Verhalten geschweißter Magnesiumlegierungen unter mechanisch‐korrosiver Komplexbeanspruchung

Corrosion and corrosion fatigue of welded magnesium alloys In addition to the prevalent use of magnesium cast alloys a high potential for lightweight constructions is offered by magnesium‐wrought alloys, in particular in the automobile industry. The use of rolled and/or extruded magnesium alloys (profiles and sheet metals) requires suitable and economic join technologies like different welding procedures in order to join semi finished parts. Thus, the realization of lightweight constructions asks for high standards of materials‐ and joining‐technologies. In this context, the mechanical properties as well as the corrosion behaviour of the joints are of large interest. During welding of magnesium alloys, influences concerning the surface, the internal stresses and the microstructure occur. These influences particularly depend on the energy input and thus, on the welding procedure as well as the processing parameters, which all affect the corrosion behaviour of the joints. Sheets of magnesium alloys (AZ31, AZ61, AZ91) were joined with different welding procedures (plasma‐, laser beam‐ and electron‐beam welding in the vacuum and at atmosphere). The corrosion behaviour (with and without cyclic mechanical loading) of the welded joints was investigated by different methods such as corrosion tests, polarisation curves, scanning electron microscopy and metallography. Furthermore, substantial influencing variables on the corrosion behaviour of welded joints of magnesium alloys are pointed out and measures are presented, which contribute to the improvement of the corrosion behaviour.     

Elektrolytische Abscheidung von Aluminium‐Magnesium‐Legierungen aus aluminiumorganischen Komplexelektrolyten

Electrolytic Deposition of Aluminium‐Magnesium‐Alloys from Electrolytes Containing Organo‐Aluminium Complexes The galvanic deposition of pure aluminium from fluoride‐containing electrolytes has been developed further and for the first time aluminium and magnesium have been deposited from a toluene‐solution of a halide‐free organo‐aluminium complex electrolyte. The rate of incorporation of magnesium can be controlled over a wide range by either adjusting the composition of the aluminium‐magnesium anode or by using separate aluminium or magnesium anodic circuits. The current efficiency for both anode and cathode approaches 100%. The resulting coating is optically attractive and, depending upon the magnesium‐content or the cathodic current density, can be formed as a dull or polished surface. Investigations using an electron microscope show that the surface, in contrast to that of pure aluminium, consists of spherical particles. The aluminium‐magnesium coating provides excellent protection against the corrosion of magnesium components. Electrochemical investigations using, for example 25% by weight magnesium incorporation, indicate a pronounced passivity interval compared to the alloy AZ91hp. In contrast to galvanic zinc‐plated and silicate‐sealed examples, cyclic corrosion tests on screws simulating 10 years of exposure, show no corrosion.     

表面处理对TC21钛合金与铝合金和钢电偶腐蚀行为的影响

通过测定TC21钛合金与铝合金和钢电偶电流的方法,研究了TC21钛合金与铝合金和钢在使用接触时发生电偶腐蚀的敏感性.结果表明:TC21钛合金与铝合金和钢形成的电偶对极易发生电偶腐蚀,不能直接接触使用;对钛合金和铝合金分别进行阳极氧化处理可以在一定程度降低电偶腐蚀敏感性.TC21钛合金与钢形成的电偶对,电偶腐蚀行为与钢的成分有很大关系,对钛合金进行阳极氧化处理,对钢进行镀镉或镀镉-钛处理可以提高表面抗腐蚀性能,降低电偶腐蚀敏感性.当TC21钛合金与铝合金和钢接触使用时,必须采取有效的防护措施.     

镁合金电偶腐蚀研究及其进展

研究了镁合金AM60与异种金属Q235碳钢、18-8不锈钢、LY12铝合金和纯铝之间的电偶腐蚀,介绍了镁合金电偶腐蚀的最新研究进展,重点探讨了影响镁合金电偶腐蚀的因素和减轻镁合金电偶腐蚀的措施,最后提出了镁合金电偶腐蚀研究中需要解决的科学问题.研究和讨论分析表明,为防止镁合金电偶腐蚀,应全面而系统地进行合理的结构设计、选择合适的匹配材料、在镁合金和异种金属表面分别涂装性能优良的耐碱性涂层体系.由于镁腐蚀发生碱化,所以防止电偶腐蚀的环境应避免选用含铝涂层体系.     

Design of wear and corrosion resistant PVD‐coatings for magnesium alloys

Although magnesium alloys became popular in the first half of the 20^th century, the bad corrosion properties prevented their breakthrough in industrial mass production. Since the technology for the production of high purity alloys was introduced in the 1970s, magnesium alloys became more and more in the focus of industrial attention. Today magnesium alloys are state of the art in structural parts in automotive industry. Despite its outstanding properties like good castability, low density and nearly unlimited availability the negative aspects like weak corrosion and wear behaviour still limit the application of magnesium in industry [1]. So, the only economic solution is the deposition of a coating or a suitable surface treatment which provides both, wear and corrosion resistance. Today, plasma electrolytic anodisations are state of the art [2–5]. They provide acceptable corrosion resistance and protect the magnesium from mechanical damage due to their high hardness. On the other hand, their high porosity limits their use in combination with electrochemically noble materials, leading to galvanic corrosion [6]. In addition, the high surface roughness of the plasma electrolytic anodisations restricts their use in tribological applications, particularly under dry sliding conditions [7]. On the other hand, due to the high life time recommendations the application of magnesium in the automotive industries motion component field is a long term process. Nevertheless, there is a quite high industrial interest to apply magnesium in the motion component field in consumer applications like do‐it‐yourself or gardenig. Some examples are motor components of lawnmovers, motor saws or drills. Especially for these fields of application there are quite high demands on the corrosion properties due to undefined storage and the conditions during usage. In order to achieve smooth surfaces with high quality, the PVD technology moves into the centre of interest. Since the 1980s PVD coatings are well established and widely used for different industrial applications, mainly for steel and tool coatings. The authors were the first who carried out serious studies on the development of PVD coatings for magnesium alloys since 1999 [6, 7]. The extensive research activities lead to the recent development of a coating system, which provides both, good wear properties as well as good corrosion behaviour.     

Aluminium‐Abscheidung aus aprotischen Elektrolyten – eine alte Idee in neuem Stil

Aluminium‐deposition from aprotic electrolyte – an old concept with new style Organo‐aluminium complex compounds have been used for many years as elec‐trolyte for the galvanic deposition of aluminium. For the galvanic deposition suitable electrolyte are aprotic complexes of type MX·2 AlR₃ solved in liquid aromatic hydrocarbons. By using aluminium‐magnesium alloy anodes or by using separate aluminium and magnesium anode circuits also AlMg‐alloy layers with high alkali resistance from aprotic electrolyte can be deposited. The aluminium layers have a microcristalline and low‐porous structure. The galvanic aluminization has become an issue of great interest for industrial applications due to its excellent corrosion behaviour and due to the fact that the resistance to hydrogen embrittlement can not be exceeded at deposition from non‐aqueous electrolyte. Therefore the galvanic aluminization from organo‐aluminium electrolyte becomes more and more important being expressed in numerous industrial examples.     

钛合金与钢的电偶腐蚀行为研究

研究了TA28、Ti6321钛合金以及921A、907A船体结构钢在海水环境中的腐蚀特性及电化学行为,对两种钛合金与船体钢之间多面积比条件下的电偶腐蚀行为进行了试验。结果表明,当钛合金与钢直接接触时,钛合金与钢制船体间存在轻微的电偶腐蚀,采用电绝缘措施可以有效控制钛合金对钢制船体的电偶腐蚀。     

Schutzsysteme für Verschraubungselemente in Kontakt mit Leichtmetall

Protective coating systems for fasteners in contact with light weight metals A review of the recent use of fasteners for light weight alloys in automotive car production is given. Mechanical design, materials selection and protective coating concepts for Magnesium- and Aluminum-alloys are presented. The specific problems caused by galvanic corrosion are discussed by the use of electrochemical data of the protective coating systems on fasteners for joining Magnesium. Experience from outdoor exposure in automotive applications up to date are considered.     

TA15钛合金与铝合金和结构钢接触腐蚀与防护研究

通过测定TA15钛合金与铝合金和结构钢组成的电偶对的电偶电流的方法,研究了TA15钛合金在使用中与铝合金和结构钢接触时产生电偶腐蚀的敏感性.结果表明:TA15钛合金与铝合金和30CrMnSiA钢接触时会产生严重的电偶腐蚀,必须进行防护处理方可使用;与30CrMnSiNi2A钢接触产生的电偶腐蚀比较轻微.对铝合金和结构钢进行表面处理可以降低电偶腐蚀敏感性,表面处理后喷涂底漆可进一步降低电偶腐蚀敏感性.     

Recent Progress in Corrosion and Protection of Magnesium Alloys

Magnesium alloys are advanced light structural and functional materials being increasingly used in the automotive, aerospace, electronic, and energy industries. However, their corrosion performance at the current stage of development is still not good enough for increasingly diverse practical applications. The Cooperative Research Centre for Cast Metals Manufacturing (CAST) in Australia is one of the most active research organizations in the world established to cope with the problems associated with the development and application of advanced light metals. Corrosion and prevention of magnesium and its alloys is an important part of CAST's research program. This paper presents a brief summary of recent research achievements by CAST and relevant research work in this area in the world. This overview covers “anodic hydrogen evolution”, estimation of corrosion rate, corrosion of aluminum‐ and non‐aluminum‐containing magnesium alloys, influences of composition and microstructure on corrosion, corrosion of a die‐cast magnesium alloy, galvanic corrosion, coolant corrosion, and an aluminum‐alloyed coating. The aim of this overview is to deepen the current understanding of corrosion and protection of magnesium and its alloys and to provide a base for future research work in this field.     

Application of the hybrid process ultrasound enhanced friction stir welding on dissimilar aluminum/dual‐phase steel and aluminum/magnesium joints

Friction stir welding as a solid‐state joining method with its comparatively low process temperatures is suitable for joining dissimilar materials like aluminum/magnesium or aluminum/steel. Such hybrid joints are of great interest regarding lightweight efforts in different industrial fields like the transportation area. The present work investigates the influence of additionally transmitted power ultrasound during the friction stir welding on the joint properties of EN AC‐48000/AZ91 and EN AW‐6061/DP600. Therefore, conventional friction stir welding was continuously compared to ultrasound enhanced friction stir welding. Light microscopic analysis and nondestructive testing of the joints using x‐ray and high frequency ultrasound show different morphologies of the nugget for the aluminum/magnesium joints as well as differences in the amount and size of steel particles in the nugget of aluminum/steel joints. Scanning electron microcopy proves differences in the thickness of continuous intermetallic layers for the aluminum/steel joints realized with and without power ultrasound. Regarding the tensile strength of the joints the power ultrasound leads to increased joint strengths for EN AC‐48000/AZ91 joints compared to a decrease for EN AW‐6061/DP600 joints. Corrosion investigations show an influence of the ultrasound power on the corrosion properties of EN AC‐48000/AZ91 joints which is attributed to a changed aluminum content in the nugget region. Because of the great potential difference between the magnesium and the nugget phase the transitional area exhibits strong galvanic corrosion. For EN AW‐6061/DP600 joints an increased corrosion caused by galvanic effects is not expected as the potentials of the EN AW‐6061 aluminum alloy and DP600 steel are very similar.     

Corrosion cracking of high-strength structural steels

A study of the mechanism of corrosion cracking of high-strength carbon steels showed that the stress dependence of time-to-rupture of steels and copper, magnesium and aluminum alloys tested in tension in various media is described by the same equation. Hence it was concluded that the basic phenomena involved in corrosion cracking are common for all alloys.     

Corrosion behaviour of some vapour deposited magnesium alloys

Abstract

Binary magnesium alloys containing chromium, manganese, or titanium were made using a physical vapour deposition technique. The corrosion resistance of the alloys was assessed in aqueous chloride solutions using total immersion tests in quiescent 600 mmol L^?1 NaCI solutions. Alloying with manganese or titanium was found to lower the corrosion rate of magnesium over most of the compositional ranges of interest, whereas addition of chromium had a detrimental effect on the corrosion resistance of magnesium. The lowest corrosion rate was recorded for a Mg–Ti alloy where the value obtained was about 80 times lower than that found for vapour deposited pure magnesium. Open circuit corrosion potential measurements conducted in 600 mmol L^?1 NaCl solution showed that additions of chromium, titanium, and manganese also produced deposits which were significantly more noble than pure magnesium, suggesting that these alloys would be less susceptible to galvanic corrosion.

MST/3064     

Effect of Thermal Oxidation on Microstructure and Corrosion Behavior of the PVD Hf‐Coated Mg Alloy

Hafnium coatings are fabricated on magnesium alloys by magnetron sputtering and are further submitted to the thermal oxidation treatment at temperature of 200, 300, and 400 °C. The thin hafnium oxide film and new grain boundaries are observed on the hafnium coatings during the appropriate treatment temperature (300 °C). These changes in microstructure result in surface densification, oxidation, and low porosity of the treated coating that significantly decrease its susceptibility to corrosion. Consequently, the thermal oxidation treatment hafnium coating exhibits a more positive corrosion potential, lower corrosion current density, and higher polarization resistance than that of the as‐deposited coating using an electrochemical system. Moreover, the enhanced adhesion of the treated coating produced by applying an appropriate treatment temperature facilitates an efficient long‐term protection of magnesium alloy.

     

Biocorrosion Zoomed In: Evidence for Dealloying of Nanometric Intermetallic Particles in Magnesium Alloys

Biodegradable magnesium alloys generally contain intermetallic phases on the micro‐ or nanoscale, which can initiate and control local corrosion processes via microgalvanic coupling. However, the experimental difficulties in characterizing active degradation on the nanoscale have so far limited the understanding of how these materials degrade in complex physiological environments. Here a quasi‐in situ experiment based on transmission electron microscopy (TEM) is designed, which enables the initial corrosion attack at nanometric particles to be accessed within the first seconds of immersion. Combined with high‐resolution ex situ cross‐sectional TEM analysis of a well‐developed corrosion‐product layer, mechanistic insights into Mg‐alloys' degradation on the nanoscale are provided over a large range of immersion times. Applying this methodology to lean Mg–Zn?Ca alloys and following in detail the dissolution of their nanometric Zn‐ and Ca‐rich particles the in statu nascendi observation of intermetallic‐particle dealloying is documented for magnesium alloys, where electrochemically active Ca and Mg preferentially dissolve and electropositive Zn enriches, inducing the particles' gradual ennoblement. Based on electrochemical theory, here, the concept of cathodic‐polarization‐induced dealloying, which controls the dynamic microstructural changes, is presented. The general prerequisites for this new dealloying mechanism to occur in multicomponent alloys and its distinction to other dealloying modes are also discussed.     

In vitro degradation behaviour of a friction stir processed magnesium alloy

In this study, the in vitro degradation behaviour of a friction stir processed AZ31 magnesium alloy was investigated. Electrochemical experiments in simulated body fluid suggest that friction stir processing marginally enhances the degradation resistance of the alloy, which could be attributed to the dissolution of secondary phase particles. Homogenisation of the microstructure reduces galvanic corrosion. It is envisaged that the beneficial effect would be more pronounced for magnesium alloys which contain high volume fraction of galvanic corrosion inducing secondary phase particles.     

Bio-corrosion behavior of magnesium-fluorapatite nanocomposite for biomedical applications

Recent studies indicate that there is a high demand for designing magnesium alloys with adjustable corrosion rates and precipitation ability of bone-like apatite layer on the surface of magnesium alloys in body. An approach to this challenge might be the application of nanocomposites based on magnesium alloys. The aim of this work was fabrication and bio-corrosion evaluation of a nanocomposite that was made of magnesium alloy AZ91 as matrix and fluorapatite (FA) nano particles as reinforcement. Magnesium-fluorapatite nanocomposite (AZ91-20FA) was made via the blending-pressing-sintering method. In vitro corrosion measurements were performed for characterization of initial materials and produced composite. The results showed that the addition of FA nano particles to magnesium alloy as reinforcement can reduce the corrosion rate and accelerate the formation of bone-like apatite layer and in turn provide better protection for matrix alloy. It is suggested that the formation of bone-like apatite layer on the surface of magnesium alloy might contribute to the good osteoconductivity of magnesium alloys.     

Corrosion of experimental magnesium alloys in blood and PBS: A gravimetric and microscopic evaluation

Corrosion tests for medical materials are often performed in simulated body fluids (SBF). When SBF are used for corrosion measurement, the open question is, how well they match the conditions in the human body. The aim of the study was to compare the corrosion behaviour of different experimental magnesium alloys in human whole blood and PBS^minus (phosphate buffered saline w/o Ca and Mg) as a simulated body fluid by gravimetric weight measurements and microscopic evaluation. Eight different experimental magnesium alloys, containing neither Mn nor other additives, were manufactured. With these alloys, a static immersion test in PBS^minus and a dynamic test using the Chandler-loop model with human whole blood over 6 h were performed. During the static immersion test, the samples were weighed every hour. During the dynamic test, the specimens were weighed before and after the 6 h incubation period in the Chandler-loop. From both tests, the total mass change was calculated for each alloy and the values were compared. Additionally, microscopic pictures from the samples were taken at the end of the test period. All alloys showed different corrosion behaviour in both tests, especially the alloys with high aluminium content, MgAl9 and MgAl9Zn1. Generally, alloys in PBS showed a weight gain due to generation of a microscopically visible corrosion layer, while in the blood test system a more or less distinct weight loss was observed. When alloys are ranked according to corrosion susceptibility, the results differ also between the test systems. The MgAl9 alloy, showing the most pronounced corrosion in PBS, was one of the least corroding alloys under simulated in vivo conditions in blood. Thus, the ranking concerning clinical suitability of the magnesium alloys tested in this study is different, depending on the used electrolyte and the kind of method. For a possible clinical use, the alloy MgAl9Zn1 might be preferable for further investigations.     

Application of a Bioactive Coating on Resorbable,Neodymium Containing Magnesium Alloys,and Analyses of their Effects on the In Vitro Degradation Behavior in a Simulated Body Fluid

Whilst bioactive coatings are commonly used as layers for non‐resorbable implant materials, such as titanium or steel to improve cell adhesion, this study investigates the application of bioactive SiO₂–CaO–P₂O₅ on resorbable magnesium alloys (Nd2 and LANd442). The bioactive coating was applied to the magnesium alloys by a dip‐coating process, where a parameter set of 20 immersions and a 10 s drying time between each immersion generated a reproducible layer with regard to its thickness and homogeneity. In vitro mass loss, strength loss, and pH value measurements were used to determine the coating's effects on the degradation behavior in a simulated body fluid. Here, it could be observed that bioactive layers on magnesium alloys lead to an increased degradation in comparison to specimens in the uncoated states. In addition to this, pitting corrosion was determined for bioactive coated magnesium samples during comparatively early periods of the investigation. Due to the decreased corrosion resistance and induced pitting corrosion of bioactive coated magnesium alloys, it is suggested that one carefully tests if the enhanced cell adhesion, which occurs with bioactive coatings, warrants the increased degradation of magnesium based implant materials.     

Surface Treatments of Automotive Parts – Research and Applications

Surface treatments used in daily manufacturing of parts for the automotive industry are selected to serve functional and decorative requirements achieved of mass production. Increased loads (mechanical, thermal etc.), longer life time, weight reduction, friction reduction, and corrosion resistance are demanded for modern automotive systems. Within the last decade, improved and new deposition techniques were developed in PVD, PECVD, thermochemical heat treatment and thermal spraying. These new treatments are becoming more and more common in powertrain and engine applications. Generating optimized surfaces for different types of substrate materials (e.g. Al‐ alloys, case hardened steels etc.) and geometries (e.g. bores) also impacts the running costs. Due to the new developments within these competing surface treatments, it becomes more and more common to substitute traditional treatment‐substrate‐systems with advanced treatments. Both the application potential and selected examples of different surface treatments will be shown. Aspects of the internal coating of bores inside the engine by plasma spraying, features of the corrosion protection of parts for the powertrain by the IONITOX process and piston ring treatments are discussed in more detail.