Improving corrosion resistance of aluminium metal matrix composites using cerium sealed electroless Ni–P coatings

Abstract

A conversion coating treatment using cerium salts was developed for the surface sealing of electroless nickel–phosphorus (Ni–P) coatings on carbon fibre reinforced aluminium (Cf/Al) composites. The corrosion resistances of uncoated and coated materials (i.e. the Ni–P coating, the Ce conversion coating and Ce sealed Ni–P coatings) were evaluated in 3·5 wt-%NaCl solution using potentiodynamic polarisation and electrochemical impedance spectroscopy. Ce sealed Ni–P coating showed the highest corrosion resistance and clearly improved the overall corrosion resistance of Cf/Al composites. Thus, the Ce sealed Ni–P coating had no obvious microcracks that were generally evident in the more conventional Ce conversion coatings. This is presumed to occur because the electroless nickel surface is relatively homogeneous, compared with the Cf/Al composite surface on which different local coating thicknesses would encourage increased microcrack formation. X-ray photoelectron spectroscopy analysis showed that the Ce conversion coating mainly contained both Ce³⁺ and Ce⁴⁺ species; however, Ce⁴⁺ species were the dominant oxidation state on Ce sealed Ni–P coatings.     

Effect on Corrosion Resistance of Ce-Rich-Sealed Ni-P Coatings on Cf/Al Composite Surface

In this study, Ni-P coatings and sealing of the coatings by Ce-rich solution on Cf/Al composite surface for enhanced corrosion resistance are investigated. The corrosion resistance of uncoating sample in 3.5 wt.% NaCl solution was investigated and a comparison with Ni-P and Ce-sealed Ni-P coatings is given. Effect of Ce-sealing on Ni-P coating is discussed. The results of electrochemical measurements of corrosion performance of Cf/Al composites show that sealing of Ni-P coatings with Ce-rich solution can improve the corrosion resistance. The Ce-rich-sealed Ni-P coating has higher corrosion resistance than the coating without Ce, and the electroless plated Ni-P coating on composite surface has higher corrosion resistance than the bare sample, as evidenced by EIS and potentiodynamic polarization measurements. The microstructure of the Cf/Al composites and the two kinds of coatings (i.e., Ni-P coating and Ce-sealed Ni-P coating) were examined by scanning electron microscopy, energy dispersive spectroscopy, and transmission electron microscopy. The Ce-sealed Ni-P coatings on Cf/Al composite surface have a total thickness of ~11 μm of which 10 μm is the thickness of the Ni-P coating and ~1 μm is the thickness of the Ce-rich sealing. It shows that the selected area electron diffraction ring pattern of Ce-rich sealing on Ni-P plated composite is consistent with Ce₆O₁₁ or CeO₂. X-ray photoelectron spectroscopy results show that Ce⁴⁺ was the dominant oxidation state for Ce-rich sealing on Ni-P plated composite. The Ce-sealing treatment on Ni-P coating has improved the corrosion resistance over and above the corrosion resistance offered by the Ni-P mono-coating to the bare substrate.     

Corrosion resistance of cerium‐based chemical conversion coatings on AA5083 aluminium alloy

In this paper the effect of several parameters, such as temperature, time of immersion, cerium ions and hydrogen peroxide concentration, pH of the conversion solution, on the composition and morphology of the conversion layer are investigated as well as on its corrosion resistance in chloride environments. The cerium‐based chemical conversion coatings ennobles the corrosion potential and inhibits both the cathodic and anodic reactions rate. Using a cerium (III) chloride solution a not homogeneous coating is obtained and agglomerates with a “dry‐mud” morphology of mixed cerium‐aluminium oxide are deposited above the cathodic intermetallic particles, while using a cerium (III) nitrate solution the coating is more uniform but thinner than that obtained with cerium (III) chloride. Solution temperature below 50°C and time of immersion of 10 minutes produces a coating with better corrosion resistance.     

The preparation and characteristics of a rare earth/nano-TiO2 composite coating on aluminum alloy by brush plating

Nano-TiO₂ modified rare earth composite coatings are prepared on 2024 aluminum alloy by brush plating. The composite coating is composed of mainly Ce(OH)₃, Ce(OH)₄, CeO₂ and TiO₂, with less cracks and lower porosity. The addition of nano-TiO₂ enhances the adhesive strength of the rare earth coating to Al substrate, results in refined coating grains and increases the micro-hardness of the coating. The nano-TiO₂ modification obviously improves the corrosion resistance of the rare earth coating. For the composite coating containing 2% TiO₂, both the corrosion current density and the impedance are reduced by more than one order of magnitude in contrast to the values for the pure rare earth coating. The higher barrier ability and increased Ce³⁺ content in the Ce oxides may explain the increase of the corrosion resistance.     

Preparation and corrosion resistance of titanium–zirconium–cerium based conversion coating on 6061 aluminum alloy

This paper aims to develop a chromium-free chemical conversion coating with good corrosion resistance. A novel chemical conversion coating was prepared on 6061 aluminum alloy by dipping in the treatment solution containing titanium/zirconium based-ions and sodium metaphosphate and cerium nitrate hexahydrate as additives. The morphology and composition of the conversion coatings were observed by scanning electron microscopy and energy dispersive X-ray spectroscopy. The microarea structure of conversion coatings at different formation stages was analyzed by electron probe microanalyzer. The electrochemical polarization curve revealed that the corrosion potential of the conversion coating was −0.577 V and the corrosion current density was 0.1148 μA/cm². The equivalent circuit fitted by AC impedance showed that the film resistance reaches 68,140 Ω. The formation of coating preferentially grows on the Al (Fe) Si intermetallic to form oxides of Ti and Zr; then TiO₂ formed by a higher concentration of Ti⁴⁺ gradually covered ZrO₂. Ce³⁺ could adsorb on the intermetallic compound, the hydrolysis of which causes the local pH of the solution to decrease and promotes the aluminum alloy dissolved.     

Cerium‐based thermal conversion treatments on silicon carbide reinforced 2009 aluminum alloy composites

The conversion coatings on SiC_p/2009 aluminum (Al) composites and 2009 Al alloy were obtained by immersing the samples into the cerium (Ce)‐based solutions with varying the chemical concentration (CeCl₃·7H₂O, NaCl), solution pH/temperature, immersing time, and drying temperature. The corrosion inhibition mechanism and kinetics were studied by using the electrochemical techniques and surface analyses. Potentiodynamic polarization tests showed that coatings improved corrosion resistance as compared to untreated samples and environmental factors played important role in the formation of conversion layer. Scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), and X‐ray photoelectron spectroscopy (XPS) analysis revealed that relatively high inhibition efficiency for composite with 25 vol% SiC_p was due to the existence of rather high surface area of cathodic intermetallics/SiC_p, which support to deposit Ce oxides/hydroxides on the entire surface.     

Improvement in the corrosion resistance of TiB2/A356 composite by molten-salt electrodeposition and anodization

This study reported a novel treatment to improve the corrosion resistance of TiB₂/A356 composites. The method was employed in combination of the molten-salt electrodeposition and subsequent electrochemical anodization technique. By means of molten-salt electrodeposition, the Al coatings were deposited on the surface of TiB₂/A356 composites. It was found that the morphology of the Al coatings is closely related to the current density. Thus, under the suitable condition, a dense and uniform Al coating can be obtained, with the crystal size in the range of 0.5–2 μm and the coating thickness of ~ 9 μm. This continuous Al layer can eliminate the adverse corrosion contribution of TiB₂ particles in Al matrix. The following step of anodization was designed to convert the Al film to an anodized Al oxide film for further corrosion protection. The electrochemical corrosion behavior was evaluated by potentiodynamic polarization curves and electrochemical impedance spectroscopy. These results showed the corrosion resistance was greatly enhanced in TiB₂/A356 composites with an anodized Al-coating than that of the anodized composites. It is evident that the new treatment of metal electrodeposition in molten salts and following anodization is an effective method of anti-corrosion in composites.     

The hot corrosion behaviour of HVOF sprayed MCrAlX coatings under Na2SO4 (+NaCl) salt films

The hot corrosion behaviour of two NiCoCrAlYTa and CoCrAlYSi HVOF sprayed coatings and a CoCrAlY VPS coating were investigated under laboratory conditions at 900°C using a synthetic gas atmosphere containing sulphur as an impurity. All the coatings tested showed good protection under Na₂SO₄ salt films. In the presence of NaCl in the Na₂SO₄ salt films, the corrosion rates of low Al containing coatings increased considerably but the NiCoCrAlYTa coating with higher Al content still revealed good performance. It is suggested that NaCl in the salt film causes premature failure of the protective scale and reduces the incubation period of corrosion in the coatings of lower Al content. Furthermore, it seems that the finely dispersed Al rich oxide particles in the sprayed and heat‐treated HVOF coating microstructure do not lead to internal corrosion. The experimental investigations include short‐term corrosion kinetic measurements and SEM analyses.     

Cerium conversion coatings for AZ91D magnesium alloy in ethanol solution and its corrosion resistance

Golden-yellow-colored cerium conversion coatings on AZ91D magnesium alloy were obtained by immersion in ethanol solution and post-treated in 3.0 wt.% Na₃PO₄ aqueous solution. SEM revealed that the coatings deposited more heavily on α phase than on β phase. XPS results showed that the coatings consist of CeO₂, Ce₂O₃, CePO₄, Al₂O₃, Mg₃(PO₄)₂ and MgO. Corrosion tests indicated that the coatings with post-treatment significantly reduced the corrosion rate of AZ91D alloy in NaCl solution. The post-treatment is necessary for better corrosion resistance. The corrosion resistance of the coatings with post-treatment is superior to that of DOW No.1 coating.     

Corrosion of alloys and their diffusion aluminide coatings by KCl:K2SO4 deposits at 650 °C in air

P91 ferritic‐martensitic steel, 17Cr–13Ni and alloy 800 austenitic stainless steels and Inconel 617 alloy have been aluminised to form Fe₂Al₅, (Fe,Ni)Al and Ni₂Al₃ aluminide coatings. These alloys and their corresponding coatings were subjected to corrosion in air by 50:50 mol/mol K₂SO₄/KCl deposits at 650 °C for 300 h. With the exception of the Inconel 617 alloy, significant metal losses (>180 µm) were recorded. These losses were planar for P91 alloy but involved internal corrosion for the two austenitic steels. The (Fe,Ni)Al and NiAl coatings on the austenitic steels and the Inconel 617 alloy were significantly corroded via intergranular and internal chloridation–sulphidation–oxidation. In contrast, the Fe₂Al₅ coating on the P91 alloy coating was virtually unattacked. For the alloys, the relative extents of corrosion damage can be explained in terms of the stability and volatility of metal chlorides formed. For the coatings, STEM/EDS analyses enable clear linkages to be made between the presence and number of Cr‐rich particles on coating grain boundaries and the corrosion damage observed for the coatings.     

Glass-like CexOy sol–gel coatings for corrosion protection of aluminium and magnesium alloys

This work reports the preparation of glass-like, environmentally-friendly, cerium-based coatings for active corrosion protection of aluminium and magnesium alloys. It describes the production of cerium sol–gel sols from cerium nitrate and their deposition by immersion and automatic spray onto aluminium and magnesium alloys to produce uniform coatings with amorphous (glass-like) structures (Ce_xO_y). The coatings have been characterised by profilometry, scanning electron microscopy (SEM), spectral ellipsometry and UV–visible, in order to analyse the structure and density of the glass-like cerium coatings as well as their redox ratio Ce⁴⁺/Ce³⁺ as a function of pH and sintering temperature. Finally, electrochemical measurements (EIS) and standard corrosion tests (neutral salt spray, filiform corrosion, immersion–emersion test and adhesion on embossing and T-bend test) have been performed to study the corrosion behaviour of the cerium glass-like coatings on aluminium and magnesium alloys. EIS measurements confirm the healing or blocking of the defects by the corrosion inhibiting species. Excellent corrosion protection is provided by cerium glass-like coatings, satisfying the most demanding industrial requirements.     

稀土铈对化学镀复合镀 Ni-P-PTFE 镀层耐蚀性能的影响

为了进一步提高化学镀Ni-P-PTFE复合镀层的耐蚀性能,采用在化学镀液中添加稀土铈的方法在45号碳钢试片表面制备了稀土铈Ni-P-PTFE复合镀层。用扫描电镜观察了镀层的表面形貌,并研究了稀土铈浓度对镀层中PTFE含量的影响,通过浸泡失重法分别研究了在3.5%NaCl和3.5%NaOH溶液中稀土铈浓度对镀层耐蚀性能的影响。结果表明:适量稀土铈的加入提高了镀层中PTFE的含量,降低了镀层的腐蚀速率,提高了复合镀层的耐蚀性能,在铈质量浓度为0.02 g/L时,在3.5%NaCl和3.5%NaOH溶液中镀层的腐蚀速率分别为0.402 mg/cm2和0.235 mg/cm2。     

Designing for the cerium-based conversion coating with excellent corrosion resistance on Mg–4Y–2Al magnesium alloy

The cerium salt chemical conversion baths containing KMnO₄ are applied to prepare protective coatings on the WA42 alloy surface, and the effect of the concentration of KMnO₄ on the microstructure and corrosion properties of the coatings is investigated by scanning electron microscopy, X-ray photoelectron spectroscopy, and electrochemical tests. The results indicate that with the addition of KMnO₄ to the conversion bath, the microstructure of the coating is more uniform and denser, and the coating with the KMnO₄ concentration of 4 g/L (4M coating) has the most uniform microstructure with the least microcracks. The 4M coating exhibits a two-layered structure, and it is mainly composed of MgO, Mg(OH)₂, CeO₂, Ce₂O₃, Ce(OH)₃, MnO, and MnO₂. In addition, as the KMnO₄ concentration increases from 0 to 6 g/L, the I_corr of the coatings in 3.5% NaCl solution decreases first and then increases, and the 4M coating shows the best corrosion resistance, which should attribute to the uniform and dense microstructure.     

Ce 盐封孔对 SiCp/ Al 复合材料表面 Ni-P 镀层的影响

为提高SiCp/Al复合材料的耐腐蚀性能,先化学镀镍,再沉积稀土封孔,讨论了稀土溶液主盐Ce(NO3)3浓度和沉积时间对镍-稀土多层膜耐蚀性能的影响。结果表明:化学镀镍的SiCp/Al复合材料在室温下沉积稀土时,采用Ce(NO3)3含量1 g/L、成膜时间2 h的条件获得的多层膜耐蚀性最好,其腐蚀电位为-0.48mV,腐蚀电流密度为3.54×10-8A/cm2;稀土在膜层中以Ce的氧化物颗粒堆积状态存在,起到了封孔的作用;膜层中的镍磷合金呈多晶态,而稀土含量少,未能测出;稀土溶液浓度越高,沉积速度越快,而在相同浓度下,膜层厚度随着时间的延长而增加,越厚则膜层结合力越差。     

Comparative study on corrosion protection properties of electroless Ni‐P‐ZrO2 and Ni‐P coatings on AZ91D magnesium alloy

Electroless Ni‐P‐ZrO₂ and Ni‐P coatings on AZ91D magnesium alloy were prepared, and their corrosion protection properties were compared in this paper. The potentiodynamic curves and electrochemical impedance spectroscopy (EIS) of the coated magnesium alloy in 3.5% NaCl solution showed that the corrosion performance of Ni‐P‐ZrO₂ composite coating was superior to that of Ni‐P coating. The same conclusion was obtained with salt spray and immersion tests. The corrosion morphologies of two kinds of coatings with various immersion time intervals in 3.5% NaCl solution indicated that most corrosion products concentrated on the nodules boundaries of Ni‐P coating and blocked corrosion pit was the main corrosion form. For the Ni‐P‐ZrO₂ coating, tortuous nodules boundaries were not the weak sites of the coating and corrosion initiated from the nickel phosphor alloy around the nanometer powders. Open corrosion pits occurred on the composite coating surface, and the coating was corroded gradually. Thus, the Ni‐P‐ZrO₂ coating exhibited better corrosion protection property to magnesium alloy substrate than Ni‐P coating.     

Phytic acid conversion coatings on magnesium surface treatment with cerium chloride solution

A chromium-free composite conversion treatment for magnesium by phytic acid and CeCl₃ solutions was studied. The composite coatings on the surface of magnesium presented network-like cracks, which was consisted of Mg, O, C, P and Ce elements. The distribution of Ce on the surface layer was non-uniform. The crack sizes of the composite coating were smaller than that of the phytic acid conversion coating. The corrosion process of the coatings was evaluated through electrochemical impedance spectroscopy in 3.5% NaCl solution, and the equivalent circuit of RL(Q(R(QR))) model was obtained through the characteristics of EIS. The average corrosion rate of the composite coating was reduced at least 50% compared with that of pure phytic acid conversion coating.     

Molten salt synthesis of perovskite conversion coatings: A novel approach for corrosion protection of stainless steels in molten carbonate fuel cells

A novel conversion coating process has been developed to meet the stability requirements of stainless steel hardware in the demanding MCFC fuel cell environments. The process applies a perovskite-based coating by exploiting spontaneous oxidizing reactions of the metallic surface with La₂O₃ in eutectic alkali carbonate mixtures. By using well controlled synthesis procedures, conversion coating layers covering the entire metallic surface with a uniform and compact structure could be obtained. The as-formed coatings with a surface morphology of agglomerated crystallite particles consisted of a thin (<5 μm) LaFeO₃ perovskite layer grown over a thicker (>5 μm) LiFeO₂-rich layer. Test coupons of 316L stainless steel with the perovskite conversion coating were analyzed for corrosion protection and interfacial resistivity properties. It was found that the conversion coating is highly conductive while showing excellent long-term corrosion stability in simulated MCFC environments. These results suggested that perovskite coatings formed by molten salt conversion reactions could be particularly attractive to confer optimal protection and electrical continuity to MCFC current collectors.     

Corrosion resistance of Mg−Al LDH/Mg(OH)2/silane−Ce hybrid coating on magnesium alloy AZ31

An environmentally-friendly hybrid coating on AZ31 magnesium alloy substrates was reported. The synergic effect was studied on Mg−Al-layered double hydroxide Mg−Al LDH/Mg(OH)₂-coated AZ31 magnesium alloy via an in-situ steam coating process and a subsequent combined surface modification of bis-[triethoxysilylpropyl]tetrasulfide (BTESPT) silane and Ce(NO₃)₃. The microstructure and composition characteristics of the hybrid coatings were investigated by means of X-ray diffraction (XRD), scanning electronic microscopy (SEM), Fourier transform infrared spectrophotometry (FT-IR) and X-ray photoelectron spectroscopy (XPS). The corrosion resistance of the coated samples was evaluated by potentiodynamic polarization (PDP), electrochemical impedance spectrum (EIS) and hydrogen evolution rate during immersion in 3.5 wt.% NaCl solution. The results show an improved corrosion resistance of the alloy in the presence of BTESPT silane and Ce(NO₃)₃. This is most likely due to the synergistic effect of steam coating and silane coating to enhance the barrier properties of hybrid coating. In addition, the formation mechanism and anti-corrosion mechanism of coatings were discussed.     

Cold-Sprayed Al Coating for Corrosion Protection of Sintered NdFeB

A protective Al coating was achieved on the sintered NdFeB magnet by cold spray. The sprayed Al particles generate plastic deformation and hang together. The thickness of the coating is about 170 μm. The corrosion currents of Al coating and NdFeB without immersion tested by potentiodynamic polarization in 3.5 wt.% NaCl solutions are 1.350 × 10^?6 and 4.361 × 10^?6 A/cm², respectively. X-ray photoelectron spectrometry results confirm that the oxide film is Al₂O₃ and the corrosion process can be derived into two different stages. The Al coating can provide long-term protection for NdFeB effectively.