Corrosion Under Insulation of Chemical Plant Pressure Vessels

Introduction

Risk exposure from equipment failures due to corrosion under insulation (CUI) failures is one of the most significant mechanical integrity issues for the chemical industry.  CUI in carbon steel pressure vessels has become a serious problem in recent years, especially in chemical plants that have been operating for 25 + years. As per stated in API 510, inspection for CUI shall be considered for externally insulated vessels and those that are in intermittent service or operate at temperatures between 10 °F (–12 °C) and 350 °F (177 °C) for carbon steel.¹ 

This article details our experience assessing the mechanical integrity of a feed pressure vessel located at a chemical plant in Pasadena, TX. This vessel was built with carbon steel and was fully insulated by design. This article describes the methodology carried out for detecting corrosion under insulation (CUI) and how results were managed for this pressure vessel.  

Corrosion Mechanisms 

Corrosion under insulation (CUI) is a particularly insidious form of localized corrosion that has plagued the chemical process industries for decades. This is a long-term damage mechanism that occurs on the external surfaces of insulated carbon steel pressure vessels operating in a temperature range from 10 °F (–12 °C) to 350 °F (177 °C) ¹. The damage mechanism is an atmospheric corrosion mechanism resulting from water-soaked insulation in contact with unprotected carbon steel.² As with any atmospheric corrosion mechanism, corrosion and metal loss rates are affected by a number of factors that include the amount, duration, and frequency of wetness and other factors such as chloride or other chemical contamination, operating temperature, and cyclic operation. External CUI corrosion rates can, therefore, vary by a significant amount, from nil in acid environments to upwards of 1 mm/y (40 mpy) in severe conditions. Corrosion rates increase with increasing metal temperature up to the point where the water evaporates quickly. For insulated components, corrosion becomes more severe at metal temperatures between the boiling point 212°F (100°C) and 350°F (177°C), where water is less likely to vaporize and insulation stays wet longer. ² Chlorides can lead to stress corrosion cracking under insulation on austenitic stainless steel equipment, within certain temperature parameters. The impact of conditions associated with intermittent operation should be considered, too.