Corrosion under insulation (CUI) is a severe form of localized external corrosion that attacks the surface of very hot or very cold insulated steel equipment. Corrosion occurs when the insulation absorbs moisture that, in turn, wets the steel surface in the presence of oxygen. Attempts to prevent water from entering insulated systems, however, are not sufficiently reliable to prevent CUI, so protective coatings are acknowledged as a highly effective method of protecting insulated carbon steel and austenitic and duplex stainless-steel surfaces.  Specifically, third generation polysiloxane coatings have shown mitigation advantages.

Polysiloxane is an inorganic polymer with superior properties that include temperature tolerance and ultraviolet (UV) resistance. These polymers have demonstrated particular characteristics that enable them to withstand the harsh conditions that can lead to CUI. 

• The polysiloxane polymer’s silicon/oxygen bonding mechanism is very resistant to heat and UV degradation due to its higher bond dissociation energy (452 kJ/mol) and oxidized state.

• The traditional carbon/carbon bonding of organic-based polymers has lower bond dissociation energy (346 kJ/mol) and the potential to oxidize further, making the organic polymers more susceptible to degradation.

• When heat and UV resistance (either individually or combined) are required, polysiloxane chemistry demonstrates a greater degree of suitability over organic-based polymers.

First Generation Polysiloxane Coatings for CUI

A first generation polysiloxane coating, comprised of an epoxy-siloxane hybrid that cures in ambient temperatures, was introduced in the mid-1990s for use as an elevated-temperature coating (1,100 °F/593 °C), either exposed or under insulation. However, in cyclic ambient-hot-ambient service on small diameter piping or convoluted small shapes, it tended to crack and disbond from the substrate due to internal stresses caused by its two-component, highly cross-linked hybrid chemistry. And while it has continued in ambient temperature service, it is rarely used on high-heat equipment.

 Second Generation 

During the last 15 years, inorganic copolymer (IC) and inert multipolymeric matrix (IMM) coatings have been widely used for CUI mitigation. Known as second generation polysiloxanes, they have a higher level of temperature tolerance (1,200 °F/649 °C) due to the elimination of organic counterparts, the high concentration of inorganic siloxane-based polymers, and higher flexibility. These coatings were a step forward for providing protection to substrates under insulation; however, there are issues with the coatings’ usage in the field, such as a soft film before they are heated to curing temperatures.

Third Generation

Now, a third generation polysiloxane for CUI mitigation is available. Launched in 2015, this coating is a single-component, fully ambient-temperature cure, inorganic polysiloxane. This coating eliminates the field issues experienced with the second generation polysiloxane coatings, including decreased film hardness, anticorrosion properties, and weathering resistance prior to post-application curing.

Additionally, a new liquid-applied thermal insulation coating (TIC) was commercialized in early 2017. The low thermal conductivity (λ) properties of the coatings reduce the hot or cold surface temperatures of processing equipment such as pipe, ductwork, or tanks to within safe-to-touch limits, maintain temperatures in storage units, and provide thermal insulation properties. This technology can be applied at ambient or elevated temperatures for in-service applications and adhere to a range of anticorrosion primers, which results in a high-performance coating system that provides both corrosion-resistance and insulation properties.

Until recently, TICs were formulated from predominately organic-based polymers (acrylics or epoxies) and filled with ceramic/glass microspheres or silica-based insulation media, which influence the resulting low thermal conductivities of TICs. Two major drawbacks with the organic-based TICs include relatively low film-build capabilities and limited temperature resistance.

Further research and development of the polysiloxane matrix resulted in a water-based, single component, ambient-cure, third generation polysiloxane TIC technology. These coatings are designed for ultra-high-build dry film thicknesses that can exceed 800 mils (20 mm) and a temperature tolerance range from –76 to 752 °F (–60 to 400 °C). Plus, the polysiloxane polymer facilitates high weathering resistance and thermal cycling properties in ultra-high film thicknesses >800 mils — film thickness capabilities needed for efficient thermal insulation and personnel protection.

With these new developments in polysiloxane coatings, assets can be protected with thermal insulation and corrosion control via a two-coat system—a third generation polysiloxane TIC applied over a third generation polysiloxane CUI primer. This system enables a coating system to replace traditional coating/insulation systems that are prone to CUI. By using a system that doesn’t need a protective cladding, asset owners can visually assess equipment during inspection programs, which eliminates the hidden corrosion threat of CUI.

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Source: MaterialsPerformance.com Article based on CORROSION 2018 paper no. 11415, “Third Generation Polysiloxane Coatings for CUI Mitigation,” by J. Reynolds and P. Bock.

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