The Engineered Repair Solution for Corrosion Under Insulation in Vertical Pressure Equipment

One of the largest problems for the refining, chemical and petrochemical industry

Corrosion under insulation (CUI) is a common issue across many industries including refining and chemical processing.  According to a study presented by ExxonMobil, the highest incidence of leaks in the refining and chemical industries are due to CUI and not process corrosion.[1] CUI is concealed from view and can remain unobserved until the insulation system is removed for inspection. Visible signs of corrosion can cascade from the jacketing or when leaks appear. CUI can be defined as localized external corrosion occurring in insulated equipment fabricated from carbon and low-carbon steels. The operating temperature of the pressure vessel is extremely important as above 350°F low carbon steels will evaporate water from the surface and under 25°F corrosion rates are dramatically reduced. CUI occurs beneath insulation and jacketing as a result of water penetration into the insulating system.  Water entering the insulating system can materialize with rainfall, steam, condensation and a variety of other areas.   Once water enters into the insulating system, contaminants such as salts, can be introduced as an accelerant the corrosion to the exterior of the pressure vessel. 

The inevitable failure of insulation systems in vertical pressure vessels

The insulating system is made up of several key elements to help slow the corrosive process.  A coating, considered the last line of defense, is first applied to the exterior base material.  Next, the insulation is applied to buffer sound, conserve heat and other energy from the vessel as well as serve as further protection in some cases.  Lastly, a jacketing is banded to the exterior of the system to protect insulation from the elements and other mechanical damage.  The materials for these systems can vary greatly and fail with as much frequency.  Mechanical damage is only one of the failure modes that allows water to enter the system. Design, maintenance and initial installation can also contribute to failure of the system.  Due to the failure modes, vertical pressure vessels tend to experience similarities in locations that CUI occurs.  Typically vertical pressure vessels experience CUI at locations such as welded clips, insulation rings and stiffener support rings as well as top head center nozzles.     

Engineering analysis to identify fatigue, mitigate risk and optimize availability

In order to determine what available options can be considered for CUI repair, the extent of the corrosion damage has to be properly assessed. Once the damage has been characterized, an engineered repair approach can be implemented to determine the optimal solution for the facility. Consideration should be paid to the area, depth, locations of the corrosion, equipment operating conditions, insulating system, failure mode and life expectancy. A fitness-for-service (FFS) assessment can predict the expected life of various repair scenarios, helping to determine the best solution.  For extensive repairs, a predictive distortion analysis can be applied to optimize the repair to minimize distortion and ovality.  In many cases, a machine welded repair by weld metal buildup is much more schedule efficient and cost effective than section replacement.

Machine Welding Approach to CUI Restoration

AZZ Specialty Welding can develop an ASME section VIII weld metal buildup (WMBU) repair strategy with machine applied deposits to restore the wall to original thickness in lieu of plate replacements and window cut outs in the vessel.  AZZ’s proprietary GMAW machine welding process is ideal for OD applications particularly CUI because of the custom automatic proportional height control.  This feature allows for our machines to electronically measure the tip to work distance over 200 times a second in order to ensure uniform deposition thickness and optimum dilution rates.  Furthermore, the weld nozzle has a stroke capability of 9” in order to address a variety of corrosion depths experienced during repair applications.  Constructed by AZZ engineers with sensor enabled parameter monitoring and digital control systems, the machine welding process has the ability to be easily field modified to handle any geometry and difficult welding position.

AZZ Specialty Welding

Whether the pressure vessel needs to be repaired emergently or during a planned turnaround, AZZ has unparalleled innovative leadership in the nuclear, refining, power generation and petrochemical industries. AZZ has over 2000 ASME qualified procedures and over 1000 active welder certifications.  Find out more about our engineered solutions and welding technologies at or follow us on LinkedIn.


[1] A.A. Mokhtar, et al. Engineering Management Asset Systems

OD Weld Metal Buildup Applied with AZZ Machine GMAW for CUI