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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 UreaKnowHow.com 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.