Hangbo Alloy Group
Shanghai · Est. 2012
Nitridation embrittles nickel alloys in ammonia, syngas, and furnace gas. Learn the mechanism and which high-chromium grades resist it.
Nitriding and nitridation are forms of high-temperature attack in which nitrogen from the process atmosphere diffuses into the alloy surface, forming brittle chromium nitrides and grain-boundary nitrogen-rich phases. In ammonia crackers, urea reactors, heat-treating furnaces, and steam-methane reformers, this can turn a ductile component into a cracked, dimensionally unstable one. This guide explains how nitridation proceeds, why chromium content governs resistance, and which nickel alloys are engineered to survive it.
Nitrogen enters the alloy as atomic N from dissociated ammonia (2NH3 to N2 + 3H2), cracked syngas, or nitrogen-bearing furnace atmospheres. At temperatures above roughly 500 C the nitrogen diffuses inward and reacts preferentially with chromium to form Cr2N and CrN precipitates in the grain boundaries and near-surface zone.
Because chromium is tied up as nitrides, the matrix loses the chromium it needs for oxidation resistance, so nitridation often travels hand-in-hand with scaling. The surface becomes hard, brittle, and prone to through-wall cracking; in thin sections the dimensional growth from nitrogen pickup can be enough to jam a furnace fixture or crack a tube.
Resistance to nitridation is, first and foremost, a function of chromium. Chromium forms a stable, adherent oxide (Cr2O3) that acts as a barrier to nitrogen ingress, and higher chromium levels extend the time before nitrogen breaks through. Nickel itself offers little direct nitridation resistance - its role is to keep the alloy austenitic and tough so the high-chromium chemistry stays fabricable.
Aluminium, and to a lesser extent silicon, further improve performance by forming protective alumina scales; alloys such as 602CA lean on an aluminium addition for exactly this reason. Below about 25 % chromium, nitridation resistance drops quickly, which is why standard 18-22 % Cr alloys are poor choices for severe nitriding service.
For ammonia crackers, urea reactors, and high-temperature furnace components, the workhorses are high-chromium nickel alloys:
Selecting the right grade means matching chromium (and aluminium) level to the nitrogen partial pressure and temperature of the actual process gas - see our guidance on reformer and pyrolysis furnace tubes and heat-treatment furnace fixtures.
The highest resistance comes from high-chromium alloys such as Alloy 602CA (approx. 25 % Cr with aluminium) and Inconel 693 (approx. 29 % Cr). For moderate service, Hastelloy X is common. Standard 18-22 % Cr alloys are only suitable where nitriding is light.
No. Nitridation is nitrogen dissolving into the alloy and forming brittle chromium nitrides; metal dusting is a carburization-type attack that converts metal to a powdery mixture of carbon and metal. They often occur in similar petrochemical environments but have different mechanisms - Inconel 693 was developed to resist both.
Typical locations are ammonia cracker tubes, urea reactor internals, ammonia-synthesis loops, and the hot zones of steam-methane reformer and pyrolysis furnace tubes. Heat-treating furnaces running in nitrogen-rich or ammonia atmospheres also see it on fixtures and muffles.
As a dedicated nickel alloy manufacturer serving chemical processing, power generation, oil & gas 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.