The properties of electrodeposited Zn-Co coatings

The possibility of increasing the corrosion resistance of automotive sheet steel by electrodepositing with Zn-Co alloy coatings was investigated. Process variables during electrodeposition such as current density, electrolyte flow rate, and pH were varied in order to examine their influence on the electroplating process. Cobalt contents varying from 0.2 to 7 wt% were easily obtained. The influence of these process parameters on the characteristics of the coating could be related to the hydroxide suppression mechanism for anomalous codeposition. The structure and the morphology of the coatings were determined using SEM and XRD analysis. Application properties important for coating systems used in the automotive industry, such as friction behavior, adhesion, and corrosion behavior, were investigated on coatings with varying cobalt content. The corrosion resistance of the Zn-Co alloy layers was found to be better than that of pure zinc coatings.     

Electrodeposition of tin–nickel alloys from oxalate–sulfate and fluoride–chloride electrolytes

The processes of electrodeposition of tin–nickel alloys from sulfate–oxalate and fluoride–chloride electrolytes are investigated. Oxalate electrolytes for the deposition of the tin–nickel alloy have a high throwing power in comparison with that of the fluoride–chloride electrolyte. Increasing the polarization in the electrodeposition of alloys from an oxalate electrolyte provides an increase in the microhardness and corrosion resistance of coatings.     

Beschichtungssysteme für Verbindungselemente in Kontakt mit Leichtmetallen

Coating systems for joining elements in contact with light metals Galvanic corrosion of light metal alloys can only be avoided, if the steel fasteners are given an electrochemically compatible protection or if the corrosion circuit is interrupted by electric insulating layers. Current density vs. potential curves of chromated zinc alloy coatings show, that by means of zinc nickel coatings a largely electrochemical adaption to the equilibrium potential of quenchaged aluminium alloys of the type AlMgSi is possible. On the other hand, the equilibrium potentials of chromated systems based on ZnFe, ZnCo respectively, are too low to avoid galvanic corrosion on aluminium. In practicerelated corrosion tests on joining elements, this result was confirmed. Besides this, zinc- and aluminium-bearing sintersystems, galvanic tin depositions as well as ternary mechanical coatings based on ZnSnAl have been proved to be compatible with AlMgSi-type alloys. For coating systems in contact with magnesium a satisfactory protection against galvanic corrosion can be achieved effectively with insulating top coats. The results with suitable duplex systems are presented and recommendations for practical applications are derived.     

The Structure and Protective Properties of Zinc and Nickel Alternate Electrodeposits

A NEW TYPE of coatings consisting of a large numberof laminar deposits has been the keen subject of muchresearch,because these layered-structure coatingspossess improved properties or novel phenomenon suchas increased mechanical strength,micro-hardness,giantmagnetoresistance and corrosion resistance[1"6'.Thesecoating systems with individual layers making upoverall structure are also called compositionallymodulated multilayer(CMM)coatings'3"5'.In view ofthe plating historical use of surface…     

Microstructure and Performances of Nanocrystalline Zinc-nickel Alloy Coatings

THE STUDY on zinc-nickel alloy coatings isdeveloped rapidly because of their higher corrosionresistance and better mechanical characteristics[1-8].The zinc-nickel coatings provide improved corrosionprotection for steels in relatively aggressiveenvironments.It has been found that the maximumprotective ability can be reached with the nickel contentbetween12%and15%[9].Recently,several newzinc-nickel alloy technologies have been developed[10-15]and further researches for better coating andchara…     

Electroplating of zinc–cobalt alloys from oxalate electrolytes

The processes of electroplating of zinc–cobalt alloys from oxalate electrolytes and the physicochemical properties of these coatings are investigated. The effect of the proportion between the alloy components on the corrosion resistance, structure, and microhardness of the coatings is revealed.     

Microbially influenced corrosion and inhibition of nickel–zinc and nickel–copper coatings by Pseudomonas aeruginosa

The present study describes the effect of Pseudomonas aeruginosa on the corrosion rate of nickel–zinc and nickel–copper alloy coatings. The presence of bacteria was associated with decreases in R_ct values, suggesting that P. aeruginosa promoted the corrosion of nickel–copper alloy coatings. However, R_ct values of nickel–zinc coatings increased in response to bacterial inoculation, corresponding to a decrease in corrosion rate for nickel–zinc alloy coatings. Our results suggest that the activity of P. aeruginosa facilitated the corrosion of nickel–copper alloy, while serving a protective function for the nickel–zinc alloy.     

The Corrosion Behavior of Cold Sprayed Zinc Coatings on Mild Steel Substrate

Zinc and its alloy coatings have been used extensively for the cathodic protection of steel. Zinc coating corrodes in preference to the steel substrate due to its negative corrosion potential. Numerous studies have been conducted on the corrosion behavior of zinc and its alloy coatings deposited using several techniques viz., hot dip galvanizing, electrodeposition, metalizing or thermal spray etc. Cold spray is an emerging low temperature variant of thermal spray family which enables deposition of thick, dense, and pure coatings at a rapid rate with an added advantage of on-site coating of steel structures. In the present study, the corrosion characteristics of cold sprayed zinc coatings have been investigated for the first time. In addition, the influence of heat treatment of zinc coating at a temperature of 150 °C on its corrosion behavior has also been addressed.     

Electrochemical Doping of Zinc Coatings with Chromium and Nickel Coatings with Phosphorus

The electrochemical doping of coatings with metals and nonmetals is discussed. Using the examples of the electrodeposition of zinc–chromium and nickel–phosphorus coatings, it has been shown that the presence of donors of the electrodeposited coating components in the inner sphere of a heteronuclear or heteroligand complex contributes to their coreduction and the formation of an alloy. The composition, morphology, and properties of the coatings have been characterized. It has been shown that the corrosion resistance of the zinc–chromium coatings is two times higher than that of the zinc coatings. After storage, the nickel–phosphorus coatings preserve their solderability with low-temperature solders.     

镁合金表面冷喷涂层防护研究进展

镁合金作为最轻质的金属结构材料,由于其密度低和比强度高等优良的物理和力学性能,在航空、航天、汽车以及电子等领域引起广泛关注。然而,镁合金化学性质活泼、耐腐蚀和耐磨损性差等缺点严重制约其进一步应用。近些年发展起来的冷喷涂技术,在固态下制备涂层,涂层致密且与基体结合良好,因此可为镁合金表面防护提供一种新的有效方法。主要综述了镁合金表面冷喷涂耐腐蚀涂层(纯铝、铝合金和复合材料涂层)和耐磨损涂层(合金和复合材料涂层),论述了影响冷喷涂层耐腐蚀、耐磨损以及其他力学性能(硬度和涂层/基体结合强度)的主要因素,包括杂质元素含量、合金种类以及复合材料涂层中陶瓷颗粒含量、尺寸和形貌等。对比了几种常用表面处理技术制备的纯铝涂层的耐腐蚀性能,并阐述了冷喷涂技术在镁合金表面防护方面的优势。此外,还分析了热处理对冷喷涂纯铝和复合材料涂层耐蚀性的影响。最后提出了目前冷喷涂技术在镁合金防护方面的局限性以及发展难题,对未来研究趋势进行了展望。     

Ti对Zn-Al合金薄膜耐腐蚀性能的影响

应用组合材料芯片技术,以离子束溅射法在低碳钢基片上制备了不同Ti掺杂量的Zn-Al-Ti薄膜(Al和Zn质量分数分别为55%和45%)样品阵列.沉积得到的多层薄膜经低温扩散和高温晶化形成合金薄膜.以电化学方法测定合金薄膜在浓度(质量分数)为3.5%的中性NaCl水溶液中的耐蚀性能,并进一步研究了优选出的组分的耐蚀性.结果表明,Ti的适量掺杂可使合金薄膜的耐蚀性能明显提高.其中,Ti的质量分数在6%左右时耐蚀性能最佳.采用XRD及SEM对6%Ti的合金薄膜的相结构和表面形貌进行了表征,并与未掺杂Ti的薄膜进行了比较.此外,分析了Zn-Al-6%Ti合金薄膜的腐蚀机理,为进一步优化薄膜体系提供了依据.     

Pulsed Electrodeposition of Composite Coatings Based on Zinc–Nickel Alloy

Composite electrochemical coatings (CECs) based on zinc–nickel alloy and modified by carbon nanotubes (CNTs) are obtained by pulsed electrolysis. The microstructure and tribological properties of these CECs are studied. It is found that addition of dispersed CNTs into the electrolyte for zinc–nickel alloy deposition results in a 1.35- to 1.65-fold decrease in the friction coefficient of the formed coatings. The electrochemical corrosion behavior of the zinc–nickel–CNT CECs is studied in 0.5 M H₂SO₄ solution.     

碱性电镀锌镍合金的研究

采用恒电流沉积方法和X射线能谱(EDS)技术,研究了碱性Zn-Ni合金的主要电沉积工艺参数对镀层组成的影响规律,获得了Ni含量稳定为(11~13)mass%的Zn-Ni合金镀层,证实了Zn-Ni合金的共沉积过程遵循异常共沉积机制。采用扫描电镜(SEM)、原子力显微镜(AFM)和X射线衍射仪(XRD)等对优化的Zn-Ni合金镀层进行了表征,发现镀层主要具有γ相(NiZn₃)结构,其表面平整、致密、光亮;腐蚀测试表明Zn-Ni合金镀层具有优良的耐蚀性能。     

Corrosion performance of Zn–Mg–Al coated steel in accelerated corrosion tests used in the automotive industry and field exposures

The corrosion performance of Zn–Mg(1–2%)–Al(1–2%) (ZMA) coatings has been compared to zinc–iron alloy (galvannealed, GA) and zinc–aluminum coating (Zn–5Al, Galfan) as well as to conventional zinc coatings produced by hot‐dip galvanization (HDG) and electrogalvanization (EG). For this purpose, cosmetic samples (painted and uncoated) and hem‐flange panels were produced. Their corrosion performance was compared in three different accelerated corrosion tests, as regularly used by the automotive industry, e.g., VDA621‐415, N‐VDA (VDA233‐102), and Volvo STD 423‐0014. As can be concluded from our results, the behavior of ZMA coatings was strongly dependent on the testing conditions as well as on the configuration of the samples. The advantageous effect of ZMA coating was more pronounced in open situations than in confined ones, irrespective of the testing conditions. ZMA coatings provided a significant improvement in comparison to conventional coatings in tests involving a significant salt load such as VDA621‐415 or neutral salt spray especially on cosmetic configurations. By contrast, the beneficial effect of ZMA coatings was less obvious in tests with lower salt load (VDA233‐102, Volvo STD423‐0014), particularly when considering cosmetic corrosion on painted samples and corrosion in confinement. Interestingly, no significant differences were observed between samples with varying Al and Mg content in the metallic coating (1–2% each). The results were compared to data from field exposure at stationary sites.     

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.     

Case study: Analysis of coating failure on hinged steel boxes

A systematic failure analysis was performed on three large steel boxes in which the coating system had blistered, extensively delaminated, and subsequently corroded. These boxes were a sample from a larger batch of boxes that had been recalled due to severe corrosion.Despite the fact that limited information on the process history was provided, we were able to determine that two out of three boxes were neither galvanized nor protected by a zinc-rich primer to provide cathodic protection. The coating system comprised an epoxy primer, zinc-rich intermediate coat followed by an acrylate urethane top coat. A zinc-rich coating should never be applied over a nonmetallic coating because cathodic protection of the underlying steel can only take place if the zinc-rich coating is in direct electrical contact with clean steel.The third box was galvanized and primed with a urethane-alkyd primer followed by an acrylate urethane top coat. Alkyd-type coatings should not be applied over zinc (galvanizing) because the corrosion products of zinc are alkaline. Alkyd-modified coatings are very sensitive to alkalinity and a saponification reaction occurs at the zinc-alkyd interface. This degrades the alkyd and causes it to peel or delaminate from the zinc alloy used for the galvanization.The epoxy primer showed unusually severe air bubbles (or pockets) that could have been due to saponification and/or solvent entrapment. In either case, these large pockets weakened the bond between the primer and galvanized layer and allowed delamination to occur.Our finding that chlorides were present on the surface of the epoxy primer indicated that the boxes might have been exposed to a marine or coastal atmospheric environment. Marine or coastal atmospheric corrosion is generally considered to be one of the most severe atmospheric corrosion environments and the presence of chlorides explains the severity of the corrosion.The boxes were presumed to have been powder coated, yet microscopic and chemical analysis showed the coatings were probably applied as liquids. This was further supported by the thinning of the coatings at the sharp edges and corners of the boxes.     

Influence of pH on the deterioration of plasma electrolytic oxidation coated AM50 magnesium alloy in NaCl solutions

The corrosion deterioration process of plasma electrolytic oxidation (PEO) coatings on AM50 magnesium alloy prepared from two different based electrolytes, i.e., an alkaline phosphate electrolyte and an acidic fluozirconate electrolyte, were investigated using electrochemical impedance spectroscopy (EIS) in a 0.1 M NaCl solution with pH of 3, 7 and 11, respectively. It was found that the PEO coating formed in alkaline phosphate electrolyte, which was composed mainly of MgO, suffered from rapid chemical dissolution and lost its protection capability very quickly in acidic NaCl solution (pH 3). The chemical dissolution of this PEO coating was retarded in neutral NaCl solution (pH 7) and the corrosion damage was localized in this environment. On the other hand, in the alkaline NaCl solution (pH 11), the MgO coating underwent only slight degradation. The PEO coating produced in acidic fluozirconate electrolyte, the failure was marked by the flaking-off of the large areas of coating in acidic NaCl solution (pH 3). However, in the neutral and alkaline NaCl solutions, the coating underwent only a slight degradation without any observable corrosion damage in the 50 h test. The results showed that the deterioration process of PEO coated magnesium alloy was governed mostly by the pH of NaCl solution and it was also strongly related to the microstructure and composition of the PEO coatings.     

镁合金表面化学镀镍前处理工艺的研究进展

镁合金作为最轻的金属结构材料,具有密度低、比强度高、弹性模量大等优势,在航天航空、汽车工业、电子通讯等领域广泛应用,但其化学性质非常活泼,在常温下很容易发生腐蚀,严重限制了其进一步推广应用。化学镀镍具有镀层致密、环境友好等优点,可有效提高镁合金的耐蚀性和耐磨性,但与普通基体相比,镁合金属于难镀金属,化学镀镍前既要去除基体表面原有的疏松多孔的氧化膜,又要生成具有保护和催化作用的新膜层,因此前处理工艺是影响镀层质量及镁合金防腐性能提高的关键因素。以化学镀镍前处理工艺为研究内容,介绍了镁合金化学镀镍前处理工艺的国内外研究现状,从除油、酸洗、活化、浸锌法、预镀层和化学转化膜等方面进行了文献综述和分析,指出相应工艺的优缺点,并探讨了研发方向。根据前处理技术的机理和不同牌号镁合金的特点,研发工艺简单、镀层性能优良、可控性强、环境友好、通用性强的低成本工艺,将是镁合金化学镀镍前处理的研究方向和发展趋势。     

Electrodeposition of zinc–nickel–carbon nanotubes composite coatings in a reversing mode

Composite electrochemical coatings (CECs) made of zinc–nickel–carbon nanotubes have been obtained in a reversing regime. The structure and functional properties of these CECs have been investigated. It has been established that introduction of carbon nanotubes into the deposition electrolyte of a zinc–nickel alloy results in a lowered friction coefficient and enhanced protective capability of the formed coatings.     

Combination of zinc alloy coating with thin plasma polymer films for novel corrosion protective systems on coated steel

Steel sheet used in automotive applications has to be corrosion protected effectively, which is usually realized by zinc or zinc alloy coatings with a thickness range of 5–10 μm. Steel sheet for areas of a car body which are exceptionally stressed by corrosion, e.g. cavity flanges or joints, may be protected additionally by a thin weldable organic coating with a thickness of 2–4 μm. A very promising approach to a significantly reduced use of resources is the combination of zinc alloy coatings with thin plasma polymer films deposited by means of plasma-enhanced chemical vapour deposition (PECVD). Such plasma polymer films of just a few 100 nm thickness show excellent barrier and adhesion properties as well as a high mechanical stability.Within this work thin plasma polymer films were deposited on zinc alloy coated steel substrates using the strip hollow cathode (SHC) method, which was modified for application on grounded substrates. A pulsed DC glow discharge in a mixture of argon and an organosilane precursor was used for the deposition of films with a thickness of 100–500 nm.The chemical compositions of the coatings were determined by means of X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The morphologies of the coating systems were studied by means of scanning electron microscopy. The performance of the coating systems has been studied in different specific tests of corrosion and processing behaviour. The investigated coating systems show a corrosion resistance comparable to reference samples of electro-galvanised steel sheet with additional organic coating even with a coating thickness less of half of the reference samples.