Kontaktkorrosion einer Magnesiumlegierung mit beschichteten Bauteilen

Galvanic corrosion behaviour of magnesium alloy in contact with coated components Electrochemical investigations were carried out to examine galvanic corrosion in preparation for the use of steering gear made from a magnesium, pressure die‐casting. The aim was to characterize various joining elements of the steering gear (e.g. hold‐down devices, threaded bushings, plugs, screws) with different types and qualities of protective coating with regard to the risk of galvanic corrosion in magnesium alloy AZ91. In addition the change in the polarization resistance was determined for one coating system after contact with AZ91 and compared with data without contact. The quality of the coatings is very important to prevent galvanic corrosion. This was demonstrated using as an example, KTL coated systems of various quality. The evaluation of current density potential curves of the coatings only allows a gradation of the top coat qualities in relatively short time, but the problems which can actually appear at contact to AZ91 aren't recognized.     

Corrosion and corrosion prevention of magnesium alloys

Introduction of high purity alloys improved the corrosion resistance of magnesium alloys significantly. This has led to an increased use of magnesium for components like valve covers, transmission housing, and gear box housing. Because of the unnoble nature of magnesium, galvanic corrosion is the main challenge when magnesium is used in corrosive environment. By proper material selection, proper design and selective use of coatings and insulation materials, the risk for galvanic corrosion is significantly reduced. Test results show that fasteners made of aluminium of the 6000 series reduce galvanic corrosion of magnesium to very low levels in salt spray tests. Combinations of plated steel fasteners and aluminium washers are very efficient for galvanic corrosion prevention. Furthermore, it seems that sealed zinc plating is the best type of plating for steel fasteners. The plating must, however, be free from defects and the bolt head design is an important factor to get a high quality plating.     

Galvanic corrosion of aluminium alloy members of bridge guiderails under severe atmospheric exposure conditions

ABSTRACT

Aluminium alloys are nowadays preferred as materials for bridge guiderails especially for bridges connecting oceanic islands or spanning inlets due to their inherent corrosion resistance. But because of the limited mechanical strength of aluminium alloy, fasteners of guiderail members are made from steel materials. It has been found that contact between bare steel fasteners and aluminium alloy members can cause galvanic corrosion in the aluminium alloy. Research was carried out to investigate the capability of different surface treatments on fasteners and aluminium alloy members to inhibit galvanic corrosion under atmospheric exposure for periods of one and three years. It was found, among other results, that stainless steel fasteners treated with zinc flake coating were the most effective inhibitors of galvanic corrosion on aluminium alloy members.     

Influence of tin addition on corrosion resistance of aluminium ion platings

Abstract

Aluminium ion platings are galvanically compatible with the aluminium alloys used in the aircraft industry and prevent galvanic attack when in contact with them. However, these platings fail to provide the good lubricity which the traditional, but toxic, cadmium platings are known to provide. Adding tin to aluminium ion platings can improve their frictional characteristics. In this work, electrochemical polarisation and metallurgical studies have been used to examine the influence qf introducing 5%Sn into aluminium ion platings on their corrosion resistance. Results have shown an increase in the corrosion tendency as indicated by a shift of the corrosion potential in the active direction and an increase in the corrosion current density which can be related to a galvanic action between two different phases of aluminium rich columnar grains. Domes observed within these grains can easily be removed during early stages of corrosion leaving cavities in the platings, thus providing another possibility of galvanic attack between the plating and substrate taking place.     

Galvanic corrosion of magnesium alloy AZ91D in contact with an aluminium alloy, steel and zinc

An investigation was carried out into the galvanic corrosion of magnesium alloy AZ91D in contact with zinc, aluminium alloy A380 and 4150 steel. Specially designed test panels were used to measure galvanic currents under salt spray conditions. It was found that the distributions of the galvanic current densities on AZ91D and on the cathodes were different. An insulating spacer between the AZ91D anode and the cathodes could not eliminate galvanic corrosion. Steel was the worst cathode and aluminium the least aggressive to AZ91D. Corrosion products from the anode and cathodes appeared to be able to affect the galvanic corrosion process through an “alkalisation”, “passivation”, “poisoning” effect or “shortcut” effect.     

Electrochemical behaviour of aluminium/steel rivet joints

The galvanic corrosion behaviour of a rivet joint of two sheets of the aluminium alloys EN AW-6014-T4 and EN AW-6082-T5 joined by an electrogalvanized steel blind rivet was investigated. The potentiodynamic polarization curves in a 5 wt.% NaCl solution show a potential reversion of the anodic and cathodic regions of the rivet joint. The surface potential was investigated with a capillary electrode before and after corrosion. From the measurements by the capillary electrode and SEM-observations an improved resistance against galvanic corrosion is expected after the dissolution of the zinc layer of the blind rivet.     

Korrosionsschutzwirkung von Beschichtungen am Defekt – Einfluss der Oberflächenvorbereitung und der Pigmentierung der Grundbeschichtung

Corrosion‐protective effect of organic coatings at the defect – influence of surface preparation and anti‐corrosive pigments of base coating In practice of corrosion protection a clear influence of surface preparation and pigments of base coating has been widely observed on defect‐induced delamination of organic coatings. In the presented paper this mechanism was studied using the Scanning Kelvin‐Probe. This instrument enables determination of the corrosion potential by high resolution contactless measurements even with presence of insulating organic coatings. During these investigations the following results were obtained: Cathodic delamination takes place in case of coatings with zinc phosphate primers on blast cleaned steel substrates. Using zinc dust primers, especially the edges of the defect are cathodically protected by anodic dissolution of zinc as part of the zinc dust primer. Because of the formation of zinc oxides, increasing load duration can lead to a deactivation of zinc dust and a progression of the corrosion process below the coating. On hand‐cleaned steel surfaces rust reduction takes place at the edge of the defect in form of a cathodic partial reaction which is independent from pigments of the base coating. This reduction is increased by the galvanic cell. In case of cyclic loading reduced rust is reoxidized during dry phases and can re‐participate in the process during wetting. In the result the thickness of the rust‐layer is growing and phase transformations in the rust lead to the loss of adhesion of the coating starting from the edge of the defect     

Novel zinc‐rich epoxy paint coatings with hydrated alumina and carbon nanotubes supported polypyrrole for corrosion protection of low carbon steel: Part II: Corrosion prevention behavior of the hybrid paint coatings

Utilization of various types of multi‐walled carbon nanotubes (MWCNTs) in zinc‐rich paints (ZRPs) is presented addressing percolation and porosity related phenomena of traditional ZRPs. Hybrid paint coatings were formulated with 3.21 wt% polypyrrole (PPy) deposited alumina‐MWCNT inhibitor particles (PDAMIPs) and 70 wt% zinc contents. Corrosion protection behavior of the hybrid coatings was investigated by electrochemical impedance spectroscopy (EIS), glow‐discharge optical emission spectroscopy (GD OES), X‐ray photoelectron spectroscopy (XPS), and FT‐Raman spectroscopy. Immersion and salt‐spray chamber tests gave evidence of improved galvanic protection and barrier nature of the hybrid coatings over the conventional ZRPs, whereas inhibited zinc corrosion and ignorable steel corrosion took place besides lower degradation of the binder. Zinc‐rich hybrid paints with either high relative amount of polyelectrolyte‐modified or low proportion of functionalized MWCNTs afforded enhanced corrosion prevention. This result is partly attributed to the nanotube volume fractions around the threshold of infinite cluster formation contributing to electrical percolation and galvanic action of the hybrids. Experimental results are discussed in a broader context on the basis of structure related findings of the PDAMIPs (described in Part I) and in the light of recent literature data. From the newly developed inhibitor particles, some of them are respected as worthy additives for application in hybrid coatings featuring high performance corrosion prevention functionality.     

Corrosion Resistance of Zinc-alloy Coated Steel in Construction Industry Environments

Considerable efforts are being made to improve the corrosion resistance of zinc coatings by alloying and application of compatible conversion coating treatments. These new systems have the potential for reliable and cost effective protective coatings on steel used in construction applications.

The present investigations have been concerned with: (i) development of a simple sensor that can be used to determine corrosion rates of zinc alloy coated steel in the macro/micro climates found in and around buildings and other structures.

(ii) determination of the compatibility and corrosion rates of zinc alloy coatings in contact with cementitious materials and preservative treated timber.

This paper describes the rationale for the investigations and the experimental techniques used, with a summary of the principle results and conclusions.     

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.     

Electrochemical corrosion behaviour of Mg–Al alloys with thermal spray Al/SiCp composite coatings

The corrosion protection of Mg–Al alloys by flame thermal spraying of Al/SiC particles (SiCp) composite coatings was evaluated by electrochemical impedance spectroscopy in 3.5 wt.% NaCl solution. The volume fraction of SiCp varied between 5 and 30%. The as-sprayed Al/SiCp composite coatings revealed a high number of microchannels, largely in the vicinity of the SiCp, that facilitated the penetration of the electrolyte and the subsequent galvanic corrosion of the magnesium substrates. The application of a cold-pressing post-treatment reduced the degree of porosity of the coatings and improved the bonding at the coating/substrate and Al/SiC interfaces. This resulted in improved corrosion resistance of the coated specimens. The effectiveness of the coatings slightly decreased with the addition of 5–30 vol.% SiCp compared with the unreinforced thermal spray aluminium coatings.     

Plating and Polishing

Laboratory tests were made to determine the behaviour of cadmium and zinc coatings at room temperature and at 60°C. in humid atmospheres in the presence and absence of various insulating materials. The results indicate that cadmium plating, particularly if chromate, should give adequate protection to steel components in electrical equipment in most conditions of exposure even when temperatures and relative humidities are high. Rather more attack may be expected if condensation occurs. In the presence of moisture and certain insulating materials, however, corrosion of cadmium plating can be greatly accelerated and a dangerous loose corrosion product may be formed. One potent accelerating agent was a sample of yellow insulating tape which evolved acid vapours and caused the formation of corrosion products containing propionates and butyrates. Vapours of the lower fatty acids were also found to increase considerably the corrosion of cadmium. A sample of phenol-formaldehyde resin also accelerated attack, but in this case alkaline vapours were evolved. The results obtained showed that zinc coatings were less resistant than cadmium coatings under the various conditions used. Attack of zinc coatings also was greatly accelerated by the lower fatty acids and by the vapours from the yellow tape and the phenol-formaldehyde resin.     

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.     

汽车表面达克罗防腐涂层的组织和性能

介绍了达克罗技术在汽车工业中的应用情况，对达克罗防腐涂层表面组织形貌和耐腐蚀性能进行了研究，并和镀锌、镀铝涂层的耐腐蚀性能进行对比。结果表明，锌、铝、铬等元素在达克罗涂层中分布均匀，片状的锌粉和铝粉层叠，而无定形的铬酸盐聚合物分布在锌、铝之间。涂层覆盖在被保护工件表面，形成牢固的防腐蚀保护涂层。     

Modelling the corrosion behaviour of Al2CuMg coarse particles in copper-rich aluminium alloys

The corrosion behaviour of 2024 aluminium alloy in sulphate solutions was studied; attention was focused on the influence of coarse intermetallic Al₂CuMg particles on the corrosion resistance of the alloy. Model alloys representative of the aluminium matrix and of Al₂CuMg coarse intermetallics were synthesized by magnetron sputtering. Open-circuit potential measurements, current-potential curve plotting and galvanic coupling tests were performed in sulphate solutions with or without chlorides. Further explanations were deduced from the study of the passive films grown on model alloys in sulphate solutions. The results showed that model alloys are a powerful tool to study the corrosion behaviour of aluminium alloys.     

超细锌粉无机硅酸锌涂层的结构及耐蚀性研究

制备了3种超细锌粉作填料的无机硅酸锌涂层,用FTIR、SEM对涂层的固化机理和结构进行了研究,对3.5%NaCl溶液浸泡腐蚀后的试样表面形貌进行了观察,通过电化学试验研究了涂层的耐蚀性.结果表明,无机硅酸锌涂层的结构为:Zn粉颗粒间以Si-O-Zn聚合物网络键合,涂层与底材钢基体间是以化学键结合的微观多孔的涂层结构;涂层的耐蚀性随Zn粉颗粒度的减小而显著提高.涂层的耐蚀机理主要是电化学阴极保护作用以及腐蚀产物填塞涂层孔隙的物理屏蔽作用.     

Mischbauweise im Automobilbau Oberflächenvorbehandlung und Korrosionsschutz

Multimetal construction for car construction Surface treatment and corrosion protection The use of aluminium beside steel and galvanized steel for carbodies leads to multimetal constructions which have to pass through a common pretreatment line. The phosphates baths which are used contain fluorid. The combination of different materials could lead to contact corrosion. The choice of alloys and coatings with similar free corrosion potentials should minimize the danger of contact corrosion of automobiles. Examples will be shown. The influence of surface treatment to filiform corrosion will be demonstrated. A procedure for the substitution of chromating will be shown.     

Advances in Replacements for Cadmium Plating in Aerospace Applications

This paper examines some of the problems associated with the replacement of cadmium plating for the protection of aircraft components and fasteners manufactured from steel. It reviews the range of coatings which are available commercially and are currently being considered as alternatives to cadmium plating. Results of research being undertaken into the corrosion behaviour and galvanic compatibility of electrodeposited zinc alloy coatings and aluminium-magnesium alloy coatings are presented. Other aspects of coating performance are also considered including frictional properties, effects of coatings on fatigue strength, hydrogen embrittlement of steels and the repair of damaged coatings. It is concluded that although the corrosion performance of cadmium plating may be achieved with several coating systems, no one coating is likely to be acceptable as a replacement for cadmium for all aerospace applications.     

Influence of alloying elements on electrochemical behaviour of ternary Al-Zn-Ga alloys for sacrificial anodes

Abstract

This work underlines the influence of alloying elements zinc and gallium on the electrochemical behaviour of aluminium alloys used as sacrificial anodes. Both the single and joint effects of these two elements on the free corrosion potential, the corrosion current density, and the development of the passive region were studied. Coupled potential and efficiency measurements were also carried out. It has been shown that the influence of gallium on the extent of the negative shift in corrosion potential of aluminium is greater than that of zinc. As regards the joint influence of the two elements, zinc was found to reduce the extent of the gallium induced negative shift in potential. Finally, for any composition the efficiencies were low, possibly because of partial segregation in these alloys.     

Untersuchungen über Inhibitoren der atmosphärischen Korrosion feinmechanischer Erzeugnisse

Investigations into atmospheric corrosion inhibitors for precision instruments Precision instruments can be protected against corrosion during shipment to tropical regions when inhibitor combinations are used formed by two components having different modes of action. Such a combination of dicyclohexylammoniumnitrite and dimethyldiethoxysilane have turned out to be efficient inhibitors. While the nitrite being a wellknown vapourphase inhibitor is deposited on metal surfaces as a film which is easily washed off the silane compound forms a hydrophobic layer protecting the inhibitor layer from being washed off by condensing moisture. According to the results obtained in condensing humidity conditions the combination tested is much more efficient than the components when used separately. The increased protection can be recognized after about 24 hours and after 96 hours 75% protection is afforded. Some metals suffer a slight decoloration (Al, Cd, Zn) while iron, copper and their alloys, nickel, chromium, anodized aluminium materials, chromated cadmium and zinc were uneffected; plastic materials and protective coatings are not altered by the inhibitors.