Hot-side SCR places the catalyst and ammonia injection upstream of the air heater, in gas that is hundreds of degrees hotter and significantly dirtier than a conventional cold-side arrangement sees. The emissions case is real. So is the materials problem: the carbon and low-alloy steels commonly specified for casings, internals, supports, and injection hardware are operating near the top of their useful temperature range — and in some installations, near their creep limits.
Materials of construction are near their limit
At hot-side gas temperatures, sustained metal temperature sits close to the threshold where creep becomes a life-governing mechanism for materials that were never intended to be creep-designed components. Local hot spots, flow maldistribution, insulation deficiencies, and transient excursions push sections over that threshold without any alarm registering it. The field consequence is showing up as distortion of catalyst support structures, cracking at stiffener and attachment welds, and — increasingly — embrittlement found during repairs, when a weld that should have been routine reveals parent material that no longer behaves like the original specification.
Where ammonia meets metal, nitrogen and hydrogen follow
The less-discussed mechanism sits at the ammonia interface. Where injection equipment, mixing hardware, or downstream surfaces operate in a zone where ammonia contacts metal at elevated temperature, ammonia dissociation gives nitrogen and hydrogen a path into the material. Nitrogen ingress nitrides and embrittles the surface; hydrogen ingress drives the hydrogen damage mechanisms familiar from ammonia process equipment — loss of ductility and cracking that arrives without external wastage to warn of it. BakerRisk has published on hydrogen damage mechanisms in ammonia service; the operating lesson for SCR systems is that the same chemistry does not check whether it is standing in an ammonia plant or a flue-gas duct before it goes to work.
Why this is going untold
These findings currently live in individual outage reports, repair files, and root-cause investigations that rarely leave the sites that produced them. OEM manuals do not headline them. EPC specifications largely predate them. The result is that each owner discovers the problem independently, at their own expense, on their own critical path. Publishing the pattern is the point of this series.
- Verify sustained and transient metal temperatures against the actual creep thresholds of the installed materials — not the original design gas path assumptions
- Map the zones where ammonia can contact metal across the full operating range, including startup, low-load, and injection-tuning conditions
- Add embrittlement screening (hardness, in-situ metallography, or sample removal) to inspections of casings, supports, and injection hardware — before a repair weld discovers it for you
- Review weld details and dissimilar transitions for thermal-fatigue susceptibility under the real cycling profile
- Baseline catalyst supports, expansion joints, and injection hardware early, so degradation rates are measured rather than guessed
A material operating at its limit does not announce it. It waits for the repair crew, the hydrotest, or the news cycle.
The mitigation is a review, done before the story
The response is a disciplined materials-compatibility and design review against the real operating profile: metal temperatures, cycling, ammonia exposure zones, and the inspection access the arrangement allows. Done early, it converts open risk into a monitoring plan with owners and trigger points. Done late, it becomes a repair program executed under outage pressure — or an incident investigation.