Alclad 2024-T3 Research Articles

Improved Corrosion Protection of Aluminum Alloys by System Approach Interface Engineering: Part 4—Spray Paint Primer-Coated Aluminum Alloys

Abstract This study investigates the development of a chromate-free coating system for the corrosion protection of aluminum alloys based on the concept of system approach interface engineering (SAIE) by chemical and plasma techniques. The aluminum alloys investigated include AA2024-T3 ([2B]), AA7075-T6 ([7B]), Alclad 2024-T3 ([2A]), Alclad 7075-T6 ([7A]), plate stock AA2124-T851 ([2P]), and plate stock AA7050-T7451 ([7P]). Direct current (DC) cathodic plasma processes were used to enhance the bonding and protection of spray paint primers on aluminum alloys. Besides anode magnetron enhancement, the DC cathodic plasma processes were also operated with the anode assembly removed, a more practical approach for industrial applications. With appropriate application, DC cathodic plasma coatings provided not only a corrosion-resistant layer on aluminum alloy surfaces but also excellent adhesion bases for spray paint primers, including both chromated and nonchromated water-borne primers. When tested by sulfur diox...

Influence of surface preparation on performance of chromate conversion coatings on Alclad 2024 aluminium alloy: Part II: EIS investigation

Chromate conversion layers, formed on Alclad 2024-T3 aluminium alloy after different surface preparation procedures, were investigated by means of EIS measurements performed in aggressive and middle aggressive electrolytes. The morphology of the chromate layer varies depending on the type of solution and on the time of duration due to deposition of corrosion products inside the defects and deterioration of the layer. Therefore, three slightly different equivalent circuits have been used to model the chromate film in the two electrolytes and for different times of exposure. The proposed equivalent circuits enable the study of the effect of variation in the aluminium surface microstructure, caused by different surface pretreatment, on the performance of the deposited chromate conversion coating. The removal of galvanic couples, mainly caused by precipitates, from the aluminium surface appears to be a necessary condition for the formation of a chromate layer with good corrosion resistance. Besides, the properties of the surface of the aluminium substrate largely influence the morphology and composition not only of the chromate film but also of the corrosion products formed at weak locations, which seem to play an important role in the protection provided by the chromate conversion coating.

Influence of surface preparation on performance of chromate conversion coatings on Alclad 2024 aluminium alloy: Part I: Nucleation and growth

The microstructure and morphology of the surface of aluminium alloys strongly affect the nucleation, growth, and protective properties of chromate conversion coatings. Samples of AA 2024-T3 cladded with AA1230 were pretreated with two different procedures before they underwent a chromating process, which consisted of dipping in a commercial Alodine 1200 solution. In one procedure, an extra step, consisting of immersion in nitric–hydrofluoric acid solution for 30 s (desmutting bath), was carried out after immersion in sulfuric –phosphoric acid solution (acid pickling bath). Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and electrochemical investigations highlighted the fact that this extra step in the pretreatment route gave rise to a more etched, but less reactive surface, due to the removal of a large amount of cathodic sites, mainly related to AlFeSi precipitates present in the clad layer. A similar effect, but on a smaller scale, was caused also by the increase of the time of immersion in sulfuric –phosphoric acid solution. The decrease of galvanic coupling gives rise to a more homogeneous nucleation of the chromate conversion coating on the surface of the Alclad 2024-T3. Moreover, the chromate film tends to be denser and with fewer defects.

Improved Corrosion Protection of Aluminum Alloys by System Approach Interface Engineering: Part 1—Alclad 2024-T3

Abstract This study investigated the development of chromate-free corrosion protection systems for Alclad 2024-T3 (UNS A92024) aluminum alloy ([2A]) based on the concept of system approach interface engineering (SAIE) by chemical and plasma techniques. Anode magnetron enhanced direct current (DC) cathodic plasmas were used to treat [2A] surface and to create interface engineered systems of cathodic electrocoat (E-coat)/plasma polymer/plasma-treated [2A]. Plasma polymer coatings provided not only a corrosion-resistant layer on [2A] surface but also an excellent adhesion base for subsequent cathodic E-coat. When tested by sulfur dioxide (SO2) and Prohesion salt spray tests, these plasma-modified systems showed excellent corrosion protection characteristics. The corrosion test results of SO2 and Prohesion salt spray tests showed that most of these plasma polymer-coated systems outperformed or performed comparably to the two types of controls used in this study (i.e., chromate conversion-coated and E-coated o...

Multiple-site damage in riveted lap-joints: experimental simulation and finite element prediction

The multiple-site damage (MSD) phenomenon is discussed, and exemplified by the behaviour of riveted lap-joint specimens of aluminium alloy 2024-T3 alclad. The tests performed, on which the paper is based, are part of the contribution of IDMEC to a project on the fatigue behaviour of ageing aeronautical structures—the BRITE-EURAM project ‘SMAAC’, partially funded by the European Union. The study involves fatigue testing under constant amplitude loading of 1.6-mm-thick riveted lap-joints, and includes examination of the specimens during and subsequent to testing (post-mortem analysis of the fracture surface in a scanning electron microscope) in order to determine the time of occurrence, location and extent of fatigue damage. Crack growth rates are determined from periodic crack length measurements with a travelling microscope. Stress measurements are made using extensometry and the SPATE infrared technique to determine loading distribution of the lap-joints and redistribution due to cracking of fastener holes. Data on the initiation and growth of cracks and on residual static strength are used to assess the predictive model based on the finite element method.

Applicability of the SPATE technique to the detection of hidden cracks

The present study evaluates the applicability of the SPATE system to the detection of hidden cracks in plates of aluminium alloy 2024-T3 alclad 1.6 mm thick, taken from riveted lap-joints. Artificial part-through cracks were used to simulate a hidden crack in one face, and SPATE readings were performed on the opposite face. The results show that the SPATE system can detect a hidden part-through crack with a minimum depth of 0.6 mm. Finite element modelling indicates a large stress gradient in the vicinity of the crack. Due to limitations of the SPATE discussed in the paper, that gradient could not be experimentally measured.

The Effect of Interfacial Tension on the Adhesion of Cathodic E-coat to Aluminum Alloys

Adhesion of a cathodically electrodeposited paint (E-coat) to aluminum alloys, Alclad 2024-T3, AA 2024-T3 and AA 7075-T6, was investigated to examine the influence of interfacial tension at the paint/metal interface. The surface energy of an aluminum plate was modified by depositing a plasma polymer of a mixture of trimethyl silane (TMS) and one of three diatomic gases (O2, N2, and H2) by cathodic plasma polymerization. The contact angle (Φ) of water on a modified surface changes as a function of the mole fraction of the diatomic gas. The plot of cos ΦPP of a plasma polymer as a function of the mole fraction of the gas crosses the plot of cosΦEC of the E-coat. The difference, ΔCosΦ = cos ΦPP - cosΦEC, is a parameter which indicates the level of interfacial tension at the paint/metal interface. ΔCosΦ = 0 represents the minimum interfacial tension. The adhesion of a cured E-coat on a panel was evaluated by the N-methyl pyrrolidinone (NMP) paint delamination time test. The maximum peak of adhesion test values plotted as a function of ΔCosΦ occurred around the zero point. ΔCos Φ = 0, indicating that maximum adhesion is obtained with minimum interfacial tension. Mixtures of TMS and N2 on all three aluminum alloys studied consistently displayed longer delamination times in the NMP test than mixtures of TMS and O2 or H2.

Influence of surface pretreatment and electrocoating parameters on the adhesion of cathodic electrocoat to the Al alloy surfaces

Adhesion of cathodic electrocoat films to the aluminum alloys 2024-T3 bare and Alclad 2024-T3 with different pretreatments and with different cathodic electrocoat process parameters was investigated. The pretreatments studied were acetone wipe and alkaline cleaning. The cathodic electrocoat process parameters studied include variation of cathodic electrocoating voltage and time. Adhesion performance was evaluated by measuring the delamination time and percent delamination of the electrocoat from the alloy surface by placing the small specimen of the sample in the N-methyl pyrrolidinone (NMP) solution at 60°C until the film lifts off or for 2 h whichever comes first. NMP times for electrodeposited film delamination from alkaline cleaned surfaces were found to be higher than the acetone wiped and or those of as-supplied metal surfaces. There was not much effect of acetone cleaning of these alloy surfaces on the adhesion performance of the cathodic electrocoat. The voltage–current (of cathodic electrocoating process) relationships for alkaline cleaned surfaces were also found to be significantly different from the other two types of surfaces. The NMP times of cathodic electrocoat delamination at lower cathodic electrocoating voltage and lower electrocoating times were higher than those at higher cathodic electrocoating voltage and electrocoating times for alkaline cleaned 2024 bare surfaces. Electrocoat thickness developed on the surfaces during the electrodeposition process increased with increasing electrodeposition voltage and time as anticipated.

Characterization of disbond effects on fatigue crack growth behavior in aluminum plate with bonded composite patch

The fatigue response of pre-cracked 2024-T3 Alclad aluminum panels repaired with bonded boron/epoxy composite reinforcements was investigated. A completely bonded reinforcement and four partially bonded reinforcement configurations were tested. The effects of various disbond locations and sizes were studied and compared to each other as well as to panels with a completely bonded reinforcement and to cracked panels without any reinforcement. The results demonstrated that there is a definite variation in fatigue life depending on the location and size of the disbonds. However, crack growth characteristics also revealed that partially bonded composite repairs are fairly damage tolerant of the pre-existing disbonds in the bondline of the repair.

Fatigue crack initiation and growth in riveted specimens: an optical and acoustic microscopic study

While optical microscopy on riveted specimens reveals only surface cracks, the acoustic C-scan images reveal subsurface and buried cracks emerging to the surface. This is of particular interest for fatigue cracks that initiate below the outer surface such as with chamfered riveted panels. Fatigue crack initiation and growth in riveted panels of Alclad 2024-T3 were characterized using optical microscopy and scanning acoustic microscopy to obtain C-scan images. The C-scan images were obtained using a focused transducer with a center frequency of ca 50MHz, and the peak value of the back surface echo of the plate with countersunk rivet holes was recorded in the C-scan images. Data on the initiation and development of fatigue cracks at rivets in riveted Alclad 2024-T3 are given.

Studies of the degradation of metal-adhesive interfaces with surface analysis techniques

The application of bonded repairs and reinforcements to aircraft components places an emphasis on adherend surface treatment procedures because these treatments can significantly change bond strength and durability. A typical minimum treatment sequence for Alclad 2024-T3 aluminium alloy adherends includes a solvent degrease, an abrasion with a Scotchbrite ® pad, a clean with a methyl-ethyl-ketone (MEK) soaked tissue and a grit-blast with 45 μm alumina powder. The adherends are treated with γ-glycidoxy-propyl-trimethoxy-silane (γ-GPTS) coupling agent then bonded with an epoxy film adhesive. The composition of the adherent, the bond durability and the locus of fracture were examined at several stages of the adherent surface treatment. Boeing wedge tests show that grit-blasting the adherends creates a more durable adhesive bond than the Scotchbrite ®/MEK treatment and that the application of γ-GPTS improves bond durability in both cases. XPS has shown that the cleaning sequence decreases the concentration of hydrocarbon contaminant on the grit-blasted adherend to an average thickness of less than 1.5 nm. XPS analyses of the fracture surfaces indicates that for the grit-blast, grit-blast plus γ-GPTS and Scotchbrite ®/MEK plus γ-GPTS treatments, failure occurs primarily in the oxide film, whereas for the Scotchbrite ®/MEK treatment failure occurs at the adhesive/oxide interface possibly due to weakness induced by contaminant. XPS measurements show that a γ-GPTS overlayer retards the growth of the oxide on aluminium in humid air, until the γ-GPTS overlayer is desorbed. The improved bond durability with the coupling agent may be due to the inhibition of hydration sites on this oxide surface.

The effect of disinfectants on 2024-T3 aluminum in the air medical helicopter

A principle structural component of helicopters is 2024-T3 aluminum alloy. This alloy has been designed for use in areas requiring high strength-to-weight ratios, but it is susceptible to corrosion damage. The air medical helicopter is frequently exposed to bloodborne pathogens, dirt, intravenous solutions and a variety of other contaminants. The amount of damage to the helicopter that can be caused by the use of cleaners and disinfectants has been raised as an area of concern for the safety of the helicopter, crew and patients. In a controlled study, 2024-T3 alclad aluminum strips were placed in 120-ml glass jars that were filled with 60 ml of solution and then sealed. The solutions used were disinfectants, cleaners and water (both tap and distilled). The strips in solution were placed in a controlled oven at 140 F for 100 hours to simulate long-term damage from immersion, vapors and heat. On examination, many strips were discolored and corroded. Only one solution caused no apparent damage, and only one caused slight vapor damage. As a result of the study, one of the solutions has been designated for use. The results have been used as examples for flight team members on the costly damage that can result from inappropriate use of these substances. A cleaning policy and procedure has been developed to ensure adequate protection from chemical exposure while protecting all team members from the dangers of bloodborne pathogens.

Fatigue of bearings (Fatigue dans les roulements): Inglebert, G., Gras, R., Jeanblanc, G., Edeline, E. and Fenzans, G. Mec. Mater. Electr. 441 (Oct–Dec 1991) 11–13 (in French)

Tests were carried out on fatigue crack growth in 2024-T3 Alclad sheet specimens reinforced with strips. Strip materials were 2024-T3, 7075-T6, Ti-6Al-4V and ARALL. Adhesive bonded and riveted strips were used. Integral strips were tested for 2024-T3 only. Several observations were made on crack growth retardation caused by the strips. Adhesive bonding gives better results than riveting. The best strip materials were ARALL and the Ti-alloy. ARALL with advanced glass fibers are the most effective solution for crack stopper bands in a pressurized fuselage. Moreover they offer less production problems.

Fatigue cracking in welded-steel shovel dipper handles in iron ore mining: Laczko, P. Aust. Weld. J. Oct.–Dec. 1989 34, (4), 29–31

Tests were carried out on fatigue crack growth in 2024-T3 Alclad sheet specimens reinforced with strips. Strip materials were 2024-T3, 7075-T6, Ti-6Al-4V and ARALL. Adhesive bonded and riveted strips were used. Integral strips were tested for 2024-T3 only. Several observations were made on crack growth retardation caused by the strips. Adhesive bonding gives better results than riveting. The best strip materials were ARALL and the Ti-alloy. ARALL with advanced glass fibers are the most effective solution for crack stopper bands in a pressurized fuselage. Moreover they offer less production problems.

The surface pretreatment of aluminium-lithium alloys for structural bonding

Transmission electron microscopy examination of ultramicrotomed sections taken through a bonded joint allows both the effect of standard chemical pretreatments and an initial adhesive/adherend interfacial analysis to be made for structural joints prepared using BA 8090C-T3 aluminium-lithium alloy substrates. These observations, when compared with earlier work using Alclad 2024-T3 adherends, show that broadly similar films are formed on the aluminium-lithium surface; a rougher topography has, though, been noted on some specimens. In general, these films appear to be wetted and, in some cases, penetrated by the adhesive used in the same manner as the Alclad controls. Initial measurement of bond strengths shows that the pretreated aluminium-lithium is amenable to adhesive bonding. However, although examination of sections through the environmentally exposed joints shows no apparent interphasial damage, pickled aluminium-lithium substrates perform better than those anodized in phosphoric acid; this is contrary to expectations based on prior work using the more conventional aluminium alloys.

Crack stoppers and arall laminates

Tests were carried out on fatigue crack growth in 2024-T3 Alclad sheet specimens reinforced with strips. Strip materials were 2024-T3, 7075-T6, Ti-6Al-4V and ARALL. Adhesive bonded and riveted strips were used. Integral strips were tested for 2024-T3 only. Several observations were made on crack growth retardation caused by the strips. Adhesive bonding gives better results than riveting. The best strip materials were ARALL and the Ti-alloy. ARALL with advanced glass fibers are the most effective solution for crack stopper bands in a pressurized fuselage. Moreover they offer less production problems.

Water-displacing organic corrosion inhibitors—their effect on the fatigue characteristics of aluminium alloy bolted joints

Some aircraft manufacturers and operators have attempted to control in-service corrosion by the use of water-displacing organic inhibitors which can be either brushed or sprayed onto corrosion-susceptible areas of the structure. However, because of the low surface tension and lubricating properties of these preparations, concern has been expressed as to their potential side-effects on the fatigue performance of bolted and riveted joints. Fatigue tests were carried out in repeated tension under both constant-amplitude and multi-load-level sequences on several types of 8-bolt double-lap joint specimens of 2024-T3 alclad aluminium alloys sheet. Tests were made on joints assembled with either ‘dry’ components or components coated with the corrosion inhibitors LPS-3 or PX-112. Contrary to the findings of previous investigations into the effect of inhibitors on riveted joints, the two corrosion inhibitors used were found, in general, to have either no effect or a beneficial effect on the fatigue lives of bolted joints. It is concluded that the specific effects of a water-displacing organic corrosion inhibitor on fatigue strength of joints are likely to be dependent on the type of joint, its configuration and on the severity of the load spectrum involved.

Durability of aluminium bonded joints in long-term tropical exposure

Abstract Aluminium bonded joints were exposed to the jungle weathering conditions near the equator in Suriname, South America, for 12 years. Distinct differences were found between different jungle locations, with open jungle exposures being more severe than more protected, secluded jungle sites. More durable bonds were made to a higher purity surface alloy, such as Alclad 2024-T3, than to a more highly alloyed surface adherend, such as 6061-T6 alloy. Deoxidizing the aluminium surfaces before bonding produced both higher initial strength and more durable joints than only vapour degreasing the aluminium. The highest strength retentions were obtained with a room temperature-curing epoxy as compared with a rubber modified heat-cured epoxy. A silicone sealant which had effectively protected similar joints from a seacoast exposure for eight years was relatively ineffectual in the jungle environment.

Properties of Large Multispot Ultrasonically Welded Joints

Using ultrasonic spotwelds, two series of overlap joints were prepared with aluminum alloys and tested in tension. In the first series between 4x12x0.063 in.-thick panels, it is shown that on a one-for-one basis ultrasonic welds produce joints superior to those made with Vi-in. steel fasteners. In the second series on 7075T6 and 2024-T3 alclad sheets 12 x 30 x 0.063 in. thick, joints were made with different numbers and arrays of spots. Joints based on row separations of 2 in. and pitches on the order of /4 in. are shown to be optimum and able to develop sheet stresses in excess of the yield strengths at failure. At pitch distances greater than A in., the higher individual spot loads can lead to shear and nugget failures as well as lower sheet stresses at failure. With a more dense population than at optimum pitch, there is a distinct reduction of sheet stress at failure. This is due, apparently, to restricted yield strain redistributions along the edges of the close packed rows of spots.

An evaluation of the reliability of Fracture Mechanics methods

Many Fracture Mechanics methods have been proposed in the non-linear range and modifications of them often suggested; the basic hypotheses of these methods are often questionable and they cannot generally be considered reliable. This note aims at assessing the reliability of certain of the more common Fracture Mechanic methods by carrying out a program of Fracture Mechanic tests. Three materials are examined, namely alclad 2024-T3 and 7075-T6 and 2219-T851 commonly utilized in aeronautical and space structures; the specimens tested were centrally cracked flat panels. The test results are evaluated according to Linear Elastic Fracture Mechanics and the following Ductile Fracture Mechanics methods: the R-Curve, the Eftis-Liebowitz method; the Bockrath Glassco method; the C.S.A. method and the Feddersen method; C.O.D. and J-Integral quantities can also be measured with little modification of the methodology used, but no results are given here. Each test was assessed by using all the aforementioned methods. The test rig is simple and described briefly here; the static characteristics of the aforementioned materials were obtained by means of a simple rig and are relevant to the plates actually examined; it should be noted that alclad 2024-T3 and 7075-T6 panels were cut off from plates produced by the same company, and the same goes for the 2219-T851 specimens. Some interesting conclusions (often unexpected) are reached and future development for research is pointed out when necessary.