UNS A92024 Research Articles

A Comparison Study of Stress Corrosion Crack Growth Rates for AA7XXX Alloys as a Function of Bulk Aqueous Chloride Concentrations

Abstract Alternative aluminum alloys are under consideration for drop-in replacement and refurbishment of corrosion and stress corrosion cracking prone AA7075-T6 (UNS A97075) components on aging military aircraft. An assessment of new materials for refurbishment and replacement of older generation, aging components is necessary for a viable life extension program. A quantitative database of stress corrosion cracking (SCC) properties of old generation materials compared to currently available materials is needed as a guideline/justification for part replacement. This effort evaluates AA7150-T7751 (UNS A97150) and AA7040-T7651 (UNS A97040), both which meet mechanical property requirements and are thought to be superior to AA7075-T6 in terms of corrosion performance. Specifically, stress corrosion crack growth rates for these plate alloys were documented and found to be similar in various bulk aqueous chloride environments and to represent a significant improvement over measured rates for AA7075-T651.

Test Environment Selection for Corrosion-Fatigue Testing of Aluminum Alloy 7075-T6

Abstract The appropriateness of a variety of test environments was determined for studies of interactions between corrosion and fatigue crack propagation (FCP) in the context of damage tolerance and durability. Because many fatigue cracks in aerospace structures involve materials in occluded geometries, measurements included occluded region potential as a function of time, boldly exposed region polarization scans, corrosion morphology comparisons, and pH. The alloy of interest was aluminum alloy 7075-T6 (UNS A97075). The test environments included four common ASTM test solutions for localized corrosion susceptibility of aluminum alloys and the lap joint simulant solution (LJSS), which was designed to reproduce in-service corrosion morphologies. The use of the ASTM standard solutions severely limited the range of potentials that can be probed during FCP testing. The LJSS was more passivating, resulting in the ability to probe a wide range of potentials without inducing localized corrosion on the boldly exp...

Critical Concentrations Associated with Cobalt, Cerium, and Molybdenum Inhibition of AA2024-T3 Corrosion: Delivery from Al-Co-Ce(-Mo) Alloys

Abstract Corrosion inhibition of aluminum alloy (AA)2024-T3 (UNS A92024) in sodium chloride (NaCl) solutions after pretreatments in solutions containing Ce(III), Co(II), or Mo(VI) ions are reported...

Influence of Salt Fog Fallout on the Failure of Chromated Aluminum Alloy 2024-T3 Panels in Neutral Salt Fog

Abstract Chromated aluminum alloy (AA)2024-T3 (UNS A92024) panels were exposed to neutral salt fog (NSS) over a range of salt fog fallout rates. Exposure was continued well beyond the standard testing periods (e.g., 168 h or 336 h) until there was general breakdown across the surface. From visual observation using the unaided eye, it was found that the development of pits on the surface went through four stages: —induction: where no pitting occurred —the development of a small number of pits —substantial growth in pit numbers —stabilization of pit numbers The number of pits on the surface was strongly dependent on the salt fog fallout rate and, hence, total chloride dose. The time to failure (appearance of a limited number of pits) as defined under various standards was essentially independent of the salt fog fallout rate.

Study of roundness on cylindrical bars turned of aluminium–copper alloys UNS A92024

Aluminium alloys are broadly employed in aerospace industry for the production of the different elements that conform the airships. Moreover, the appearance of emergent government legislations in environmental matters has forced the replacement of machining methods in the last years, which prevents or minimizes the employment of substances or fluids. In this sense, the turning has been carried out in dry for the performance of the experimental part of this work. In this work, macrogeometric deviations of UNS A92024 (Al–Cu) cylindrical bars are analyzed (roundness), and turned dry under some conditions of imposed cutting (cutting speed and feed). Also, an exponential relation between roundness and the cutting parameters is established for the dry turning process of UNS A92024 alloy. The obtained relationship allows us to predict the behaviour of these deviations in the range of cutting speed and feed considered.

Performance Assessment and Prediction of Corrosion Prevention Compounds with Electrochemical Impedance Spectroscopy

Abstract The performance of corrosion prevention compounds (CPC) on aluminum alloy (AA)7075-T6 (UNS A97075) was assessed using electrochemical impedance spectroscopy (EIS). The good correlation bet...

Effect of Reinforcement Coating on Corrosion Behavior of AA6061/SiC/20p Composite in High Relative Humidity Environments

Abstract A study of the influence of silicon dioxide (SiO2) active coatings on silicon carbide (SiC) reinforcement particles of an aluminum metal composite, AA6061 (UNS A96061)/SiC/20p (Al-Mg-Si alloy reinforced with 20 vol% of SiC particles), exposed to a high relative humidity environment (98% RH and 50°C), was carried out under simulation in a climatic chamber. To improve processibility, SiC particles were coated prior to composite manufacture by a casting procedure, using the sol-gel technique. The corrosion resistance of the composites was analyzed to determine the influence of the manufacturing variables. The nature of corrosion products before and after accelerated testing was analyzed by scanning electron microscopy (SEM) and low-angle x-ray diffraction (XRD), determining the influence of microstructural changes on corrosion behavior during exposure to the aggressive environment. The kinetics of the corrosion process was characterized on the basis of gravimetric tests. The corrosion damage to comp...

Corrosion Behavior of Nanocrystalline Bulk Al-Mg-Based Alloys

Abstract The corrosion behavior of a series of nanocrystalline Al-Mg alloys has been investigated and compared to that of conventional aluminum alloy 5083 (AA5083 [UNS A95083]). Experimental result...

Inhibition of Filiform Corrosion on Organic-Coated Aluminum Alloy by Hydrotalcite-Like Anion-Exchange Pigments

Abstract Novel anion-exchange anticorrosion pigments consisting of nitrate-, carbonate-, and chromate-exchanged hydrotalcite (HT) are shown to inhibit the propagation of filiform corrosion (FFC) effectively on organic-coated aluminum alloy 2024-T3 (AA2024-T3 [UNS A92024]) alloy. The HT-pigments are dispersed in polyvinyl butyral (PVB) coatings applied to the AA2024-T3 surface. Following FFC initiation by application of controlled amounts of aqueous hydrochloric acid (HCl) to a penetrative coating defect, the time-dependent extent of coating delamination is determined both optically and by repeated in-situ scanning using a scanning Kelvin probe (SKP) apparatus. All the HT-pigments profoundly reduce rates of coating delamination by comparison to unpigmented samples. However, inhibitor efficiency is dependent upon the exchangeable anion and increases in the order: carbonate < nitrate < chromate. It is also shown that by using SKP potentiometry, exchangeable chromate anions interact specifically with the metal surface to depress free corrosion potential values.

Cerium Chemical Conversion Coating for Aluminum Alloy 2024-T3 and Its Corrosion Resistance

Abstract New golden, yellow-colored cerium chemical conversion coatings on aluminum alloy 2024-T3 (AA2024-T3 [UNS A92024]) surface at room temperature were obtained by immersing the alloy into a cerium solution containing zinc chloride (ZnCl2) and hydrogen peroxide (H2O2). Electrochemical methods and immersion tests were used to study the dynamics of the coatings formation and their corrosion resistance in 3.5% sodium chloride (NaCl) solution. The morphologies of the coatings were recorded by scanning electronic microscopy (SEM). Energy-dispersive x-ray (EDX) analysis and x-ray photoelectron spectroscopy (XPS) were used to analyze the chemical composition and the oxidation state of the elements in the coatings. Polarization experiments and immersion tests in 3.5% NaCl solution showed that the sensitivity to pitting corrosion for the conversion-coated AA2024-T3 was greatly lower than that of the untreated specimens, and the corrosion resistance improved markedly. SEM photographs showed that the coatings consisted of a lot of spherical particles. EDX and XPS experimental results showed that the coatings were made up of oxygen, cerium, and aluminum, and the spherical particles contained higher contents of cerium and oxygen than the other sites. Cerium was mainly in the form of Ce4+. The mechanisms of conversion coatings formation and improvement on corrosion resistance also are discussed.

Corrosion Protection of Aluminum Alloy 2024-T3 by Vanadate Conversion Coatings

Abstract In this paper, the formation, chemistry, morphology, and corrosion protection of a new type of inorganic conversion coating is described. This coating, referred to as a vanadate conversion coating (VCC), forms on aluminum alloy substrates in a matter of minutes during simple immersion in aqueous vanadate-based solutions at ambient temperatures. VCCs are yellow in color and conformal across the surface of aluminum alloy 2024-T3 (AA2024-T3 [UNS A92024]) substrates. Auger electron sputter depth profiles and x-ray absorption near-edge spectroscopy show that VCCs formed by a 3-min immersion are 300 nm to 500 nm thick and consist of a mixture of vanadium oxides and other components in the coating bath. In anodic polarization experiments conducted in aerated chloride solutions, VCCs increase the pitting potential and decrease the rate of oxygen reduction. When characterized by electrochemical impedance spectroscopy, VCCs demonstrate a low-frequency impedance between 1 MΩ-cm2 and 2 MΩ-cm2 after 24 h exposure to aerated 0.5 M sodium chloride (NaCl) solutions. In salt spray testing conducted according to ASTM B117, VCCs suppress formation of large pits for more than 168 h. VCCs also appear to be self-healing. Analysis of solution in contact with VCCs by inductively coupled plasma emission spectroscopy indicates that vanadate is released into solution upon exposure. Vanadium deposits were identified by x-ray microchemical analysis on a bare alloy substrate held in close proximity to a vanadate conversion-coated surface, and corrosion resistance of this bare surface was observed to increase during exposure. An important component of VCC formation appears to involve inorganic polymerization of V5+, which leads to the buildup of a film that passivates the surface and inhibits corrosion.

In Situ X-Ray Radiographic Study of Intergranular Corrosion in Aluminum Alloys

Abstract In situ x-ray radiography was used to characterize intergranular and exfoliation corrosion in high-strength Al alloys. The samples were either exposed to sodium chloride (NaCl) solution at a controlled potential or to high humidity after an electrochemical pretreatment in NaCl solution. Intergranular corrosion (IGC) growth rates in solution for samples encased in epoxy and exposed to solution were distributed over a range with an upper limit equal to the rates determined by the foil penetration technique. Unencased AA2024-T3 (UNS A92024) samples that were held at a controlled potential in solution exhibited IGC and then exfoliation corrosion. AA2024 and AA7178 (UNS A97178) samples encased in epoxy developed sharp intergranular fissures during exposure to 96% RH following an electrochemical pretreatment. Unencased samples given the same treatment exhibited exfoliation in the high-humidity environment.

Impedance Characterization of Crevice Corrosion of an Aluminum 2024 Structure

Abstract The impedance behavior of aluminum 2024 (UNS A92024), bare and with a MIL-P-23377 epoxy primer/MIL-C-85285 polyurethane topcoat, in aqueous 0.5 M sodium chloride (NaCl) was investigated in experiments that included a simulated crevice. Impedance spectra of the bare alloy were effectively modeled by the Randles circuit; the coated alloy required the addition of a pore resistance in series with, and a geometric coating capacitance in parallel with, the parallel RC circuit representing the metal/coating interface. A coated structure containing a corroding crevice was modeled by a simplified Randles circuit element, to represent the crevice, in parallel with the foregoing circuit. Experiments with a corroding system showed this set of circuit models to be self-consistent and to yield reasonable values for the parameters characterizing the model circuits. Using these models and parameter values, it was shown that, for the 2024/0.5 M NaCl system, a corroding crevice can be detected in the context of a ...

Photo-Corrosion of Different Metals during Long-Term Exposure to Ultraviolet Light

Abstract The effect of ultraviolet light on the corrosion rates of different metals was studied in two separate investigations. Metal samples were immersed in a flowing fresh water discharge stream for either three or five months under both ultraviolet (UV) light and dark conditions. Weight-loss results demonstrated that pure zinc, carbon steel, aluminum 6061 (UNS A96061), pure copper, and pure silver all experienced photo-corrosion, that is, a greater weight loss under exposure to UV light vs in the dark. Titanium, zirconium, Type 304 (UNS S30400) stainless steel, and pure nickel showed a weight gain during immersion, but the cause of the weight gain was uncertain. Pure nickel and Al 5052 (UNS A95052) did not show any reproducible trend. Zirconium, titanium, and brass showed a photo-effect in one investigation, but not in the other. The photo-corrosion is explained with models based on the photo-voltages measured on corroded metal samples.

Topographical Analysis of Pitting Corrosion in AA7075-T6 Using Laser Profilometry

Abstract Laser profilometry (LP) is explored as a means for mapping and quantifying the corrosion damage of AA7075-T6 (UNS A97075) specimens exposed to cyclic prohesion spray for various times. Analysis tools were developed and used to provide quantitative measurements of the distribution of damage site shape, area and volume, and depth. Evolution of these parameters with exposure time was examined. The measured maximum damage site depths of 22 μm to 55 μm agree well with values obtained using other techniques. Modeling the damage sites as elliptical surface flaws yielded maximum ΔK values for the individual fatigue specimens (2 MPa√m to 5 MPa√m) that are equal to or higher than typical fatigue crack growth thresholds available in the literature. The maximum (or average) ΔK values calculated from the corrosion damage distributions correlate well with measured fatigue life.

Improvements in Strength and Stress Corrosion Cracking Properties in Aluminum Alloy 7075 via Low-Temperature Retrogression and Re-Aging Heat Treatments

Abstract The susceptibility of aluminum alloy 7075 (AA7075 [UNS A97075]) to intergranular stress corrosion cracking (SCC) in the peak strength T6 temper is alleviated through the use of the T73, or over-aged temper, which provides improved SCC resistance with a 10% to 15% strength loss compared to the T6 temper. Previous research has indicated that retrogression and re-aging (RRA) heat treatments reduce the trade-off between T6 strength and T73 SCC resistance. The short-term heat treatment they used, however, limited the applicability of RRA to thin sections of material. The primary goals of this research effort were to determine if lower retrogression temperatures could be used in the RRA process to extend the applicability of this heat treatment to thick section aircraft components and to quantify any observed improvements. Alternate immersion (AI) and double-cantilever beam (DCB) tests were conducted in a 0.6-M sodium chloride (NaCl) solution to evaluate the SCC resistance of various tempers. Improveme...

Effect of Ambient Aging on Inhibition of Oxygen Reduction by Chromate Conversion Coatings

Abstract Cathodic polarization curves were collected from chromate-conversion-coated 2024-T3 (UNS A92024) electrodes subject to prior aging under ambient indoor conditions. Selected exposures at el...

Corrosion Protection of an Aluminum Alloy with Nontoxic Compound Inhibitors in Chloride Media

Abstract In this study, the properties of some compound inhibitors were measured on aluminum alloy (AA)7075-T76 (UNS A97075) in chloride media. The compound inhibitors were composed mainly of phosp...

Pitting Corrosion Control of Aluminum 2024 Using Protective Biofilms That Secrete Corrosion Inhibitors

Abstract The corrosion behavior of Al 2024-T3 (UNS A92024) exposed to artificial seawater (AS) and Luria Bertani (LB) medium has been studied using electrochemical impedance spectroscopy (EIS). Tests were performed in sterile media and in the presence of three different biofilm-forming bacteria: a Bacillus subtilis biofilm that was genetically engineered to produce either polyaspartate or polyglutamate, a Bacillus licheniformis bacterial biofilm that naturally produces γ-polyglutamate, and an Escherichia coli biofilm that was genetically engineered to produce polyphosphate. A significant reduction in active pit growth rates and an ennoblement of the corrosion potential (Ecorr) were observed in both media in the presence of these biofilms, which produce corrosion inhibitors. The lowest corrosion rates of Al 2024 exposed to the LB medium were observed in the presence of the B. subtilis bacterial biofilm-producing polyaspartate and the E. coli bacterial biofilm that produced polyphosphate. In the presence of...

Hydrotalcite Coating Formation on Al-Cu-Mg Alloys from Oxidizing Bath Chemistries

Abstract The effects of bath oxidizing power on hydrotalcite coating formation and breakdown were examined in this study. Overall, results indicate that increasing the oxidizing power of the coating bath shortens the immersion time required to form highly corrosion-resistant coatings. Hydrotalcite conversion coatings were formed on Al 2024-T3 ([UNS A92024] Al-4.4%Cu-2.5%Mg-0.6%Mn) panels by immersion in alkaline lithium salt solutions containing either carbonate, nitrate, or nitrate/persulfate additions. The resulting coating structure was characterized by scanning electron microscopy. Corrosion resistance was characterized by electrochemical impedance spectroscopy (EIS) and salt spray exposure testing. When nitrate and persulfate oxidizers were present in the bath, coating formation time and hydrogen evolution were sharply reduced while coating weight was increased, indicating an overall increase in coating efficiency. A decrease in pinhole-type coating defects was attributed to early cessation of hydrog...