Question: We are about to braze a high-temperature nickel-bearing alloy, and we have been warned about a potential problem that is called sulfur embrittlement. We have not heard of this before. What is it and what causes it?
Answer: If nickel alloys are brazed in the presence of compounds that contain sulfur, such as grease, oil, paint and deep-drawing compounds, the compounds will evaporate and leave a sulfurous gas. The gas will diffuse into the surface grain boundaries and form nickel sulfides. These sulfides are extremely brittle.
The newly formed nickel sulfide will lead to premature failure by surface cracking when stressed. If the nickel sulfide is formed at the grain boundaries, the part should be scrapped because early failure will occur (particularly if a safety-critical item). Therefore, it is mandatory that an effective pre-clean wash and rinse system be utilized to ensure surface cleanliness.
Please do not underestimate the value of pre-cleaning prior to brazing. This applies to all brazing and metallurgical processing techniques. Successful brazing requires successful pre-cleaning to ensure a surface that is free from contamination and potential contamination.