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Executive Summary of SM AG-17 on Cadmium substitution
Contents Background Background Cadmium plating is the preferred protective treatment for use on aerospace components and fasteners manufactured from steel. Cadmium plating provides a high level of corrosion protection to steel being both a good barrier coating and a sacrificial coating and is galvanically compatible with aerospace aluminium alloys. Its low coefficient of friction makes it an attractive coating for use on fasteners and threaded parts. Concerns about the toxicity of cadmium and its harmful effects on humans and the environment in general has led to European legislation banning the use of cadmium plating for many engineering applications. Efforts are being made to identify alternatives to cadmium plating for aerospace applications. This report describes the results of research undertaken as part of a GARTEUR collaborative programme to evaluate a number of commercially available coatings. The Organizations participating in the programme were DERA (now QinetiQ), Short Brothers and British Aerospace from the United Kingdom, Aerospatiale (France), SAAB-SCANIA AB (Sweden), NLR and Fokker Aircraft BV (The Netherlands) and Daimler Benz Aerospace Airbus (Germany). The work was carried out under Action Group AG17 "Cadmium Substitution" set up under the Structural Materials Panel of GARTEUR. Coatings evaluated Coatings evaluated in the programme were either aluminium or zinc based and were produced commercially. A reference electro-deposited cadmium coating was included in the programme together with an experimental aluminium - magnesium coating produced by unbalanced magnetron sputtering. Two aluminium coatings were selected; one produced by ion vapour deposition and the other prepared using a non-aqueous electro-deposition process. A zinc-nickel coating deposited using an acid electroplating process and an electro-deposited zinc-cobalt-iron coating were both evaluated in the programme. In addition two metallic - ceramic coatings were investigated one containing aluminium flakes in an oxide matrix and one consisting of aluminium and zinc particles in an inorganic matrix. For most of the studies conducted the coatings were applied to simple steel test panels or aerospace grade fasteners. Each of the coatings have been evaluated to determine their suitability for use on aerospace components and in addition to properties such as corrosion resistance, lubricity, galvanic compatibility and the effect of coatings on fatigue strength the resistance of coatings to aircraft fluids and paint adhesion have been studied. Optical and scanning electron microscopy techniques were used to determine coating thickness and microstructures. The results obtained indicated that the coatings had been applied in accordance with the procedures recommended by the various coating suppliers. Electrical conductivity measurements were made
using two simple lap joints one employing Hi-Lok fasteners and one countersink
screws. With the exception of a metallic-ceramic coating incorporating
zinc particles, all the coatings tested gave electrical resistances of
less than 1m The corrosion resistance of each of the coatings was determined using a range of accelerated corrosion tests, electrochemical measurements and outdoor exposure trials. The barrier properties of the coatings were determined from electrochemical measurements. These established that the zinc alloy coatings and zinc based metallic-ceramic coatings were similar in performance to electroplated cadmium. The aluminium based coatings all gave much lower corrosion currents implying that they were more effective barrier coatings. The sacrificial properties of the coatings were assessed from open circuit potential experiments and from the use of scratch model specimens and protection distance measurements. The main conclusions were that pure aluminium coatings and an aluminium based metallic-ceramic coating were less effective than cadmium plating. Overall the zinc alloy coatings were more effective than the aluminium coatings. Two methods were employed to study the galvanic compatibility between coatings and aerospace aluminium alloys. In the first coated bolts inserted into aluminium alloy blocks were exposed to neutral salt fog and at an outdoor exposure site. The results obtained indicated that the zinc based metallic-ceramic coatings were the most promising as no rusting was detected. The second approach used was to measure the galvanic current developed between coated panels and aluminium alloy. The results obtained indicate that ED aluminium and ED zinc-nickel coatings lower the corrosion rate of the aluminium alloy. Other coatings studied were found to accelerate the rate of corrosion above that found for cadmium plating. The effects of coating on fatigue performance were assessed using notched specimens tested under constant amplitude tests. It was established that for the ED aluminium, the two metallic-ceramic coatings and UBMS aluminium - magnesium coatings the reduction in fatigue strength was less than 5%. ED zinc-cobalt-iron and cadmium gave similar reductions of ~10% whilst the ED zinc-nickel coatings had the most detrimental effect being ~25%. Sustained load tests conducted on coated notch specimens, exposed to sodium chloride solution, indicated that the zinc based metallic-ceramic and ED zinc-cobalt -iron coatings may promote stress corrosion cracking in high strength steels. Additional tests carried out including the slow bend test suggest that any susceptibility to hydrogen embrittlement may be minimised by heat treatment after electroplating. Most of the replacement coatings examined failed to show any significant degradation on exposure to a range of chemicals used on aircraft. Exceptions were ED zinc-nickel coatings in contact with Turco 5948 and ED zinc-cobalt coatings immersed in Skydrol hydraulic fluid. Cadmium plating was also found to be degraded by these fluids. Cross-cut tests show that good paint adhesion may be achieved with metal coatings. An important factor is the time delay between passivation and the application of a primer. Data obtained indicate that if the passivated surfaces are exposed to the atmosphere for too long, poor paint adhesion may be obtained. Tribological studies were conducted to allow the suitability of the different coatings for use on fasteners to be established. The coefficient of friction of several of the coatings has determined and the torque-tension characteristics of coated fasteners after repeated tightening and untightening has been compared. It is concluded that only the zinc based metallic-ceramic coatings have a coefficient of friction comparable to cadmium plating. All the coatings examined resulted in pre-loads on Hi-Lok fasteners greater than the minimum 4kN required. With the ED cadmium, ED aluminium, ED Zn-Co-Fe and ED Zn-Ni coated steel fasteners the maximum pre-load of 10kN was exceeded. The use of brush plating to repair several of the coatings was investigated. Simulated corrosion damage and re-plating tests showed that brush plated zinc-cobalt and zinc-nickel coatings could be used to re-protect a range of GARTEUR coatings including zinc-nickel, zinc-cobalt-iron, PVD aluminium and cadmium. Environmental and cost studies A environmental study was undertaken to establish the effect of the coatings evaluated on workers involved in the manufacture and maintenance of aerospace components and the impact of the coatings on the environment. It is concluded that none of the alternative coatings considered in this programme present major environmental problems or offer a health risk to workers involved in the application of coatings or in the assembly and maintenance of aerospace components. Several of the processes however involve the use of post plating treatments containing hexavalent chromium compounds, which could cause handling problems. Some tentative costs have been calculated for the replacement coatings and these are generally similar to the cost of cadmium plating. Conclusions and future work It is concluded that none of the coatings evaluated gave an overall performance equivalent to cadmium plating. Several of the coatings may have uses as substitutes for cadmium plating in particular applications. The major problem areas remain the replacement of cadmium on fasteners and threaded parts and it is recommended that further work should focus on these requirements. The use of multilayered coatings based on electro-deposited zinc alloy coatings and unbalanced magnetron sputtered aluminium alloy coatings is suggested. |
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The data indicate that it is possible to achieve good electrical conductivity
with the other coatings. A low temperature tensile test method indicated
that the adhesion of the coatings to grit blasted steel substrates was
good.