Al Alloy Panels Research Articles

Combination of Galvanic Interaction and Acoustic Emission Measurements on Damaged Painted Al Alloys: Towards Instrumented Witness Coupons for Outdoor Corrosion Testing

After real environmental exposures, the presence of corrosion damages on aircraft structures may be detected by means of non-destructive techniques (NDT). However, corrosion initiation and mechanism behind the defect cannot be distinguished by currently used physical methods. Therefore, aircraft industry needs sensors able to detect both corrosion initiation and its propagation.Few years ago, a new test sample design providing accelerated response during atmospheric corrosion tests was reported by G.S. Frankel research group [[1],[2]]. A painted Al alloy panel, uncoated through-hole noble fasteners and scribes, was specially designed to measure the galvanic current between fasteners and scribes during exposure in a salt fog chamber using a zero-resistance ammeter.But galvanic current measurement remains an electrochemical technique which could be difficult to be implemented in environments expected to be encountered in service, even if some attempts were described in literature [[3]]. More robust NDT methods, like acoustic emission was suggested to evaluate corrosion damage on Al alloys [[4]] but their application has not been yet transferred to instrumented witness coupons.Recently a European program [[5]] was launched to validate the application of ultrasonic corrosion sensors for real time detection of early stages of localized corrosion on Al alloys. Following our previous work on laboratory-scale tests based on mass transport control from inhibitors leaching from primer exposed at scribes [[6]], this talk describes the laboratory validation of acoustic emission combined with galvanic corrosion current measurements to evaluate the role of depletion of available corrosion inhibitors in the paint on scratched plates, mimicking riveted aircraft panels, exposed to chloride environments, in various conditions, i.e. immersion and salt spray conditions. [1] C.A. Matzdorf, W.C. Nickerson,B.C. Rincon Troconis,G.S. Frankel, Longfei Li, R.G. Buchheit, Galvanic Test Panels for Accelerated Corrosion Testing of Coated Al Alloys: Part 1—Concept, Corrosion, 69 (2013) 1240-1246. [2] Z.Feng and G.S. Frankel, Galvanic Test Panels for Accelerated Corrosion Testing of Coated Al Alloys: Part 2—Measurement of Galvanic Interaction, Corrosion,70 (2014) 95-106. [3] Z. Feng, G.S. Frankel,W.H. Abbott,C.A. Matzdorf, Galvanic attack of coated al alloy panels in laboratory and field exposure, Corrosion, 72 (2016) 342-355. [4] A. Prateepasen, C. Jirarungsatian, Implementation of acoustic emission source recognition for corrosion severity prediction, Corrosion, 67 (2011), 056001/1-056001/11. [5] Early detection and progress monitoring and prediction of corrosion in aeronautic Al alloys through calibrated Ultrasonic-CorROSion Sensor application, EU H2020 Grant agreement ID: 864905, https://cordis.europa.eu/project/id/864905 [6] R. Oltra and F.Peltier, Laboratory-scale testing of the anti-corrosion effectiveness of a primer coating on a bare AA2024 surface, Surface Interface Analysis, 48 (2016) 775–779.

Characterization of the Surface Chemistry of Corroded and Non-Corroded Sites on Aluminum Alloys 2024-T3 and 7075-T6 Exposed at a Coastal Location

The last few decades have seen an improvement in the characterization of bare metal surfaces undergoing atmospheric corrosion, leading to a better understanding and consideration of the environmental parameters involved. However, in the quest for more reliable accelerated laboratory corrosion test methods, knowledge of the corrosion processes and the role that the environmental parameters play in those processes is critical in replicating the field exposure environment. The corrosion process occurs within a multiphase system that is rather complex involving chemical reactions and equilibria, ionic transport phenomena, and gaseous, aqueous and solid phases.1 Various corrosion products, specific to the metallic substrates in the system, and the corrosive species present (anions, cations, acidic and basic salts, particulates, etc.) which interact with each other all vary in amounts and residence time. It is well documented that corrosion behaviors of metal substrates in accelerated laboratory testsdo not correlate with the observed behaviors in an outdoor exposure environment. 2-6 Atmospheric constituents found on metal surfaces are a function of the atmosphere itself, with sulfates, nitrates, nitrites, chlorides, carbonates, hydrogen ions, ammonium, metal ions, particulates and organic compounds commonly found in electrolyte chemistries or corrosion layers. Each of these have an effect on the corrosion processes on exposed surfaces.7 These result either directly from the deposition processes or from aqueous phase reactions of the deposited atmospheric constituents. Sulfates, chlorides and cations such as sodium, magnesium, calcium, potassium and bromine can originate either directly from the wet deposition process, from particle deposition or from aerosol reactions of the gaseous air pollutants in the aqueous phase of the adsorbed electrolyte.1 In the present work, a matrix of epoxy-mounted AA2024-T3 and AA7075-T6 alloy samples were exposed to outdoor ambient environmental conditions at the Naval Research Laboratory-Key West (NRL-KW) facility for 18 months. Samples were retrieved for analysis at 3 month intervals. The field exposed samples were subjected to SEM/EDS for image and elemental analysis using a Zeiss Environmental Model EVO-50XVP scanning electron microscope integrated with an Energy Dispersive Analysis X-Ray (EDAX) Genesis 2000 EDS system. Images of the metal surfaces and compositional elemental data for the control (non-exposed) alloy samples were compared to the field exposed metal alloy samples in both the “as received” condition and after cleaning by sonication in deionized (DI) water. Pitted and non-pitted sites on each sample were analyzed for compositional elements of the alloy as well as non-compositional (i.e. environmentally-derived) elements. Any elements remaining on the surfaces after cleaning were considered to be well adhered, non-water-soluble corrosion products. The resulting concentrations of various environmental elements and their presence within or outside of pit sites were calculated as a percentage of their composition in natural seawater at NRL-KW and entered into a database. It was found that the elements chlorine, sulfur, potassium and calcium were present at higher concentrations on the metal surfaces than in natural seawater at NRL-KW; it was also found that sulfur concentrations were higher in the pits of the AA7075-T6 metal substrates than the surrounding non-pitted surfaces (Figure 1). Data analytics was performed to determine what environmental factors, if any, could be correlated with the observed presence of elements derived from atmospheric-seawater interaction and deposited onto the metal substrates. This work is currently supporting the development of an accelerated corrosion test protocol on identical bare metal alloy specimens using a modified laboratory atmospheric exposure chamber.References Cited Oesch, M. Faller, “Environmental Effects on Materials: The Effect of the Air Pollutants SO2, NO2, NO and O3 on the Corrosion of Copper, Zinc and Aluminum. A Short Literature Survey and Results of Laboratory Exposures,” Corrosion Science 39:9 (1997): p. 1505-1530. 2. Y. Wan, E. Macha, R. Kelly, “Modification of ASTM B117 Salt Spray Corrosion Test and Its Correlation to Field Measuremnets of Silver Corrosion,” Corrosion 68:3 (2012): p. 036001-1 to 036001-10. 3. Z. Feng, G. Frankel, W. Abbott, C. Matzdorf, “Galvanic Attack of Coated Al Alloy Panels in Laboratory and Field Exposure,” Corrosion 72:3 (2016): p. 342-355. 4. A. King, B. Kannan, J. Scully, “Environmental Degradation of a Mg-Rich Primer in Selected Field and Laboratory Environments: Part 1-Without a Topcoat,” Corrosion 70:5 (2014): p. 512-535. 5. D. Liang, H.C. Allen, G.S. Frankel, Z.Y. Chen, R.G. Kelly, Y. Wu, B.E. Wyslouzil, “Effects of Sodium Chloride Particles, Ozone, UV and Relative Humidity on Atmospheric Corrosion of Silver,” J. Electrochem. Soc. 157:4 (2010): p. C146-C156. Figure 1

Effect of Al alloy back panel on the dynamic penetration and damage characteristics of ceramic composite armors

This paper mainly focuses on the investigation of dynamic penetration and damage characteristics of a hybrid ceramic composite armor normally impacted by 12.7 mm armor piercing incendiary projectiles. The hybrid ceramic composite armor was composed of a ceramic cylinder layer, a Ti–6Al–4V plate, an ultrahigh molecular weight polyethylene (UHMWPE) composite layer, and an Al alloy panel. Three different areal densities of composite laminates with 82, 87, and 92 kg/m2 were tested. 3D finite element model of the ceramic composite armor was generated in ABAQUS, and the simulation results were employed to study the damage evolution. The effect of alumina ceramic cylinders layer on the ballistic performance and the failure mechanisms of Ti–6Al–4V and UHMWPE after ballistic impact were examined by experimental and simulative results. According to the numerical and analytical models, an optimal thickness range of Al alloy back panel was found in minimizing areal density of the ceramic composite armor.

Fundamental Study of Corrosion Preventive Compounds: Part II—Effects on Galvanic Corrosion of Coated Al Alloy Panels Coupled to Noble Fasteners

The effects of corrosion preventive compounds (CPCs) on galvanic corrosion of coated Al alloy panels coupled to noble material fasteners were studied using a test specimen consisting of a painted AA7075-T6 panel and uncoated through-hole noble material fasteners. This specimen provides accelerated degradation of multi-layered coatings through galvanic attack. Homemade hard film CPCs formulated based on the general composition of a commercial hard film CPC were applied on the test specimens. The galvanic current between the panel and each fastener was monitored using a zero-resistance ammeter during exposure of the test panels in an ASTM B117 chamber for 3 weeks. The corrosion morphology, depth, and volume of the test panels after exposure and removal of the coating systems were investigated using optical profilometry (OP). The equivalent mass losses determined by two means, OP analysis and charge calculations by integration of galvanic currents with exposure time, were compared. For comparison with the homemade CPCs, test specimens without CPC and with commercial hard and soft film CPCs were also studied. CPCs reduced the galvanic current to a large extent and correspondingly mitigated galvanic attack. In particular, the commercial CPCs and a homemade CPC without corrosion inhibitor exhibited the best performance. This homemade CPC was also applied on a pre-corroded test panel. The galvanic current decreased after the CPC was applied on the pre-corroded test panel, indicating that the homemade CPC inhibited both the initiation and growth of galvanic corrosion.

Accelerated Corrosion Degradation of Coated Aluminum Alloy Systems through Galvanic Interactions

The use of structural aluminum alloys for aircraft is common due to its low density and relatively high strength. While exhibiting significant general corrosion resistance, these alloys are susceptible to various forms of localized corrosion, such as pitting, intergranular corrosion, etc. When in galvanic contact with fasteners noble to the aluminum alloy panel, these phenomena can be much more aggressive. In an effort to mitigate these problems, multi-layer coating systems are utilized to protect the aluminum alloy from aggressive environments and to prevent galvanic contact with dissimilar metals. However, defects in these coatings may lead to galvanic interaction with the fastener and localized attack of the underlying panel. In addition, delamination of coating at these sites can result in accelerated attack as the environment becomes more aggressive in these shielded areas. Previous work for this project has focused on the galvanic interaction of aluminum alloy 7075-T6 panels with stainless steel 316 fasteners when coated with either a chromated (chromate conversion coating, chromate-rich primer) or non-chromated (adhesion promoting pretreatment, praseodymium-rich primer) system in two environments: immersion in five weight percent sodium chloride and a continuous fog salt spray chamber. A test panel configuration using a scribed Al alloy panel and isolated fasteners was used. Galvanic current data can be acquired during exposure to determine total charge passed due to galvanic interaction between the fastener and panel for the purpose of approximating the extent of aluminum dissolution and comparison of the two coating systems. Post-exposure optical profilometry has been conducted on samples to gain insight on morphology of the attack, as well as to quantify the corroded volume of material. Analyses of this galvanic current data and morphology have been coupled to survey what coating strategy could be more beneficial. Current work focuses on identifying the susceptibility of certain areas of the test panel to localized attack and development of a damage function for galvanic corrosion degradation in coated aluminum alloy systems. Coating performance can be compromised at the fastener hole due to mechanical damage from the fastener and from inadequate coating coverage. Electrochemical impedance spectroscopy (EIS) has been utilized to determine the extent of coating protection at the fastener hole and assess the effects of defects that may serve as initiation sites for accelerated attack. Various fastener hole conditions have been compared to the protection afforded by the coating in areas away from the fastener hole. Impressed current has also been used to determine the feasibility of attack initiating when the unintentional coating defects within the fastener hole are not a factor. This process involves current being passed from a counter electrode to a panel without fasteners in solution, allowing control of the amount of charge passed, as well as the rate at which it is passed. With the absence of fasteners, current may be passed to any defect, regardless of proximity to the fastener hole. This technique of supplying current via a potentiostat can also be used in conjunction with a controlled, artificial defect (scribe) as a means to compare various coating systems. Contrary to the aforementioned technique, during this galvanostatic stressing, a fastener is used as the counter electrode and the exposure is carried out in a salt spray chamber, not in bulk solution. This allows for analysis of the coating systems under wetting and drying cycles that more closely resemble in-service conditions, while utilizing impressed current to better control and accelerate the charge passed from fastener to panel compared to simple exposure of galvanic panels. Post-exposure, optical profilometry analysis will be conducted to ascertain volume, depth, and area of attack, which may then be utilized in the development of damage functions for galvanic corrosion degradation based on a variety of variables. Acknowledgement: This work was supported by the Office of Naval Research through award number N00014-13-1-0738.

Galvanic Attack of Coated Al Alloy Panels in Laboratory and Field Exposure

A galvanic test panel design incorporating a painted and scribed Al alloy panel and uncoated through-hole noble fasteners has recently been utilized to compare effects of different surface pretreat...

Effect of Surface Pretreatment on Galvanic Attack of Coated Al Alloy Panels

A test assembly consisting of a coated and scribed aluminum alloy 7075-T6 panel galvanically connected to uncoated through-hole noble fasteners has recently been utilized to quantify corrosion attack during corrosion testing in laboratory chambers. In this work, the corrosion morphology and extent of attack of Al alloy panels with five different surface pretreatments prior to painting and scribing were compared after exposure to ASTM B117 with galvanic connection to uncoated stainless steel fasteners. The nature of the attack for samples with different surface pretreatment samples was found to exhibit two different trends: they either penetrated deeply into the substrate at the scribes or spread out under coatings. The galvanic currents between the coated Al alloy panel and the bare 316 stainless steel fasteners were monitored during 21 days exposure in an ASTM B117 chamber. The current trends measured for different surface pretreatment panels were consistent with optical profilometry (OP) results. However, the galvanic current and OP techniques both underestimated the extent of corrosion attack because of local H2 evolution and undercut attack, respectively. The results from both techniques were modified to compensate for these deficiencies. Acceleration factors associated with the galvanic interaction for the different surface pretreatment panels were determined.

Galvanic Test Panels for Accelerated Corrosion Testing of Coated Al Alloys: Part 2—Measurement of Galvanic Interaction

A test sample incorporating a painted Al alloy panel, uncoated through-hole fasteners, and scribes has recently been shown to provide an accelerated response during atmospheric corrosion testing in...

Quantification of Accelerated Corrosion Testing of Coated AA7075-T6

A test sample incorporating a painted Al alloy panel, uncoated through-hole fasteners, and scribes has recently been shown to provide accelerated response during atmospheric corrosion testing in the field and in laboratory chambers. In this paper, the galvanic current of an AA7075-T6 panel coupled with mixed SS316 and Ti-6Al-4 V fasteners was monitored using a zero-resistance ammeter during 3 weeks exposure in an ASTM B117 chamber or immersed in 5 wt% NaCl solution. SS316 fasteners provided more cathodic current than Ti in both environments and the current in ASTM B117 is higher than in 5 wt% NaCl solution due to greater oxygen availability. The integral of the anodic current with time and optical profilometery (OP) analysis were used to assess the corrosion attack quantitatively for two different coating systems. An acceleration factor was defined to represent the extent of accelerated corrosion for galvanically-connected fasteners. The acceleration factors were in the range of 20–50 for panels with SS316 fasteners and two different coating systems, both with and without a topcoat. The effects of SS316 fasteners were similar for the different coating systems even though the attack morphology was very different.

Galvanic Test Panels for Accelerated Corrosion Testing of Coated Al Alloys: Part 1—Concept

A test sample incorporating a coated and scribed Al alloy panel and uncoated through-hole fasteners was designed to provide accelerated response during atmospheric corrosion testing in the field or in laboratory chambers. Protective coatings that would take many months to show signs of degradation on normal scribed samples with no galvanic connection exhibited corrosion in much shorter times when uncoated Ti alloy or stainless steel fasteners were used, particularly in conjunction with scribes through the coating. The results after three weeks of exposure to ASTM B117 correlated well with long-term beach exposure, allowing rapid ranking of different coating systems. Of the fastener materials studied, visual observation indicated that the worst attack occurred near Type 316 (UNS S31600) stainless steel (SS316) fasteners even though Ti-6Al-4V fasteners are more noble. The volume of AA2024-T3 (UNS A92024) lost during the exposure, as determined by an optical profilometer, was also largest for samples with SS316 fasteners. Scanning Kelvin probe measurements across the scribes on one of the coating systems showed that, after exposure in an ASTM B117 salt fog chamber, the potential was lowest for scribes next to SS316 fasteners. The difference between SS316 and Ti-6Al-4V was explained by the available cathodic current, which was measured in chloride solution.

Model Study Investigating the Role of Interfacial Factors in Electrochemical Impedance Spectroscopy Measurements

Abstract To observe the effect of interfacial factors on electrochemical impedance spectroscopy (EIS), model systems consisting of Parylene C (PC) film coated on Alclad 7075-T6 (UNS A97075) sheets and freestanding PC films were investigated. To modify the top surface of PC coating, the coating on Al alloy panels were treated with two plasmas: trimethylsilane (TMS) and a mixture of TMS + O2. Plasma polymerization of TMS followed by hexafluoroethane (HFE) also was applied to the substrate surface prior to the deposition of PC film to improve the adhesion of the film to the surface. Coating performance was evaluated using EIS, and interfacial factors produced by plasma treatments were studied by applying suitable equivalent circuit models, which take into account all the interfacial parameters and bulk coating. Simulations performed with circuit models proposed in this work indicated that top surface modification with plasma treatment influenced the salt intrusion property of the top layer as well as of the ...