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:   CI- - ions
 - copper alloys:  NH3

In the Mechanical Paper of December 2009, an example of Nitrate Stress Corrosion Cracking has been discussed.  The Mechanical Paper of January 2010 discussed two examples of overall corrosion, which in fact were a crater-like attack at critical areas such as at the lower elbow of a vertical line or under  behind a clamp. This Paper again discusses a case of Nitrate Stress Corrosion Cracking, now in an ammonia plant.