CHARACTERIZATION OF OXIDE LAYERS FORMED ON ALUMINUM ALLOYS DURING NEW PPG ELECTRODEPOSITED STRUCTURAL PAINT ECODESIGN - GRANT AGREEMENT N 267285 Dr Marion Collinet Ecole Nationale Supérieure de Chimie de Lille UMET Dr Didier Labouche PPG Aerospace Europe Bringing innovation to the surface. INTRODUCTION TO PPG AEROCRON TECHNOLOGY PPG has been deeply involved in the development of a breakthrough concept in order to protect structural components of aircrafts against electrochemical corrosion. That concept is based on the electrodeposition of a REACh compliant anaphoretic paint which has been especially designed for the alloys commonly used in Aerospace. The electrodeposition process of our AEROCRON technology is described hereafter. When an electrical field is applied, and in parallel to water electrolysis, negatively charged paint particles move to the anode which is the part to be painted. The electro osmosis process leaves a paint film deposited at the surface which contains ar. 5% of water. A thermal cure is then applied to obtain final properties of the paint film.
This new concept will provide many manufacturing process advantages compared to conventional sprayable primers like: Conventional Solvent Spray Process Use of PPE required Labor intensive Increased waste disposal Increased worker exposure Over spray / material usage Air cure dry time restrictions Standard Electrocoat Process Aqueousbased coating chrome free Automated process minimizes worker exposure No over spray / increased material usage Consistent film build over surface (complex shapes) overall weight reduction Thermal cure part fully cured after bake The design of AEROCRON allows the elimination of the current anodizing pretreatment. A typical process consists of: Alkaline degreasing Hot water cleaning Acid pickling Deionized water cleaning Paint electrodeposition (*) Thermal curing 3mn at 11 C (*) in order to achieve 2 microns of dry film thickness, a standard lab process consists of applying 12volts during 2mn (ramp up time included). A high corrosion protection is achieved on such surface preparation even in the absence of anodizing pretreatment. The corrosion test results obtained are significantly above the existing specifications with 2microns of dry film thickness. Standard test duration is 96hours for filiform corrosion test and 3hours for Neutral Salt Spray test. In our case, even after tripling exposure to filiform cabinet and doubling it for Neutral Salt Spray, we obtained a very high resistance to corrosion as shown hereafter: 3 hours Filiform test EN3665 15 hours Alternate Emersion Imersion 6 hours Neutral Salt Spray ISO9227 Application Substrate (*) Length average } Clad 224-T3 1mm Anaphoretic paint >> Bare 224-T3 <,5mm (2 microns) Clad 224-T3,5mm Anaphoretic paint (2 microns) Bare 224-T3 <,5mm Clad 224-T3,5mm Anaphoretic paint (2 microns) Bare 224-T3 <,5mm } >>above above current spec current spec *Length filament average done on 6 panels
Due to the nature of our technology, we already know that an oxide layer is formed at the surface of the aluminum alloy concomitant to the paint electrodeposition: TEM 5nm That oxide layer certainly plays a significant role in the final properties of the coating. In the following example we have compared, with electrochemical impedance spectroscopy (EIS), the evolution of Rt charges transfer resistance of the oxide layer formed on 775 clad during paint electrodeposition vs 775 clad which was only solvent degreased. The anaphoretic paint was deposited at two different voltages V1 and V2. The paint applied was voluntarily not cured and was totally removed from the surface in order to compare properties of oxide layers only. There is evidence of a significant contribution to Rt -and by consequence to the corrosion protectionof the oxide layers formed during the anaphoretic paint electrodeposition process. EIS- NaCl 5% - Rt charges tranfer resistance as function of time of immersion 6E+5 4E+5 2E+5 E+ Charge transfer resistance (Ω.cm²) 9,8E+5 1 3 7 14 immersion time (days) 775 Clad degreased 775 Clad Voltage V1 775 Clad Voltage V2 Clad metal was only cleaned with solvent Clad metal e-coated at voltage V1 E-coat paint not cured, then stripped and panel solvent cleaned Clad metal e-coated at voltage V2 E-coat paint not cured,then stripped and panel solvent cleaned
PPG has been granted by Clean Sky (Grant Agreement n 267285) in particular for understanding some fundamental aspects of our anaphoretic paint and for the characterization of the oxide layer formed during the paint electrodeposition process. We have been actively working on that subject with our partner Ecole Nationale Supérieure de Chimie de Lille (France). Clean Sky is the most ambitious aeronautical research programme ever launched in Europe. Its mission is to develop breakthrough technologies to significantly increase the environmental performances of airplanes and air transport, resulting in less noisy and more fuel efficient aircraft, hence bringing a key contribution in achieving the Single European Sky environmental objectives. OXIDE LAYER - CHARACTERIZATION The research strategy consisted in developing several characterization methods based on the various analytical techniques available, then applying the most relevant and reliable ones to the study of the oxide layer and the parameters that could influence its growth. Together with the development of characterization methods based on conventional techniques like FEG-SEM and TEM, we have developed a method based on Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) for a deep characterization of the oxide layer. FEG-SEM and TEM analysis provided clear images of the coating/aluminum interface and allowed us to determine thickness and structure of the interfacial oxide layer. We observed that the oxide layer formed during the paint electrodeposition on different aluminum alloys was composed of a dense sub-layer on the substrate side covered by oxide filaments on the coating side: Interpenetration of coating and oxide filaments contributes to the strong coating adhesion observed for AEROCRON. However, these techniques require time consuming specific and complex sample preparations. In the case of FEG-SEM analysis, correct observation of the oxide layer was achieved on cross sections prepared with a triple ion beam cutter (TIC 3X from Leica) that provides high quality crosssections within 1-12 hours per.8mm-thick sample. TEM analysis required a sample preparation by ultra-microtomy, a technique that requires a high level of skill and know-how. Interesting data are obviously obtained with these techniques but they are not suitable for the study of numerous samples. ToF-SIMS method appeared particularly attractive for this purpose.
ToF-SIMS is a powerful and sensitive technique for surface analysis. Combined with in-situ sputtering it permits simultaneous measurement of many elements during one single positive or negative depth profile, with a good depth resolution, and is particularly suitable for analyzing multilayer systems. The effectiveness of using ToF-SIMS analysis for the characterization of the oxide layer formed during paint electrodeposition is described hereafter through the determination of the oxide layer s thicknesses. The following graph shows the ToF SIMS depth profiles obtained for the Al 224 clad specimen after the paint electrodeposition. The sample preparation for ToF SIMS analysis is very simple. It only consists of removing the coating part (voluntarily not cured) from the panel by solvent cleaning and then cutting a sample of the panel for analysis. We can differentiate in the depth profiles the oxide layer part, which can be characterized by an intense AlO - 2 signal, and the substrate core that can be characterized by an intense Al - 3 signal. intensity 25 2 15 1 5 Al_3-2 4 6 8 1 sputter time (s) A calibration of the sputtering rate was necessary to convert the sputter time to depth in order to determine the oxide layer thicknesses of the different specimens. 25 2 15 intensity 1 5 Al_3- AlO_2- AlO_2-1 2 3 4 5 depth (nm) The oxide layer thickness is then deduced from these calibrated depth profiles. In this example it is [2 ± 3] nm determined from the 1st derivative of the smoothed Al - 3 profile.
ToF-SIMS depth profiling combined with high lateral resolution imaging mode also provides information about both the 2D distribution of the elements along depth and allows the 2D and 3D reconstruction of the sample section. ToF-SIMS has been applied for the characterization of the oxide layer formed on several substrates: 224 clad, 224 bare, 775 clad and 775 bare. A typical paint electrodeposition process was used as previously described. The paint film was voluntarily not cured and was totally removed from the surface. In order to validate the results obtained from ToF-SIMS, the oxide layer thicknesses have been compared to those obtained from TEM micrographs. They are in good agreement. ToF-SIMS TEM 1 nm 18 (± 2) nm 2 (± 3) nm 224 clad 9 (± 3) nm 75 (± 2) nm 224 bare 22 (± 3) nm 2 (± 2) nm 775 clad 13 (± 3) nm 11 (± 2) nm 775 bare
Example of ToF-SIMS practical application In the following example of the use of ToF-SIMS, the goal was to study the influence of the time of paint electrodeposition on the thickness of the oxide layer formed. The voltage was fixed at 12volts and the ramp up time was adjusted at 3s. Different samples were taken at different times of electrodeposition. As previously described, the paint was not cured and was totally removed from the surface. ToF-SIMS analysis was launched and allowed a quick determination of the evolution of the thickness as a function of time of electrodeposition. 25 voltage (V) 12 Oxide thickness (nm) 2 15 1 5 5 1 15 time (s) 2 25 3 Certainly TEM would continue to provide complementary information with regard to the morphology of the oxide layer as seen hereafter, where we can see that the filament part of the oxide forms first when the voltage is applied and that the dense part then grows over time. 25 voltage (V) 12 Oxide thickness (nm) 2 15 1 5 5 1 15 time (s) 2 25 3
CONCLUSION We are currently conducting many process trials as part of the industrialization of our AEROCRON technology Different variables have to be screened like: Degreasing type and process Acid pickling type and process Nature of the alloy Ramp up time Voltage Bath conductivity Ageing. This represents thousands of panels The characterization of the oxide layer formed with different process conditions is important to understand. FEG-SEM and TEM can provide reliable info when there is no time constraint When quick information related to the oxide layer is required a new technique has to be used Thanks to Clean Sky Grant Agreement we have developed a method based on ToF-SIMS well adapted to get quick reliable information on a significant number of panels This is now very helpful to support the industrialization of our AEROCRON technology