Surface Conversion Of Aluminum And Ferrous Alloys For Corrosion Resistance

"These proceedings represent contributions to the Symposium on the Corrosion and Protection of Light Metals, held at the 204th Meeting of the Electrochemical Society, October 12th through 17th, 2003, in Orlando, Florida."--P. iii.

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.

Investigations in this laboratory have shown that immersion of Al 6061-T6 and Al 7075-T6 in 1000 ppm CeCl3 produces very corrosion-resistant surfaces. For Al 6061-T6 that had been treated in 1000 ppm CeCl3 for one week, pitting did not occur in 0.5N NaCl for two months. Similar results have been obtained for immersion in NdCl3 and PrCl3, but not in LaCl3 and YCl3. The effects of the concentration of the passivation solutions, the length of exposure time and type of surface preparation have been investigated in more detail using electrochemical impedance spectroscopy (EIS) and d-c techniques. EIS data and d-c polarization curves have been determined during the passivation process and during subsequent exposure to NaCl. In addition, surface analysis has been performed using scanning electron microscopy with energy-dispersive X-ray analysis (SEM/EDAX) and scanning Auger electron spectroscopy (AES). More recent efforts have concentrated on shortening the passivation time. Comparisons with the corrosion behavior of chromate conversion coated aluminum alloys suggest that passivation in rare earth metal chlorides can produce surface films with a corrosion resistance similar to that provided by chromate conversion coatings.

This memorandum deals with the corrosion protection of magnesium and magnesium alloys. The corrosion resistance of these alloys in many natural environments, while not as good as that of copper, nickel, stainless steel, and aluminum, is in the same range as that of the iron and plain carbon steels. Some type of additional corrosion protection is often necessary and, as with steel, the protective measures usually involve some type of coating system and/or surface treatments. Also, magnesium can suffer accelerated attack when coupled, in the presence of a conductive electrolyte, to most metals below it in the galvanic series. This galvanic effect complicates the problem of corrosion protection. A number of coating systems have been proposed and used satisfactorily. Depending upon the application, these coating systems include the use of conversion coatings, organic coatings, metallic coatings, and others. In addition, special designs can be employed to improve the over-all corrosion resistance of the systems. This memorandum describes many of the coating systems and design methods which are used to reduce corrosive attack on both galvanically coupled and uncoupled magnesium assembliesd (Author).

This specification covers the requirements for two classes of chemical conversion coatings formed by the reaction of chemical conversion materials and the surfaces of aluminum and aluminum alloys. This specification is intended specifically to provide components of military weapon systems with maximum corrosion resistance. The coating also provides a surface having better paint adhesion than uncoated aluminum. It is not intended as a general purpose coating for commercial and decorative applications, (see 6.1).

A program in which corrosion-resistant aluminum alloys were dispersion- strengthened by powder metallurgical methods is described. The alloy was obtained in powder form, mixed with hard, stable dispersant powders of Al2O3, AlPO4, SiO2, B4C, and SiC by various methods, compacted, and extruded. Tensile strengths of extruded rods were, in some cases, triple that of the extruded, dispersion-free powder product. Corrosion resistance of these materials was observed to be equivalent to that of the wrought dispersion-free alloy.

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