Chromic Acid Anodizing Research Articles

Effect of chromic acid anodization on the corrosion resistance of laser cladding austenitic stainless steel coatings based on orthogonal tests

In this paper, Ni transition layer and Fe-based alloy coatings were prepared on HT250 by laser cladding technology, and the corrosion resistance of the coatings was further enhanced by the chromic acid anodization (CAA) surface treatment process. The effects of time (t), current intensity (I), and temperature (T) on the microstructure and corrosion resistance of the coating after CAA were explored based on the three-factor and three-level orthogonal test method. The results show that after CAA, the intergranular morphology of the coating is corroded, the intragranular organization is exposed and protrudes, and the micro-morphology shows a multi-layer "skeleton" overlapping structure. The main phases were transformed from γ-Fe (FCC) to CrNi (FCC) and FeNi (BCC). The corrosion residue of the original coating is a lamellar metal-carbide weave, whereas the CAA coating is fibrous. The order of influence of CAA process parameters on corrosion depth and maximum impedance value is I >t >T. I on the degree of densification of the passivated film is 1.03 and 1.81 times higher than T and t, respectively. The best corrosion resistance of the sample was obtained at t = 30 min, I = 0.1 A, and T = 40 ℃, and its corrosion rate (2.83 × 10−3 g/m2·h) was 29.88 % lower than the original coating (4.03 × 10–3 g/m2·h).

Effect of chromic acid anodization on the corrosion resistance of Fe-based alloy coatings by high-speed laser cladding

Fe-based alloy coatings were coated on surface of ductile cast iron by high-speed laser cladding, and the effects of chromic acid anodization on corrosion resistance of coatings were analyzed. The results show that as chromic acid anodization time increases, corrosion rate falls and then rises. After 20 min, the coating had Cr and Fe oxide passivation film, maximum impedance value (5.40 × 105 Ω·cm2) and polarization resistance (50.6 kΩ·cm2), and minimum self-corrosion current (0.77 μA/cm2). Its corrosion resistance had improved significantly.

Hybrid PEO/sol-gel coatings loaded with Ce for corrosion protection of AA2024-T3

Plasma electrolytic oxidation (PEO) has been targeted as an eco-friendly alternative technology to conventional chromic acid anodizing (CAA) for corrosion protection of aluminium alloys in the aircraft industry. However, conventional PEO technology implies high energy consumption. Flash-PEO coatings (≤10 μm) produced in short treatment times (≤ 5 min) constitute a feasible way to overcome this limitation. Nevertheless, the long-term corrosion resistance is compromised, thus requiring novel sealing post-treatments. The present work studies the effect of stand-alone hybrid sol-gel (HSG) and Ce-doped hybrid sol-gel (HSGCe) coatings as a sealing post-treatment to evaluate the long-term corrosion resistance of Flash-PEO coatings on aluminium alloy (AA) 2024-T3. The characterization of the PEO, HSG, and HSG-Ce coatings was performed by scanning electron microscopy, X-ray diffraction, water contact angle, dry adhesion tests (ISO 2409), optical profilometry and Fourier transform infrared spectroscopy. The corrosion behaviour was assessed by electrochemical impedance spectroscopy up to 21 days (3.5 wt% NaCl). Active corrosion protection was assessed by immersion tests of artificially scratched coatings. Present findings reveal that low-energy-cost Flash-PEO coatings were successfully formed on AA2024-T3 alloy. Both HSG and HSG-Ce coatings were homogeneously formed on Flash PEO coating. Regarding the corrosion resistance, HSG-Ce showed significant scratch protection during 21 days of immersion in 3.5 wt% NaCl. The results suggest that, while the release of Si and Ce from the coating provided corrosion protection, NO3−release promoted localized corrosion phenomena in the scribe. This was associated with the preferential pitting corrosion phenomena at the Cu-rich intermetallic compounds instead of forming a thick and stableNO3−-rich passive layer.

Investigation of failure and development of mitigation techniques of a cracked aircraft wing spar cap

The research is aimed to identify the failure mechanism which leads to cracks development in the wing spar cap of a transport aircraft and subsequently devise a mitigation strategy to prevent such defects. The research objective was accomplished in two stages. In the first stage, failure analysis of the spar cap was carried out through a hierarchal experimental approach where a combination of various experimental techniques (i.e., compositional analysis, stereoscopy for dimensional measurement of crack, and fractographic analysis through SEM/EDX) was employed to ascertain the cause of failure. Microscopy revealed multiple cracks originating from the hole edge which were studied in detail to ascertain the cause of failure. The failure analysis revealed the presence of corrosion products inside the hole due to crevice corrosion as a prime contributor to crack initiation. Further, cyclic loading caused the crack to propagate, while the presence of undiffused lead particles in the cracked surface confirms the occurence of deposition corrosion and crack branches at various places suggested Stress Corrosion Cracking (SCC) . The numerical analysis helped to identify that although the hole was a stress concentration site yet it was material weakening due to corrosion which primarily caused the failure . The second stage entails the development of a mitigation strategy to avoid corrosion-assisted crack development. For this purpose, two different types of coatings, i.e., Sulphuric Acid Anodizing (SAA) and Chromic Acid Anodizing (CAA) were applied to the specimens and then subjected to salt spray testing as well as exposed to environmental conditions. SAA coating appeared to be more corrosion resistant as it showed little signs of corrosion as compared to original (uncoated) and CAA-coated samples.

Evaluation of Surface Treatment for Enhancing Adhesion at the Metal-Composite Interface in Fibre Metal-Laminates.

The paper presents the issues of metal surface treatment in fibre metal laminates (FML) to obtain high adhesion at the metal–composite interface. Aluminium 2024-T3 and titanium Grade 2 were analysed. The metal surface modifications were carried out by mechanical (sandblasting, Scotch-Brite abrasion), chemical (P2 etching, phosphate-fluoride process), electrochemical (chromic and sulphuric acid anodizing), and plasma treatment, as well as the application of sol-gel coatings. In terms of surface geometry, the analysis included roughness and 3D surface topography examination. The morphology was examined using scanning electron and atomic force microscopy. The surface free energy and its components (polar and dispersive) were determined using the Owens–Wendt method. The novelty of this study is the determination of the effect of different surface treatments on the surface free energy, topography, and morphology in terms of the possible appropriate adhesion in fibre metal laminates. Chromic acid anodizing is still the most effective in enhancing the expected adhesion. A suitable technique may be the use of P2 etching of aluminium. It results in low roughness, numerous micro-irregularities, and the presence of porosity. The obtained test results show that the application of sol-gel coating increases the surface free energy and may increase the adhesion.

Influence of and differences between Chromic and Sulphuric acid anodising on the fatigue properties of 7050 T7451 aluminium alloy

The aim of the present study was to investigate the effect two commonly used anodising processes, i.e. Chromic acid anodising (CAA) and Sulphuric acid anodising (SAA) had on the fatigue life of 7050-T7451 (AMS4050). The fatigue results show that regardless of the anodising process employed, reductions in fatigue were observed when compared to the as machined condition. However, based on topographical, residual stress and surface roughness observations combined with the metallographic features observed from micro-sections taken through the anodised layers it would appear that mechanism by which fatigue was influenced was different between the two anodising processes. For example with regard to CAA the reduction in fatigue was due to a combination of grain boundary attack and dissolution of the non metallic precipitates from pickling and the CAA process in which fatigue cracks initiated in the substrate. Whereas, in the case of SAA the reduction in HCF was primarily considered to be due to the presence of surface cracking which initiated from pits caused by the dissolution of the non metallic precipitates during the anodising process. Differences in loss of fatigue and failure mode were also observed between the two anodising processes as a result of stress concentration in which fatigue loss was greater and the number of multi-failure sites higher when the SAA samples were notched, inferring the Kt factor was exerting a greater influence on SAA by effectively lowering its strain to fracture.

Review of Cr-Free Coatings for the Corrosion Protection of Aluminum Aerospace Alloys

Aluminum alloys are known to have many advantages (e.g., light weight and low cost) but they are not immune to corrosion. So, it is important to assess their corrosion behavior, in particular under atmospheric conditions. To protect aluminum alloys against corrosion, paints are generally applied onto the materials. Corrosion protection in the aerospace industry consists of a conversion or anodized coating, an inhibited primer, and a top-coat. Chromate conversion coating (CCC) and primers containing chromate pigments have been widely used in the aerospace industry over the last decades. However, new environmental regulations have led to major changes for aluminum corrosion protection. By limiting or prohibiting some chemicals, for instance Cr(VI), the European regulation REACH (Regulation on Registration Evaluation, Authorization and Restriction of Chemicals) has induced major changes to some of the finishing processes of aluminum alloys (e.g., chromate conversion, chromic acid anodizing, and chromate sealing). Interesting results have been obtained while seeking replacements for Cr(VI), for example, with the incorporation of cerium, lithium salt, or nanocontainers loaded with corrosion inhibitors in organic coatings. For several years, hybrid sol–gel coatings able to replace the pre-treatment and primer steps have been under development, showing interesting results. New prospects for the future involve the use of photopolymerization to reduce the energy-intensive heat treatment needed in sol–gel technology. It will also be necessary to test these new technologies in service conditions or in accelerated corrosion tests before being able to conclude on the real effectiveness of these coatings. This review summarizes the recent developments in Cr-free coatings for aluminum alloys. Their advantages and drawbacks are also discussed.

Multifunctional, environmental coatings on AA2024 by combining anodization with sol-gel process

The present work proposes the development of multifunctional composite coatings on AA 2024 by combining anodization and sol-gel process. To render the surface of AA 2024 with maximum corrosion resistance, eco-friendly citric-sulfuric acid (CSA) electrolyte with low sulphur content was used for anodization at 20 V for a duration of 30 min. The obtained anodized layer was porous. Ambient curable hybrid sol-gel coating with 8-hydroxy quinoline (8-HQ) as corrosion inhibitor was used as a sealant for the porous anodized layer to enhance the corrosion resistance with self-healing properties. Surface pre-treatments were carried out using sand blasting and alkaline etching to ensure high surface activity prior to anodization. The surface morphology and chemical composition of samples with and without coatings were characterized by field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopic analysis (EDX). Adhesion strength and wettability of the coatings were measured by tape adhesion test and water contact angle analysis respectively, which revealed excellent binding strength and hydrophobic nature. The corrosion resistance of the coatings was evaluated using electrochemical impedance spectroscopic (EIS), potentiodynamic polarization and salt spray tests. The results revealed improved corrosion resistance of anodized + 8-HQ sealed AA2024. Moreover, when the coated samples were scribed and exposed to the corrosive medium, the SEM/EDX mapping confirmed presence of corrosion inhibitors at the location of the defect, thereby confirming the self-healing property. Hence, the proposed system is a chromium-free, environment friendly multifunctional system exhibiting excellent self-healing corrosion protection and can be a promising substitute for chromic acid anodization.

Effect of cerium (IV) on thin sulfuric acid anodizing of 2024-T3 alloy

Chromic acid anodizing (CAA) is still being used today for corrosion protection of fatigue-critical components in the aeronautic industry due to the lack of feasible alternatives. Ce-containing sulfuric acid anodizing (SAA) has been identified as a promising strategy for the development of alternatives to toxic CAA. This work explores thin sulfuric acid anodizing (TSAA) focusing on the following effects: (i) current density and voltage values; (ii) concentration of Ce(IV). Screening of the optimum combination in search of minimum thickness and the best corrosion resistance was performed using electrochemical impedance spectroscopy (EIS). Two Ce-containing anodic films were selected and further investigated in comparison with the inhibitor-free film in terms of morphology (FEG-SEM, TEM), composition (RBS), corrosion resistance (EIS, NSST), high-cycle fatigue and paint adhesion. The results indicate that the path to approach the CAA performance lies through thin (<1 μm) SAA-Ce films formed at low current density.

Effects of chromic acid anodizing on fatigue behavior of 7050 T7451 aluminum alloy

Abstract The effect of chromic acid anodizing (CAA) surface treatment on 7050 T7451 aluminum alloy was presented in this study in terms of fatigue behavior. CAA is a treatment against corrosion by producing aluminum oxide layer (Al2O3) at the surface. However, fatigue performance of 7050 T7451 is affected by the coating. In this study, eight different CAA processes were examined with regard to etching stage of pre-treatments by using an alkaline etchant and/or acid etchants with various immersion times. Optical microscopic examinations were applied in order to determine pitting characteristics for the selection of CAA process parameters before fatigue tests. A CAA process was selected among eight processes in terms of pitting characteristics in order to apply fatigue specimens. Four fatigue test groups were determined to investigate bare condition of 7050 T7451 and sub-stages of the CAA particularly. Constant amplitude axial fatigue tests were conducted on specimens at 91 Hz at stress ratio (R) -1 until run-out criteria, which was 106 cycles. Fatigue life reduction was determined due to pretreatments of CAA. Fracture surfaces of the specimens were examined by scanning electron microscope (SEM) to investigate morphology and crack initiation sites.

Development and certification of chromic acid-free anodizing process for aircraft grade aluminium alloys

Chromic acid (Cr 6+ ) anodization process is widely used for the corrosion protection of aircraft aluminium alloys. Hexavalent chromium being toxic in nature need to be phased out by eco-friendly alternatives. In the present study modified tartaric-sulphuric acid (TSA) process has been developed followed by sealing in permanganate based bath to obtain 4 to 6 µm thick anodic oxide layer on 2024-T3, 6061-T6 and 7075-T6 aluminium alloys. The process was carried out using a pilot scale anodizing plant. The anodized specimens were characterized for visual observation, thickness, adhesion, electrical breakdown voltage, corrosion resistance and tensile behaviour. All the tests were carried out as per MIL-A-8625F specifications. The specimens were also subjected for about 800 hrs to real time corrosion testing, 200 metres away from sea shore at Mandapam Camp, Rameshwaram, India. The performance of the permanganate sealed TSA anodized aluminium alloys are comparable with that of the conventional chromic acid anodized coatings. This chromic acid-free anodization process has been qualified to airworthiness regulating standards by Indian military certification authorities. Efforts are in progress to commercialize this technology for use on aero platforms.

A Review on Anodizing of Aerospace Aluminum Alloys for Corrosion Protection

Aluminum alloys used for aerospace applications provide good strength to weight ratio at a reasonable cost but exhibit only limited corrosion resistance. Therefore, a durable and effective corrosion protection system is required to fulfil structural integrity. Typically, an aerospace corrosion protection system consists of a multi-layered scheme employing an anodic oxide with good barrier properties and a porous surface, a corrosion inhibited organic primer, and an organic topcoat. The present review covers published research on the anodic oxide protection layer principles and requirements for aerospace application, the effect of the anodizing process parameters, as well as the importance of process steps taking place before and after anodizing. Moreover, the challenges of chromic acid anodizing (CAA) substitution are discussed and tartaric-sulfuric acid anodizing (TSA) is especially highlighted among the environmentally friendly alternatives.

Optimization of preparation technology on fibre metal laminates (FMLs) for high-temperature applications

Fibre metal laminates based on titanium and carbon fibre reinforced polyimide (TCP-FML) are designed and manufactured to meet the requirements of high temperature applications. In this work, the preparation technology of the TCP-FML was optimized through the investigation of prepreg manufacture as well as surface treatment of titanium. Prepreg were manufactured using a solution dip method. The effects of step width, fibre tension and translational speed on the performance of prepreg were studied. Surface treatments were performed on titanium using sandblasting, chromic acid (CAA) anodization and NaTESi anodization methods. SEM observation and contact angle were used to analyze the surface characteristics of titanium. The effects of surface treatment on the interlaminar properties of titanium/polyimide were investigated by single-lap shear test and double cantilever beam (DCB) tests. This careful surface preparation, to ensure a good interfacial strength, is crucial for good mechanical properties.

Effect of shot peening and anodizing on fatigue crack growth of aircraft material Al 7050-T7651

The rate of fatigue crack propagation of the 7050-T7651 aircraft material subjected to automatic shot peening machine and chromic acid anodizing (CAA) has been studied and compared with previous research results. Al 7050-T7651 is made into specimens according to ASTM E647 standard treated with automatic shot peening machine (with steel shot material, 0.017inch diameter, shot flow 30%, 3 bar pressure, Almen intensity 0.0082 A with 100 mm shooting range and 90° angle of shot) and CAA (operating standard) in surface treatment area of Indonesian Aerospace. The fatigue cracking test with a load of about 200 kg, with a stress ratio of R = 0.1. The results of the fatigue crack growth test resulted in the constant of Paris A = 3.654 × 10−9 and n = 1.67, indicating that the automatic shot peening machine and chromic acid anodizing treatment decreased the fatigue life and increased the fatigue crack growth rate compared to the automatic shot peening machine, manual machine with or without CAA, and without treatment.

Tritium Retention in Hexavalent Chromate-Conversion–Coated Aluminum Alloy

In this work, Aluminum 6061-T6 samples were subjected to MIL-DTL-5541F type-I, class-3 anodic coatings, where a yellow irradiate finish was achieved. Both chromate-conversion coatings (CCCs) and unmodified samples were exposed to deuterium-tritium (PT = 0.51 atm) gas for 24 h at room temperature. Following loading, the samples were subjected to one of two desorption techniques: temperature-programmed desorption or a surface stripping technique. The results show that chromic-acid anodizing of aluminum dramatically increases the total quantity of tritium retained by the treated surface as compared to unmodified aluminum. X-ray photoelectron spectroscopy and scanning electron microscopy studies of both treated aluminum and unmodified samples indicate that the CCCs contain significant quantities of hydrated chromium. Using transmission electron microscopy, the surface is shown to have significant cracking and fracturing of the film and leads to a highly grained and porous surface. Such surface defects coupled with the vast quantity of hydration sites are likely reasons for the increased retained tritium inventory observed for CCC samples. Because of the physical and chemical properties of unmodified CCC samples, they are not suitable for use in tritium environments.

Effect of Potential on Corrosion Behavior of Tartaricsulphuric Acid Anodized 7075 T6 Aluminum Alloys

Anodizing of aluminum alloy in tartaric-sulphuric acid (TSA) is studied as an alternative to replace chromic acid anodization (CAA) to obtain more environmentally-friendly process. Bare 7075 T6 aluminum alloy specimens were anodized in TSA and subsequently protected by boiled water sealing treatment. The TSA solution used contains 86,76 gpl tartaric acid and 44 gpl sulphuric acid at 37°C. Some specimens are anodized in CAA as comparison. In this research, the effect of anodization potential of 7075 T6 aluminum alloy in TSA on the thickness, weight, and corrosion resistance of anodize layer are studied. Corrosion resistance test was carried out by conducting salt spray test for 96 hours and corrosion potential and current density measurement using potentiostat. The morphology and chemical composition of the sealed anodize layer were evaluated by scanning electron microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The results showed that the thickness and weight of the anodize layer increases as the anodization potential increases. The best corrosion resistance is achieved by 7075 T6 aluminum alloy specimen with potential anodization of 15 V.

Surface and Electrochemical Characteristics of Novel Chromate-Free Mn-V Oxyanion Sealed Tartaric–Sulfuric Acid Anodized Coating

In the present investigation, a novel chromate-free Mn and V oxyanions sealed tartaric–sulfuric acid anodic oxide coating (TSA) was developed on airframe grade aluminum alloy (AA2024). The results obtained from field emission scanning electron microscopy and energy-dispersive x-ray analysis showed effective surface modification of TSA after Mn-V oxyanions sealing (TSAMnVO). Potentiodynamic polarization results showed no significant difference in corrosion current density value (0.01 µA/cm2) after 1 h and 336 h of immersion in 3.5% NaCl solution. EIS result exhibited barrier layer resistance (Rb) value of TSAMnVO > 106 kΩ cm2 even after 336 h of immersion in NaCl solution. The 3D profile and Raman analysis showed no significant difference in the topography and composition, respectively, on the surface of TSAMnVO even after 1000 h of salt spray test. TSAMnVO also showed excellent adhesion with aircraft grade epoxy primer as per the ASTM 3359. Tensile and constant amplitude fatigue test results of TSAMnVO exhibited ultimate tensile strength and number of cycles to failure comparable with chromic acid anodization (CAA) process. Hence, the developed novel TSAMnVO coating system eliminates the usage of chromate ions completely in anodization process since it provides comparable corrosion resistance and mechanical properties with CAA.

Morphology perspective on chromic acid anodizing replacement by thin film sulfuric acid anodizing

Conventional chromic acid anodizing (CAA) with dichromate seal provides both superior corrosion protection and strong adhesion without significant fatigue debit, yet hexavalent chrome is toxic and needs to be replaced. Nickel acetate seal removes most of Cr (VI) from the CAA coating. Thin Film Sulfuric Acid Anodizing (TFSAA) replacing CAA eliminates Cr (VI) usage in the anodizing process. TFSAA coating with non-Chromated seal is the most desirable for Cr (VI) elimination. An Electrochemically stressed (EC-stressed) cyclic test, designed for fast screening by us, was used to compare CAA with dichromate seal, CAA with nickel acetate seal, and TFSAA with an environmentally friendly two-step seal. In EC-stressed test, CAA with dichromate seal performed worst, CAA with nickel acetate seal provided much better barrier properties and TFSAA with two-step seal performed the best for both barrier properties and corrosion resistance. Coating morphology and barrier properties are important factors for different coating applications.

Effect of surface treatment on adhesively bonded aluminium-aluminium joints regarding aeronautical structures

Abstract The structural integrity of several structures could be determined by their joints strength. Over the years, adhesively bonded joints have been often chosen to achieve a compromise between mass reduction and higher mechanical strength. Among others, the reduction in stress concentrations, the ability of producing smooth surfaces with no discontinuities and the reduced weight penalties are some of the factors that make this type of joints so attractive. Normally, to increase the bond strength, the materials to be bonded must be subjected to a kind of surface treatment. For metals, and more specifically, for aluminium alloys, phosphoric acid anodizing and chromic acid anodizing have been the most used treatments worldwide. However, recent investigations show that these kinds of anodizing are detrimental to health due to the release of carcinogenic substances. With this in mind, it is of the utmost importance to find alternative surface treatments that can ensure an effective bond. In this paper, a vast experimental study was performed based in the single lap joint ASTM D 1002 standard method, with the objective of determining the best alternative surface treatment (Sulfuric Acid Anodizing and Boric-Sulfuric Acid Anodizing), for aluminium-to-aluminium joints, using two types of adhesives, namely the AF 163 and the EA 9658 AERO. Results show that the optimum surface treatment is different for each type of adhesive and this fact has a huge influence on mechanical behavior of this type of aeronautical adhesive joints.

Malic-Sulfuric Acid Anodizing of AA 2024 Compared to Other Dicarboxilic-Sulfuric Acid Anodizing Processes

Aiming to develop hexavalent chromium (CrVI)-free anodizing baths for Al-alloys, the porous anodizing of AA2024 was studied in different mixtures of sulphuric acid-organic acid formulations with similar concentrations. Mixtures of sulphuric with dicarboxylic acids as malic, malonic, oxalic and tartaric and with the tricarboxylic citric acid were tested. The corrosion behavior of the unsealed anodized layers exposed to 0.1M NaCl were compared by voltammetry and chronoamperometry at polarizations slightly above the pitting potential of blank AA2024 with the behaviour of standard chromic acid anodizing. The obtained oxide layers were characterized by SEM, FTIR and micro-Raman spectroscopy, which confirmed the incorporation of organic acids in relative different amounts. Best corrosion resistances expressed in terms of pitting nucleation time were observed for malic-sulphuric, followed by tartaric-sulphuric anodizing. However, none of the formulations achieved the resistance conferred by the chromic acid anodizing. A direct correlation between mesoporosity of the anodizing layer and corrosion resistance was verified.