ICMCTF2004 Session H4: Novel Approaches to Nonchromate Corrosion Protection

Friday, April 23, 2004 8:30 AM in Room Royal Palm 1-3

Friday Morning

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8:30 AM H4-1 Developing and Qualifying Nonchromate Conversion Coating for Aerospace Applications
Joseph Osborne (The Boeing Company)
Chromate conversion coatings (CCC) are widely used on aluminum aerospace hardware to provide corrosion protection and to promote adhesion of organic coatings to the metal substrate. The CCC are used to provide stand-alone corrosion protection during manufacturing, as a corrosion protective conductive film in electrical bonding and grounding connections, and in low corrosion risk areas where weight is a concern. Most CCC are used as part of a finishing system where versatile processing characteristics allow use on a wide variety of applications ranging from detail parts to whole aircraft. The versatility of CCC makes finding a single drop-in replacement nonchromate conversion coating (NCCC) very difficult. Select applications have transitioned to NCCC. This talk will review the design requirements where CCC are used, outline the process by which new materials can be tested and qualified for use on existing vehicles, and give examples of where NCCC have been successfully introduced into production.
9:10 AM H4-3 Non-traditional Methods for Chromate-free Protection of Aerospace Alloys
G.P. Bierwagen, V.J. Gelling, D.E. Tallman (North Dakota State University)
There has been considerable research performed recently addressing the problem of protecting Al-based aerospace alloys from corrosion without the use chromates either in coating pigmentation or in metal pretreatment. We have been addressing this problem here at NDSU since 1996 and have lately been pursuing two non-traditional routes to accomplishing this difficult problem. What is meant in this context as non-traditional are that the protection schemes utilize neither soluble metal oxide pigments for passivation and elimation of cathodic reactions nor do they utilize an acid, passivating bath to form a protective passivating layer on the alloy surface prior to repainting. The two schemes for Cr-free corrosion protection that we have been pursuing are: (1) the use of Mg metal pigment particle for a primer that provides cathodic protection of Al-alloys; and (2) the use of conductive polymer films as primer layers to provide corrosion inhibition and control. Both routes to Cr-free corrosion protection will be described and recent results describing the present status of our efforts will be given.
9:50 AM H4-5 Recent Progress Using "Smart" Corrosion Inhibitors as Replacements for Chromate
M.W. Kendig, M. Hon (Rockwell Scientific Company LLC)
The demand to eliminate hexavalent chromium from corrosion protective coatings has lead, over the years, to a better understanding of the chromate corrosion inhibition mechanism for high strength aluminum alloys. The understanding of the chromate inhibition mechanism suggests novel approaches for alternative coatings. The "active", and perhaps even "smart", release of a corrosion inhibitor, chromate, by chromate conversion coatings suggests a strategy for alternative coatings. This strategy has very recently lead groups at Rockwell Scientific and elsewhere to consider a class of coatings that have the possibility of releasing an inhibitor (oxygen reduction reaction (ORR) inhibitor) depending on the corrosive conditions seen by the environment of the coated surface. As an example, inherently conducting polymers (ICPs) can retain inhibiting ions as dopants that can be released by environmental chemistries through chloride ion exchange and chemical and electrochemical reduction. Other approaches for 'smart' release of an inhibitor as demanded by the environment surrounding the coated surface may also be considered. Such approachs will be reviewed and the Rockwell approach will be presented in detail. .
10:30 AM H4-7 Sol-Gel-Derived Corrosion-protective Coatings with Controllable Release of Incorporated Organic Corrosion Inhibitors
A.N. Khramov (University Technology Corporation); N.N. Voevodin (University of Dayton Research Institute); V.N. Balbyshev (Universal Technology Corporation); M.S. Donley (Air Force Research Laboratory)
Improved corrosion protection performance of sol-gel-derived organic-inorganic hybrid coatings on high-strength aluminum alloy substrates has been achieved by incorporation of potent organic corrosion inhibitors into the coating material. The inclusion of organic corrosion inhibitors has been done as a result of physical entrapment of the inhibitor within coating material by adding it in the coating application solution with subsequent crosslinking and formation of the host network structure. Once trapped within the coating material, the organic corrosion inhibitor becomes active in corrosive electrolyte when the entrapped inhibitor can slowly diffuse out of the host material. However, in order to provide long-term corrosion protection, a sustained release of the corrosion inhibitor assisted by a secondary chemical equilibrium is needed to ensure continuing delivery of the inhibitor to corrosion sites. Several organic corrosion inhibitors, such as mercaptobenzothiazole (MBT), mercaptobenzimidazole (MBI), mercaptobenzimidazolesulfonate (MBISA), and thiosalicylic acid (TSA), were selected as model compounds to investigate two possible strategies for inhibitor storage/delivery - a) anchoring of the inhibitor by the ion-exchange with coating material or b) cyclodextrin-assisted molecular encapsulation. Effectiveness of these two approaches to achieve a sustained release of organic corrosion inhibitors has been discussed.
10:50 AM H4-8 Corrosion/Erosion Properties of Oxide Coatings on Mg Alloys
L. Wang, H. Hu, X. Nie, Y. Ma (University of Windsor, Canada)
The chromate-based corrosion protection on such as aluminum (Al), magnesium (Mg), titanium (Ti) and other alloys has been used with some success. However considering the pollution problem of chrome it is necessary to search an alternative process to replace conventional chromate coating technology. Electrolytic plasma processing (EPP) is an emerging, environmentally-friendly surface engineering technique. The study in this article is to utilize EPP to deposit oxide coatings (Al2O3, MgO or SiO2) on magnesium-aluminum alloys for corrosion and erosion protections. X-ray diffraction (XRD) is used to characterize the coating microstructure. Anodic polarization measurement is employed to investigate corrosion properties of the coated samples. A device is also designed for corrosion and erosion testing. A 3.5% NaCl solution with SiO2 particle (~50 micron) suspension is used as a testing medium in which the treated and untreated samples are immersed and rotated at various speeds. The weight loss after certain revolutions is counted for evaluation of the corrosion/erosion properties. Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX) are used to study surface morphology and corrosion effects after the testing. Keywords: coatings, tribology, plasma, corrosion, oxide.
11:10 AM H4-9 Corrosion Protection of Aluminum Alloys by Al-based Nanostructured Quasicrystal Thin Films
V.N. Balbyshev (Universal Technology Corp.); A.N. Khramov (University of Dayton Research Institute); D.J. King, M.J. Daniels (Technology Assessment & Transfer, Inc.); M.S. Donley (Air Force Research Laboratory)
We have recently shown that magnetron-sputtered Al-based quasicrystal thin film coatings deposited on Al substrates exhibit exceptional corrosion protection properties under exposure conditions closely emulating atmospheric environment. Superior resistance to electrolyte attack makes these coatings strong candidates for corrosion protection surface treatment of Al alloys. Further investigation of these coating systems reveals that elemental composition of the coating material and deposition condition are critical to their corrosion protection performance. Nanostructured quasicrystal films were magnetron sputtered from Al-transition metals targets with various elemental compositions. It was found that quasicrystal films with higher atomic concentration of Al (~80 at.%) outperformed those with lower Al content (~70 at.%) and deposited under the same conditions. Corrosion-related properties of aluminum-based quasicrystal thin film coatings on high-strength Al alloys (AA2024-T3 and AA7075-T6) were studied by electrochemical impedance spectroscopy (EIS) and potentiodynamic scan (PDS) techniques. Mechanism of corrosion protection in relation to microstructure and film morphology has been discussed.
11:30 AM H4-11 Corrosion and Tribological Investigations of CrN Coated, Wet-stripped and Recoated Functional Substrates used for Cutting and Forming Tools
J Housden (Tecvac Ltd., United Kingdom); G.G. Fuentes (Asociación de la Industria Navarra, Spain); N. Watson (Tecvac Ltd., United Kingdom); F Montalá (Tratamiento Tármicos Carreras, Spain); T. Tate (Imperial College, United Kingdom); E. Spain (Tecvac Ltd., United Kingdom); B. Lerga (Asociación de la Industria Navarra, Spain); A. Conde, A. Cristóbal, J. de Damborenea (CENIM-CESIC, Spain); R.J. Rodríguez (Asociación de la Industria Navarra, Spain)
We report recent breakthroughs in the field of wet-stripping processes of Physical Vapour Deposited (PVD) CrN coatings on standard high speed and stainless steels, and hard metal substrates. The investigations have been carried out within the consortium CREST funded by the European Eureka program for R&D. The validation of the stripping processes have been evaluated in terms of the corrosion of the steel substrates after exposure to the chemical agents and also in terms of their tribological properties before and after stripping. In particular, we focus our investigation on the influence of the stripping processes on the hardness, roughness, adherence and wear resistance of the re-deposited CrN coatings after stripping. Analysis of the treated substrates has shown that: a) hardness and elastic modulus of the re-deposited CrN coatings do not change significantly with respect to these for the first CrN deposit, i.e. untreated substrate, and b) surface roughness of the re-deposited CrN coating is greater by a factor of 10% than the initial, regardless the deposition method. This latter effect is probably due to the increase of the substrate roughness as a consequence of the chemical attack from the reactants. In the case of the hard-metal substrates, the surface damage and its effect on the re-deposited coating performance after the stripping process is also discussed in terms of the amount of Cobalt leaching as a function of the different stripping processes. We have found that, with the applied stripping treatments, surface Cobalt leaching in hard-metal/WC can be optimised to cause the loss of less than 10% of the cobalt binder at the surface of the WC.
11:50 AM H4-12 Corrosion Protection for Aerospace Aluminum Alloys by Modified Self-assembled Nano-phase Particle (SNAP) Sol-Gel
L.S. Kasten, N.N. Voevodin (University of Dayton Research Institute); JW Kurdziel (University of Dayton Chemical Engineering); M.S. Donley (Air Force Research Laboratory)
The Air Force Research Laboratory is developing environmentally benign alternatives to the traditional chromated aircraft coating for aircraft corrosion protection, targeted to a 30+ year performance life cycle. The Self-assembled NAno-phase Particles (SNAP) process is a new method of forming functionalized silica nanoparticles in-situ from hydrolyzed tetramethoxysilane (TMOS) and glycidoxypropyltrimethoxysilane (GPTMS) in an aqueous sol-gel process, and then crosslinking the nanoparticles to form a thin, fully dense, protective film on aluminum aerospace alloys. These nanostructured coatings have been shown to provide an excellent barrier to corrosion in aluminum aerospace alloys, and other applications are envisioned. Much work has been done on characterization and performance of these SNAP coatings. This paper discusses a modification of the SNAP formulation with tetraethoxysilane (TEOS). Films were formulated and developed to produce a dense barrier sol-gel coating on AA2024-T3. Barrier properties of the films were evaluated with potentiodynamic scan (PDS) electrochemical technique. Corrosion behavior of the film has been investigated using electrochemical impedance spectroscopy (EIS). Scanning vibrating electrode technique (SVET) was used to study local electrochemical activities under the coatings. X-ray photoelectron spectroscopy (XPS) and angle-resolved x-ray photoelectron spectroscopy (ARXPS) have been utilized to investigate the surface chemistry. Morphology of the films has been studied with Scanning Electron Microscopy (SEM).
12:10 PM H4-13 Plasma Polymerized Thin Films Grown by PECVD Method and Comparison of Their Electrochemical Properties
I.-S. Bae, S.H. Cho, Z.T. Park, J.H. Boo (Sungkyunkwan University, South Korea)
Plasma polymerized organic thin films were deposited on Si(100) glass and Copper substrates at 25-100°C using cyclohxeaneand ethylcyclohexane precursors by PECVD method. In order to compare physical and electrochemical properties of the as-grown thin films, the effects of the RF plasma power in the range of 20-50 W and deposition temperature on both corrosion protection efficiency and physical properties were studied. We found that the corrosion protection efficiency (Pk), which is one of the important factors for corrosion protection in the interlayer dielectrics of microelectronic devices application, was increased with increasing RF power. The highest Pk value of plasma polymerized ethylcyclohexane film (92.1% at 50 W) was higher than that of the plasma polymerized cyclohexane film (85.26% at 50 W), indicating inhibition of oxygen reduction. Impedance analyzer was utilized for the determination of I-V curve for leakage current density and C-V for dielectric constants. To obtain C-V curve, we used a MIM structure of metal(Al)-insulator(plasma polymerized thin film)-metal(Pt) structure. Al as the electrode was evaporated on the ethylcyclohexane films that grew on Pt coated silicon substrates, and the dielectric constants of the as-grown films were then calculated from C-V data measured at 1MHz. From the electrical property measurements such as I-V and C-V characteristics, the minimum dielectric constant and the best leakage current of ethylcyclohexane thin films were obtained to be about 3.11 and 5 x 10--12 A/cm2and cyclohexane thin films were obtained to be about 2.3 and 8 x 10-12- A/cm2.
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