Abstract
This failure case analysis highlights the impact of silicon content on the corrosion resistance of Ni-Mo alloy piping welding joints, known as Hastelloy B-2 (UNS N10665), in high concentration sulfuric acid (98 wt. % H2SO4) medium. A damaged piping weld joint is analyzed against an accelerated corrosion mechanism which was found to shorten the joints lifetime drastically. Superiority of B-2 alloys is largely attributed to the high concentration of molybdenum (26–30 wt. %) that gives its corrosion resistance to highly reducing acidic environments. However, certain deficiencies, such as short-range ordering (SRO) and intermediate phases formations (Ni4Mo transformation) in its metallurgical structure, in the as-welded conditions, was found to hinder its thermal stability. In this piping failure case analysis, close examination including optical assessment, X-ray fluorescence (XRF) spectrometry, and in-situ replication metallography were performed on a defected weld neck butt-fusion welding joint. XRF revealed a silicon content of almost four times (0.38% against 0.1%) the ASTM maximum requirements, which appears to have lowered the alloy’s thermal stability and increased its kinetics to rapid phase transformation (from α to β) along the grain boundaries. Extensive second phase precipitates were observed along the domain boundary interfaces in the microstructure, which is suggested to have resulted in Mo-depleted zones in the matrix. In this case analysis, the welding temperature (650–750°C) of NiMo alloys butt-joints along with the silicon content (0.1% to 0.3%) present were found as main contributors to the reduction of the welding corrosion resistance and hence to the shortening of the piping lifetime.
