Hangbo Alloy Group
Shanghai · Est. 2012
Sensitization causes intergranular corrosion in nickel alloys when chromium carbides form at grain boundaries. Learn detection and how to prevent it in welds.
Intergranular corrosion (IGC) is a localized attack that runs along the grain boundaries of a metal rather than through the grains themselves. In nickel alloys it almost always traces back to sensitization: a heat exposure that lets chromium-rich carbides precipitate at the grain boundaries, starving the adjacent metal of chromium and leaving it vulnerable. The failure mode is deceptive because the surface can look sound while the material loses cohesion from the inside out. This guide covers why sensitization happens, where it shows up in real equipment, how it is detected, and how it is prevented in both welded and heat-treated components.
Sensitization is driven by carbon. Nickel-chromium alloys contain a few tenths of a percent of carbon in solution, and at elevated temperature carbon has a strong affinity for chromium. When the material sits in roughly the 425–870 °C (800–1600 °F) window for any appreciable time, chromium carbides of the M23C6 type nucleate preferentially at grain boundaries because diffusion is fastest there.
Each carbide that forms pulls chromium out of the neighboring narrow zone. Chromium is the element that gives the alloy its passive, corrosion-resistant film, so the chromium-depleted zone can no longer protect itself. An aggressive environment then eats along the boundary network while the grain interiors remain untouched. In molybdenum-bearing grades, molybdenum-rich carbides and a molybdenum-depleted zone can compound the effect, and in some austenitic nickel alloys sigma or other intermetallic phases may also appear in the same temperature band and worsen embrittlement.
The key variables are time and temperature. Short exposures just above the range may not sensitize at all, while hours at the peak of the range can fully sensitize thin section. Cooling rate through the danger zone and the starting carbon level decide how much carbide actually forms.
The classic case is “weld decay” in the heat-affected zone (HAZ) a few millimetres from a weld. The weld itself cools fast enough to stay unsensitized, but the HAZ passes through the sensitization window and then sits there while the joint is large enough to act as a heat sink. A subsequent corrosive service then attacks exactly that band.
A sharper variant is knife-line attack: a very narrow, highly localized groove right next to the weld in stabilized grades such as 347-type or 625 that contain niobium or titanium. The weld thermal cycle can dissolve those stabilizing elements locally, then the boundary carburizes on cooling, producing a thin but deep attack line. It is easy to miss in visual inspection.
Sensitization is not only a welding problem. Furnace fixturing, solution-annealing overshoot, repeated stress-relief cycles, and process upsets in digesters, reformers, and heat exchangers can all sensitize a component that was never welded. Any operation that holds the metal in the critical band for minutes to hours is a candidate.
Field judgment starts with service history and a visual or dye-penetrant check for crazing along boundaries, but the definitive screen is metallurgical. The family of tests in ASTM A262 is the standard reference:
Cross-section metallography under an optical microscope remains the clearest proof: a continuous chain of grain-boundary carbides plus a chromium-depleted band is the signature. For incoming material, verifying the heat-treatment certificate and carbon level before fabrication prevents most surprises.
Prevention is mostly about carbon control and thermal control. Three proven routes dominate:
In fabrication, controlling weld heat input, using low-interpass-temperature procedures, and avoiding unnecessary post-weld stress relief in the sensitization band all reduce risk. Note that the safe stress-relief band differs from the sensitization band, which is why a dedicated post-weld heat treatment plan matters for nickel alloys.
As a dedicated nickel alloy manufacturer, Hangbo Alloy Group supplies both ultra-low-carbon and stabilized chemistry grades with certified heat treatment, so specifiers can match the material to the fabrication route and service environment rather than gambling on a generic stock grade.
The dangerous window is roughly 425–870 °C (800–1600 °F). Holding the alloy in this band for minutes to hours lets chromium carbides precipitate at grain boundaries. The exact peak depends on carbon content and section thickness; this is also why post-weld heat treatment must avoid the same band unless the grade is stabilized or ultra-low-carbon.
Weld decay is a wider sensitized heat-affected zone a few millimetres from the weld, caused by the base metal passing through the sensitization window. Knife-line attack is a very narrow, deep groove immediately adjacent to the weld in stabilized grades, where the thermal cycle locally dissolves titanium or niobium and lets the boundary carburize on cooling. Both are covered in our nickel alloy welding guide.
They greatly reduce it but do not make a part immune under all conditions. With very little carbon available, carbide precipitation is minimal, yet extreme or prolonged exposures can still sensitize, and molybdenum-rich carbides or intermetallics may also form. The dedicated Hastelloy C-276 discussion on sensitization of Hastelloy C-276 shows how even a low-carbon grade is qualified by test, not assumed safe.
ASTM A262 is the reference family, with the oxalic acid etch (Practice A) as the screening step and the ferric sulfate, Huey (nitric), or Strauss (copper-copper sulfate) tests as quantitative follow-ups. Accepting material against a verified mill test certificate and ASTM A262 result before fabrication is the most reliable defense.
As a dedicated nickel alloy manufacturer serving chemical processing, oil & gas, power generation, marine sectors, Hangbo Alloy Group supplies the grades discussed here across all standard product forms with full ASTM/ASME documentation and EN 10204 3.1 certification. Our material is specified for applications where corrosion resistance, high-temperature strength, and traceable quality are non-negotiable. See our production & testing facilities for more on how every heat is verified.