Pretreatments for metal-to-metal bonding

Pretreatments for metal-to-metal bondin

Novel methods, incorporating pre- and post-anodising steps, for the replacement of the Bengough–Stuart chromic acid anodising process in structural bonding applications

The present investigation focuses on novel anodising processes which are being developed for the replacement of the hexavalent chromium containing 40/50V Bengough-Stuart process, with particular emphasis on their resultant performance in structurally bonded systems used in the demanding and harsh environments encountered on operational aircraft. An electrolytic phosphoric acid based deoxidiser (EPAD)has been studied in combination with a standard sulphuric acid anodise. It has been shown that the EPAD provides an open porous structure in order to enhance adhesion to the modified sulphuric acid anodised (SAA) surface. Additionally, a post anodising (PAD) treatment has been used to aid structural adhesion in combination with the SAA processes. 2 As a control, the standard 40/50V Bengough-Stuart chromic acid anodising (CAA) has been used as a baseline performance indicator in adhesion tests. Single lap shear (SLS) and modified Boeing wedge tests were used to determine adhesion performance. SLS tests were used to determine initial, dry joint strengths whilst wedge test joints immersed in deionised water for up to 100 hours gave a measure of joint durability. Overall, excellent initial joint strengths and durability have been found with both EPAD plus SAA and PAD plus SAA processes suggesting that these environmentally benign treatments may be used as possible drop-in replacements for the currently used hexavalent chromium process. Electron microscopy has been used to investigate the topographical changes introduced to the surface by the various surface pretreatments under investigation to provide an explanation for the observed adhesion test results

Friendlier surface treatments - for metals

With only a few exceptions, some degree of surface treatment is applied to all metal surfaces prior to adhesive bonding. The surface treatment applied will depend upon the requirements of the bond and service conditions that it will see and will generally be chosen on a “fit-for-purpose” basis. The minimum preparation which is usually carried out might include a simple degrease to remove processing aids, such as oils and waxes, and contaminants. However, it is recognised that the current state-of-the-art processes for structural or semi-structural metal bonding are highly complex, multi-stage treatments including conversion coating and anodic oxidation. Alternatives to the commonly-used degreasing processes are sought for many reasons, for example: established processes may not be adequate for difficult-to-remove materials; the processes may use VOCs; they can be carcinogenic or ozone depleting. Regarding the higher treatments, the anodising processes, in particular, are difficult, time consuming and costly to carry out. There are also legislative drivers which make utilisation of the more complex processes, especially those which utilise hexavalent chromium, highly undesirable. Other factors such as energy and chemical disposal costs also deserve consideration when considering the need for environmental or operator “friendly” processes. This paper will consider a number of alternative friendly surface treatments which might be considered as drop-in replacements for the current industrial standards. The friendlier surface treatments include two simple cleaning methods, namely: seaweed-based cleaners and CO2 laser ablation. In addition, to cover the spectrum of processes, two novel anodising methods will also be discussed. These are based upon electrolytic phosphoric acid deoxidising plus sulphuric acid anodising (EPAD+SAA) and alternating currentdirect current (ACDC) anodising

State of the art hexavalent chromium free surface pretreatments for aluminium alloys

At the present time, the phosphoric acid anodising (PAA) process [1] is widely used in America for the pretreatment of aluminium alloys used in adhesively bonded structures. With the replacement of the Forest Products Laboratory (FPL) etch in the anodising line with a hexavalent chromium free alternative [2], this has solved many of the environmental problems associated with the ever increasing regulations enforced by both national and local authorities. However, this route has never been favoured in the European aircraft industry due to the superior bond durability offered by chromic acid anodising (CAA) relative to PAA in corrosive environments. Furthermore, CAA has been shown to have twice the anodising throwing power and generally higher peel strengths to that of PAA [3]

Use of carbon nanotubes reinforced epoxy as adhesives to join aluminum plates

Carbon nanotube (CNT)-reinforced epoxy was developed to use as adhesives. Mechanical stirring with ultrasonication was a simple and effective approach to obtain the adhesives with rather uniformly dispersed CNTs in the epoxy matrix. It was found that the thermal stability and electrical conductivity increased with the addition of CNT. The thermal degradation temperature of the adhesive with 1 wt.% of CNT was about 14 degrees C higher than that of neat epoxy. The percolation threshold for surface electrical conductivity was less than 0.5 wt.% of CNTs. In order to study the adhesion properties, two aluminum alloy plates were joined together with the adhesives with different amount of CNTs. The bonding strength and durability of the joints were studied in terms of Boeing wedge test under water of 60 degrees C, and both of them were found to increase significantly with the incorporation of CNTs. The specimen with neat epoxy was failed after immersion into water of 60 degrees C for 3 h, while as the specimen with 1 wt.% CNT-filled epoxy still showed fracture toughness as high as about 6.3 x 10(5) J/m(2) under the same testing conditions. Nevertheless, the initial fracture toughness decreased with CNT fraction when CNT fraction was greater than 1 wt.%

Surface cleaning technologies for the removal of crosslinked epoxide resin

This study provides details of the use of laser ablation and sodium hydride cleaning processes for the removal of crosslinked epoxide and other residues from resin transfer moulding (RTM) tool substrates, as used in the aerospace industry. The requirement for removal of such contamination is so that the mould can be re-used, following the subsequent application of an external release agent. These tools are, typically, fabricated from steel, nickel or CFRP composite materials; this paper focuses on the use of nickel substrates. The requirement to clean large surface areas quickly to satisfy commercial restraints, compromises the degree of absolute cleanliness that can be obtained. However, in applications where cleaning time is not a constraint, laser cleaning can be a very gentle and efficient process; typically Nd:YAG lasers find application in this area. In contrast, high power lasers are desirable for industrial scale applications where large areas need to be cleaned quickly. In this instance pulsed CO2 lasers can be used. The use of sodium hydride was also found to be highly successful in removing crosslinked organic contamination providing that suitable hard rinse and drying operations were also carried out

Fatigue and fracture assessment of toxic metal replacement coatings for aerospace applications

The use of corrosion resistant and adhesion promoting films and coatings is established industrial practice for many fatigue sensitive components and structures. However, recent environmental legislation restricting the use of a range of toxic heavy metals and their derivative processes, such as chromic acid anodising (CAA), has meant that a number of new coatings systems and pretreatments are currently being developed to replace the traditional processes still in use. Typical of these new systems are the boric–sulphuric acid anodising (BSAA) process, which can be modified to provide excellent adhesive bonding properties, the sulphuric acid anodising process, which includes an additional electrolytic phosphoric acid deoxidising stage (EPAD) to produce a duplex oxide layer, and the recently patented ACDC sulphuric acid anodising process which produces a two layered oxide film which can be tailored to produce different porosity volume fractions within each layer. This communication reports the preliminary findings of a study carried out to assess the fatigue response of Al2618:T6 aluminium alloys to these new processes. In contrast to CAA anodising, the initial results indicate that the EPAD and ACDC processes do not appear to have a significant effect on fatigue

Modelling cyclic moisture uptake in an epoxy adhesive

This paper presents a methodology for predicting moisture concentration in an epoxy adhesive under cyclic moisture absorption-desorption conditions. The diffusion characteristics of the adhesive were determined by gravimetric experiments under cyclic moisture conditions and the dependence of diffusion coefficient and saturated mass uptake on moisture history was determined. Non-Fickian moisture absorption was observed during absorption cycles while moisture desorption remained Fickian. The diffusion coefficient and saturated moisture content showed variation with absorption-desorption cycling. A finite element-based methodology incorporating moisture history was developed to predict the cyclic moisture concentration. A comparison is made between the new modelling methodology and a similar method that neglects the moisture history dependence. It was seen that the concentration predictions based on non-history dependent diffusion characteristics resulted in over-prediction of the moisture concentration in cyclic conditioning of adhesive joints. The proposed method serves as the first step in the formulation of a general methodology to predict the moisture dependent degradation and failure in adhesives

Electrodeposition of zinc-manganese alloy coatings from ionic liquid electrolytes

Electrodeposited zinc-manganese alloys have been found to have potentially attractive corrosion resisting characteristics for ferrous substrates. However, researchers have found their formation problematic from aqueous electrolytes, due particularly to the reactive nature of manganese and its low reduction potentials. The present investigations examine the possibility of electrodepositing this alloy utilising an ionic liquid electrolyte. In this manner electrolytes based upon manganese and zinc chlorides and boric acid were made by dissolution in a 2:1 molar ratio urea: choline chloride solvent. Physical measurements of electrolyte characteristics as well as pertinent electrochemical information on alloy electrodeposition were obtained to examine the efficacy of this metal/electrolyte combination

Development of a superhydrophobic polyurethane-based coating from a two-step plasma-fluoroalkyl silane treatment

A method of achieving a superhydrophobic surface based upon a highly filled polyurethane (PU) paint coating has been demonstrated through the use of a combined oxygen/argon plasma pretreatment and a fluoroalkyl silane (FAS) final treatment. The combined plasma-FAS treated PU surface has been investigated and characterised using: field emission gun secondary electron microscope (FEG-SEM); X-ray photoelectron spectroscopy (XPS); energy-dispersive X-ray spectroscopy (EDX); water contact angle analysis (WCA); atomic force microscopy (AFM), and; Fourier transform infrared spectroscopy (FTIR). It was found that the oxygen/argon plasma treatment increased both the surface roughness (Ra) and surface free energy (SFE) of the PU paint coating from approximately 60-320 nm, and, from ~52 to ~80 mN/m respectively. It was also found that the plasma process created a multiscale roughened texture through the process of differential ablation between the PU polymer and the barium sulphate solid content, which is present in the paint as an extender, and other additives. In addition, the process also imparted favourable polar groups into the PU surface from the ionised and radical oxygen species in the plasma. When the FAS coating was subsequently applied to the PU without prior plasma treatment, there was a significant increases in water contact angles. This parameter increased from approximately 60°on untreated PU to around 130°with FAS applied. In this case, the SFE decreased to ~7.5 mN/m and showed 42.0 at% fluorine present as indicated by XPS. However, subsequently applying the FAS polymer after plasma pretreatment takes advantage of the known synergistic relationship that exists between surface roughness and low surface free energy coatings. The two processes combined to create superhydrophobicity with a surface that exhibited water contact angles up to 153.1°. With this optimised process, the apparent SFE was 0.84 mN/m with a more highly fluorinated surface present. In this case 47.2 at% surface fluorine was observed by XPS. In addition to changes in SFE, plasma treatment was also observed to alter levels of surface gloss and colour. After exposure to 600 s of plasma gloss levels are shown to reduce from values of from ~50 to ~21 (GU), with small but significant corresponding increases in the lightness and yellowness of the surface