Corrosion Under Insulation (CUI) Guidelines

S. Winnik

• Publisher: Woodhead Publishing
• Year: 2014
• eBook ISBN: 9781845694272
• Topic: Technology & Engineering
• Subtopic: Mining Engineering
• Index: Technology & Engineering

Corrosion under insulation (CUI) refers to the external corrosion of piping and vessels that occurs underneath externally clad/jacketed insulation as a result of the penetration of water. By its very nature CUI tends to remain undetected until the insulation and cladding/jacketing is removed to allow inspection or when leaks occur. CUI is a common problem shared by the refining, petrochemical, power, industrial, onshore and offshore industries.The European Federation of Corrosion (EFC) Working Parties WP13 and WP15 have worked to provide guidelines on managing CUI together with a number of major European refining, petrochemical and offshore companies including BP, Chevron-Texaco, Conoco-Phillips, ENI, Exxon-Mobil, IFP, MOL, Scanraff, Statoil, Shell, Total and Borealis. The guidelines within this document are intended for use on all plants and installations that contain insulated vessels, piping and equipment. The guidelines cover a risk-based inspection methodology for CUI, inspection techniques (including non-destructive evaluation methods) and recommended best practice for mitigating CUI, including design of plant and equipment, coatings and the use of thermal spray techniques, types of insulation, cladding/jacketing materials and protection guards. The guidelines also include case studies.

• Guidelines cover inspection methodology for CUI, inspection techniques, including non-destructive evaluation methods and recommended best practice
• Case studies are included illustrating key points in the book

1

Introduction

Corrosion-under-insulation (CUI) refers to the external corrosion of piping and vessels fabricated from carbon–manganese, low-alloy and austenitic stainless steels that occurs underneath externally clad or jacketed insulation owing to the penetration of water. By its very nature, CUI tends to remain undetected until the insulation and cladding or jacketing are removed to allow inspection or when leaks to atmosphere occur. CUI is a major common problem on a worldwide basis that is shared by all the refining, petrochemical, power, industrial, onshore and offshore industries. It is not a new problem, but it can be a serious problem. CUI has been responsible for many major leaks that lead to health and safety incidents, result in lost production and are responsible for the large maintenance budgets which are required to mitigate the problem.

Corrosion of austenitic stainless steels usually manifests itself as chloride external stress corrosion cracking (Cl-ESCC). Although Cl-ESCC [1] was first reported in 1965, not many references are available on the CUI of carbon–manganese steels and low-alloy steels up to 1980 when a meeting was held in November 1980 [2]. A review of this very successful 2 day meeting was given by Richardson [3] during a symposium which was held in 1983 [4], and was sponsored by the Association for Testing and Materials (ASTM), the National Association of Corrosion Engineers (NACE) and the Materials Technology Institute (MTI). It would appear that, when reviewing the literature from that meeting today, the problems reported in 1980 mirror the experiences currently being reported today.

Although numerous instances of CUI are reported annually, this has not been reflected in the production of many industry standards for insulation or measures to mitigate CUI. The first ASTM standard on thermal insulation materials relevant to CUI was adopted in 1971 [5]. NACE Task Group T-6H-31 first issued a report on CUI [6] in 1989 and later Task Group T-5A-30 was formed, which became an open forum for CUI problems and solutions. This led to the publication of a NACE recommended practice RP0198-98 [7] which was revised in 2004 [8]. A number of conferences and initiatives covering CUI and insulation materials have taken place since 1983 but the problem remains unresolved. It would seem that the incidence of CUI examples is not diminishing and would appear to be increasing, given the number of instances being reported. An NACE conference in 2003 reviewed similar topics covered back in 1983 which were illustrated by Delahunt [9] who presented an excellent historical perspective of the occurrence of CUI. A conference held in the UK in 2004 [10] had a similar theme and again suggested that CUI had not been mitigated and that instances of CUI were actually increasing. These instances led to the formation of an informal group (UK CUI Forum) [11] by corrosion and materials engineers from a number of major oil and gas producers in the UK specifically to share CUI-related information. The Forum has since expanded and now includes representatives from other industries. Collaboration between the UK CUI Forum and the European Federation of Corrosion (EFC) led to the development of this document, which hopefully will be regularly updated to reflect any major advances in the mitigation of CUI.

Why does CUI occur? CUI of carbon–manganese steels and low-alloy steels usually occurs when a number of conditions are fulfilled.

• Water or moisture must be present on the substrate in order to allow oxygen corrosion to occur. Water ingress is due to breaks in the insulation, cladding or jacketing which may have resulted as a consequence of poor installation or damage during service or simply be a result of deterioration over time. The principal sources of water are as follows.

External sources which include rainwater, deluge systems and process liquid spillage.

Condensation.

This water may be retained depending on the absorption properties of the insulation material and the operating temperature. Depending upon process conditions, saturated insulation may never have the opportunity to dry out completely.

Contaminants that can cause problems on both carbon–manganese steels and low-alloy steels as well as on austenitic stainless steels need to be present. Chlorides and sulphides make up the bulk of the contamination and generally increase the corrosivity of the water. The source of the contaminants can be external such as environmentally borne chloride sources include sites situated in a marine environment (e.g. offshore), or windborne salts from cooling tower drift, or from periodic testing of firewater deluge systems. Contaminants can also be produced by leaching from the insulation material itself. In the presence of an applied or residual stress and temperatures exceeding 60 °C (140 °F), high chloride contents of water contribute to Cl-ESCC.

The operating temperature range of the piping or vessels should be between −4 °C (25 °F) and 175 °C (347 °F). This temperature range reflects the experience from the contributors to this document and is meant as guide to enable mitigation procedures to be developed. CUI problems have been reported outside this range; the majority of CUI occurrences are, however, within the specified range from −4 °C (25 °F) to 175 °C (347 °F). In general, the metal temperature will be approximately the same as the process operating temperature (for insulated equipment). However, if the insulation is damaged and/or highly humid conditions commonly exist, a process temperature significantly above 121 °C (250 °F) can result in metal temperatures low enough to cause CUI; therefore the CUI range is extended to 175 °C (347 °F). In addition, equipment subject to cyclic temperatures even outside this range (e.g. regeneration equipment) or dead legs (including ‘cold’ dead legs nominally operating below −4 °C and warming up to ambient temperatures) should be considered to be subject to CUI. Systems which utilise heat tracing require careful consideration.

The insulation type may only be a contributing factor since CUI has been reported under all types of insulation. However, the individual insulation characteristics can influence the rate at which CUI occurs. These include the following.

• Water-leachable contaminants such as chlorides and sulphates are present.

• There is water retention, permeability and wettability of the insulation.

• Any residual compounds may react with water to form hydrochloric or other acids.

• It provides an annular space or crevice for the retention of water and other corrosive media.

• It may absorb water.

• It may contribute contaminants that increase or accelerate the corrosion rate.

• Anodic reactions at the substrate surface may be caused by the presence of anode/cathode corrosion cell activity in a low-resistance electrolyte which may be at an elevated temperature or subject to cyclic temperature variations.

• CUI initiates owing to the presence of water, oxygen and other contaminants. Once water and oxygen are present on the steel surface, corrosion occurs through metal dissolution.

It follows that the insulation system that holds the least amount of water and dries most quickly should result in the least amount of corrosion damage to equipment. The absence or the presence of a damaged barrier coating will permit direct contact between the water and the piping or vessel surface which will permit corrosion to occur.

The rate of CUI is determined by the availability of oxygen, the contaminants in water, the temperature, the heat transfer properties of the metal surface and the wet or dry condition of the surface. This in turn is influenced by the properties of the insulation materials. Damage can be general or localised. Service temperature is an important property as illustrated by Fig. 1.1 [12], which shows the effect of temperature of the corrosion rate of insulated carbon–manganese steels and introduces the concept of a closed system in which oxygenated water evaporation is limited, resulting in increased corrosion rates at higher temperature. This is the reason why CUI is such a problem as corrosion rates are often greater than anticipated.

1.1 Corrosion rate as a function of temperature.

In...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• European Federation of Corrosion (EFC) publications: Series introduction
• Volumes in the EFC series
• Abbreviations
• Dedication
• 1: Introduction
• 2: Economic consideration
• 3: Ownership and responsibility
• 4: The risk-based inspection methodology for corrosion-under-insulation
• 5: Inspection activities and strategy
• 6: Non-destructive examination and testing screening techniques for corrosion-under-insulation
• 7: Recommended best practice to mitigate corrosion-under-insulation
• 8: Design for the prevention of corrosion-under-insulation
• 9: Appendix overview
• Appendix A: Cost-economic evaluation
• Appendix B: Quality assurance
• Appendix C: Additional guidelines on the implementation of corrosion-under-insulation best practice
• Appendix D: Coatings
• Appendix E: Application of thermally sprayed aluminium
• Appendix F: Types and forms of insulation material
• Appendix G: Cladding and jacketing materials
• Appendix H: Use of protection guards
• Appendix I: Non-destructive examination and testing techniques
• Appendix J: Case studies
• Index