For corrosion to occur, four conditions must be met simultaneously...

1. Operating temperatures in the range of 100°F – 300°F (38°C – 149°C)
2. Oxygen
3. Water must be present.  This water must be liquid water, not water vapor.  Water vapor does not contribute to corrosion.
4. Corrosive chemical(s)

If one of the above conditions is absent, corrosion will not occur; all four are required simultaneously. In a very large number of cases in industrial settings, the operating temperatures of pipes and equipment are above the 300°F (150°C) range.  Therefore, corrosion is not a worry on a day-in and day-out basis.  But the reality is that industrial equipment is periodically shut down for maintenance.  The shutdown process will take the equipment through the critical temperature range window of 100°F – 300°F (38°C – 149°C) initially as the unit is shut down, idled, and then again, as it is restarted and brought back up to its normal operating temperature range.  And since oxygen is approximately 21% of the air that we breathe, exposure to it cannot be avoided.

The bottom line is that we really cannot do too much to control the existence of the key temperature range, or the presence of oxygen.  But we can exercise control over the two remaining conditions:  the presence of water and chemicals.Typically, where water is present, it contains some chemical elements like chlorides, which might be dissolved salts found in rainwater.  So, of the four necessary conditions for corrosion to occur, the most obvious condition we can focus our efforts on preventing is the presence of water in contact with both the steel requiring protection and the insulation material it touches. There are four general areas in which the insulation industry and specifiers direct their efforts in order to control the presence of water:

• Selection of the insulation material
• Protective jacketing design
• Protective coatings
• Insulation system maintenance

Each of these areas has its limitations, and more often than not a combination of one or more of the methods is used. 

Source: http://www.sangwonkorea.co.kr/#!1/c172g