High corrosion and wear resistant electroless Ni–P gradient coatings on aviation aluminum alloy parts

A Ni–P alloy gradient coating consisting of multiple electroless Ni–P layers with various phosphorus contents was prepared on the aviation aluminum alloy. Several characterization and electrochemical techniques were used to characterize the different Ni–P coatings’ morphologies, phase structures, elemental compositions, and corrosion protection. The gradient coating showed good adhesion and high corrosion and wear resistance, enabling the application of aluminum alloy in harsh environments. The results showed that the double zinc immersion was vital in obtaining excellent adhesion (81.2 N). The optimal coating was not peeled and shredded even after bending tests with angles higher than 90°and was not corroded visually after 500 h of neutral salt spray test at 35℃. The high corrosion resistance was attributed to the misaligning of these micro defects in the three different nickel alloy layers and the amorphous structure of the high P content in the outer layer. These findings guide the exploration of functional gradient coatings that meet the high application requirement of aluminum alloy parts in complicated and harsh aviation environments.

Corrosion resistance of cerium-doped zinc calcium phosphate chemical conversion coatings on AZ31 magnesium alloy

Zinc calcium phosphate （Zn-Ca-P） coating and cerium-doped zinc calcium phosphate （Zn-Ca-Ce-P） coating were prepared on AZ31 magnesium alloy. The chemical compositions, morphologies and corrosion resistance of coatings were investigated through energy-dispersive X-ray spectroscopy （EDS）, X-ray photoelectron spectroscopy （XPS）, X-ray diffraction （XRD）, electron probe micro-analysis （EPMA） and scanning electron microscopy （SEM） together with hydrogen volumetric and electrochemical tests. The results indicate that both coatings predominately contain crystalline hopeite （Zn3（PO4）2·4H2O）, Mg3（PO4）2 and Ca3（PO4）2, and traces of non-crystalline MgF2 and CaF2. The Zn-Ca-Ce-P coating is more compact than the Zn-Ca-P coating due to the formation of CePO4, and displays better corrosion resistance than the Zn-Ca-P coating. Both coatings protect the AZ31 Mg substrate only during an initial immersion period. The micro-galvanic corrosion between the coatings and their substrates leads to an increase of hydrogen evolution rate （HER） with extending the immersion time. The addition of Ce promotes the homogenous distribution of Ca and formation of hopeite. The Zn-Ca-Ce-P coating has the potential for the primer coating on magnesium alloys.

Microstructure and Performances of Nanocrystalline Zinc-nickel Alloy Coatings

Nanocrystalline zinc-nickel alloy coatings were deposited from an alkaline zincate bath contained an organic additive that can reduce polarization and a complexing agent. SEM and TEM observations and XRD analysis were performed to examine the microstructure and phase composition of the coatings. The nickel content in deposits is 12.0-14.7% and the coating is consisted of single nanocrystalline y-phase structure (Ni5Zn2i), with grain average grain size about 15nm. The nanocrystalline zinc-nickel alloy coatings have better corrosion resistance, less bnttleness and higher microhardness than the conventional zinc coatings.

Effect of Rare Earth on the Corrosion-Resisting Performance of Zinc-Based Alloy Coatings

The corrosion behavior of coatings of pure zinc and Zn-Al,Zn-Al-RE alloys in NaCl solu- tions was studied by salt-spray experiments,even corrosion experiments and electrochemical measurements of bi-directional polarization curves and a.c.impedance in weak polarization region consistent regularities were obtained by these different methods,viz.,the corrosion resistance of Zn could be enhanced by alloying it with Al,and particularly with Al-RE.The causes of enhancement of corrosion resistance by RE were also discussed.

Effect of Rare Earth Elements on Depositing Rate of Nickel Alloy Brush Plating Coatings

The effect of four kinds of rare earth elements on the depositing rate of Ni-based alloy brush plating coatings was investigated. The results indicate that all of the selected rare earth elements increase the depositing rate of Ni-based alloy coatings, and Sm increases the depositing rate most obviously. There is an optimum amount of rare earth addition in the plating solution. With the change of plating voltage to a certain extent, the results reveal no differences. The mechanism of the increase of the depositing rate was analyzed.

Dual Bath Electrodeposition of Alternate Multilayer Coatings of Zinc and Nickel Deposits

The synthesis of zinc and nickel alternate multilayer coatings produced by successive deposition from dual baths containing a revised zinc sulphate electrolyte and a new developed nickel bath has been investigated. Smooth and uniform zinc-nickel compositionally modulated multilayered (CMM) coatings with different multilayer configurations were obtained. The surface and cross-sectional morphologies of samples were examined using scanning electron microscopy (SEM). Cross-sectional morphology showed the layered structure of the coatings clearly.

Review on the phosphate-based conversion coatings of magnesium and its alloys

Magnesium(Mg)and its alloys are lightweight as well as biocompatible and possess a high strength-to-weight ratio,making them suitable for many industries,including aerospace,automobile,and medical.The major challenge is their high susceptibility to corrosion,thereby limiting their usability.The considerably lower reduction potential of Mg compared to other metals makes it vulnerable to galvanic coupling.The oxide layer on Mg offers little corrosion resistance because of its high porosity,inhomogeneity,and fragility.Chemical conversion coatings(CCs)belong to a distinct class because of underlying chemical reactions,which are fundamentally different from other types of coating.Typically,a CC acts as an intermediate sandwich layer between the base metal and an aesthetic paint.Although chromate CCs offer superior performance compared to phosphate CCs,yet still they release carcinogenic hexavalent chromium ions(Cr^(6+));therefore,their use is prohibited in most European nations under the Registration,Evaluation,Authorization and Restriction of Chemicals legislation framework.Phosphate-based CCs are a cost-effective and environment-friendly alternative.Accordingly,this review primarily focuses on different types of phosphate-based CCs,such as zinc,calcium,Mg,vanadium,manganese,and permanganate.It discusses their mechanisms,current status,pretreatment practices,and the influence of various parameters-such as pH,temperature,immersion time,and bath composition-on the coating performance.Some challenges associated with phosphate CCs and future research directions are also elaborated.

ELECTRODEPOSITION OF AMORPHOUS Ni-P ALLOY COATINGS

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Nickel and nickel-phosphorous matrix composite electrocoatings

Nickel and nickel-phosphorous matrix composite coatings reinforced by TiO2,SiC and WC particles were produced under direct and pulse current conditions from an additive-free Watts' type bath.The influence of the variable electrolysis parameters(type of current,frequency of current pulses and current density) and the reinforcing particles properties(type,size and concentration in the bath) on the surface morphology and the structure of the deposits was examined.It is demonstrated that the embedding of ceramic particles modifies in various ways the nickel electrocrystallisation process.On the other hand,Ni-P amorphous matrix is not affected by the occlusion of the particles.Overall,the imposition of pulse current conditions leads to composite coatings with increased embedded percentage and more homogenous distribution of particles in the matrix than coatings produced under direct current regime.

STUDIES OF LOW-CONCENTRATION CHROMATE PASSIVE FILM FOR ZINC-BASED ALLOY COATED STEEL WIRES

The corrosion resistance of a low concentration chromate passive film for zinc based alloy coated steel wires was assessed by salt spray and electrochemical corrosion tests. XPS and AES analyses showed that the composition of such chromate passive film was S 5 5, Na 3 4, C 11 8, Ti 7 9, O 41 6, Cr 13 7, Zn 16 0.

Electroplating zinc transition layer for electroless nickel plating on AM60 magnesium alloys

Electroplating zinc coating as transition layer of electroless nickel plating on AM60 magnesium alloys was investigated. The zinc film can be deposited in a pyrophosphate bath at 50-60℃under current density of 0.5-1.5 A/dm2. A new fore treatment technology was applied by acid cleaning with a solution containing molybdate and phosphorous acid, by alkaline cleaning in a bath containing molybdate and sodium hydroxide. The subsequent electroless plating was carried out in nickel sulfate bath. The SEM observation shows that the deposition is uniform and compact. The polarization curve measurements show that the corrosion potential of the zinc plating in 3.5% NaCl is about -1.3 V(vs SCE) which is noble than that of magnesium substrate. The zinc electroplating can be applied as the pretreatment process for electroless nickel plating on magnesium alloys.

Effect of heat diffusion on properties of zinc-aluminum coating on AZ91D magnesium alloys

A protecting zinc and aluminum coating on the surface of AZ91D magnesium alloys was obtained by thermal spraying to improve the corrosion and wear resistance performances. In order to enhance the combination between magnesium alloy matrix and zinc and aluminum coating, the sample was heat-treated at 300℃for 2 h, then, the cross-section patterns, XRD pattern, micro-hardness, wear and corrosion resistance abilities were researched. The results indicate that the interface between the coating and substrate is metallurgical bond, and a transitional fusion layer is formed by diffusion. The micro-scale abrasion test and polarization test in 3% NaCl solution show that the diffusion-treated specimen has better wear and corrosion resistance performances in comparison with the undiffusion-treated and substrate magnesium alloys; in addition, it has relatively higher micro-hardness than the undiffusion-treated magnesium alloys.

Interface characteristics of Al_2O_3－13%TiO_2 ceramic coatings prepared by laser cladding

Al2O3-13%TiO2 （mass fraction） coatings, prepared by laser cladding on nickel-based alloy, were heated using high frequency induction sources. The coating microstructure and the interface between bond coating and ceramic coating were characterized by SEM, XRD and EDS. The results show that two-layer substructure exists in the ceramic coating： one layer evolving from fully melted region where the sintered grains grow fully; another layer resembling the liquid-phase-sintered structure consisting of three-dimensional net where the melted Al2O3 particles are embedded in the TiO2-rich matrix. The mechanism of the two-layer substructure formation is also explained in terms of the melting and flattening behavior of the powders during laser cladding processing. The spinel compounds NiAl2O4 and acicular compounds Cr2O3 are discovered in the interface between bond coating and ceramic coating. It proves that the chemical reactions in the laser cladding process will significantly enhance the coating adhesion.

Investigation on a Non-cyanide Plating Process of Ni-P Coating on Magnesium Alloy AZ91D

In this research we presented a non-cyanide plating process of Ni-P alloy coating on Mg alloy AZ91D. By applying a new process flow of electroless nickel plating in which zinc coating is used as transition of Ni-P coating on Mg alloy AZ91D, the process of copper transition coating plated in the cyanides bath can be replaced. A new bath composed of NiSO4 was established by orthogonal test. The results show that zinc transition coating can increase the adhesion and protect the Mg alloy substrate from the bath corrosion. The optimal plating bath composition is NiSO4·6H2O 20 g/L, NaH2PO2·H2O20g/L and C6H8O7·H2O 2.5 g/L, and optimal bath acidity and optimal plating temperature are pH 4.0 and 95℃, respectively. The present process flow is composed of ultrasonic cleaning→alkaline cleaning→acid pickling→activation→double immersing zinc→electroplating zinc→electroless nickel plating→passivation treatment. The present non-cyanide process of electroless nickel plating is harmless to our surroundings and Ni-P coating on Mg alloy AZ91D produced by present process possesses good adhesion and corrosion resistance.

Oxidation and hot corrosion behaviors of HVAF-sprayed conventional and nanostructured NiCrC coatings

The oxidation and hot corrosion behaviors of HVAF-sprayed conventional and nanostructured NiCrC coatings were studied.The oxidation experiment was conducted in air,and the hot corrosion was conducted in the Na2SO4-30%K2SO4 environment, in the temperature range of 550-750℃for periods up to 160 h.The corrosion kinetics was tested with the thermogravimetric method. The corrosion products were characterized by scanning electron microscopy(SEM),energy dispersive X-ray spectroscopy(EDX)and X-ray diffractometry(XRD).As indicated by the results,both types of coatings possess high corrosion resistance,especially the nanostructured NiCrC coating.The enhanced grain boundary diffusion in the nanostructured coating not only promotes the formation of a denser Cr2O3 scale with a higher rate,but also helps to mitigate the Cr depletion at the metal/scale interface.The less porosity of the nanostructured coating is also thought to be beneficial to the anti-corrosion properties.

Study on the formation of intermetallic phases in the new type of Zn-Mg coatings on steel sheets

Zn-Mg alloy coatings were obtained by physical vapor deposition （PVD） on electroplated steel sheets. Study on the formation of intermetallic phases in the coatings was conducted through the use of X-ray diffraction （XRD） ,scanning electron microscope （SEM） and glow discharge optical emission spectrometry （GDOES）. It is found that MgZn2 is the main Zn-Mg alloy phase formed after heat treatment. The formation of Mg-Zn intermetallic phases is controlled not only by thermodynamics, but also by kinetics. MgZn2 has different morphologies, such as laminar structure,porous structure and floc-like structure,which are mainly determined by the annealing temperature. Obvious diffusion of Mg starts at 350 ℃, and the diffusion of iron increases significantly when the temperature is elevated to 380℃.

Corrosion behavior of NiCrBSi coatings deposited by HVOF spraying

The corrosion resistance of NiCrBSi coating deposited on steel substrate by HVOF was examined using electrochemical tests and immersion tests so as to offer an experimental basis to expand a promising applied field of HVOF in aqueous medium, comparing with those of coatings deposited by oxyacetylene flame spraying and flame cladding. The results show that the general corrosion rate of HVOF sprayed coatings is quite bigger than that of clad coatings, but it is less sensitive to local corrosion. There is less and smaller porosity in the coatings deposited by HVOF than that in flame sprayed coatings. The effects of porosity on the corrosion current density was indistinctive, but the existence of large amount of defects in the coatings damaged the cohesion of the coatings, causing the metallic particles drop off from the coatings under the influence of corrosive medium. Improving the quality and reducing the porosity of coatings is the key to get the coatings with high corrosion resistance.

Study of a New Chromium-Free Conversion Coating Formed on ZnAl Alloy

A new chromium-free conversion film was obtained on surface of a ZnAl alloy by chemical conversion process. Influence of the additives in treating solution containing cerium salt on the corrosion protection of the conversion film formed on zinc alloy was investigated. Corrosion tests and electrochemical measurements in sodium chloride solution were performed . The microstructure and composition of the coatings were examined by means of SEM, EDS and XRD. It was found that the corrosion protection capabilities of the conversion film are markedly increased with the cerium nitride plus additives (hydrogen fluoride acid and an organic inhibitor) treating process. The modified conversion film is an organic/inorganic composite coating and is much more corrosion resistant than the conventional chromate conversion coating and the single cerium conversion coating.

不同镀层板的电阻点焊腐蚀行为

[目的]研究点焊对不同镀层板腐蚀行为的影响。[方法]对镀锌板和镀锌铝镁板进行电阻点焊,采用扫描电镜和能谱仪分析了点焊部位的微观形貌和元素分布。通过中性盐雾试验和电化学阻抗谱测试研究了两种板材点焊前后在相同环境下的腐蚀行为。[结果]点焊镀锌板的焊点边缘存在大量凸起和凹坑,而点焊镀锌铝镁板的焊点边缘无明显的凸起和凹坑。二者的焊点位置Zn元素挥发明显,均有部分基体裸露。无论点焊与否,镀锌板在中性盐雾试验中的耐腐蚀能力都不如镀锌铝镁板。[结论]电阻点焊容易导致镀层局部破损,影响镀层的耐蚀性。

碳钢表面电镀锌镍/有机富铝复合涂层的制备与性能

为提高碳钢表面电镀锌镍的防护性能和应用限制,采用电镀锌镍层作为底层,有机富铝涂层作为顶层,设计出一种复合涂层防护体系。采用扫描电镜(SEM)、能谱(EDS)、中性盐雾试验、电化学测试等手段对电镀锌镍/有机富铝复合涂层的表面/截面形貌及耐蚀性进行表征与分析。结果表明:电镀锌镍/有机富铝复合涂层表面光滑平整,无明显缺陷,复合涂层体系界面结合良好,无分层现象,经1500 h中性盐雾试验后复合涂层无红锈。复合涂层较单独的电镀锌镍合金层腐蚀电流密度降低了2个数量级,表现出受控的牺牲阳级作用,并实现了在海洋防护领域的应用。复合涂层有效提升了电镀锌镍层的耐腐蚀性能,拓宽了电镀锌镍层在腐蚀防护领域的应用范围。