The growth of zinc phosphate coatings on 6061-Al alloy

Zinc phosphate coating was formed on 6061-Al alloy through a phosphating bath containing mainly ZnO, H₃PO₄, NaF. Yttrium oxide (Y₂O₃) was used as an accelerator of phosphatization to replace nitrite. The morphology, composition and the growth process of the zinc phosphate coating were investigated by SEM, EDX, XRD, FTIR and electrochemical measurements. The phosphate coating formed is composed of hopeite and metallic zinc. The formation and morphology of the zinc phosphate coating were strongly influenced by the presence of yttrium oxide (Y₂O₃) in the phosphating bath. The formed zinc phosphate coatings exhibited high corrosion resistance in 3% NaCl solution as shown by polarization measurement.     

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.     

The effects of nano-SiO2 additive on the zinc phosphating of carbon steel

In this paper, nano-SiO₂ was used as an accelerator for improving the microstructure and the corrosion resistance of phosphate coating on carbon steel. The chemical composition and microstructure of the coatings were analyzed by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). The effects of nano-SiO₂ on weight, roughness and corrosion resistance of the phosphate coatings were also investigated. Results show that the compositions of phosphate coating were Zn₃(PO₄)₂·4H₂O (hopeite), and Zn₂Fe(PO₄)₂·4H₂O (phosphophylite). The phosphate coatings became denser due to the addition of nano-SiO₂ which reduced the size of the crystal clusters. The average weight of phosphate coatings approximately linearly increased with the nano-SiO₂ content in the bath from 0 to 4 g/L, and the phosphate coatings formed in bath containing 2 g/L nano-SiO₂ showed the highest corrosion resistance in 5 wt.% sodium chloride solution at ambient temperature. Nano-SiO₂ would be widely utilized as a phosphating additive to replace the traditional nitrite, due to its less pollutant and its better quality of the coating.     

Development of permanganate assisted manganese phosphate coating on mild steel

Manganese phosphate coatings are widely used as pretreatment for corrosion and wear resistance of numerous automotive components. Manganese phosphating formulations are established with various additives to improve the coating weight besides achieving the required amount of iron dissolution in a shorter time. The aim of our investigation is to use permanganate as an additive to increase iron dissolution and to achieve subsequent improvement in the quality of manganese phosphate coating. Manganese carbonate, phosphoric acid, nitric acid are used to create a basic formulation in which permanganate concentration is varied. The composition of the final formulation is optimised by giving due consideration to higher iron dissolution and improved coating weight. The resultant coating is characterised with polarisation, electrochemical impedance spectroscopy (EIS), FT-IR, XRD, SEM and EDX. The kinetics of the coating formation is also followed using potential–time measurement for all the experimental baths. Kinetic studies showed earlier attainment of point of incipient precipitation of manganese phosphate coating in the optimised formulation when compared to other formulation under investigation. The study revealed that the addition of KMnO₄ in the manganese phosphating bath enabled an increase in the rate of metal dissolution, and enrichment of ferrous ion concentration at the metal/solution interface thus, favoured precipitation of corrosion and wear resistant Hureaulite ((Mn,Fe)₅H₂(PO₄)₄.4H₂O). For a given coating weight, addition of KMnO₄ substantially reduced the processing time.     

Characterization of phosphate films on aluminum surfaces

A thin layer of phosphate conversion coating was formed on pure aluminum in a commercial zinc-manganese phosphating bath. A number of surface analytical techniques were used to characterize the phosphate thin films formed after immersion times ranging from 30 s to 10 min. The coating contained mainly a crystalline structure with dispersed micrometer-scale cavities. The major constituents of the phosphate film were zinc, phosphorus, and oxygen; a small amount of manganese was also detected. Based on these results, a three-stage mechanism was proposed for the formation and the growth of phosphate conversion coatings on aluminum. Electrochemical impedance spectroscopy was used to evaluate the corrosion performance of phosphated and uncoated aluminum samples in 0.50 M Na₂SO₄ and 0.10 M H₂SO₄ solutions. Both types of samples exhibited a passive state in the neutral solution and general corrosion behavior in the acid solution.     

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.     

Characteristics of a fast low-temperature zinc phosphating coating accelerated by an ECO-friendly hydroxylamine sulfate

A fast low-temperature phosphating processing accelerated by an ECO-friendly hydroxylamine sulfate (HAS) is developed. The zinc phosphate coating was fast formed on high-carbon steel in a low-temperature phosphating bath. Growth stages and characteristics of the phosphate coating were investigated by open circuit potential (OCP), SEM, EDS and XRD techniques. The phosphating process can be divided into three stages, namely amorphous precipitation, anodic depolarization and growth of phosphate coating. The phosphate coating consists of Zn₃(PO₄)₂ · 4H₂O and Zn₂Fe(PO₄)₂ · 4H₂O phases. The addition of HAS makes the three stages' time shorten to 53%, 31% and 50%, respectively, while markedly reduces the size of phosphate crystals from 100 µm to about 50 µm, and increases the Zn₂Fe(PO₄)₂ · 4H₂O content from 30% to 44% in the coating. HAS would be widely used as a low-temperature phosphating accelerator to replace the traditional nitrite, due to its less pollutant, higher phosphating rate and better quality of the coating.     

Duplex-layered manganese phosphate conversion coating on AZ31 Mg alloy and its initial formation mechanism

A duplex-layered phosphate conversion coating was obtained on AZ31 Mg alloy by substituting NaF bath with a citric bath. The morphology, composition and corrosion resistance of the coating were investigated using SEM, EDS, SPM and electrochemical methods. A three-stage mechanism for initial formation of the coating was proposed: Dissolution of the loose oxide film and deposition of Mg₃(PO₄)₂ and AlPO₄, formation of a composite intermediate layer of Mg₃(PO₄)₂, AlPO₄ and Mg(OH)₂, and deposition of manganese phosphate nuclei followed by growth and lamination of the nuclei. The nuclei preferentially deposit at the Al–Mn phase surface and near the grain boundary.     

The EIS investigation of powder polyester coatings on phosphated low carbon steel: The effect of NaNO2 in the phosphating bath

The effect of different type of iron-phosphate coatings on corrosion stability and adhesion characteristic of top powder polyester coating on steel was investigated. Iron-phosphate coatings were deposited on steel in the novel phosphating bath with or without NaNO₂ as an accelerator. The corrosion stability of the powder polyester coating was evaluated by electrochemical impedance spectroscopy (EIS), adhesion by pull-off and NMP test, while surface morphology of phosphate coatings were investigated by atomic force microscopy (AFM).The adhesion and corrosion stability of powder polyester coatings were improved with pretreatment based on iron-phosphate coating deposited from NaNO₂-free bath.     

Enhanced corrosion performance of magnesium phosphate conversion coating on AZ31 magnesium alloy

Magnesium phosphate conversion coating (MPCC) was fabricated on AZ31 magnesium alloy for corrosion protection by immersion treatment in a simple MPCC solution containing Mg²⁺ and PO^3?₄ ions. The MPCC on AZ31 Mg alloy showed micro-cracks structure and a uniform thickness with the thickness of about 2.5 µm after 20 min of phosphating treatment. The composition analyzed by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy revealed that the coating consisted of magnesium phosphate and magnesium hydroxide/oxide compounds. The MPCC showed a significant protective effect on AZ31 Mg alloy. The corrosion current of MPCC was reduced to about 3% of that of the uncoated surface and the time for the deterioration process during immersion in 0.5 mol/L NaCl solution improved from about 10 min to about 24 h.     

Phosphate-permanganate conversion coatings on the AZ81 magnesium alloy: SEM, EIS and XPS studies

Conversion coatings on the magnesium alloy AZ81 were prepared using the phosphate-permanganate baths differing in composition. The corrosion behavior of the coated and uncoated alloys has been investigated by electrochemical impedance spectroscopy (EIS) and linear polarization methods. The choice of proper electric equivalent circuit (EEC) is discussed. The effect of temperature, bath composition and time of conversion as well as etching in acids before application on the corrosion resistance of the coated alloy has been evaluated. The best corrosion resistance was obtained for the samples coated in the bath containing 25 g KMnO₄, 150 g Na₂HPO₄ and 50 ml H₃PO₄ in 1 dcm³, applied at 80 °C. Differences in the morphology and composition of coated surfaces were investigated by the scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) techniques and correlated with the corrosion resistance of the samples.     

Microstructure and Corrosion Resistance of Electrodeposited Ni-Cu-Mo Alloy Coatings

This paper deals with the electrodeposition of Ni-Cu-Mo ternary alloy coatings on low-carbon steel substrate from an aqueous citrate sulfate bath. The structures and microstructure of coatings were characterized by scanning electron microscopy and x-ray diffractometry. The corrosion resistance of coatings was investigated by potentiodynamic polarization (Tafel) and electrochemical impedance spectroscopy techniques. The results show that the Ni-Cu-Mo coatings are mainly composed of fcc-Ni phase and a small amount of NiCu phase. Ni-Cu-Mo coatings exhibit a nodular surface morphology, and the roughness of electroplated coating increases with the increasing of Na₂MoO₄·2H₂O in the bath. The corrosion performance of the coatings is significantly affected by the Mo content of the alloy coating and their surface morphology. The coating prepared in bath containing 40 g/L Na₂MoO₄·2H₂O has the highest corrosion resistance in 3.5 wt.% NaCl solution, while that prepared in bath containing 60 g/L (or more) Na₂MoO₄·2H₂O shows a lower corrosion resistance due to the presence of microcracks on the coating surface.     

Corrosion and protection characteristics of phosphate coatings: effect of yttrium oxide as additive

Abstract

The corrosion and protection characteristics of phosphate coatings formed in a phosphating solution containing mainly ZnO, H₃PO₄ and NaF, using Y₂O₃ as an additive, were investigated through SEM, polarisation curves and EIS diagrams. The results show that the corrosion protection of phosphate coatings has been improved when Y₂O₃ is added to the phosphating solution, making the free corrosion potential shift to the positive direction and causing the corrosion current to decrease. The protection ability of phosphate coatings depends mainly on their barrier performance. The phosphate coatings formed in the phosphating solution with 10 and 20 mg L^–1 Y₂O₃ have finer crystal structures and smaller porosity; therefore, they exhibit better corrosion resistance and adhesion properties than those without Y₂O₃ and with 40 mg L^–1 Y₂O₃.     

Synthesis and evaluation of corrosion resistance of molybdate-based conversion coatings on electroplated zinc

Three molybdate-based conversion coatings on electroplated zinc have been prepared and the composition, morphology, and structure of these coatings are measured by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and X-ray diffraction (XRD), respectively. It was found that these coatings with ‘meshwork’ surface were complex coatings composed of multiple compounds. Molybdenum species were present in the conversion coating as Mo (VI) and Mo (IV) compounds. The results of neutral salt spray test showed that molybdate-based conversion coatings with the addition of H₃PO₄, SiO₂ and TiOSO₄ in the passivation baths possess higher corrosion resistance compared with chromate conversion coatings, which was due to the compactness and anti-corrosion essence of the conversion coating.     

Ceria/stannate multilayer coatings on AZ91D Mg alloy

In this work, CeO₂/stannate multilayer coatings on AZ91D magnesium alloy were successfully obtained by chemical conversion and sol–gel dip coating. The stannate conversion coatings were prepared from a stannate aqueous bath containing Na₂SnO₃, CH₃COONa, Na₃PO₄ and NaOH at different temperatures and immersion times. Ceria films were produced on stannate/AZ91D starting from Ce(III) nitrate solutions in H₂O. In some cases, the PVA was added as chelating agent. Ceria top coatings were fired at 200 °C for 1 h. Coating microstructure was examined by FE-SEM. Finally, the corrosion resistance features of the coatings were tested by the electrochemical impedance spectroscopy (EIS) in 3 wt.% NaCl solution. The effect of PVA addition was evaluated in terms of microstructure and corrosion resistance features. CeO₂/stannate multilayer films, 3 μm thick, uniform, well adherent and nearly crack free were obtained. The formation of CeO₂ phase was confirmed by XRD and XPS analyses. The XPS depth profiles showed a limited diffusion of Mg towards the ceramic film. The EIS tests showed a significant improvement of corrosion resistance of the multilayer coatings (~ 16.6 kΩ after 48 h in NaCl solution) with respect to the blank alloy (~ 2.4 kΩ after 48 h in NaCl solution).     

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…     

Corrosion protection of magnesium alloys by cerium, zirconium and niobium-based conversion coatings

A new Ce, Zr and Nb-based conversion coating was designed for AZ91 and AM50 magnesium alloys. The corrosion protection provided by this coating was evaluated by electrochemical measurements (polarization curves, electrochemical impedance spectroscopy) in Na₂SO₄ electrolyte, and accelerated atmospheric corrosion tests (humid, SO₂ polluted air, and salt spray). Its chemical composition was characterized by X-ray photoelectron spectroscopy (XPS). Electrochemical measurements showed that Mg alloys treated during 24 h in the Ce-Zr-Nb conversion bath exhibit: (i) increased corrosion potential, (ii) decreased corrosion and anodic dissolution current densities, and (iii) increased polarization and charge transfer resistances. The accelerated corrosion tests revealed excellent atmospheric corrosion resistance for all Ce-Zr-Nb-treated samples, with or without an additional layer of epoxy-polyamide resin lacquer or paint. XPS analysis showed that the coating includes CeO₂, Ce₂O₃, ZrO₂, Nb₂O₅, MgO, and MgF₂ as main components. No significant modification of the chemical composition was observed after cathodic and anodic polarization in Na₂SO₄. This new coating provides improved corrosion resistance, and excellent paint adhesion. It offers an alternative to the chromate conversion coating for magnesium alloys.     

6061 铝合金表面无铬稀土镧转化膜性能的研究

利用极化曲线方法,研究了以La(NO3)3.6H2O为促进剂的磷酸盐转化膜的耐蚀性,同时与铬磷化膜及无稀土促进的单纯磷酸盐膜的耐蚀性进行了对比;通过划格法和全浸腐蚀试验,研究了这三种转化膜与有机涂层间的结合力。结果发现:与单纯磷酸盐膜相比,稀土促进生成的磷酸盐膜中的传输阻力增加,耐蚀性明显增强,而与铬磷化膜相比,二者在弱极化区的耐蚀性能相近;稀土促进生成的磷酸盐膜与有涂层间的结合力明显优于铬磷化膜。     

Effect of ecofriendly sealing coat against corrosion protection of steel rebars in concrete

Phosphating is one of the most important chemical conversion treatments for steel, mainly to improve corrosion resistance and paint adhesion and as an absorbing layer for waxes, oils, lacquers, etc. However, phosphate coating are crystalline and porous and need a sealing treatment after phosphating. Chromate sealing is a well known practice, and due to its toxicity, development of an ecofriendly sealing treatment is very essential. This paper focuses on the effect of zinc phosphate chemical conversion coating with the addition of nano-SiO₂ to protect the mild steel rebars against corrosion in chloride contaminated concrete. The coated surfaces were characterised by scanning electron microscopy and X-ray diffraction. Corrosion resistances of coated and uncoated rebars were evaluated by anodic polarisation, linear polarisation resistance, Tafel and alternating current impedance spectroscopy and cyclic polarisation technique. The results indicated that the coated rebars have considerably reduced the corrosion rate even in the presence of 3% chloride environments.     

不锈钢电化学着黑色工艺与成膜机理研究

采用电化学着色法对不锈钢着黑色进行了研究,讨论了钝化处理、着色液组成等因素对着色的影响,测定了着色膜的耐磨性和耐蚀性,并根据着色膜的组成、微观结构分析了成膜机理.结果表明:钝化和封闭处理能明显提高着色膜的耐磨性和抗色变性;电化学分析表明在1 mol/L H2SO4溶液、3.5%NaC l溶液和10%NaOH溶液中,着色膜腐蚀电位比不锈钢基体分别正移1130、565和790 mV,且腐蚀电流密度都比相应介质中的小;扫描电镜和能谱结果显示膜层为封闭块状结构,着色膜主要成分是Fe、Cr、Mn等元素,封闭处理能明显减少其裂纹数目.该成膜反应机理为:1)不锈钢基体的溶解形成大量的M e2+;2)金属/溶液界面上的M e2+与Cr3+水解形成合金氧化膜沉积在基体表面上;3)电化学致密过程中4H2MoO4+2SO42-+4H+2(MoO)2SO4+6H2O+6[O]和M e+[O]=M eO反应是着色膜致密的主要原因.