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What are the major causes of frequent pitting corrosion failure of carbamate pumps?
It is in 25-22-2 metallurgy and lately been reported with these failures on frequent basis.

Even though 25-22-2 has good carbamate resistance, it is still vulnerable to localized pitting when chlorides contaminate the system.
Typical chloride sources:
Cooling water leakage
Steam condensate contamination
Hydrotest water
Improper flushing water
Contaminated passivation chemicals
Process carryover
Atmospheric contamination during maintenance

This becomes even serious when:
Chlorides locally destroy the passive oxide layer.
Once passivity breaks down, pits grow.
Carbamate environment accelerates propagation.
This is extremely common in urea plants. Multiple failure studies identify chloride-assisted pitting as the initiation mechanism.
(better someone from materials background comment more)

Suggest you to add this topic under Maintenance Engineers forum to get even better responses.

25.22.2 is the guy here to work all well. However its performance depends on maintaining a stable passive oxide layer.
In carbamate service, small oxygen content is intentionally maintained for passivation. If oxygen distribution becomes poor, localized active corrosion begins.
Common reasons include poor air dosing, maldistribution in circulation loop, dead legs, stagnant areas inside pump casing, and shutdown/startup instability.
This is even important in older urea plants.

So the material is all good to sustain.
Thanks. Let me check these pointers if we are missing out on any of them.