TWART CORROSION UNDER INDUSTRIAL INSULATION

"Chemical Engineering Progress" – October 1999

The article analyses problems associated with corrosion under insulation that can cause failures of mechanical systems and possibility of fire due to leakages.

The dissolved oxygen in water creates the potential for corrosion. Water can be introduced from outside of the insulation or from the inside (condensate). Common external sources are from atmospheric condensation or rain, cleaning of equipment, deluge systems and internal leaks or spillages. Absorption properties of the insulation play a major role in this instance. Depending upon process conditions, saturated insulation may never get the opportunity to completely dry out.

The chemical content of water can cause problems on both carbon and stainless steel. Acidic moistures can result in pitting and general corrosion of carbon steel. Chloride content of water can contribute to stress corrosion cracking (SCC) of austenitic stainless steel.

Correct design of atmospheric protection of insulation (jacketing) can minimise water intrusion as well as nonabsorptive insulation (closed cellular glass). To be considered that calcium silicate can absorb up to 400% of its own weight when immersed in water. When wet calcium silicate is alkaline and be detrimental to coatings such as alkyds and inorganic zinc. Key properties of materials for insulation are water vapour permeability, absorption of moisture and combustibility.

Details of jacketing closures are critical, pay attention to expansion due to temperature (insulation materials all have different coefficient of expansion). The role of accessory materials is also considered to provide wetherproofing.