Conversion Coatings For Aluminum Alloys By Chemical Vapor Deposition Mechanisms

With the rise of environmental awareness and the renewed importance of environmentally friendly processes, the United States Environmental Protection Agency has targeted surface pre-treatment processes based on chromates. Indeed, this process has been subject to regulations under the Clean Water Act as well as other environmental initiatives, and there is today a marked movement to phase the process out in the near future. Therefore, there is a clear need for new advances in coating technology that could provide practical options for replacing present industrial practices. Depending on the final application, such coatings might be required to be resistant to corrosion, act as chemically resistant coatings, or both. This research examined a chemical vapor deposition (CVD) mechanism to deposit uniform conversion coatings onto aluminum alloy substrates. Robust protocols based on solutions of aryl phosphate ester and multi-oxide conversion coating (submicron) films were successfully grown onto the aluminum alloy samples. These films were characterized by X-ray Photoelectron Spectroscopy (XPS). Preliminary results indicate the potential of this technology to replace aqueous-based chromate processes. Reye, John T. and McFadden, Lisa S. and Gatica, Jorge E. and Morales, Wilfredo Glenn Research Center NASA/TM-2004-212905, E-14328

Providing in-depth coverage of the technologies and various approaches, Luminous Chemical Vapor Deposition and Interface Engineering showcases the development and utilization of LCVD procedures in industrial scale applications. It offers a wide range of examples, case studies, and recommendations for clear understanding of this innovative science.

This book is a guide to all new and presently existing processes available to chemically modify the surfaces of industrially used metals. The modifications described here will produce hard scratch-resistant surfaces, corrosion-resistant surfaces, and surfaces that will easily accept applied coatings, such as industrial paints. Included in the book are processes for aluminum, magnesium, titanium, iron, copper, and silver and their respective alloys, as well as a number of other metals and their related alloys.

"An autodeposition coating process was developed for aluminum alloys; which has different resin chemistry and deposition mechanism as compared to the autodeposition process reported to date. Autodeposition of organic polymer films onto aluminum 7075 alloy surface was developed with formation of an in-situ conversion coating underneath the organic layer. In the second part of the project, autodeposition coating process for aluminum alloy was developed with out the use of oxidizing agents in coating bath. The deposition mechanism was depended on destabilization of polymer latex with manganese cations generated from a previously formed manganese oxide layer. Miniemulsion polymerization method was used to synthesize epoxy-polyurethane-acrylic hybrid latexes. In third part of the study, a free radically polymerizable carbonate functional macromonomer was synthesized from epoxidized soybean oil by reaction with carbon dioxide followed by transesterification with hydroxyl ethyl methacrylate. Copolymers containing the macromonomer were synthesized through a miniemulsion approach. Cross-linking reactions of these carbonate functional copolymers with diamines to produce a hydroxyl functional polyurethane net work were studied. In the final part of the study we have investigated conversion of the soybean oil or its methyl ester into free radically polymerizable monomer. Soy acrylate was prepared by base catalyzed transesterification reactions of soybean oil or methyl soyate with hydroxyl ethyl methacrylate (HEMA)"--Abstract, leaf iv.

This dissertation focuses on a fundamental understanding of the formation mechanism, chemical structure, and basic electrochemical properties of the TCP coating on three high strength aluminum alloys: AA2024-T3, AA6061-T6, and AA7075-T6. The formation of the TCP coating is driven by an increase in the interfacial pH. The coating is about 50-100 nm thick and has a biphasic structure consisting of a ZrO 2 /Cr(OH)3 top layer and an AlF63- /Al(OH)3 interfacial layer. The coating contains hydrated channels and or defects. -- Abstract.

Abstract: Chromate conversion coatings (CCCs) are applied to aluminum alloys to enhance their resistance to localized corrosion and to increase paint adhesion. However, chromate is toxic and suspected carcinogen. To develop environmentally friendly alternative coatings, a detailed and accurate understanding of CCC formation and breakdown is needed. Several studies on CCC formation and breakdown were conducted in this regard. A first set of experiments was aimed at studying CCC formation and breakdown on 25-element Al electrode arrays. The coating process occurs in two stages. The first stage is characterized by intense electrochemical activity on the array and last from 20 to 30 seconds. The second stage occurs under electrochemical quiescence and little measurable current flows among elements in the electrode array. Raman spectroscopy shows that the coating continues to adsorb Cr6+. Anodic polarization of conversion coated arrays in chloride solutions led to several important findings. It was found that pitting potential increases as coating time increases through both stage one and stage two. Changes in coating structure and chemistry occur during the electrochemically quiescent second stage of coating formation. Pitting potentials were higher on electrode elements that were net cathodes during first stage CCC formation than on electrode elements that were net anodes. Related experiments were conducted by forming CCCs on electrode arrays in conversion coating baths where the activating agent, NaF, and the accelerating agent K3Fe(CN)6 were withheld either individually or together. Coatings formed in these modified solutions were then subject to anodic polarization in chloride solution. These supplemental ingredients are essential to CCC formation and contribute greatly to increasing the corrosion protection provided by the coating. A second set of experiments characterized the effect of aging on CCC structure and properties. CCCs continue to polymerize after they are removed from the coating bath. Using cathodic polarization experiments carried out in aerated chloride solutions, it was found that CCCs less than 48 hours old inhibited cathodic reactions. With increased aging time in ambient lab air, cathodic inhibition was lost. a loss attributed to coating dehydration and continued polymerization, which led in turn to the development of shrinkage cracking and loss in Cr6+ leachability. The relative humidity of the environment in which coatings aged also had a significant effect on the CCC aging process. electrochemical testing. CCCs aged in ambient lab air (RH 50%) exhibited less shrinkage cracking, a 2-order of magnitude increase in Cr6+ release, and considerably greater corrosion resistance.

The aerospace industry uses a variety of metal alloys as structural components in aircraft. Corrosion and degradation of these metals in service is typically inhibited using a multi-layer coating system (conversion coating + primer + topcoat). Historically used conversion coatings and primers contain Cr(VI) as the active corrosion inhibitor. Due to human health and environmental concerns, there is a world-wide effort to eliminate chromate from use and to replace these coatings with more environment-friendly and equally-as-effective coatings. The trivalent chromium process (TCP) conversion coating is the leading replacement candidate. A challenge with implementing this conversion coating for protecting high Cu aluminum alloys is that it is not as good as the legacy chromate coatings for reasons that remain poorly understood. In this dissertation project, fundamental research was conducted to better understand how to improve the anti-corrosion properties of TCP on AA2024-T3, a high Cu containing aerospace alloy. A focus was on understanding system-level interactions between the coating and alloy surface, and how it correlates with early-stage failure of alloys. The overall objective of this dissertation project was to improve the corrosion performance of TCP conversion coatings through: (i) process parameter optimization; (ii) a better understanding of early-stage failure mechanisms, specifically, the sites where alloy corrosion initiates and the coating composition and structure on and around these sites. The final chapters of this dissertation also focus on the use of TCP as a sealant for surface modified (anodized) AA2024-T3 and comparing the effectiveness of a TCP seal against other industrially used sealing methods.

The aerospace industry uses a variety of metals and alloys, primarily aluminum alloys, for the different structural components in aircraft including the fuselage, landing gear, tail fins, and the many other parts. As these components are metal based, the control and mitigation of corrosion in service is of paramount importance. The military and civilian aviation sectors spend considerable sums of money annually on corrosion prevention and maintenance. Components made with aluminum alloys are generally placed in service with a multilayer coating system to prevent corrosion. This coating system consist of a conversion coating, primer, and topcoat. The coating system can inhibit corrosion in multiple ways, but the general mechanism involves barrier layer protection that reduces contact of the environment with the underlying metal. Legacy conversion coatings and primers have chromate (Cr(VI)) as a component. While chromate is an excellent corrosion inhibitor, it is toxic and constitute a significant environmental hazard. There is a current technological need to (i) replace chromate conversion coatings and primers with nonchromateor zero-chrome coating systems and (ii) understand how to properly pretreat the aluminum alloys surfaces for application of such surface finishes. The trivalent chromium process (TCP) coating is the leading replacement non-chromate conversion coating and praseodymium and new aluminum-based coatings are replacement primers being investigated. There is also a scientific need to better understand how to properly pretreat aluminum alloys in order to properly form conversion coatings and primers that effectively prevent environmental degradation and corrosion. These surface pretreatments typically include abrasion and polishing, wet chemical cleaning, and deoxidation or desmutting.In this dissertation project, fundamental research was conducted to better understand how surface pretreatments of aluminum alloys impact the formation of TCP conversion coatings and the mechanisms by which TCP conversion coatings inhibit electrochemical corrosion in laboratory measurements and during accelerated degradation testing. Research was also conducted to learn how effectively TCP coatings can seal porous anodic oxide coatings on aluminum alloys thereby improving the barrier properties and electrochemical corrosion resistance. The specific surface pretreatments investigated included laser cleaning and hyperpassivation of aluminum alloyAA2024-T3, in comparison with conventional wet chemical processing. Additionally, studies were performed to learn the mechanisms and effectiveness of TCP sealants for anodic coatings formed on this aluminum alloy during sulfuric acid (SA) and sulfuric acid/boric acid (SABA)anodization.