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.

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