What does high temperature range mean in CUI considerations?

Corrosion under insulation (CUI), like other forms of corrosion, is electrochemical in nature and therefore is significantly influenced by the temperature of the surrounding environment. The higher the temperature during the electrochemical reaction, the greater the amount of energy delivered into the system. This increased energy results in more successful collisions between particles (bonds formed between reactants) and therefore faster reaction rates.

The temperature of the metal surface is a critical factor in CUI. Higher temperatures translate to faster and more aggressive corrosion of the metal substrate. Conversely, at low temperatures, corrosion rates are vastly reduced as a result of relatively low energy levels and limited oxygen availability due to ice formation.

However, as temperatures continue to rise, the moisture within the insulation system can evaporate before it reaches the metal surface to initiate corrosion. At sufficiently high temperatures, the corrosion rate decreases due to the lack of a corrosive environment. (For more about wet insulation, read The Detrimental Effects of Wet Insulation in the CUI Range.)

It is generally accepted that carbon steel and low-alloy steels are more susceptible to CUI at temperatures between 32ºF (0ºC) and 300ºF (149ºC) while stainless and austenitic steels are vulnerable within the 140ºF (60ºC) to 300ºF (149ºC) temperature range. From this data, it can be concluded that the optimum temperature range for CUI in most steels is around 200ºF (93ºC) to 240ºF (116ºC). In this range, enough heat energy is delivered to the system to increase the rate of the chemical reactions without evaporating the moisture before it reaches the surface of the equipment.

In summary, CUI prevention relies on ensuring that there is no moisture in the insulation system at temperatures below 300ºF (149ºC) because intermittent boiling and flashing processes can generate an aggressive CUI environment.