The use of Al-Al2O3 cold spray coatings to improve the surface properties of magnesium alloys

Pure Al and 6061 aluminium alloy based Al₂O₃ particle-reinforced composite coatings were produced on AZ91E substrates using cold spray. The strength of the coating/substrate interface in tension was found to be stronger than the coating itself. The coatings have corrosion resistance similar to that of bulk pure aluminium in both salt spray and electrochemical tests. The wear resistance of the coatings is significantly better than that of the AZ91 Mg substrate, but the significant result is that the wear rate of the coatings is several decades lower than that of various bulk Al alloys tested for comparison. The effect of post-spray heat treatment, the volume fraction of Al₂O₃ within the coating and of the type of Al powder used in the coatings on the corrosion and wear resistance was also discussed.     

High‐temperature corrosion behavior of some post‐plasma‐spraying‐gas‐nitrided metallic coatings on a Fe‐based superalloy

The objective of the present study is to propose a cost‐effective process for modifying commercially available coatings by gas nitriding using commonly available equipment and starting materials. Al–Cr and Ti–Al metallic coatings were deposited on Superfer 800H (Fe‐based superalloy) using a plasma spray process. Then the gas nitriding of the coatings was done in the lab and the parameters were optimized after conducting several trials on plasma‐sprayed‐coated specimens. Characterization and high‐temperature corrosion behavior of coatings after exposure to air and molten salt at 900°C were studied under cyclic conditions. Techniques like XRD, SEM/EDX, and X‐ray mapping analysis were used for the characterization of the coatings and analysis of the oxide scale. Both the coatings successfully protected the substrate and were effective in decreasing the corrosion rate when subjected to cyclic oxidation (Type‐I hot corrosion) at 900°C for 50 cycles in air and molten salt (a salt mixture of Na₂SO₄–60%V₂O₅ dissolved in distilled water). Based on the findings of the present study, the coatings under study are recommended for tapplications to super‐heater and reheater tubes of boilers and all those surfaces that face fireside corrosion, such as fluidized beds, industrial waste incinerators, internal combustion engines, gas turbines or steam turbines, to provide protection against degradation in these environments. The cost of the product/process is approximately Rs. 0.62 per mm² in case of Al–Cr coating and Rs. 1.86 per mm² in case of Ti–Al coating.     

Characterization of the corrosion products of electrodeposited Zn, Zn–Co and Zn–Mn alloys coatings

The morphology, composition, phase composition and corrosion products of coatings of pure Zn (obtained from two types of electrolytic bath: an acidic bath (Zn_acid) and a cyanide-free alkaline bath (Zn_alkaline)) and of Zn–Mn and Zn–Co alloys on steel substrates were studied. To achieve this, diverse techniques were used, including polarization curves, atomic force microscopy (AFM), scanning electron microscopy (SEM), glow discharge spectroscopy (GDS), X-ray diffraction (XRD), and the salt spray test. In the salt spray test, the exposure time required for the coatings to exhibit red corrosion (associated with the oxidation of steel) decreased in the following order: Zn–Mn_(432h) > Zn–Co_(429h) > Zn_{alkaline(298h)} > Zn_acid(216h). The shorter exposure times required for corrosion of the pure Zn coatings are related to the coating composition and the crystallographic structure. Analysis of the corrosion products disclosed that Zn₅(OH)₈Cl₂·H₂O was a corrosion product of all of the coatings tested. However, the formation of oxides of manganese (MnO, Mn_0.98O₂, Mn₅O₈) in the Zn–Mn coating, and the formation of the hydroxide Zn₂Co₃(OH)₁₀·2H₂O in the Zn–Co coating, produced more compact and stable passive layers, with lower dissolution rates.     

Comparative study between galvanised steel and three duplex systems submitted to a weathering cyclic test

Two types of samples, uncoated galvanised steel (UGS) and galvanised steel coated with chlorinated rubber paint (DS1), alkyd paint (DS2) and water-borne acrylic paint (DS3) were tested in a weathering cyclic chamber. The behaviour evolution of these systems was tracked using electrochemical impedance spectroscopy. For the duplex system, regardless of the type of paint, high impedance values were obtained, even at long exposure time. The UGS samples exhibited lower impedance values. Such behaviours could be interpreted in terms of different nature of the corrosion products generated on zinc surface. Using scanning electron microscopy, energy dispersive X-ray and X-ray diffraction techniques the identification of those compounds was possible. The main corrosion product generated on UGS was hydrozincite, Zn₅(CO₃)₂(OH)₆, in the case of the duplex systems, the corrosion product formed was zincite, ZnO. Such difference suggests that the zinc oxide formed under the coating is the responsible of the better duplex system behaviour and so, it leads a better passivation of zinc surface than the obtained with hydrozincite, which is the main corrosion product formed on UGS. In that sense, the coating acts as a selective membrane, avoiding the CO₂ uptake and so, hydrozincite formation is inhibited. This behaviour was observed even for coatings with poor barrier properties, such as water-borne acrylic one.     

热喷涂钢柱10年海水腐蚀行为

采用直流电弧喷涂和火焰喷涂技术,在碳钢管表面沉积了１２种类的Ｚｎ、Ａｌ和Ｚｎ－１３Ａ１涂层在日本的千仓海岸进行海水腐蚀实验,结果表明,在５年暴露期内,所有的涂层均没有发生明显的腐蚀;经７年暴露后,未经封闭和经封闭处理的Ｚｎ涂层在浸泡区出现严重的锈蚀,电弧喷涂并经封孔处理的Ａ１涂层,火焰喷涂并经封孔处理的Ｚｎ－Ａ１涂层和火焰涂并经重涂装的Ａ１涂层显示出优良的耐蚀性能,相比之下,未经封闭处理的火焰喷涂     

Surface characterization and corrosion behavior of a novel gold-imitation copper alloy with high tarnish resistance in salt spray environment

A novel gold-imitation copper alloy (CuZnAlNiSnBRe) was designed and its corrosion behavior in salt spray environment was investigated. The new alloy has better tarnish resistance and corrosion resistance than the current coinage alloy used in China (H7211). A multi-layer film formed on the surface of the new alloy after a period of exposure to salt spray was responsible for the good resistance of the alloy. The corrosion products were a mixture of CuO, Cu₂O, ZnO, Al₂O₃ and Al(OH)₃, with the transition from Cu₂O to CuO occurring during the corrosion process.     

不同环境介质中热喷涂Al和Zn的腐蚀行为

采用极化曲线、盐雾试验和自然暴晒等方法,研究了热喷涂Al或Zn层在3%NaCl溶液、西安工业区降水(相当于酸雨)和海南海洋大气条件下的耐腐蚀性能。提出了喷涂层出现不同面积剥落时的腐蚀速率和低碳钢基体允许裸露的平均面积比率,为设备在不同地区和不同使用寿命要求下的涂装工艺设计、使用维护提供依据。     

Chemistry of corrosion products on Zn-Al-Mg alloy coated steel

Zn-Al-Mg alloy (ZM) coating provides a decisively enhanced corrosion resistance in a salt spray test according to DIN EN ISO 9227 (NSS) compared to conventional hot-dip galvanised zinc (Z) coating because of its ability to form a very stable, well adherent protecting layer of zinc aluminium carbonate hydroxide, Zn₆Al₂(CO₃)(OH)₁₆·4H₂O on the steel substrate. This protecting layer is the main reason for the enhanced corrosion resistance of the ZM coating. Surface corrosion products on ZM coated steel consist mainly of Zn₅(OH)₆(CO₃)₂, ZnCO₃ and Zn(OH)₂ with additions of Zn₅(OH)₈Cl₂ · H₂O and a carbonate-containing magnesium species.     

Electrochemical Characteristics in Seawater for Cold Thermal Spray-Coated Al-Mg Alloy Layer

For corrosion protection of carbon steel in a marine environment,cold arc thermal spray coating was applied to the surface with Al and Al-Mg alloy wires.The surface hardness of Al and Al-Mg thermal spray coatings increased with Mg content.And the various electrochemical experiments were carried out to evaluate corrosion damage characteristics of the thermal spray coating layers.The Al and Al-Mg thermal spray coating layers presented negative potentials compared to carbon steel in corrosion potential measurements.And an anodic polarization experiment revealed a tendency of activation polarization with no passivation.Furthermore,the corrosion damage of the thermal spray coating layer in galvanostatic experiment was observed mainly at the defect area,and the Al-3Mg thermal spray coating layer presented less surface damages than others.In addition,the Al-3Mg thermal spray coating layer showed the lowest corrosion rate while having a sufficient driving voltage for cathodic corrosion protection.Therefore,it is an optimal thermal spray material for sacrificial anode.     

Improving the corrosion protection of aluminium alloys using reactive magnetron sputtering

Most high strength aluminium alloys used in the aircraft industries are susceptible to corrosion. Up to now hexavalent chromium is the conventional corrosion inhibitor. Because chromium in hexavalent state is carcinogenic, it is necessary to develop effective alternative inhibitor systems. We investigated magnetron sputtered substoichiometric and stoichiometric aluminium nitride (AlN_x with x ? 1) coatings for corrosion protection of aluminium alloys 2024-T3, 6061-T4 and 7075-T6. The corrosion behaviour of the treated surfaces was tested by anodic polarization scanning and salt spray testing.From the polarization curves it can be concluded that magnetron sputter coating with AlN_x leads to a higher pitting potential of the three aluminium alloys investigated. The salt spray tested samples also confirm the protective effect of the coatings. In addition we found that AlN_x layers with high nitrogen content lead to a stronger shifting in pitting potential than those with low nitrogen content. Anyway, the results of the salt spray testing show that particularly nitrogen-rich AlN_x layers are less stable towards NaCl electrolyte.     

Corrosion resistance of painted zinc alloy coated steels

Organic coating in combination with sacrificial metal coating is the most popular method of protecting steel strips against atmospheric corrosion. Experiences over the years have proven that such duplex coating systems are best suited for the coil industry for the long term corrosion protection of steel. The excellent corrosion resistance of such systems has been attributed to the synergy between the cathodic protection provided by the sacrificial coating of zinc alloys and the combined barrier resistance of the metal and organic coatings. Traditionally continuously hot dip zinc-coated steels are used for such applications. However, off late the quest for further extending the longevity of the coil coatings has led to the replacement of the zinc coating with a host of other hot dip zinc–aluminium alloy coatings such as Galvalume^®, Galfan^®, ZAM^®, SuperDyma^®, etc. Each of these metal coatings has its own unique metallurgical features in terms of flexibility, bonding, microstructure and electrochemical characteristics which may significantly influence the performance of the organic coatings applied over it. This paper looks into the various aspects of these features of the hot dip coatings on the corrosion performance of the pre-painted steel strips. For simplicity only polyester paint system, the work horse of the coil industry, is considered.     

电弧喷涂Zn-Al伪合金涂层耐腐蚀性能研究进展

通过在钢基体表面制备涂层可以很好地延长钢铁材料的服役时间,减少因腐蚀造成的重大事故和人员伤亡。相较于传统的纯Zn涂层、纯Al涂层以及Zn-Al合金涂层,Zn-Al伪合金涂层能够为基体材料提供长久有效的腐蚀防护,在钢铁材料的腐蚀防护中具有巨大的应用潜力。简述了Zn-Al伪合金涂层电弧喷涂制备工艺的特点;介绍了Zn、Al、Zn-Al合金及Zn-Al伪合金涂层在模拟海洋环境下的腐蚀防护原理;在此基础上从组分、喷涂工艺参数(喷涂距离、喷涂电流和喷涂电压)、元素掺杂(Mg、Si及Re)及后处理工艺(封孔、激光重熔)等角度,论述了其对Zn-Al伪合金涂层耐蚀性的影响;讨论了Zn-Al伪合金涂层防腐体系在桥梁、海洋钢结构件、地下运输管道中的应用现状;最后总结了目前研究工作中存在的挑战,提出了电弧喷涂Zn-Al伪合金涂层尚需深入研究的重点问题,为提高钢铁材料使用寿命提供了参考。     

300M钢表面冷喷涂锌镍复合涂层性能研究

目的 研究300M高强钢表面的冷喷涂锌镍复合涂层的涂层性能.方法 采用机械混合的方式将锌粉和镍粉进行混合,利用低温气动喷涂技术在300M高强钢表面制备锌镍复合涂层,采用扫描电子显微镜、能量色散谱和显微硬度测试仪研究涂层的微观组织结构.采用普通中性盐雾加速实验、涂层破损盐雾加速试验及户外暴晒试验,对冷喷涂锌镍涂层的抗腐蚀性能进行综合评价.考核冷喷涂对300M钢基材疲劳性能的影响.结果 根据SEM的表面及截面图片分析,冷啧涂锌镍复合涂层十分致密,且无孔隙及裂纹,涂层的孔隙率平均为0.4％,结合强度40 MPa左右.根据EDS结果分析,锌镍复合涂层中锌的质量分数为85％左右,镍的质量分数为15％左右.锌镍复合涂层的显微硬度为70.8HV0.49.锌镍复合涂层中镍粒子的强化作用,提高了涂层的硬度.冷喷涂锌镍复合涂层具有非常好的抗腐蚀性能,盐雾实验超过770h,即使涂层破损也可以达到600h.户外暴晒实验12个月表面无明显腐蚀,并且冷喷涂对300M钢基材的疲劳性能没有影响.结论 冷喷涂锌镍复合涂层作为抗腐蚀涂层,可以对300M高强钢提供保护.     

涂膜破坏后膜下镀锌钢板大气腐蚀扩展研究

    通过SEM和EDS研究了带有划痕的涂装镀锌钢板在海洋大气环境下暴露一年半后,划痕周围涂膜下腐蚀产物的扩展行为,并分别对电镀锌、热镀锌、电镀锌预磷化的材料进行了耐蚀性比较.结果表明,在划痕周围涂膜下镀层的耐蚀性与镀层制作工艺无关,而与镀层厚度有关;涂膜前的预磷化对耐蚀性产生了不利影响;虽然涂膜与镀锌层间发生分离,但对镀层的耐蚀性没有产生影响;海洋大气环境存在的氯容易富集在腐蚀产物的前端,而硫则较均匀的分布在整个腐蚀产物转化区.     

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.     

Electrophoretic painting on AZ31 Mg alloy pretreated in cerium conversion coating solutions prepared in ethanol-water mixtures

Electrophoretic painting (E-paint) was prepared on AZ31 Mg alloy samples pretreated in cerium conversion coating (CeCC) solutions with various ratios of ethanol and water mixture and its characteristics, adhesion and corrosion resistance were investigated. It was found that CeCC formed on AZ31 Mg alloy in a CeCC solution without ethanol was partly cracked structure and mainly consisted of Mg(OH)₂/MgO, which exhibited weak adhesion with E-painting layer after water immersion test, and low corrosion resistance, as indicated by rapid formation of blisters and paint delamination during salt spray test. The addition of ethanol promoted the growth of a fine nano-crystalline CeO₂ layer over the entire substrate surface. The E-paint on AZ31 pretreated in the CeCC solutions with addition of ethanol showed also improved corrosion resistance, as represented by the delayed time for paint delamination and blister formation. The E-paint layers on the CeCC layers formed in solutions containing 50–80 vol% ethanol showed stronger adhesion and better corrosion resistance than those formed on the samples treated in a non-ethanol containing CeCC solution.     

Corrosion performance of anodic films containing polyaniline and TiO2 nanoparticles on AA3105 aluminium alloy

The corrosion protection afforded to an AA3105 aluminium alloy supporting an anodic film with incorporated polyaniline and TiO₂ nanoparticles has been examined. The films were synthesised by simultaneous anodizing and electropolymerisation of aniline in the presence of nanoparticles. The morphology and composition of the films were probed by TEM, SEM, rf-GDOES and XPS. The resultant coatings comprised a thin porous anodic film of 2-3 µm thickness, with an outer hybrid polyaniline/TiO₂ layer of several tens nanometres thickness, with the dimensions of TiO₂ nanoparticles being below 10 nm. Electrochemical impedance spectroscopy analysis and salt spray testing revealed that TiO₂ containing films provide improved corrosion protection to the AA3105 aluminium alloy compared with the film without nanoparticles. The improved protection provided by the coatings containing TiO₂ nanoparticles is attributed to the TiO₂ particle-rich thin film layer formed on the outer part of the coating that acts as a blocking barrier layer for the anodic porous aluminium oxide film.     

Corrosion Inhibition of 304 Stainless Steel by Nano-Sized Ti/Silicone Coatings in an Environment Containing NaCl and Water Vapor at 400–600°C

The corrosion behavior of 304 stainless steel (SS) and its corrosion inhibition by brushing nano-sized Ti/silicone coatings on its surface in an environment containing a solid NaCl deposit and water vapor at 400–600°C was studied. Results indicated that water vapor or NaCl, especially water vapor plus NaCl accelerated the corrosion of the steel markedly. The corrosion scales of the uncoated steel had a duplex structure at 400–500°C and internal oxidation occurred for the uncoated steel at 600°C in an environment containing NaCl and water vapor. The corrosion of the 304SS was inhibited efficiently by the coatings at 400–500°C, and the coated steel suffered corrosion to some extent and most of the coatings were destroyed at 600°C. X-ray diffraction (XRD) indicated that the corrosion products of the uncoated steel were mainly Fe₂O₃, Cr₂O₃, NiO or Na₂CrO₄, and the coatings consisted mainly of TiO₂ and SiO₂ after exposure at 400–500°C. The good corrosion resistance of the nano-sized Ti/silicon coatings was attributed to the formation of SiO₂, and TiO₂ that resulted from the decomposition of the organic components in the coating and fast oxidation of nano-Ti powder respectively during the experiments, TiO₂ mixed together with SiO₂ and formed a new coating on the steel surface that played an important role in the protection of the steel.     

Effect of surfactant on the alumina dispersion and corrosion behavior of electroless Ni?P?Al2O3 composite coatings

Electroless Ni? P? Al₂O₃ composite coatings have been synthesized on mild steel shafts using surfactant cetyltrimethyle ammonium bromide (CTAB) as dispersant. The effects of the surfactant on the alumina dispersion, weight fraction in coatings, and corrosion resistance of the composite coatings under salt spray test were investigated. Results showed that alumina dispersion was improved, whereas weight fraction was decreased, with the increasing concentration of the dispersant CTAB. The corrosion resistance of the composite coatings was found to increase with the increase in CTAB concentration up to a certain optimum of 20 mg/L, beyond which a decreasing trend of corrosion resistance was observed under salt spray test. Compared with Ni? P coating, all the Ni? P? Al₂O₃ composite coatings showed improved corrosion‐resistant properties.     

Enhanced corrosion protection by powder coatings on hot rolled steel (HRS) for high-temperature applications

Hot rolled steel (HRS) has long been a major product to the motor industry for bodywork, as galvanised steel (zinc coatings), and it is widely used in building and as tinplate (including tin and chrome oxide coatings) for food, etc. can-making. Among more specialist uses its moderate production costs makes it suitable for the manufacturing process. In this study, newly developed corrosion protection enhanced silicone based powder coating was evaluated on hot rolled steel (HRS). The powder coating in this study was developed to withstand high temperatures up to 550°C without any degradation. In the study silicone resin was incorporated with fillers, corrosion inhibitors and fibres at various compositions to achieve maximum heat resistance and improved corrosion resistance. Protective behaviours of the differently formulated powder paints were investigated before and after heat exposure. The developed paint system was evaluated using various test methods such as heat resistance, salt spray, Electrochemical Impedance Spectroscopy (EIS), adhesion test and microstructure study using SEM. The evaluation of test results revealed that powder coating combining zinc dust particles and mineral fibres have better performance due to better corrosion and adhesion stability in the corrosive environment before and after heat exposure up to 550°C.