One main condition for atmospheric corrosion  or corrosion under insulation to take place is moisture. Moisture which penetrates the insulation may originate from:
• Rain water
• Vapour resulting from “breathing” due to cyclic temperature changes, followed by dew formation
• Increased water exposure resulting from:
- nearby cooling towers
- water-jetting of heat exchangers
- fire-fighting drills
- sprinkler installations
- leaking trace lines

Atmospheric corrosion or corrosion under insulation can have the following forms:
• Overall corrosion (crater-like attack at critical area’s)
• Stress Corrosion Cracking
- carbon steel:  NO3- - ions
- austenitic SS:  Cl- - ions
- copper alloys:  NH3

This Paper discusses a severe incident of nitrate stress corrosion cracking in a HP reactor of a urea plant.