disc sliding tests. All tests were performed under gross slip conditions. Heat treatment decreases the wear volume loss during fretting in ambient air for all coatings investigated. Heat-treated NiP coating has a lower wear volume loss compared to composite NiP-SiC coatings for all sliding tests. The wear rate at the bi-directional sliding test was found to be lower relative to the wear rate at uni-directional sliding test

Electrophoretic impregnation of porous anodic aluminum oxide film by silica nanoparticles

In this paper, it is proposed to study the deposition of nanoparticles by electrophoretic deposition (EPD) inside a porous anodic aluminum oxide film. Despite the presence of a highly resistive barrier layer at the metal-anodic film interface, porous anodic films on AA 1050A were successfully filled by 16-nm, surface modified silica particles. During this study it was shown that both the colloidal suspension conductivity and the applied electric field drive the penetration into the porous film. FEG-SEM observations showed that large (130-nm diameter), linear pores of 10 μm in length can be completely filled in 1 min. These results attest that porous anodic films can be efficiently filled with nanoparticles by EPD despite the presence of the barrier layer

Sol–gel processing and characterization of (RE-Y)-zirconia powders for thermal barrier coatings

The effect of doping on the structural, morphological and thermal properties of ZrO2–XO1.5 (X=Y, La, Sm, Er) solid solutions for thermal barrier (TBC) applications was investigated. Oxide powders of various compositions from 9.7 to 40 mol% XO1.5 (X=Y, La, Sm, Er) were synthesised by the sol–gel route. The structural analysis of the powders was performed using X-ray diffraction analysis coupled with Rietveld refinements and the measurement of their specific surface area with the BET method. For each rare earth dopant, the morphology of the powders varies from monoliths to agglomerates of thinner particles when the doping amount increases. In order to determine the specific heat, the thermal diffusivity at room temperature and the thermal expansion coefficient of some selected compositions, DSC, laser thermal diffusivity and hightemperature dilatometry measurements were performed on samples densified by Spark Plasma Sintering. Working thermal characterisation indicated that zirconia doped with 30 mol% SmO1.5 and ErO1.5 have better insulation properties and a lower thermal expansion coefficient than our reference YSZ ceramic. These various compositions are very promising for the elaboration of multilayer TBCs by the sol–gel process

New sol-gel formulations to increase the barrier effect of a protective coating against the corrosion of steels

Films were deposited onto AISI 430 stainless steel substrates by dip-coating technique. The aim is to reach the AISI 304L stainless steel anti-corrosion properties by a coated AISI 430 stainless steel system. Sol formulation is done from the starting precursors tetraethylorthosilicate (TEOS) and 3(trimethoxysilyl) propyl methacrylate (MAP). After the hydrolysis of these precursors, sol-gel reactions occur before the addition (or not) of a controlled quantity of cerium nitrate. The addition of the PEG (polyethylene glycol), used as plasticizer has been studied in this paper and both physical and chemical properties of the synthesized hybrid films were studied by varying PEG ratios. Based on SEM observations and mass gain measurements, the thickness of the films has been adjustable. Another parameter plays a key role: the drying step of the whole system. It has been investigated and optimized in this paper to lead to coatings with a high barrier effect. The efficiency of the anti-corrosion protection of hybrid-coated stainless steel was investigated by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) after immersion of the material in a 3.5% NaCl solution. Double-layered systems were successfully developed and a good compromise between PEG content and drying conditions has been found. Potentiodynamic polarization curves showed that the hybrid coating prepared using a TEOS/MAP/PEG yielded the best anti-corrosion performances. It acts as an efficient barrier similar to AISI 304 stainless steel used as reference, increasing the total impedance and significantly reducing the current densities

Effect of thermal treatment on mechanical and tribological properties of hybrid coatings deposited by sol–gel route on stainless steel

This paper deals with the effect of thermal treatment on the mechanical and tribological properties of an organic–inorganic hybrid coating deposited on stainless steel 430. Organic–inorganic coating derived from glycidoxypropyltrimethoxysilane (GPTMS) and aluminum tri-sec-butoxide Al(OsBu)3 were prepared via sol–gel route and deposited by dip-coating process with various thicknesses. A preliminary thermal analysis (DTA, TGA) of xerogel obtained by hydrolysis and condensation reaction of sols, highlighted three characteristic domains of temperature (110–200°C, 250–300°C, 400–500°C). When thermal treatments were applied to the coated stainless steel in these temperature domains, the tribological behavior (wear and friction) underwent strong changes, analyzed from linear ball/plane tribometry. The tribological tests showed a lower friction coefficient and wear after thermal treatment at a temperature in the domain 250–300°C. In order to explain this phenomenon, xerogel structure was studied from XRD and Raman spectroscopy and correlated to the mechanical and adhesive properties and to the tribological behavior

Improvement of barrier properties of a hybrid sol-gel coating by incorporation of synthetic talc-like phyllosilicates for corrosion protection of a carbon steel

Sol–gel coatings for corrosion protection of metals are a good alternative to toxic chromate treatments. The present work focussed on the incorporation of inorganic fillers in a sol–gel coating to improve the barrier properties of the film. Talc-like phyllosilicates obtained by hydrothermal synthesis at 160°C, 260°C and 350°C, called T160, T260 and T350 respectively, were selected as inorganic fillers. The synthetic materials showed talc lamellar structure but, in contrast with natural talc, their smaller size (about 300 nm) and their hydrophilic character allowed easier dispersion of the particles in the sol–gel matrix. Electrochemical impedance measurements performed on the sol–gel coatings deposited on XC35 carbon steel showed that the incorporation of T260 and T350 at a concentration of 20 g L− 1 strongly enhanced the barrier properties of the coating by comparison with the filler-free system. As a consequence, the corrosion protection of the metal substrate was improved