The problem of corrosion under insulation (CUI) is one of the greatest threats to industries worldwide. The use of the outer metal shell protects and isolates the surface of the material from the environment. However, the operation process and ambient temperature change have facilitated the formation of water and corrosion under insulation.
The Metal Coating technology has become the first choice to prevent/ reduce corrosion under the insulation layer with protective coating systems. This method creates advanced coatings that prevent CUI and shows excellent performance through cost-saving and downtime.
So, where exactly does the corrosion under the insulation come from and why are Thermal spray aluminium coating methods the optimal solution for modern industries? Find out more in the article below.
Corrosion under insulation (CUI): an overview of what you need to know
Corrosion Under Insulation (CUI) is one of the most common causes of failure in the refining, petrochemical, power, and other industries, both onshore and offshore. It is also the most challenging form of corrosion to monitor and control because problems are not shown in the insulation structure, usually only detected when it is removed for inspection or maintenance. Thermal spraying is the current leading solution to address this concern. To better understand this corrosion process, below will present the concept, mechanism and causes of CUI.
What is Corrosion Under Insulation (CUI)?
In simple terms, corrosion under insulation (CUI) occurs due to the accumulation of moisture on the outer surface of insulated equipment. This problem leads to significant losses such as high maintenance and repair costs, time-consuming temporary production shutdowns.Many factors lead to CUI, and most are similar to other types of corrosion except in the environmental conditions in which they occur. Moisture accumulation and water ingress contribute largely to corrosion-related problems under CUI insulation.
Outside water enters the insulation system mainly due to breakage or damage in the insulation system. In addition, water from the inside due to the sealed environment of the insulation creates conditions that easily accumulate moisture on the surface of the insulation when the temperature changes higher or lower.
Because the insulation does not allow evaporation and acts as a carrier, water in one area moves through the insulation to another place, causing corrosion to spread faster. If not detected, the result of this CUI corrosion can lead to failure and shutdown a part/ the entire system.
The CUI mechanism and the urgent need for a modern metal coating solution
Insulated corrosion is a term that refers to several different corrosion processes such as:
- Electrochemical corrosion: involves the transfer of charge-carrying ions between the anode and cathode through the pore fluid of the insulation. The principles of electrochemical corrosion for a primary corrosive component require four essential elements: the anode (where corrosion occurs and current flows) and the cathode (where no corrosion occurs). wear and current flows), electrolyte (a medium capable of conducting current by ionic current, in this case, an insulating layer containing water) and the metal path where the anode and terminal are connected negative allows current to return and complete the circuit.
- Acidic/Alkaline Environment: Occurs due to a combination of internal moisture and alkaline or acidic environment in the protective layers of the fibre or grain type. When the temperature exceeds 121 degrees Celsius, most of the water will evaporate and condense at the surface of the insulating materials. The water then dissolves the alkalis/acids to form a corrosive solution.
- Chloride: occurs on 300-series stainless steel surfaces when chloride-containing insulation materials combine with water at 60 degrees Celsius. These chloride ions usually arise from the evaporation of rainwater or water in curing systems on fire.
Determining the cause of CUI to choose the proper metal coating technology
The appearance of CUI can be a challenge to predict, but basically, the effect of the equipment on corrosion under insulation depends on several critical factors as outlined below:
Water source
The two primary sources associated with CUI are intrusion from external sources and internal condensation. Water enters from an external source such as rainwater, stream discharge, fire hydrant spray, or water flowing from a cooling tower. External water enters the insulation system through cracks that form over time. Condensation occurs when the temperature of the metal surface is below the dew point of the atmosphere and causes liquid trapping between the metal and the insulating material.
Temperature
The metal corrosion process will be accelerated by the increasing temperature, causing the metal coating to be destroyed early. The influence of temperature on metal corrosion is complicated. On the one hand, raising the temperature speeds up chemical reaction processes, whereas electrochemical reactions degrade materials. Increased temperature, on the other hand, accelerates the evaporation of the solution layer on the material’s surface, lowering moisture retention time on the metal surface. The solubility of oxygen and other corrosive gases decreases as the temperature rises. As a result, the rate of oxidation and corrosion of the metal surface beneath the insulation layer is accelerated.
Insulation solutions
The ability to absorb moisture, chemicals, and insulation is directly related to corrosion under the insulation system. As a result, choosing the suitable insulation material for the system is critical to minimizing the risk of insulation corrosion.The insulation provides an annular void or crevice to trap water and other corrosive media, especially chloride. In alkaline environments with a pH between 7 and 11, chloride ions (CI-) tend to disrupt the passivation (minimum corrosion rate) of carbon steels and alloy steels and initiate pitting corrosion in the acid solution. If the pH is less than 5.5, corrosion will tend to increase rapidly in the presence of chloride.
External environment
The rate of corrosion is also affected by the external environment. The marine environment, hot or humid weather, and wet weather all contribute to a higher CUI rate.
Methods of metal coating to prevent corrosion under insulation
Anti-corrosion metal coating under insulation was created as a “savior” to solve corrosion-related issues such as maintenance costs and machine downtime. Thermal spray aluminum is a technology used in many critical industrial sectors, including aerospace, automotive, power generation, petrochemical, and offshore structures, for wear and corrosion-resistant coatings.Among them are some exemplary thermal spraying methods for preventing corrosion under the stabilizing layer – CUI, such as:
Thermal spray aluminium coating – Advanced metal coating technology
Thermal Spray Aluminium (TSA) creates a protective coating of materials when exposed to harsh environmental conditions with excellent anti-corrosion properties. This metal coating provides corrosion protection at temperatures as high as 660 degrees Celsius (the melting point of aluminium). Outstanding properties such as high strength, long service life and low maintenance requirements make TSA coating the ideal choice for marine and offshore constructions, such as the offshore oil and gas industry.
The highly durable thermal spray aluminium coating anti-corrosion solution provides comprehensive protection, preventing corrosion and significantly improving service life compared to conventional protective coatings. According to reports, a TSA coating with a thickness of 200µm will provide a service life of more than 30 years in the corrosive environment located directly under the water of marine constructions.Corrosion protection mechanism: The aluminium spray coating has the structure of an aluminium sheet, surrounded by aluminium oxide – the thin oxide layer functions as a barrier coating that is resistant to pitting and erosion damage.
Thermal Spray Zinc – The metal coating method with optimal surface protection
With its impressive anti-corrosion properties, zinc is often chosen as a protective coating for various coatings and materials under harsh environmental conditions. Zinc coating is a highly effective metal coating anti-corrosion method. It also allows for a thicker coating, which contributes to the longer life of components, helping them to perform well in harsh environments.
Corrosion Protection Mechanism: Zinc acts as a sacrificial electrode to protect the internal material. On the other hand, it acts as a barrier to the corrosive elements. The highly sacrificial property of the zinc coating provides cathodic protection on the steel surface. In addition, it also produces insoluble corrosion products, preventing porosity of the heat sprayed coating, thereby limiting substrate corrosion by preventing corrosive media from penetrating the exposed coating with the metal background.
Flame Spraying – Thermal spray aluminium with a flame of the combustible gas
The flame spraying process uses chemical energy due to the high combustion temperature from acetylene with oxygen, leading to high-temperature sparks. Materials including spray powder and combustion fuel are injected, melted, and accelerated to the material’s surface by an expanding gas stream. The jet velocity is typically below 100 m/s giving rise to particle velocities up to 80 m/s. The temperature of the spark is usually above 2500 degrees Celsius. This metal coating process typically allows for lamella coatings to have a density of 85 – 90%.
The flame will be adjusted during the spraying process, and usually, only a few square meters of coating is done at a time for easy temperature control. When the coating is finished, it will be fired with a flame. Flamethrower systems are usually manually operated but can be semi-automated or fully automated if required. The cost of materials and techniques of flame spraying is generally lower than that of arc spraying, but the cost of implementation is usually higher. The amount of material that Thermal Spray Aluminium with the flame process can spray is also limited by the size of the wire and the material.
Arc Spraying – Metal coating technology recovers the surface of the material
Arc spraying involves the creation of an electrically deflected arc between the ends of a molten conductor. The molten material is atomized using a high-speed flow of compressed air to accelerate the particles towards the surface to be coated. The system consists of the spray gun, feed system and power supply.The combination of high arc temperatures (6000 K) and particle velocities over 100 m/s give arc spray coatings superior bond strength and lower porosity compared to thermal spray aluminium coating by the flame mentioned above.
With valuable benefits and outstanding advantages, metal coating methods by thermal spraying are being widely applied in mechanical engineering, anti-corrosion, functional surface coating, machine manufacturing, machining, decoration, fine arts, etc. In addition to preventing corrosion, thermal coating technology also helps businesses to save materials, prolong the life of metal surfaces, and cut shutdown time and transportation costs operation and maintenance.
The thermal spray coating method provided by VIVABLAST has outstanding advantages such as saving materials, effectively extending metal lifetime, and reducing downtime machine stop and warranty cost will be the best choice for you.
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