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. This Paper discussed two examples of overall corrosion, which in fact is a crater-like attack at critical area’s such as at the lower elbow of a vertical line or under  behind a clamp.

It is strongly recommended to always coat not-insulated equipment in unalloyed and low alloy steel.