Corrosion resistance of lamellar aluminium pigments coated by SiO2 by sol–gel method

The aluminium pigments were coated with SiO₂ by sol–gel method to improve their stability. The effects of formulation factors, such as medium of reaction, adding sequence of catalyst and number of coating, were investigated. The stability of the coated aluminium pigments in acid was examined by measuring the hydrogen generation amount. It was found that the coating layer formation is due to the condensation of tetraethyl orthosilicate (TEOS) to form a dense 3D cross-linked layer on the surface of aluminium. The optimized sequence of adding catalysts would be hydrochloride first, then ammonia. Stability tests confirmed that the aluminium pigments have better corrosion resistance after coating with SiO₂.     

Cyclic carburizing behaviour of Al modified high Si-containing HP40 alloy

The chromia layer, which is formed during the pre-oxidation process, can suffer from the reduction during the carburizing. However, the oxide coating retards the growth rate of the external carbide, and thus permits the development of the inner silica layer. Nevertheless, wedging crack of the silica layer during cyclic carburizing facilitates the internal carburization. For the comparison, the high grain boundary density within the Ni, Al rich layer of aluminium adopted alloy promotes the severe internal carburization. However, as the ion diffusion has been blocked by the integrity outer alumina layer, the pre-oxidized Al-adopted alloy almost unaffected during cyclic carburizing.     

Surfactant inhibition of aluminium pigments for waterborne printing inks

Micrometre-sized aluminium particles are used as pigments in silver inks and for environmental reasons it is desirable to develop water-based formulations of such pigments. In waterborne coatings, however, aluminium is prone to react with water, with subsequent hydrogen gas evolution and loss of the silvery lustre. The protection against water of aluminium pigments by adsorption of either a nonionic or an anionic surfactant has been evaluated. Phosphate esters with different lengths of the hydrocarbon tail have been synthesised and tested, and were found to provide very effective protection of the pigments. Sodium dodecyl sulphate (SDS), on the other hand, was completely ineffective as inhibitor, even though the adsorption isotherm was about the same as for the structurally similar sodium dodecyl phosphate (SDP). This difference may be explained by formation of different types of complexes with the aluminium oxide surface. Outer-sphere complexes are suggested for SDS, whilst SDP can form more stable inner-sphere complexes. Tests with the non-surface active methyl phosphate as inhibiting agent gave insufficient protection, indicating that surface activity is crucial for the inhibition, and that the use of the phosphates as such is not enough to give proper protection. The work shows that amphiphilic compounds having phosphate as anchoring group are efficient in providing water resistance to aluminium pigments.     

Corrosion and electrochemical behavior of aluminium treated with high-temperature pulsed plasma in CsCl–NaCl–NaNO3 melt

Dense protective layers of aluminium corrosion products, whose composition depends on the oxidation temperature, are formed on the surface of aluminium treated with high-temperature pulsed plasma (HTPP) without visible remelting and then held in a chloride–nitrate melt in conditions of anodic polarization. Modification of aluminium treated with HTPP changes the properties of 20 μm layer under its surface and the oxide layer formed by such treatment has different morphology: it consists of smaller crystals and so has the other protective properties as compared with aluminium untreated by plasma.     

Corrosion protection of Zn-phosphate containing water borne dispersion coatings: Part 2: Investigations of the corrosive de-adhesion of model latex coatings on iron

Defined mono- and multilayer styrene/acrylate latex films were applied on model oxide covered iron substrates. Unmodified and chemically modified Zn-phosphate pigments were investigated with regard to corrosion inhibiting properties in combination with the model latex films. The nature of these pigments and their location in the coating influencing the de-adhesion kinetics in corrosive environments were studied by means of in situ scanning Kelvin probe measurements, which were correlated with the composition of the coating/metal interface. As shown by means of ATR-FTIR spectroscopy, Zn-phosphates improve the barrier properties. Due to the application of Zn-phosphate containing latex coatings an about 2-3 nm thin inorganic interface layer was formed containing Zn-hydroxides, phosphates, chromates and molybdates in different quantities, depending on the chemical modification of the pigment. The release of pigment ions through the binder matrix is responsible for the inhibition of the cathodic de-adhesion process at the interface. At distances larger than few tenths of nanometers the release effect of the pigments is significantly reduced. Chromate and molybdate modified Zn-phosphates showed a fast release and interfacial enrichment of these ions thereby leading to improved corrosion resistance.     

Resistance to oxidation and ablation of SiC coating on graphite prepared by chemical vapor reaction

A thick SiC coating was prepared on graphite by chemical vapor reaction. The coating reveals a typical crystalline structure with limited porosity and combines well with the substrate. Oxidation tests demonstrate that the coating has a weak self-healing ability at 1100 K and good self-healing ability at temperatures from 1623 to 1823 K. An oxyacetylene torch test verifies that the prepared coating can effectively protect graphite from ablation for 50 s. After the ablation test, the silica microspheres and other interesting silica structures such as microwires, microparticles, microflowers, nanowires and nanoparticles are formed at the ablation center and its surroundings.     

Electrochemical behaviour enhancement of stainless steels by a SiO2 PACVD coating

Amorphous silica coatings have been deposited by plasma assisted chemical vapor deposition (PACVD) process on M2 steel and 304 stainless steel substrates. The chemical inertia of the coating has been established by resonant quartz crystal microbalance (RQCM), and no porosity has been observed at the AFM scale. The barrier effect has been evaluated on M2 steel by applying the concept of dipolar relaxation. The protection is very efficient, and no deleterious galvanic coupling with the uncoated metallic surface can occur.

A coated stainless steel was also investigated in NaCl solution in order to understand how the passive properties are influenced by such a barrier coating. When deposited on stainless steel, coated part keeps the beneficial passive behaviour inherent to the substrate. They evidence almost no corrosion. The silica based layer behaves as a quasi perfect dielectric. The corrosion rate is then greatly reduced, and the pitting resistance is improved. The excellent localized corrosion protection has been explained by a tiny porosity rate, and a limited access of chloride ions through open pores.     

Inert anode composed of Ni–Cr alloy substrate, intermediate oxide film and α-Al2O3/Au (Au–Pt, Au–Pd, Au–Rh) surface composite coating for aluminium electrolysis

An inert anode composed of alloy substrate, intermediate oxide film and surface composite coating for aluminium electrolysis has been fabricated. The intermediate oxide film (ZrO₂/Y₂O₃) provides good adhesion and mutual diffusion resistance between the substrate and the surface coating which consists of α-Al₂O₃ particles embedded in Au (Au–Pt, Au–Pd, Au–Rh) matrix. The results of electrolysis test revealed that aluminium with high purity (>99.999%) can be produced. It is demonstrated that the inert anode exhibits superior erosion and corrosion resistance during aluminium electrolysis, especially in low-temperature (800 °C) electrolytes.     

Plasma electrolytic oxidation of pre-anodized aluminium

The influence of an anodizing pre-treatment in sulphuric acid is investigated on plasma electrolytic oxidation (PEO) of aluminium in silicate electrolyte under constant rms current. The presence of the anodic film is shown to promote the establishment of a micro-arc regime that is favourable for growth of the PEO coating. The incorporation of the pre-formed film into the coating appears to proceed by thermal transformation of the anodic alumina, accompanied by formation of oxide beneath the pre-formed layer. The final coatings contain α- and γ-Al₂O₃, with increased concentrations of silicon, sodium and potassium in an outer region of the coating.     

Protective action towards aluminium corrosion by silanes with a long aliphatic chain

A noticeable and persistent protective action towards aluminium corrosion can be obtained by treatments with silane molecules containing a long aliphatic chain (e.g., n-octadecyl-trimethoxy-silane, C18), which can markedly increase the barrier properties of the silanic coating. At variance with other silanic compounds, the layer of C18, although not uniform, tends to cover the whole aluminium surface completely and, in fact, its EIS spectra initially present one time constant only. It is likely that v.d.Waals interactions between the alkyl chains of C18 can lead to the formation of a polymolecular layer. The aqueous salt solutions may permeate through this layer, but with greater difficulties than in the case of an efficient di-silyl derivative (e.g., 1,2-bis-trioxymethyl-silyl-ethane, BTSE). C18 coatings noticeably retard not only the cathodic oxygen reduction reaction, but also the anodic metal oxidation process. A very prolonged action is also found towards the pitting process in the particularly aggressive chloride solution.     

A modified dual-layer SiC oxidation protective coating for carbon/carbon composites prepared by one-step pack cementation

To improve the oxidation resistance of SiC coating produced by pack cementation for carbon/carbon composites, a modified SiC coating has been produced by one-step pack cementation by adding ferrocene in pack compositions. The as-received coating exhibited a dual-layer dense structure, and oxidation protective ability of SiC coating could be improved by introducing ferrocene. The modified coating could protect C/C composites from oxidation for more than 100 h at 1673 K in air. The weight loss of the coated C/C composites was considered to arise from deflection of penetrating cracks formed in outer layer from inner layer to C/C matrix.     

A two step anodizing process of aluminium as a means for improving the chemical and physical properties of oxide films

An anodizing process was developed to form corrosion resistant and hard oxide films on aluminium. The process consists of two steps: first the formation of chromate/phosphate treated layer on the surface of aluminium and secondly anodizing in a sulphuric acid solution. The anodic oxide films formed by the present process contain Cr(III) and phosphate species mostly in the outer part of the porous layer. The films formed by the present process provided a better corrosion resistance to the substrate aluminium from pitting in a chloride medium than the films formed by conventional anodizing and sealed in a boiling chromate solution. Further, Vickers hardness on the cross section of film increased compared with the films formed by conventional anodizing. This two step process can be developed to form other composite oxide films by using different treatments for the first step.     

Effect of placement of aluminium foil on growth of etch tunnels during DC etching

The tunnel growth behaviour of aluminium foil etched in different placement has been investigated. The limiting length of tunnel of specimens in increasing order is: face-down, vertical, and face-up placement. Many sub tunnels branch on the wall surface of the main tunnel when aluminium is etched in face-down placement, while much fewer sub tunnels are formed when aluminium is etched in face-up placement. A novel etching model has been proposed to interpret the phenomenon, by considering the electrocapillary effect. It reveals that the status and transport of hydrogen bubbles inside the tunnel has a great influence on the tunnel growth.     

Anodic oxides on InAlP formed in sodium tungstate electrolyte

Amorphous anodic oxide films on InAlP have been grown at high efficiency in sodium tungstate electrolyte. The films are shown to comprise an outer layer containing indium species, an intermediate layer containing indium and aluminium species and an inner layer containing indium, aluminium and phosphorus species_. The layering correlates with the influence on cation migration rates of the energies of In³⁺-O, Al³⁺-O and P⁵⁺-O bonds, which increase in this order. The film surface becomes increasingly rough with increase of the anodizing voltage as pores develop in the film, which appear to be associated with generation of oxygen gas.     

Evaluation of the corrosion resistance of an epoxy-polyamide coating containing different ratios of micaceous iron oxide/Al pigments

The corrosion resistance of an epoxy coating reinforced with different ratios of MIO/Al pigments was studied. The coatings properties were investigated by an electrochemical impedance spectroscopy (EIS), salt spray test, cathodic disbonding and a scanning electron microscope (SEM). The corrosion resistance of the epoxy coating was improved using MIO (micaceous iron oxide) and Al pigments. The corrosion resistance of the purely Al pigmented coating was considerably greater than the purely MIO pigmented coating. The cathodic disbonded area of coating was decreased using MIO and Al pigments. The decrease in disbonded area was more pronounced in the presence of Al particles.     

Microstructure and oxidation behaviour of an AlSiY/NiCrAlYSi composite coating at 1150 °C

A composite coating consisting of an outer AlSiY layer and an inner NiCrAlYSi layer has been prepared by a two-step arc ion plating method. The isothermal and cyclic oxidation behaviour of the composite coating at 1150 °C, including the growth of oxide scale and the microstructure transformation of the coatings, have been investigated comparing with the reference coating, NiCrAlYSi. The results show enhanced oxidation performance of the composite coating, which is concerned with its abundant possession of β-NiAl phase reservoirs, providing it the long term healing capacity of re-growing the α-Al₂O₃ scale.     

Low-temperature atomic layer deposition of Al2O3 thin coatings for corrosion protection of steel: Surface and electrochemical analysis

ToF-SIMS, XPS, voltammetry and EIS investigation of the anti-corrosion properties of thin (10, 50 and 100 nm) alumina coatings grown by atomic layer deposition at 160 °C on steel is reported. Surface analysis shows a thickness-independent Al₂O₃ stoichiometry of the coating and trace contamination by the growth precursors. The buried coating/alloy interface has iron oxide formed in ambient air and/or resulting from the growth of spurious traces in the initial stages of deposition. Electrochemical analysis yields an exponential decay of the coating porosity over four orders of magnitude with increasing thickness, achieved by sealing of the more defective first deposited 10 nm.     

Microstructure and High Temperature Oxidation Performance of Silicide Coating on Nb-Based Alloy C-103

The microstructure and oxidation resistance of NbSi₂ coating formed on Nb-based alloy C-103 by a pack siliconization process have been studied. The as-formed coating consists of an outer NbSi₂ layer and an inner Nb₅Si₃ layer. A NbSi₂–Nb₅Si₃ two-phase zone is also present between the above two layers. Weight-change data obtained under isothermal and cyclic oxidation in air at 1100 and 1300°C, suggests that the coating gives oxidation protection up to about 4 h. The oxide scale that formed on the coating during oxidation exposure consists of an outer glassy silica layer and an inner Nb₂O₅-silica mixed layer. Nb₂O₅ phase is also present in the outer silica scale in the form of elongated particles. Oxidation protection is achieved primarily by the presence of the glassy silica layer on the surface. Spallation of this layer during thermal cycling causes significant reduction in the protective life of the coating.     

Cyclic oxidation and interdiffusion behavior of a NiAlDy/RuNiAl coating on a Ni-based single crystal superalloy

A novel NiAlDy/RuNiAl coating was deposited onto a Ni-based single crystal superalloy DD3. The cyclic oxidation and interdiffusion of the NiAlDy/RuNiAl coating and a NiAlDy coating at 1100 °C were investigated. For the NiAlDy coating, secondary reaction zone (SRZ) consisting of γ′, γ and σ-topologically-closed-packed phase (σ-TCP) needles was formed in the superalloy after 100 h annealing. The NiAlDy/RuNiAl coating effectively suppressed the formation of SRZ as the RuNiAl layer worked as a buffer layer. Premature scale spallation was observed on the NiAlDy coating after about 80 h cyclic oxidation, whereas the NiAlDy/RuNiAl coating exhibited improved oxidation-resistance property.     

Corrosion behaviour of molybdate–phosphate–silicate coatings on galvanized steel

In this work, a Cr-free conversion layer based on molybdate–phosphate–silicate (MPS) was synthesised on a galvanized steel by simple immersion and its corrosion behaviour was compared to that of a typical chromate layer. Stationary electrochemical techniques and electrochemical impedance spectroscopy (EIS) were employed to highlight the corrosion mechanisms of both coatings in different NaCl concentrations, immersion times and pH. Contrary to the chromate layer, the MPS coating showed good electrochemical stability even in concentrated NaCl solutions and remarkable electrochemical efficiency. With increasing time, two corrosion stages were associated with the two likely sublayers of the MPS coating. Furthermore, the MPS coating behaved better than the chromate layer in acidic and alkaline pH, especially the latter as a compact corrosion product layer formed. Finally, each coating/electrolyte interface was characterised by an electrical equivalent circuit giving a satisfactory correlation between the experimental and the calculated impedance. It derived that the MPS could be an environmentally friendly alternative to chromating.