Sulfur Hexafluoride-Resistant Coating System

Question:

Our company manufactures electron beam accelerators. We use carbon steel pressure vessels for our accelerators. We have the vessels painted inside and out. First the vessels are sandblasted inside and out to remove mill scale, etc. The inside gets a primer and a finish coat and the outside only a primer. Paint thickness is a minimum of 4 mils. I have been getting conflicting information from the pressure vessel fabrication vendors on the best kind of paint to use, how thick the paint finish should be, etc. None like the brand of paint we specify. We use two pressure vessels for each machine. Both contain SF6 gas. One vessel is only a gas storage tank that is used to store the SF6 gas and is usually outside the buildings. The other vessel contains electronic equipment and SF6 gas and is always inside a building. Do you have any suggestions on how we should finish these vessels? T.C.

Answer:

You are on the right track, T.C. A dry film thickness of 4 mils should be adequate for this application. There are two important things to remember. First, the inside surface must be clean-free from corrosion and oily soils. Second, proper drying between coats is always difficult when painting enclosed surfaces. Often, it is necessary to force air through an enclosed vessel to remove solvent vapors between coats and after the final coat. Exposure to sulfur hexafluoride (SF6) is considered harsh service. My personal experience with coatings has shown that a two-component epoxy finish system will provide excellent protection to metal surfaces exposed to SF6. My recommendation for the finishing procedure on interior surfaces is:

1. Degrease to remove oily soils.
2. Blast clean to remove rust, mill scale and other corrosion products.
3. Apply a two-component epoxy primer to a dry film thickness of 2 mils.
4. Apply a two-component epoxy enamel to a dry film thickness of 2 mils.

It is also important to follow paint suppliers' directions for mixing and drying between coats when applying these finishes.