The impact of silicon content on the corrosion resistance of nickelmolybdenum alloy in high concentration sulfuric acid transport

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 shortrange ordering (SRO) and intermediate phases formations (Ni4Mo transformation) in its metallurgical structure, in the aswelded 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 a to ß) 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.