Zeolite Coatings by In-Situ Crystallization as an Environmentally Benign Alternative to Chromate Conversion and Anodization CoatingsEPA Grant Number: R828134
Title: Zeolite Coatings by In-Situ Crystallization as an Environmentally Benign Alternative to Chromate Conversion and Anodization Coatings
Investigators: Yan, Yushan
Institution: University of California - Riverside
EPA Project Officer: Richards, April
Project Period: August 1, 2000 through July 31, 2003 (Extended to July 31, 2004)
Project Amount: $250,316
RFA: Technology for a Sustainable Environment (1999) RFA Text | Recipients Lists
Research Category: Nanotechnology , Sustainability , Pollution Prevention/Sustainable Development
Chromate conversion and chromic acid anodization are widely used on aluminum and its alloys for corrosion protection. In both processes, hexavalent chromium is used and subsequently released to the environment. Hexavalent chromium is a proven carcinogen and known to cause many serious health problems. As the environmental and occupational health and safety regulations on chromium use and release become increasingly strict, it is clear that post-treatment is inadequate and a novel chromium-free surface finishing process will have to be developed so that pollution is avoided at the source. Some non-chromate conversion and anodization processes are already available, but they are all somewhat inferior in performance to their chromate counterpart. Obviously there is still a great need for a novel coating that not only is chromium-free, but also has equivalent or superior performance to chromate conversion and anodization coatings.
The objective of this proposed project is to develop a chromium-free zeolite coating that has comparable thickness to chromate conversion and anodization coatings and equivalent or superior performance in coating adhesion, corrosion protection, abrasion resistance, and paint adhesion. An intrinsically inexpensive, safe, and non-polluting in-situ crystallization process that is capable of coating large surfaces with complex shape and in confined spaces also will be developed. Zeolites are microporous crystalline silicate materials and have widely been exploited for their microporosity (<15?) as catalysts and separation media. However, many high silica zeolites are non-porous in their as-synthesized state because of the organic molecules occluded in their pores during crystallization. High-silica zeolites are also known for their thermal and chemical stability and high mechanical strength. The goal of this project is to explore these dense polycrystalline high silica or pure silica zeolite films in their as-synthesized state for corrosion protection. Specifically, as-synthesized high-silica zeolite ZSM-5 and ZSM-39 coatings will be studied and performance tested as a potential alternative to chromate conversion and anodization coatings. The in-situ crystallization process will be optimized so that high-quality zeolite coatings can be produced with high throughput. The project will also seek to obtain fundamental insights on the in-situ crystallization process as well as on the corrosion and erosion behavior of aluminum and its alloys under the new oxide barrier coating ? zeolite coatings. The success of the proposed research has broad implications in many important fields including metal surface finishing, membranes, separation, adsorption, catalysis, fuel cells, and advanced microelectronics.
The in-situ crystallization process proposed here is based on a similar coating process that the PI previously developed for condensers in space applications. Using this method, uniform dense polycrystalline zeolite coatings were successfully formed with excellent adhesion on a variety of metals including aluminum and its alloys and on surfaces with complex shape and in confined spaces. As-synthesized high-silica ZSM-5 coating on aluminum alloy 6061-T4 showed superior corrosion resistance than chromate conversion coating and passed salt fog corrosion test ASTM B117. Hardness data suggest that zeolite coatings potentially can be much more abrasion-resistant than chromate conversion and anodization coatings. The in-situ crystallization process used aqueous aluminosilicate solutions, and thus is intrinsically inexpensive, safe, and non-polluting.