Stainless steel is a low vapor pressure material and is frequently used in very high vacuum applications. The heating of SST causes Chromium Oxide to form at the surface of SST components. This creates a poor wetting surface that prevents braze filler-alloys from flowing uniformly.
The decision to braze in “wet” or “dry” Hydrogen can depend on a few important aspects such as the base or substrate materials being used and/or the filler alloy type, as well as the application or performance requirements. In cases where the removal of oxides is predominantly important or necessary, dry Hydrogen is used and if the user is more concerned with the removal of hydrocarbon contaminants, it is advisable to braze in wet Hydrogen.
The chart below shows the temperature and/or dew point where the native oxides for various metals can be reduced [dew points below -60 °C are not achievable) [Source: Based on Bredzs, N., and C. C. Tennenhouse, 1970, Metal-metal oxide equilibria in pure hydrogen atmosphere, Welding Journal 49(5): 189-s-193-s].
- Cr2O3stable even in Dry H2
- Stainless Chart
- Metal-Metal Oxide Equilibria in H2/H2O Atmospheres
Chromium (Cr), which is a large constituent of stainless steel, occurs near the middle of the oxidation/reduction equilibrium space that hydrogen furnaces can produce. As desired, we can form Chromium Oxide or reduce that oxide by the atmosphere’s dew-point for temperatures greater than 800 ºC. For Chromium rich stainless (SST), we braze in a dry Hydrogen atmosphere which acts as a fluxing agent to reduce native oxides and remove hydrocarbon contamination, producing an ultra-clean raw metal surface. SST can also be plated with Nickel or another suitable metal and then be alternately brazed in a wet Hydrogen atmosphere.
For very high vacuum applications, assemblies containing a significant amount of stainless steel are sometimes run through a high-vacuum clean fire to help remove excess Hydrogen which certain vacuum systems find it difficult to pump. Hydrogen (H2) gas acts as a deoxidizing agent. Many oxides, like Fe, Cu, Ni, and Co are easily reduced by H2 whereas many others like Al, Be, Ti & Si can be very tenacious and will not braze or reduce properly in wet or dry H2. Here we have a reactive family of elements that may form undesirable compounds and are therefore typically brazed in high vacuum or with other inert gases like Helium or Argon.