In approximately 1929, a new Ni-Mo alloy was introduced (1) that reportedly resisted the corrosive effects of strong reducing acids. Since that date, large quantities of that alloy, HASTELLOY alloy B, and similar alloys have been utilized by the chemical processing industry. In areas such as hydrochloric acid production, butane isomerization, alkylation, ethyl chloride production, and urea production, alloy B has provided the corrosion resistance necessary with one significant limitation: weld decay. Following the weld fabrication of process equipment, a solution heat treatment followed by rapid cooling was required to restore the weld areas to their optimum corrosion resistance. The need for this heat treatment severely limits the size of welded fabrications that can be utilized due to the practical problems of furnace capacity, cooling facilities, and distortion of the part. Without the solution heat treatment, however, alloy B may exhibit knife-line attack (KLA) immediately adjacent to the weld bead and/or heat-affected zone (HAZ) attack at some distance from the weld. Flint (2,3) in investigating these phenomena in Ni-Mo alloys attributed the susceptibility of the KLA region to the formation of Mo2C and of the HAZ region to the formation of M6C.
