Optimization of deposition conditions for bright Zn-Fe coatings and its characterization

Sulfate bath having ZnSO₄ · 7H₂O, Fe₂(SO₄)₃ · H₂O and thiamine hydrochloride (THC) and citric acid (CA) in combination, represented as (THC + CA) was optimized for deposition of bright Zn-Fe alloy coating on mild steel. Bath constituents and operating parameters were optimized by standard Hull cell method, for peak performance of the coating against corrosion. The effect of current density (c.d.), pH and temperature on deposit characters, such as corrosion resistance, hardness and glossiness were studied and discussed. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods were used to assess the corrosion behaviors. Surface morphology, and composition of the coatings were examined using Scanning Electron Microscopy (SEM), interfaced with EDXA facility, respectively. The Zn-Fe alloy, with intense peaks corresponding to Zn(100) and Zn(101) phases, showed highest corrosion resistance, evidenced by X-ray diffraction (XRD) study. A new and cheap sulfate bath, for bright Zn-Fe alloy coating on mild steel has been proposed, and results are discussed.     

Characterisation and corrosion resistance of Zn–Mn coatings electrodeposited from acidic chloride bath

Zn–Mn alloy coatings were galvanostatically electrodeposited from an acidic chloride bath. Effects of deposition current density, pH and temperature on surface morphology, microstructure and corrosion resistance of Zn–Mn coatings were studied. The coatings deposited at 10, 50 and 100 mA cm^?2 had a single η-Zn phase structure. However, a dual phase structure of η-Zn and ?-Zn–Mn with higher Mn content was found for the coatings deposited at 200 mA cm^?2. The dual structure degraded the corrosion resistance of the coatings. The highest corrosion resistance was achieved for the Zn–Mn coating deposited at 100 mA cm^?2, pH 4·9 and 25°C. This coating contained 4·1 wt-%Mn and showed a unique surface morphology consisting of randomly arranged packs of very thin platelets, laid perpendicular to the surface and provided a high compactness deficient free structure.     

A study of chromium-free pickling process before electroless Ni-P plating on magnesium alloys

A chromium-free pickling process of magnesium alloys in H₃PO₄ + Na₂MoO₄ solution for electroless Ni-P plating was described. The dosage of Na₂MoO₄ was established by detecting adhesion and corrosion resistance of chemical nickel coatings. Electrochemical behaviors of pickling solution of H₃PO₄ + Na₂MoO₄ and NH₄HF₂ activation solution were investigated with the open circuit potential curves and the polarization curves. The results show that the Na₂MoO₄ has strong inhibition ability. Na₂MoO₄ in H₃PO₄ solution can reduce active sites of microcathodic and microanodic zones in the corrosion cells of the substrate surface of magnesium alloys and plays an inhibition role. The activation film with some oxides and fluorides can prevent the substrate magnesium from the fierce displacement and corrosion reaction of electroless plating bath. The chemical Ni-P coating with good adhesion and corrosion resistance was obtained by the pretreatment of 200 cm³ dm⁻³ 85% H₃PO₄ + 5 g dm⁻³ Na₂MoO₄ pickling bath and activation in 200 g dm⁻³ NH₄HF₂ solution. This procedure of surface pretreatment before electroless nickel plating can replace the existing acid pickling containing chromium and HF activation.     

A Comparative Study on the Effect of MWCNT as Reinforcement on the Corrosion Parameters of Different Ni–W/MWCNTs Nanocomposite Coatings in Various Corrosive Media

Nickel–tungsten multi-walled carbon nanotubes (Ni–W/MWCNTs) nanocomposite coatings were co-electrodeposited in the ammonium-free bath by means of constant direct current coulometry. The results indicate that the amount of MWCNTs incorporated into the nanocomposite coatings has a key role in the improvement of their microhardness and corrosion resistance. The corrosion behavior of the coatings was evaluated using potentiodynamic polarization and electrochemical impedance spectroscopy methods in three corrosive media of 3.5 wt% NaCl, 1.0 M NaOH, and 0.5 M H₂SO₄. The experimental data of the corrosion current density (j_corr), corrosion rate (CR), the polarization resistance (R_p), and microhardness indicate that the presence of MWCNTs in coatings improves the quality of those coatings. The surface morphology of the coatings and the elemental analysis data were obtained by scanning electron microscopy and energy dispersive X-ray microanalysis respectively. As the results showed, the coatings were uniform and crack-free in the presence of 5.3 wt% carbon. Also, a microhardness test revealed that the nanocomposite coating containing 5.3 wt% carbon obtained in an ammonium-free bath which provided the higher content of tungsten had the highest hardness value among others.     

Study on corrosion behaviour of nanocrystalline and amorphous Co–P electrodeposits

Abstract

In this study, corrosion properties of nanocrystalline and amorphous Co–P coatings prepared using dc electrodeposition were investigated. The morphology of amorphous coatings was smoother and brighter than nanocrystalline coatings with 'cauliflower' morphology. Tafel polarisation tests of the coatings in 0·1M H₂SO₄ solution revealed that the amorphous coating had better corrosion resistance than the nanocrystalline one. Corrosion resistance of both coatings decreased after annealing. On the nanocrystalline coating, corrosion attacks were localised around the nodules on the surface while a more uniform type of attack was observed on amorphous coatings. Neither nanocrystalline nor amorphous coatings showed passivation in 0·1M H₂SO₄ solution, but both of them showed an active–passive behaviour in 10%NaOH solution due to the formation of Co(OH)₂ as fine hexagonal shape products which acted as a passive layer. The passive current density of the amorphous coating was higher than that of the nanocrystalline one, but it was decreased markedly by annealing. However, annealing had no significant effect on the passivation behaviour of the nanocrystalline coating.     

Ni-Zn-P合金镀层在人工模拟海水中腐蚀行为的研究

目的 提高金属材料在海洋环境中的耐腐蚀性和使用寿命.方法 采用碱式化学镀方法 在Q235碳钢表面施镀Ni-P镀层和Ni-Zn-P合金镀层,镀液配方NiSO4·6H2 O 20~25 g/L,C6 H5 O7 Na3·2H2 O 50~70 g/L,NH4Cl 25~30 g/L,NaH2PO2·H2O 15~25 g/L.制备Ni-Zn-P合金镀层时,在以上配方中加入0.4~0.8 g/L ZnSO4·7H2 O.采用金相显微镜和扫描电子显微镜(SEM)观察镀层在人工模拟海水中腐蚀前后的组织形貌,用能谱分析仪(EDS)分析镀层腐蚀前后表面成分.结果 Ni-P镀层和Ni-Zn-P合金镀层中的P质量分数分别为11.26%和9.97%.从P含量和镀层组织形貌,可以确定得到的两种镀层是连续致密的非晶镀层.Ni-Zn-P合金镀层比Ni-P镀层的胞状组织更加均匀平滑,胞与胞的边界结合更加连续致密.在人工模拟海水中腐蚀144 h后,Ni-P镀层出现明显的点蚀坑,Ni-Zn-P合金镀层仍然连续完整.Ni-Zn-P合金镀层腐蚀后,Zn含量明显下降,并出现少量的Fe和O,表明合金镀层腐蚀过程是Zn优先被腐蚀,然后镀层逐渐被腐蚀破坏,最后基体发生腐蚀.Ni-Zn-P合金镀层的腐蚀速率明显低于Ni-P镀层的.结论 Ni-Zn-P合金镀层的胞状组织比Ni-P镀层的更加均匀平滑,胞与胞的边界结合更加连续致密,Ni-Zn-P合金镀层腐蚀速率明显低于Ni-P镀层.     

Zinc phosphate conversion coatings on magnesium alloys: A review

Phosphating is one of the most widely used surface treatments of steels and aluminum due to its low-cost, easy mass production, good corrosion resistance and good adhesion with paint. Many researchers have tried to expand applications of the phosphating process, especially to magnesium alloys for automobiles and aerospace applications. Recently, the coatings on magnesium alloys by zinc phosphate conversion coatings (Zn₃(PO₄)₂·4H₂O) have been intensively studied. This paper reviews the state-of-the-art of phosphate conversion coatings developed for magnesium alloys, in terms of coating properties, phosphate conversion coatings processes and compositions of phosphating bath.     

A Black Phosphate Conversion Coating on Steel Surface Using Antimony(III)-Tartrate as an Additive

A novel black phosphate conversion coating was formed on steel surface through a Zn-Mn phosphating bath containing mainly ZnO, H₃PO₄, Mn(H₂PO₄)₂, and Ca(NO₃)₂, where antimony(III)-tartrate was used as the blackening agent of phosphatization. The surface morphology and composition of the coating were characterized by scanning electron microscopy, energy dispersion spectroscopy, and x-ray photoelectron spectroscopy. Corrosion resistance of the coating was studied by potentiodynamic polarization curves and electrochemical impedance spectroscopy. The pH value of the solution had significant influence on the formation and corrosion resistance of the coating. The experimental results indicated that the Sb plays a vital role in the blackening of phosphate conversion coating. The optimal concentration of antimony(III)-tartrate in the phosphating bath used in this experiment was 1.0 g L^?1, as higher values reduced the corrosion resistance of the coating. In addition, by saponification and oil seals, the corrosion duration of the black phosphate coating in a copper sulfate spot test can be as long as 20 min.     

ZM5镁合金无铬前处理化学镀镍层的性能

采用优化的Na4P2O7+Na2SO4+NaNO3体系的化学蚀刻无铬前处理化学镀镍工艺,在ZM5镁合金上制备Ni-P镀层。利用扫描电镜、能谱仪、X射线衍射仪分析镀层的微观形貌、成分和相结构。通过电化学方法和摩擦磨损试验评价了镀层的耐蚀性和耐磨性。结果表明:无铬前处理工艺制备的镀层中P的质量分数为12.90%。与ASTM标准的含铬前处理工艺得到的镀层的耐蚀性和耐磨性相比,无铬前处理得到的镀层的自腐蚀电位为-0.506V,腐蚀电流密度为2.132×10-6 A/cm2,接近ASTM工艺含铬前处理得到的镀层的耐蚀性能;同时其磨损率为3.056×10-4 mg/s,与ASTM工艺的1.778×10-3 mg/s相比,其抗摩擦磨损性能明显优于含铬前处理的镀层。无铬前处理化学镀镍显著提高了ZM5镁合金的耐蚀性和耐磨性。     

Corrosion properties ofAZ31 magnesium alloy and protective effects of chemical conversion layers and anodized-coatings

The corrosion properties of AZ31 magnesium alloys were studied by potentiodynamic polarization curves and electrochemical impedance spectroscopy（E1S） techniques, meanwhile, the protective properties of two environmentally protective types of chemical conversion layers and anodized coatings of AZ31 magnesium alloys were also discussed. The component of chemical conversion bath is NaH2PO4·12H2O 20 g/L, H3PO4 7.4 mL/L, NaNO2 3 g/L, Zn（NO3）2·6H2O 5 g/L and NaF 1 g/L, and components of the anodization bath is Na2SiO3 60 g/L, C6H5Na3O7·2H2O 50 g/L, KOH 100 g/L and Na2B4O7·2H2O 20 g/L. The results show that the corrosion resistance of AZ31 magnesium increases with the increase of pH value of the corrosive medium. For the chemical conversion layer acquired at 80 ℃, 10 min is the best processing time and the charge transfer resistance of the chemical conversion layer is enhanced nearly by 10 times. The optimum processing time for the anodization of AZ31 is 60 min, the charge transfer resistance value of the anodized sample at the early immersion stage is nearly 26 times of that of the blank sample and the corrosion type of the anodized samples is pitting.     

Electrodeposition of high performance multilayer coatings of Zn–Co using triangular current pulses

Abstract

Compositionally modulated alloy (CMA) coatings of Zn–Co were electrodeposited on to mild steel from an acid chloride bath containing thiamine hydrochloride, as an additive. Electroplating was carried out galvanostatically from a single bath containing Zn²⁺ and Co²⁺ ions. Gradual change in composition in each layer was effected by triangular current pulses, cycling between two cathode current densities. Compositionally modulated alloy coatings were developed under different conditions of cyclic cathode current density and number of layers, and their corrosion resistances were evaluated by potentiodynamic polarisation and electrochemical impedance spectroscopy. The formation of multilayer and corrosion mechanism was analysed using scanning electron microscopy. The corrosion resistances of CMA and monolithic alloy coatings were compared with that of the base metal. Compositionally modulated alloy coating at optimal configuration, represented as (Zn–Co)_{2·0/4·0/300}, was found to exhibit ～80 times better corrosion resistance compared with monolithic (Zn–Co)_3·0 alloy, deposited for the same length of time from the same bath. Improved corrosion resistance was attributed to the formation of n-type semiconductor film at the interface, supported by Mott–Schottky plots. Decrease in corrosion resistance at high degree of layering was found, and is due to lower relaxation time for redistribution of solutes in the diffusion double layer, during plating.     

Phosphating process of AZ31 magnesium alloy and corrosion resistance of coatings

1 Introduction Magnesium alloys are relatively light structural materials, with excellent physical and mechanical properties,such as low densityand high specific strength, excellent castability and good machinability. These properties make them ideal cand…     

An Investigation into the Corrosion Behavior of MgO/ZrO<Subscript>2</Subscript> Nanocomposite Coatings Prepared by Plasma Electrolytic Oxidation on the AZ91 Magnesium Alloy

Plasma electrolytic oxidation (PEO) of AZ91 Mg alloys was performed in ZrO₂ nanoparticles containing Na₂SiO₃-based electrolytes. The phase composition and the microstructure of PEO coatings were analyzed by x-ray diffraction and scanning electron microscopy followed by energy dispersive spectroscopy. Pitting corrosion properties of the coatings were investigated using cyclic polarization and electrochemical impedance spectroscopy tests in a Ringer solution. The results showed the better pitting corrosion resistance of the composite coating, as compared to the oxide one, due to the thickened inner layer and the decrease in the surface defects of the composite coating. Also, the PEO process decreased the corrosion current density from 25.06 µA/cm² in the Mg alloy to 2.7 µA/cm² in the oxide coating and 0.47 µA/cm² in the composite coating.     

Study of the structure and corrosion behavior of PEO coatings on AM50 magnesium alloy by electrochemical impedance spectroscopy

In this work coatings were developed on the surface of AM50 magnesium alloy using four different electrolytes containing 10 wt.% each of K₃PO₄ and Na₃PO₄ in combination with either potassium or sodium hydroxides. Electrolyte conductivity and breakdown voltage were measured in order to correlate the property of the coating to the nature of electrolyte. Further, the coatings were examined using scanning electron microscopy for surface morphology and cross sectional investigation, X-ray diffraction for phase determination, and electrochemical impedance spectroscopy for corrosion resistance evaluation. The effect of employing different ions in the electrolytes results in different surface morphologies, chemical phases and, consequently, the corrosion resistance of the coatings. The EIS results indicate the presence of porous and compact layers in the structure of the PEO coatings, whilst the overall coating resistance mainly results from the compact layer, the role of the porous layer as a barrier against corrosion is negligible. Finally, a correlation between the passive current density of the bare alloy and the corrosion resistance of the PEO coating is proposed.     

The examination of corrosion behaviors of hap coated Ti implant materials and 316L SS by sol-gel method

In this research, hydroxyapatite (HAP) coatings have been produced on Ti, Ti6Al4V alloy and 316L stainless steel substrates by sol-gel method. (NH₄) · H₂PO₄ is taken as P precursor and Ca(NO₃)₂ · 4H₂O is taken as Ca precursor to obtain HAP coating. Additionally, three different pretreatment processes (HNO₃, anodic polarization, base-acide (BA)) have been applied to Ti, Ti6Al4V alloy and 316L stainless steel substrates. The corrosion behaviors of bare and HAP coated samples are examined in Ringer and 0.9% NaCl. HAP coated Ti have showed over 87.85% inhibition. HAP coated Ti6Al4V alloys have showed over 87.33% inhibition. In Ringer solution, 99.24% inhibition has been showed in HAP coated anodic pretreatment for 316L stainless steel. All pretreatment processes are effective on clinging of HAP coating to the surface. It is seen that impedance values have increased in HAP coatings (Ti and Ti6Al4V). HAP coatings have raised the corrosion resistance of Ti and Ti6Al4V. The values of polarization resistance in HAP coated samples have increased for 316L stainless steel in 0.9% NaCl and Ringer solutions. It is seen in SEM images that open pores and attachments among pores have been observed in the coating, which increases osteointegration. It is noted in EDX analyses of the surfaces of the HAP coated samples that there is only Ca, O, and P on the surface. Ca/P ratio varies in 1.84–2.00 ranges. As Ca/P ratio increases, the inhibition increases too. It is seen in XRD images of HAP powder that there are HA ate structures. Additionally, it is seen in FTIR analysis, characteristic HA absorption bands have occurred in sintered powders.     

Characterization of calcium-modified zinc phosphate conversion coatings and their influences on corrosion resistance of AZ31 alloy

Two kinds of phosphate conversion coatings, including zinc phosphate coating and zinc-calcium phosphate coating, were prepared on the surface of AZ31 alloy in phosphate baths. The morphologies of these coatings were observed using scanning electron microscopy. Their chemical compositions and structures were characterized using energy-dispersive X-ray spectrum, X-ray photoelectron spectroscopy and X-ray diffraction. The corrosion resistance of the coatings was evaluated by potentiodynamic polarization technique. The results show that the flowerlike Zn-Ca phosphate conversion coatings are mainly composed of hopeite (Zn₃(PO₄)₂·4H₂O). They have a quite different morphology from the dry-riverbed-like Zn phosphate coatings that consist of MgO, MgF₂, Zn or ZnO and hopeite. Both of the zinc and zinc-calcium phosphate coatings can remarkably reduce the corrosion current density of the substrates. The Zn-Ca coating exhibits better corrosion resistance than the Zn coating. Introduction of calcium into the phosphate baths leads to the full crystallinity of the Zn-Ca coating.     

Corrosion behavior of thermal-sprayed WC cermet coatings in SO4 2− environment

A series of the electrochemical and long-term corrosion tests was carried out in a 3.5 wt% Na2SO4 solution on thermal-sprayed WC-17Co and WC-10Co-4Cr cermet coatings in order to examine the effect of composition of binder materials on the corrosion behavior. The results reveal that the overall corrosion resistance of the WC-17Co coating is inferior to that of the WC–Co–Cr coatings due to the corrosion of binder materials which induce WC particles to fall off. CoO and WO3 oxide films form on the surface of WC-17Co coating in Na2SO4 solution electrochemical corrosion process, which will protect the coating in the process of corrosion. Cr2O3 oxide film formed on the WC-10Co-4Cr coating surface has a strong hindered role to corrosion. The corrosion mechanism of WC-17Co coating in Na2SO4 solution is entire corrosion of Co matrix, while it is film-hole corrosion mechanism for WC-10Co-4Cr coating.     

化学镀Ni-P和Ni-Cu-P合金耐蚀性研究

研究了硫酸铜加入量对化学镀Ni-Cu-P合金的镀层成分、组织及热稳定性影响,用中性盐雾实验和在20%H2SO4+20 g/LAl2O3溶液中的冲刷腐蚀实验研究了Ni-P与Ni-Cu-P合金的耐蚀性和耐冲刷腐蚀性能.结果表明,Ni-Cu-P合金镀层具有比Ni-P合金镀层更好的热稳定性、耐蚀性和耐冲刷腐蚀性能.     

Hot Corrosion Behavior of Low-Pressure Cold-Sprayed CoNiCrAlY Coatings

CoNiCrAlY coatings were deposited by low-pressure cold spraying and pre-oxidized in a vacuum environment, and its hot corrosion behavior in pure Na₂SO₄ and 75 wt.% Na₂SO₄ + 25 wt.% NaCl salts was investigated. The pre-oxidation treatment resulted in the formation of a dense and continuous α-Al₂O₃ scale on the coating surface. After being corroded for 150 h at 900 °C, the pre-oxidized coating exhibited better corrosion resistance to both salts than the as-sprayed coating. The presence of preformed Al₂O₃ scale reduced the consumption rate of aluminum, by delaying the formation of internal oxides and sulfides and promoting the formation of a denser and more adherent Al₂O₃ scale. Moreover, we investigated the corrosion mechanism of cold-sprayed CoNiCrAlY coatings in the two salts and discussed the effect of the pre-oxidation treatment.     

Electrochemical characteristics of 316l stainless steel processed with a thermally treated Ni- and Cr-rich surface coating

The influence of a thermally treated Ni-Cr rich protective coating on the corrosion behavior and contact resistance of stainless steel in a 0.1NH₂SO₄+ 2 ppmHF electrolyte at 80 °C was evaluated using electrochemicals, interfacial contact resistance (ICR) measurements and X-ray photoelectron spectroscopy. Through a low-cost, continuous production line, the Ni-rich coating film for stainless steels was developed by dipping steel samples in acrylic resin and CrO₃ solutions with different levels of NiSO₄·6H₂O added as a nickel source. Each sample was then heated at 800 for 10 min in a hydrogen-reducing environment. It was shown that an increase in residual Ni content in the surface coating noticeably lowered the interfacial contact resistance and raised the corrosion resistance, depending on the remaining nickel content and the thickness of the surface coatings. In support of the XPS depth profile, this was ascribed to the relative enrichment of the Ni element and the detectable reduction of oxygen content in the coating, which could be associated with the significant evaporation of acrylic resin that occurred during thermal treatment. The optimum Ni composition in the resultant coating film, achieved through the addition of 15 wt.% NiSO₄·6H₂O to the acrylic resin and CrO₃ solution, was estimated to be about 10 wt.%.