| 摘要 |
<p>The inner surface of a high temperature nickel chromium alloy product such as ethylene furnace tubes is cleaned with high temperature hydrogen to prepare the surface for the deposition, diffusion and modification of metals. A first layer of chromium or chromium and silicon is deposited and diffusion heat treated or covered by the second layer of aluminum, magnesium, silicon and manganese and third layer of rare earth metals such as yttrium and zirconium. Each layer or the combination of layers is diffusion heat treated at sufficient time and temperature to cause a diffusion depth ranging from 50 microns to 150 microns with a maximum of 250 microns. The surface is then heated to convert the immediate surface to a spinel and further pretreated with argon and nitrogen to stabilize the surface oxides. The surface of the final coating can be polished to minimize sites for carbon buildup. When ethylene is produced using furnace tubes which are coated in this manner less coking occurs.</p> |
