Phosphoric Acid Anodizing Research Articles

Characterization of Adhesive-Primer Penetration into Porous Oxides Formed on Anodized Aluminum Using Energy Electron Loss Spectroscopy (Eels)

Abstract The purpose of this investigation is to evaluate the effects that relatively minor changes in phosphoric acid anodizing (PAA) parameters (voltage, temperature, time) have on oxide morphology and the subsequent penetration of an applied adhesive-primer into the porous oxide. This preliminary study examines two cases, one in which industry-standard PAA parameters have been followed, and a second in which the PAA parameters are slightly above industry limits. Baseline PAA parameters have been developed in the aerospace industry from extensive empirical testing in order to maximize adhesive bond strength and durability in the presence aggressive environments. It has been noted that for effective mechanical strength of structurally bonded elements, penetration of the adhesive primer into the porous oxide formed by an anodizing pre-treatment is critical. Verifying primer penetration is somewhat problematic due to the high spatial resolution required to both structurally and chemically characterize an oxide film on the order of one micron in thickness.

Plasma Spray Coatings as Treatments for Aluminum, Titanium and Steel Adherends

Plasma spray coatings have been evaluated as surface treatments for aluminum, titanium and steel substrates prior to adhesive bonding. These treatments are environmentally benign in that they involve no chromates and emit no liquid or gaseous wastes. The coatings can be engineered for specific applications and are better suited for localized repair than chemical processes. For aluminum adherends, a 60Al-Si/40polyester coating gives a performance equivalent to that of the best chemical treatment (phosphoric acid anodization) for some epoxy adhesives. With stronger, tougher adhesives, a Ti-6Al-4V coating provides improved performance to match that of phosphoric acid anodization. A Ti-6Al-4V coating on titanium substrates exhibits identical initial strength and durability to the best chemical controls under moderate temperature conditions. At high temperatures, the plasma spray coating continues to exhibit excellent durability while oxide-based treatments readily fail due to oxygen dissolution into the metal. For steel adherends, an Ni-Cr-Zn coating provides enhanced corrosion resistance and bondability even after exposure to aggressive environments or ambient conditions over long periods of time. Additionally, rubber bonds with the plasma spray coating are more tolerant to surface contamination than those with grit-blasted surfaces. These investigations indicate that the plasma spray process is more robust than conventional processes and can give equivalent or (in some cases) superior performance.

SnO 2:Sb dip coated films on anodized aluminum selective absorber plates

In this paper solar selective coatings have been prepared on aluminum substrates using a two phase eloxal technique. Porous anodic films were formed by dc anodization in phosphoric acid and followed by black coloration via ac electrolysis in nickel sulfate electrolyte. To increase the thermal, chemical and mechanical stability, and also to provide low emissivity, a protective layer of tin oxide was coated on the surface by the sol-gel process. Conditions required to produce good quality coatings having the best spectral selectivity and thermal stability were determined from spectral reflectance measurements and temperature tests. Microstructure and chemical composition of the best samples were investigated using SEM, EDX, AAS. The resulting technology has been formed into a process which suits the facilities and skills of Turkish industry.

Experimental Investigation of Aluminum/Epoxy Interfacial Properties in Shear and Tension

Abstract This paper presents the results of comprehensive testing to characterize the effect of several different surface treatments on shear and tensile bond strength between 7075-T6 aluminum and two epoxy systems: EPON 815/V40 and EPON 828/Z. A rod pull-out test was used to determine interfacial shear strength, modeled after similar tests on reinforced concrete. The tensile bond strength was characterized using a tension test fixture designed in this study. Overall, the interfacial shear strengths were higher than the tension strengths. Surface knurling gave the highest interfacial shear strength, representing a 72% increase over untreated specimens. Phosphoric acid anodization (PAA) was also quite effective in shear. In tension, the highest strength was obtained from specimens treated with the PAA process along with a silane coupling agent. These specimens showed an increase in interfacial tensile strength by a factor of 5.6.

Commercially Pure Titanium and Ti6Al4V: XPS Comparison Between Different Commercial Ti Dental Implants and Foils Prepared by Various Oxidation Procedures

Commercially pure (c.p., 99.6%) titanium and the Ti6Al4V alloy are widely used in biomedical applications. The surface (oxide) properties of these materials play a key role for their function in biomaterial applications, and it is therefore important to analyze their surface composition. XPS survey and high resolution spectra (Ti 2p, O 1s, and C 1s) are presented from three commercially available dental implants, all manufactured by machining but with different post treatments (grit blasting, cleaning, sterilizing, and packaging). Comparison is made with a TiO2 single crystal (rutile (110)) reference sample, and with Ti and Ti6Al4V foils prepared by the following treatments commonly used for titanium biomaterials: ASTM F86/B600 passivation, electropolishing in perchloric acid based electrolyte, anodization in sulfuric acid and phosphoric acid, and glow discharge plasma oxidation. Survey spectra from the implants are qualitatively similar but quantitatively different, with Ti, O, and C as the main elements and trace amounts of preparation-specific impurities. The high resolution spectra show differences in oxide composition (varying deviations from pure TiO2 stoichiometry), thickness, and carbon overlayer composition. Spectra from the differently prepared Ti and Ti6Al4V foils showed preparation-specific differences mainly with respect to oxide composition, oxide thickness, and trace impurities. For the Ti6Al4V the different preparations lead to various amounts of detected Al. A general difference between the differently prepared Ti and Ti6Al4V foils was that Al and V were always detected on the latter, but only sporadically on the Ti. These results clearly show that the composition of titanium surfaces varies considerably depending on the surface preparation used. Important implications are: (i) surface preparation and characteristics need to be considered when results from biological studies on Ti surfaces are interpreted and compared, (ii) Ti and Ti6Al4V form oxides with different composition, and (iii) different commercial Ti implants cannot a priori be assumed to have similar surface composition.

Pretreatment of aluminium: topography, surface chemistry and adhesive bond durability

This paper reports on work in which bonded joints between a clad aluminium alloy (L165, comprising Cu 4.4%, Mg 0.5%, Si 0.8%, Mn 0.8%), with different pretreatments, and an epoxy resin (Ciba-Geigy's Redux 312/5) have been examined. The datum for comparison of the pretreatments is the Boeing phosphoric acid anodization (BAC 5555). Other treatments include sulfuric acid anodizing in combination with a phosphoric acid dip and a sulfuric acid/ferric sulfate etch. The topographical structure of the surface layers formed has been examined using scanning and transmission electron microscopy in conjunction with ultramicrotome sectioning. The chemical composition of pretreated surfaces has been characterized by X-ray photoelectron spectroscopy and depth profiling using argon-ion etch performed. Adhesive bonds have been prepared and their durability assessed using wedge test specimens. The data collected have been used to calculate crack growth and strain energy release rate as functions of time for the bonds produced. Surfaces of failed specimens have been examined to establish the locus of failure.

Plasma-sprayed coatings as surface treatments of aluminum and titanium adherends

Plasma-sprayed coatings have been evaluated as surface treatments for aluminum and titanium adherends. The best aluminum treatment is found to be a 50-μm thick blend of an aluminum-12% silicon alloy and polyester with 20-40% polyester. For FM-300M film adhesive, this coating gives a wedge test performance equivalent to phosphoric acid anodization (PAA). For FM-123 film adhesive, its performance is between that of the Forest Products Laboratory (FPL) etch and PAA. The aluminum in the composite coating provides structural strength and adhesion to the substrate; the polyester serves to toughen the coating and perhaps supplement the physical bonding (mechanical interlocking) provided by the microscopically rough morphology with chemical bonding to the adhesive. For titanium, a 50-μm plasma-sprayed Ti-6Al-4V coating provides an identical performance to the best chemical treatment with crack propagation entirely within the adhesive. These plasma-sprayed coatings avoid the disposal and environmental costs of con...

Quantitative acoustic microscopy of anodized and coated aluminium at frequencies up to 1 GHz

Quantitative acoustic microscopy (QAM) has been used to measure surface wave velocities on polished, anodized and coated aluminium substrates, these materials being representative of those used for adhesive bonding in the aerospace industry. Good quality acoustic measurements were obtainable at frequencies between 225 and 980 MHz, despite the inhomogeneous nature of the oxide layer produced by phosphoric acid anodization (PAA). Good agreement was obtained between the surface acoustic wave dispersion measured on aluminium coated with 0.2 and 1.0 μm PMMA, and that calculated by a simple isotropic layer model. The anodized aluminium was modelled as a transversely isotropic oxide layer on an aluminium substrate. At 0.2 μm, the oxide layer was too thin for the comparison between measurement and calculation to be conclusive, but the calculations suggest that a change in porosity of 10% in a 0.6 μm oxide layer, as obtained with an industry standard PAA treatment, should be readily detectable. The highly dispersive nature of some of the surface acoustic wave modes makes QAM extremely sensitive to small changes in the material parameters.

Fatigue Crack Growth in Epoxy/Aluminum and Epoxy/Steel Joints

Abstract The fatigue crack growth rate within epoxy/aluminum and epoxy/steel joints was evaluated as a function of a) type of surface pretreatment, b) water soak, c) fatigue cycle rate (Hz), d) adhesive thickness and e) type of epoxy adhesive. For both adherends, aluminum and steel, a significant improvement in the fatigue behavior was obtained by use of a mercaptoester coupling agent. After an 8-day, 57°C water soak, the metal surfaces which were pretreated with coupling agent (CA) or by phosphoric acid anodization (PAA) still resulted in cohesive failure, while the controls had higher crack growth rate and showed greater scatter. The room-temperature cure matrix with CA-treated aluminum showed a less dramatic improvement, probably because of a known difference in the application procedure. For the steel joints and room-temperature adhesive the improvement in the fatigue behavior of CA-treated samples was maintained after the 8-day hot water soak. No significant change was found in the fatigue crack grow...

Environmental durability of aluminium joints with different pretreatments

Clad aluminium was pretreated by three methods: chromic acid pickle, chromic acid anodize and phosphoric acid anodize. Double lap joints were exposed to various environments with and without stress. Stress level affected time to failure but not residual strength, the rate of loss of which depended on environment. Anodize pretreatments gave similar durability in terms of strength retention and were superior to pickle. There was no consistent difference between the two 120°C-curing epoxy adhesives used.

Evaluation of primers for adhesively-bonded aircraft repair

This paper contains the results of an investigation to determine the effectiveness of primers in promoting adhesion and improving the environmental durability of epoxy adhesive bonds on grit-blasted aluminum substrates. The control for this study consisted of phosphoric acid anodization of substrates followed by the application of EC 3924 B primer. The following primers were evaluated on grit-blasted substrates: (1) EC 3924 B; (2) Metlbond 6726; (3) Dow Corning Z-6040; and (4) EA 9203. In addition, grit blasting followed by Alodine application was compared with the other primers. Studies include shear strength evaluations of each primer/adhesive system at 23 and 105°C. In order to assess the relative durability of each system, shear strength testing at 93°C of samples exposed for 2 weeks to a 93°C/condensing humidity environment and wedge crack extension testing of samples exposed for a 2-week period to a 60°C/condensing humidity environment were performed. The peel strengths of the systems were determine...

Future Design Concepts for the Development of New Pretreatments of Aluminium Alloy/Metal Matrix Composites for Adhesive Bonding

Abstract The present paper is concerned with the identification of reasons for the environmental failure of adhesive joints using aluminium alloy substrates and the potential for improvement in performance by the development of modified anodising treatments. The attention of the work is focused on the behaviour of the oxide region of the joint and its potential influence on overall bonding performance. The scanning electron microscope (SEM) is used to study the oxide region within aluminium alloy adhesive joints which have been prepared by phosphoric acid anodising (PAA) and chromic acid anodising (CAA). Static stress durability tests are used to assess the performance of the joints and the resulting failure surfaces were investigated. Finally, the importance of the presence of a “micro-composite” interphase consisting of anodic oxide penetrated by adhesive, in the joint, is advanced and its significance in future pretreatment design outlined.

Study of aluminium bonded joints by analytical electron microscopy: Microstructural changes with aging

This study concerns the influence of aging on the microstructural characteristics of each component in bonded joint systems consisting of an aluminium alloy substrate (2024) clad with an aluminium sheet, an oxide layer obtained by surface treatment ('optimized' chromic-sulphuric acid etching, chromic acid anodizing, or phosphoric acid anodizing), an epoxy-novolac-based composite primer, and an epoxy-based adhesive film. The samples studied were obtained by ultrathin cross-section microtomy of the bonded joints (before and after aging). The microcharacterization was carried out using a transmission electron microscope (TEM) equipped with an X-ray energy-dispersive spectrometer (EDS), an electron energy-loss spectrometer (EELS), and an energy-filtered imaging system. Detailed analyses revealed that microstructural changes occur during aging in the primer in contact with the oxidized substrate and that the thickness of the modified zone depends on the underlying oxide. In addition, it is shown that a 'true' ...

Composite patch reinforcement of cracked aircraft upper longeron: analysis and specimen simulation

The use of composite patches on cracked portions of metallic aircraft structures is an accepted means of improving fatigue life and attaining high structural efficiency. As more and more advanced composite materials are beng developed, the wider use of the repair technology is anticipated even for the reinforcement of primary aircraft structure. The objective of this work is to illustrate how the composite patch repair technology can be successfully applied to restore the structural integrity of cracked components. The Phosphoric Acid Anodize (PAA) surface treatment on aluminum when applied in conjunction with the AVI13/HV998 adhesive were essential for achieving the appropriate patch bonding strength. Such a process was done without immersing the component into the PAA tank; dismantling the component from the aircraft was not necessary. Boron/epoxy and carbon/epoxy patches were applied at room temperature to the 7075-T6511 cracked specimens and tested under fatigue simulating the load spectrum for the upper longeron attached to the access door of the electronic equipment bay. Considerable improvement in the fatigue life was observed after the repair. Equivalent flight test hours were increased from approximately two thousand hours at which the component fractured completely when not repired to twelve thousand hours when the repair was made with only a small amount of crack growth. A six times increase fatigue life is obtained. The laboratory developed technique has been applied to several in-service aircraft which have now been flown for more than 700 h without detection of crack growth.

The failure mode of adhesively bonded aluminium following aqueous exposure

AbstractAdhesively bonded aluminium sheets pretreated by phosphoric acid anodizing (PAA) or chromic acid anodizing (CAA) were fabricated into 180° T‐peel test pieces. These were then exposed to cycles of aqueous exposure at elevated temperature, followed by a drying procedure at ambient or elevated temperatures. In the case of the CAA‐pretreated substrate, the aqueous exposure brings about a drastic reduction in joint strength and the locus of failure (as assessed by XPS) moves from the centre of the glue line to very close to the oxide/adhesive interface. The drying procedure increases the peel strength, and the failure path once again moves towards the bulk of the adhesive. This behaviour is ascribed to the agregation and to the subsequent removal of water molecules close to the interface. The behaviour of the PAA‐pretreated joints is completely different. The short water immersion times used in this work have little effect on peel strength but subsequent drying at elevated temperature does reduce the joint strength considerably. It is postulated that this may be the result of leaching and subsequent deposition of a component of the adhesive, perhaps some unreacted curing agent. This behaviour is also reversible and the peel strength can be increased by re‐exposing the joint to water. Thus, for both CAA and PAA pretreatments the strength can be affected greatly by environmental exposure, but both phenomena show a degree of reversibility.

The metal/polymer interface in aluminium adhesive joints: A microanalytical study

Aluminium adhesive joints have been prepared with metal pretreatments used in the aerospace industry, namely chromic sulphuric acid pickling (CSA, FPL-etch), phosphoric acid anodizing (PAA), and chromic acid anodizing (CAA). Chemical and physical techniques are discussed which render the interfaces accessible to electron microscopy and microanalysis. These comprise fracturing, selective removal of metal or metal plus oxide through chemical etching, angle lapping, and argon ion sputtering. The applied analytical techniques are high resolution SEM, scanning Auger, and ESCA. It is found that the porous oxide layers are completely invaded by the adhesive primer, which cures into a fibrous structure. The porous region contains more nitrogen compounds than the primer itself, possibly owing to the migration of the reactive part of the primer into the pores and to the adsorption and interphase segregation of low-molecular-weight constituents. The early stage of corrosion in one case is studied. The techniques discussed here are suitable for quality control or failure analysis. The work supports the concept of a mechanical anchoring of the adhesive within the oxide pores as the predominant adhesion mechanism.

The electronic structure of anodized and etched aluminum alloy surfaces

Specimens of 6061 and 5052 aluminum alloys which had been anodized and etched by several commonly used procedures were examined by means of bias-reference X-ray photoelectron spectroscopy (XPS). The spectra were compared with those obtained from single crystals of pure aluminum oxides. The chemical shifts observed from the A12p surface oxide lines were interpreted as differences in the Fermi energy levels relative to those in the bulk oxide crystals, and the Fermi energy levels of the surface oxides were thus determined. Using an earlier experimental correlation obtained for values of the point of zero charge (pzc) with Fermi energy levels in aluminum oxide powders, a value of the pzc of the surface oxide was then determined. The surface exhibited the maximum alkalinity, pzc = 8.9, after a caustic etch, and the maximum acidity, pzc = 3.6, after a phosphoric acid anodizing treatment. The significance of these pzc values in the adhesive bonding of aluminum alloys is discussed.

Water of crystallization pressure at phosphate deposits

A Clapeyron equation has been used to make an order of magnitude estimate of the internal pressure generated during the volume expansion that accompanies hydration of di-sodium hydrogen phosphate dihydrate (Na 2HPO 4·2H 2O) to higher hydrates. Na 2HPO 4·2H 2O is the product of neutralization of traces of caustic soda during phosphoric acid anodising of aluminium adherends. The pressure is large (>0.5 kbar), exceeding the adhesive strength of aluminium joints, and it is proposed that generation of such pressure, at Na 2HPO 4·2H 2O or at any other hydratable inclusion, is a physical mechanism responsible for the degradation of joints in aqueous environments.

Characterization of Surface Pretreatments on Al/Li Alloy and Related Mechanical Properties of Polysulfone Adhesive Bonds

Abstract An investigation of polysulfone-Al/Li alloy interaction involved single lap shear joints and wedge samples following an FPL etch, sulfuric acid anodization (SAA) and phosphoric acid anodization (PAA). The study of the Al/Li surfaces involved the determination of the elemental composition and morphological features of the pretreated adherend before bonding and following fracture. When polysulfone was either thermally pressed or primed onto the microporous surface, the polysulfone indeed penetrated into the porous oxide and thereby provided a mechanical means of adhesion. The wedge test results for the adherend pretreated by PAA and SAA were superior to those for the FPL etched adherend. The failure path for the FPL etched samples was at the adhesive/oxide interface whereas the failure path for the PAA samples was within the adhesive but with occasional divergence of the crack into the oxide. The porous oxides on Al/Li alloy formed after PAA and SAA treatment were shown to undergo dramatic changes in morphology on short term (< 95 hrs) exposure to 71°C and 100% R.H. environment. The mechanism of failure was due to moisture which caused hydration and subsequent weakening of the surface oxide layer and the bonded joint. Lithium was not surface concentrated in the PAA treated Al/Li alloy as shown by AES depth profiling and therefore the effect of Li on the durability of the bonded alloy is considered minimal.

Influence of pretreatment rinse waters on the durability of adhesive-bonded aluminium alloys

The affect of rinse water type and temperature on the durability of adhesive-bonded alloy pretreated by either the Forest Products Laboratory (FLP) etch, chromic acid anodizing or phosphoric acid anodizing has been investigated. Durability increased when tap water was used for rinsing instead of deionized water for (FPL) etched samples. Strength retention of FPL etched lap shear joints decreased with increasing temperature of the tap water rinse. Phosphoric acid anodizing produced marginally better durability than chromic acid anodizing.