Corrosion Protection Provided By Trivalent Chromium Process Conversion Coatings On Aluminum Alloys

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.

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.

The exposure and EIS characterization of the chromate-free coatings systems enabled a ranking of the coatings systems in terms of corrosion protection provided. Coating systems were ranked according to several different methods described in the literature. Among the coatings evaluated, Deft 02GN084, a high solids, solvent-borne and Pr-containing primer coating showed best protection when used in conjunction with a number of different conversion coatings and surface pretreatments. Several different trivalent chromium conversion coatings and pretreatment were used. This general type of conversion coating appeared to provide better corrosion protection than other pretreatments whose functions were primarily surface cleaning or adhesion promotion.

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.

Chromium conversion coatings (CCC) based on Cr(VI) are widely used enhance corrosion resistance of aluminum alloys. However, the conventional CCC system consists of various Cr(VI) compounds, which are toxic and carcinogenic, leading to increasingly restricted usage. The Trivalent Chromium Process (TCP), for example, has proven to be a promising alternative to CCC. Although significant research on the application and performance of TCP has been done, questions such as the film microstructure, the nature of the TCP/substrate interface and the evolution of the TCP film in response to different environments have not been answered. Neutron reflectivity (NR) and x-ray reflectivity (XRR) are used to determine the structure and composition of TCP films on aluminum alloy 2024-T3. A new electro-assisted deposition method was designed. Quantitative NR and XRR analysis confirmed linear film growth. The film composition was determined to be Cr2O3·iH2O·x(ZrO2·jH2O) (i =2.10 ± 0.55, j = 1.60 ± 0.45 and x = 0.85 ± 0.14). In-situ neutron reflectivity was used to observe the structure and evolution of a TCP passive film with a simplified formulation on Al in a NaCl-D2O solution. We observed the evolution of the TCP film on the Al anode and compared the degradation of the Al with and without TCP protection. A dramatic improvement in corrosion resistance of AA2024-T3 is achieved by anodic hardening TCP passive films with Ce(III). The anodic current density is suppressed by the factor of 500 after exposure in the presence of Ce(III) at potentials in metastable pitting region ( -580 mV vs. SCE for 4

Inorganic polycrystalline hydrotalcite, Li2[Al2(OH)6]2·CO3·3H2O, coatings can be formed on aluminum and aluminum alloys by exposure to alkaline lithium carbonate solutions. This process is conducted using methods similar to traditional chromate conversion coating procedures, but does not use or produce toxic chemicals. The coating provides anodic protection and delays the onset of pitting during anodic polarization. Cathodic reactions are also inhibited which may also contribute to corrosion protection. Recent studies have shown that corrosion resistance can be increased by sealing hydrotalcite coated surfaces to transition metal salt solutions including Ce(NO3)3, KMnO4 and Na2MoO4. Results from these studies are also reported.

Abstract: Chromate conversion coatings (CCCs) have been employed in the surface finishing process for AA2024-T3 for their excellent ability to resist localized corrosion and to promote paint adhesion. However, due to the toxic effects of chromium compounds, a significant amount of effort has been extended to develop alternative corrosion inhibitor systems. Trivalent Chrome Process (TCP) coatings recently have gained wide acceptance and are considered an environmentally friendly replacement for chromate conversion coating, because the TCP bath and the resulting film contain no Cr (VI) species.

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 objective of this project was to develop an environmentally compliant conversion coating for use on aerospace aluminum alloys (e.g., AA2024-T3). This conversion coating was to replace the current chromate conversion coating processes in both mode of application (bath or spray applied in the depot) and function (stand alone corrosion protection, adhesion to organic layers, self-healing, and low electrical contact resistance). Hydrotalcite (HT) was developed within this program as a replacement to chromate conversion coatings. HT coatings are formed by exposure of aluminum and its alloys to alkaline lithium salt solutions. The coating chemistry used to form these conversion coatings has many processing variables (e.g., time, temperature, anion, etc.). A Fractional Factorial Design was used to determine that temperature was one the more critical processing variables. The FFD study also determined that HT coatings formed from nitrate-based chemistries had consistently better stand-alone corrosion protection properties. Through the use of additional oxidants within the coating bath, HT coatings with the ability to withstand 168 hours of salt spray could be formed in less than 6 min. HT conversion coatings could also be post-treated (e.g., hydrothermally aged, surfactant) to revert the hydrotalcite to aluminum oxide, or augmented to include high valence-state rare earth cations (e.g., cerium). Hydrothermal aging allowed a procedure to chemically anodize aluminum, while incorporation of cerium into the molecular gallery of the hydrotalcite structure provided a means to develop self-healing characteristics, a highly sought property characteristic of chromate-based coatings. Self- healing was indeed demonstrated by the cerium doped HT coatings. The adhesion of epoxy coatings to the hydrotalcite coating was studied in detail. The Lewis-base nature of HTs makes them intrinsically less able to be wet by the Lewis- base nature of epoxy.