Origins and Cures For Corrosion under Insulation




Chloride and galvanic and acidic or alkaline corrosion could create Corrosion in the Insulation (CUI). However, an understanding of the types of Corrosion and preventative measures can reduce the risk.






Corrosion is a major cost to the global economy, costing approximately $2.2 trillion every year as per the World Corrosion Organization. From that figure, it's estimated that between 40 and 60 percent of maintenance costs result from Corrosion in the Insulation (CUI).

Corrosion in Insulation is a mysterious phenomenon that is often not noticed. It is due to practical reasons. (For more information on inspection methods, refer to The 4 Most Effective Nondestructive Inspection Methods to Check Corrosion under the Insulation.) It isn't always possible to use the ideal insulation coating combination or apply the most efficient inspection methods.

The intruding of water is the main cause of the development of CUI. Corrosion can attack or damage the jacketing and the insulation hardware or even the underlying equipment or pipes. The Corrosion could manifest in several kinds, like galvanic, chloride acidic, and alkaline Corrosion. When you know the different types of Corrosion that could occur in Insulation, appropriate materials and construction methods can be utilized to avoid these types of Corrosion.

Environment-related Conditions that Cause the occurrence of Corrosion under Insulation

Predicting CUI rates isn't easy as they are often localized or more general in the natural world. Here are the most common conditions that can lead to greater 


CUI rates:

Marine environments

Humidor hot environments

Climates that have higher rainfall

Leaks in steam traced.

Intermittent wet-dry weather

Contaminants in the layer of Insulation and the air (such as sulfides and chlorides) dissolve in the water.

Systems that operate at a lower temperature than the atmospheric dew points

Systems for Insulation that do not allow water drainage

Materials for Insulation that hold in moisture

CUI is a risk to numerous industries. If left untreated, it can remain concealed under the Insulation and only becomes apparent following severe failures.

CUI can be caused by the infiltration of moisture or water and contamination through condensation or other sources (e.g., rain or sprinkler system).CUI is often very localized, with the majority of the equipment in good shape. That is why samples of inspections are not likely to uncover any occurrences.


Preventing Galvanic Corrosion

A galvanic attack occurs due to the presence of the salt electrolyte, which allows the flow of current between different metals. The extent of the assault on the lesser valuable metal is not solely based on the difference in the two materials' power and on their respective area.

Because galvanic Corrosion is caused by water intrusion in a humid environment, the cellulose insulation could be the best solution. It is also suggested to use an artificial or plastic jacket that is both fires - and weatherproof.

It is also possible to paint the metallic base can also be coated with paint to prevent the cathodic or anodic reactions and offer a highly resistant path blocking the flow of current. However, certain pigments in paint, such as red oxides or gypsum, may encourage Corrosion, particularly in the initial coating applied to the primer.

Preventing Acidic or Alkaline Corrosion

The alkaline and acidic Corrosion is caused when acids or alkalis and moisture are present in some fibrous or granular insulation material. When a hot service is used, water vapor could condense near the edges of the Insulation and then dissolve the acidic or alkaline chemicals, leading to Corrosion of the steel or aluminum jacketing.

Certain alkaline water with aluminum can cause etching as well as pitting. Pitting can be very severe, particularly if the chloride anion exists. To protect yourself, metal jackets should have an internal barrier to moisture. (Related article: The Role of Metal Jacketing in CUI.) If Corrosion of the jacket is a problem and the weathering type is plastic, it can be an effective solution. Additionally, the internal mounting of an anode under the primary weathering barrier and over the second coating was shown to be efficient in addition to the primary method.

Preventing Chloride Corrosion

Chloride corrosion is usually caused due to the combination of Insulation with leachable chlorides and 300 Series austenitic stainless steel surfaces with temperatures over 60degC (140degF). The chloride ion concentration typically results from the evaporation process of rainwater, water used to battle fires, or water used for processing. In coastal areas, stress corrosion cracking of jackets used for Insulation is a typical problem because of salts in the air. (See Chloride Stress Corrosion Cracking in Austenitic Stainless Steel for more details.)

The likelihood of failure and how fast crack development are determined through the steel's temperature and the concentration of chloride at the surface of the metal. Solutions that contain less than 1 per milligram are considered safe. At temperatures below 80degC (176degF), concentrations of 100ppm aren't especially dangerous when persistent surface-wetting takes place, however at higher temperatures; low levels could cause failure.

The water that enters the Insulation and then diffuses outward eventually will reach a dry zone at the pipe's hot point or wall of the equipment. The next dry-out area is a region where the Insulation's pores contain a saturated chloride solution. If a shutdown happens and the wall's temperature is lower to a certain point, the saturated salt solution is absorbed into the wall of the metal. After reheating, the metal wall will be temporarily within contact with a solution of saturated chloride, and stress corrosion cracking could start.

The image alt="Stress corrosion cracking on a condensate line made of stainless steel that is insulated. Stress corrosion cracking of an insulated stainless steel condensate line. The Insulation was wet with water and caused chlorides to be released from the line.

The Insulation on the surface of hot metal.

Source: NASA Corrosion Engineering Laboratory

To protect 300 series stainless steel, the Insulation should comply with the MIL-I-24244 and ASTM C-795 specifications. Wrapping the equipment in aluminum foil before applying Insulation will reduce the possibility of Corrosion since the foil creates a physical barrier that blocks the solution of saturated chloride from reaching the surface of the metal. Because of its superior thermoelectric conductivity, the aluminum will be around a similar temperature to the machine. The chloride solution will shift towards the foil instead of the stainless steel.

Aluminum that is thermally sprayed to provide a CUI Solution

Thermo sprayed aluminum (TSA) is employed to shield materials from Corrosion. TSA can be applied using either oxy-fuel or electric flame spraying, employing the solid metal wire. This TSA coating method is usually advised for carbon steel applications that require more than 25 years of longevity.

TSA is also effective in the most extreme conditions in the absence of organic coatings, such as temperatures that vary between 149degC (300degF). In the galvanic coupling Al-Fe, the steel is shielded anodically by aluminum. It functions as a barrier and acts as an anode sacrificial so that the substrate is protected in the event of chips or cracks within the coating.


Detection of Corrosion under Insulation (CUI)

Understanding where Corrosion is most likely to develop, for instance, in areas of low temperatures or certain temperature ranges, can help create an effective preventive maintenance program. The API's API 509 requires the removal of certain Insulation every five years for all vessels in which external Corrosion may be possible. It is required by the National Board Inspection Code 4. Maintaining records of the number of trouble spots may help in the establishment of an effective inspection schedule.

Multiple Inspection Methodologies to CUI

There are just a few methods for determining whether there is CUI without having to remove the Insulation. However, they all have their limitations. Some nondestructive methods include:

The Pulsed Eddy Concurrent Test

Pulsed Eddy Current Testing is a method to identify CUI and examine GI, SS, and Al cladding. The Insulation may be up to 300 millimeters thick, and the thickness of the metal could be as high as 100 millimeters. A new method, called that is known as pulsed eddy current array, is extremely efficient. The main drawback to this method is the precision of the reading, which could vary by 10%. However, the main advantage is that it can be carried out even while the piping is operating.

Long Range Ultrasonic Testing

Ultrasonic testing with long-range is possible on pipes with a diameter of two inches or larger. It requires a part of the Insulation to be removed to allow the instrument's collar to connect with the pipeline. Ultrasonic waves can detect Corrosion within the piping between 5 and 200 meters away from the collar, based on the coating the Corrosion is in, the kind of Corrosion, and if the pipe is submerged. The system can detect Corrosion that is greater than 3percent of the cross-sectional area.

Computed Radiography

Radiography computation tests can be conducted at the bends in the pipes to test for erosion or Corrosion. (Related article: The Benefits of Digital Radiography for corrosion Inspection.) It is a precise technique. However, it takes a significant amount of time to test every bend due to the radiography. Furthermore, pipes with larger diameters require a cobalt source, which makes using this method for a running plant impossible for pipes with larger diameters.

Infrared Thermography

Thermography infrared could be of tremendous assistance in finding moisture underneath Insulation. This can help locate CUI.

Recommendations for Preventing Corrosion in Insulation (CUI)

Corrosion in Insulation is an electrochemical type of Corrosion that is manageable like other kinds of Corrosion. In the end, there are two strategies to avoid CUI:

"One Line of Defense" is the "one line of defense," which is essentially the application of Insulation to the equipment.

It is the "two-line of defense strategy," which applies coatings under the Insulation. coating underneath the Insulation.


A coating beneath the Insulation is thought to be the most effective method to reduce CUI.

One of the most popular coating alternatives is one of the most popular coating options is thermal sprayed aluminum (TSA). The aluminum not only protects the metal substrate but also acts as an anode sacrificial.

The design layout is an additional element to be taken into consideration. It is essential to consider when designing the layout to avoid promoting Corrosion by allowing water to infiltrate a system directly or indirectly through Capillary actions. Suppose the equipment is subjected to a hostile environment or its shape causes difficulties when applying insulation materials. In that case, it will increase the risk of CUI, and necessary countermeasures must be implemented.

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