Pipework and vessels, which operate at high or very low temperatures, are predominantly covered by insulation to prevent heat and energy loss and for health and safety issues. The problem of Corrosion under Insulation (CUI) however can occur in both carbon steel and stainless steel. This is mainly as a result of water ingress into thermal insulation on pipework and vessels leading to rapid corrosion.

Some of the causes for the presence of water may be;

• Water trapped in the insulation during construction, e.g. storage in places where moisture exists, application carried out during poor weather.
• Joints in traditional claddings as they can form an inherent design weakness
• Mechanical damage of traditional cladding
• Spillage of liquid in low lying areas
• Condensation if there are cyclic temperature fluctuations
• Leaks in the trace heating system
• Poor fabrication standard of the protective cladding

Other factors that affect and accelerate the onset of CUI are ions such as chlorides and sulphides. Under certain circumstances these ions can accelerate the corrosion and can lead to different forms of corrosion such as pitting corrosion or stress corrosion cracking. These ions can either be sourced from the insulation material itself, or from the local environment e.g. acid rain and decomposition of organic matter such as dead vegetation or spillages.

Temperature fluctuations also play a major part in the severity of the corrosion taking place. In general all chemical reactions (of which corrosion is a form) are accelerated by increases in temperature.

The worst form of attack is a combination of cyclic temperature and the presence of ions as this can create a concentration mechanism whereby the repeated condensing and evaporation of liquid deposits increases more and more the concentration of the ion species at the point of corrosion, massively accelerating the rate of corrosion.

These forms of corrosion are of particular concern as they are often unforeseen and hidden from view until premature and potentially catastrophic equipment failure occurs.