Characterization of plasma electrolytic oxide formed on AZ91 Mg alloy in KMnO4 electrolyte

The aim of this work is to investigate microstructure, corrosion resistance characteristics and nanohardness of the oxide layer on AZ91 Mg alloy by applying different voltage with KMnO₄ contained solution. There are lots of closed pores that are filled with another oxide compound compared with the typical surface morphology with pore coated until 350 V of coating voltage. The thickness of oxide layer increases with increasing coating voltage. The oxide layer formed on AZ91 Mg alloy in electrolyte with potassium permanganate consists of MgO and Mn₂O₃. Corrosion potential of the oxide layer on AZ91 Mg alloy obtained at different plasma electrolytic oxidation(PEO) reaction stages increases with increasing coating voltage. The corrosion resistance of AZ91 Mg alloy depends on the existence of the manganese oxide in the oxide layer. The inner barrier layer composed of the MgO and Mn₂O₃ may serve as diffusion barrier to enhance the corrosion resistance and may partially explain the excellent anti-corrosion performance in corrosion test. Nanohardness values increase with increasing coating voltage. The increase in the nanohardness may be due to the effect of manganese oxide in the oxide layer on AZ91 Mg alloy coated from electrolyte containing KMnO₄.     

Effects of phosphate pretreatment and hot-humid environmental exposure on static strength of adhesive-bonded magnesium AZ31 sheets

An innovative phosphate–permanganate surface treatment (PPT) was developed to improve the static strength of adhesive-bonded 4 mm thick magnesium AZ31 sheets. The phosphate coating having the chemical composition of 1.43% P, 1.63% F and 0.15% Mn (in mass %) was formed after the treatment with PPT solution which has the formulation of KMnO₄, K₂HPO₄, Na₂SiO₃ and NaF. The combination of additives NaF and Na₂SiO₃ and the pH values in the range of 5–6 for a phosphate–permanganate solution was found to be the key elements for the formation of the phosphate coating. The appearance of the phosphate coating and corrosion resistance to 3.5%NaCl solution was assessed. To study the durability of the coating, the effect of an exposure in a hot-humid environment (96% R.H. at 40 °C) on the static strength of adhesive-bonded magnesium AZ31 was investigated. Test results showed that the phosphate coating improved not only the static strength of bonded magnesium AZ31 joints in an ambient condition but also the durability in a hot-humid environment. These results suggest that PPT surface pretreatment is capable of improving the static strength and thermal durability of adhesive-bonded magnesium AZ31 sheets.     

铝活塞的磷化工艺

研究了铝活塞的锰系磷化工艺.以外观和耐磨性为定量指标,讨论了主要成分、添加剂、温度和时间对磷化膜质量的影响,确定了最佳磷化工艺.结果表明:该磷化工艺稳定,维护简单.所得磷化膜为灰黑色,均匀光亮,耐蚀性、耐磨性和耐热性优良.由磷化机理分析表明,磷化膜的组成为AlPO4和Mn3(PO4)2.     

Effect of permanganate concentration on the formation and properties of phosphate/permanganate conversion coating on AZ31 magnesium alloy

This study investigated the role of permanganate in phosphate solutions in the formation and corrosion resistance of phosphate/permanganate coatings on AZ31 magnesium alloys. Experimental results showed that permanganate was reduced to manganese (IV) oxide together with the dissolution of magnesium during the conversion coating treatment. Adding more permanganate to the phosphate solution resulted in a thinner coating with a compact magnesium oxide layer contacting the AZ31 plate. Moreover, the thinner coating had fewer cracks and displayed higher polarization resistance and corrosion resistance than the thicker counterpart formed in the solution with less permanganate.     

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.     

Microstructural characterization and surface properties of zinc phosphated medium carbon low alloy steel

The present study concerns an in-depth investigation of the influence of process parameters on zinc phosphating of medium carbon steel to have an understanding of nucleation and growth mechanism and the microstructures developed thereafter. Furthermore, the wear and corrosion behavior of phosphated steel were evaluated in details. Phosphating was carried out in zinc phosphate bath (6–8% ZnPO₄) using 0.43 wt.% Fe as catalyst at 90 °C with variable time ranging from 60 s to 1800 s followed by water rinsing and oil sealing. The microstructural evolution with time was recorded to understand the nucleation and growth phenomena. The morphology of coating was both plate and needle shaped. The microhardness of the coated layer is significantly lower than that of as-received steel. Fretting wear behavior against hardened steel ball showed that phosphating reduced the wear resistance which was possibly attributed to the presence of phosphate needles causing an increased coefficient of friction and the reduced hardness of the phosphate layer. Corrosion potential of the phosphated steel shifted towards noble direction as compared to the as-received one. The study of the corrosion rate also shows that the rate is considerably reduced by phosphating. Pitting corrosion resistance was also found to be increased (in terms of increased critical potential for pit formation) due to phosphating as compared to as-received steel.     

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.     

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.     

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.     

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.     

磷酸盐涂层研究进展及其应用

随着世界环保压力与节能压力的剧增,绿色涂层已经成为金属腐蚀防护的发展趋势.磷酸盐涂层是一种机械强度高、热稳定性好、耐腐蚀且无毒环保的无机涂层.相较于金属涂层和无机硅涂层,磷酸盐涂层因其制备方法简便以及特殊的性能优势,受到越来越多的关注,在建筑、汽车、航海、航空航天、医疗等领域已取得广泛运用.分别介绍了反应固化型磷酸盐涂...     

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.     

Effect of surface mechanical attrition treatment (SMAT) on zinc phosphating of steel

The effects of surface mechanical attrition treatment (SMAT) of EN8 steel on the growth of phosphate coatings, morphological features and corrosion resistance of the resultant coatings have been studied. SMAT enabled the formation of a uniform surface profile although the average surface roughness is increased after treatment. SMAT increased the extent of metal dissolution and the rate of growth of phosphate coating. In spite of the similarity in phase composition, the phosphate crystallite size is relatively high for samples treated by SMAT. Compared to the untreated one, a cathodic shift in E_corr with a corresponding decrease in i_corr is observed for SMAT treated EN8 steel after phosphating. For all tested samples, zinc phosphate coatings deposited on EN8 steel after SMAT using 8 mm Ø balls for 30 min offers the highest corrosion resistance. The increase in surface roughness by SMAT is partly compensated by the expected improvement in corrosion resistance.     

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.     

稀土CeCl_(3)复合镀铁层耐磨耐腐蚀性能EI北大核心CSCD

电镀技术常用来对已破损的零件进行修复与再制造,然而其耐磨性及耐腐蚀性能需要进一步改善。在电镀技术的基础上向镀液中添加不同浓度的CeCl_(3)化合物(0、0.5、1、1.5 g/L)制备出复合镀层以探究其对电镀层性能的影响。结果表明:在镀液中添加CeCl_(3)化合物会使复合镀层中铁晶粒呈现较好的择优取向性;随着镀液中CeCl_(3)化合物浓度的逐渐提升,复合镀层的显微硬度、耐磨性、耐腐蚀性均呈现出先提高后减弱的状态;当镀液中CeCl_(3)化合物浓度为0.5g/L时,复合镀层的表面结构更为均匀、镀层中铁晶粒的排列更加致密,其显微硬度达到611.4HV,具有最佳的耐磨性和耐腐蚀性能。研究成果表明镀液中添加稀土CeCl_(3)化合物可以大幅度提高镀层的耐腐蚀性,并在一定程度上提高镀层的耐磨性,可为实际生产中提高镀铁层的耐磨性及耐腐蚀性能提供理论指导。     

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.     

免水洗常温热镀锌表面磷化技术研究

以磷酸、氧化锌、磷酸二氢锰、钼酸铵和硝酸钙等为原料,通过正交试验等方法开发了一种磷化后免水洗的常温热镀锌表面磷化液。研究了磷化液的pH值、磷化温度、磷化时间以及自干时间等对磷化膜质量的影响。结果表明:磷化液pH值为2.6～3.3,在5～40℃浸渍磷化7～10min,自然干燥3h可获得磷化后工件免水洗的磷化膜。磷化膜的耐蚀时间超过50s,喷涂铁红环氧底漆后的漆膜附着力达1级。     

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.     

Investigation of wear and corrosion resistance of nanocomposite coating formed on AZ31B Mg alloy by plasma electrolytic oxidation

Ceramic-WC coatings were prepared on AZ31 B Mg alloy by plasma electrolytic oxidation (PEO) from a phosphate based bath containing suspended tungsten carbide nanoparticles at various process times. Scanning electron microscope results indicated that increase of coating time and incorporation of tungsten carbide into the ceramic coating during the PEO process led to a decrease in the number and diameter of coating pores. Phase analysis showed that the nanocomposite coating was composed of MgO, Mg₃(PO₄)₂ and WC. Tribological properties and corrosion behaviour of uncoated AZ31 B Mg alloy and ceramic coatings were evaluated using a pin-on-disc tribometer and potentiodynamic polarisation technique in 3.5% NaCl solution, respectively. The wear and electrochemical tests showed that wear and corrosion resistance of ceramic-WC nanocomposite coatings were better than ceramic only ones. In addition, wear and corrosion behaviour of coatings improved with increasing the coating time.     

钼酸钠常温磷化液的研究

    选择钼酸钠作为磷化液的主促进剂.对其所得磷化膜的耐蚀性和膜重做了检测;用扫描电子显微镜对磷化膜的微观结构进行分析;同时研究了磷化时间和磷化温度对磷化膜性能的影响.