Methods for mitigation of corrosion under insulation and other crevice corrosion

Corrosion of steel operating equipment and piping under insulation has been recognised as an important problem in the ammonia refrigeration, chilled water, chemical and petroleum industry.

Insulation is a necessary component and there to function in three ways: save energy, control process temperatures, and protect workers from high wall temperatures. The environment under insulation, the corrosion under insulation (CUI) environment, can be hot, wet, and promotes aggressive corrosion.

The American Petroleum Institute has directives that address the CUI problem and detail a program of identification, maintenance, and remediation. These directives, as well as efforts by professional societies (NACE and ASTM), promote the development of new solutions. The issue in achieving a good end result is that no clear solution exists for new installed piping as well as maintenance and remediation of existing installations.

NACE Standard RP0198-98 is an excellent source of information for preventing corrosion under insulation, but many corrosion engineers would agree that electrolytes will eventually find their way into even the best system. Selecting the right coating is extremely important. The coating is the last line of defence for keeping the electrolyte from the metal surface and preventing corrosion.

Recent coating innovations include a hydrophobic anti-corrosion gel that is tolerant of less than optimal surface preparation, is designed to keep the electrolyte away from the surface of the substrate, and also has the ability to neutralise the electrolyte if it breeches the vapour barrier and insulation.

Technology

The reactive anti-corrosion gel utilises mineralisation technology. Mineralisation is the ability to grow very thin minerals on metal surfaces for useful purposes.

The minerals are formed when reactants are delivered to the surface of the substrate, as shown in Figure 1.

How the reactive gel corrosion treatment works:

When the ferrous (steel) surface (1) is covered with a layer of reactive gel (2), the metal surface reacts with components in the gel to form a mineral layer (3). This thin glasslike layer (3) acts as a barrier between chlorides and the metal surface, thus providing corrosion resistance.

The mineral layer (3) has a thickness of 50–200 angstroms, only 0.01 per cent, or as thick as a piece of paper.

Although the thin mineral layer can be damaged by mechanical abuse, there is extra protection built into the system.

The presence and uniqueness of the mineralised layer can be confirmed by conventional analytical surface methods such as x-ray photoelectron spectroscopy or atomic force microscope (Figure 2 and Figure 3).

The anti-corrosion gel works in three basic ways:

1. Barrier system – the specially formulated products have great adhesion characteristics and are hydrophobic to help keep moisture away from the substrate.
2. Buffering system – if moisture migrates through the gel, it is buffered to a high pH which is protective to steel piping.
3. Mineralisation – growing an engineered surface, or surface conversion – creating a surface which resists corrosion even if moisture gets to it.

The anti-corrosion gel has a maximum service temperature of 350°F (177°C).

The mineralisation technology in the anti-corrosion gel has a history of solving unique corrosion problems. The first application of the mineralisation technology was by a major automotive supplier in a crevice corrosion application on strand of brake cables. The strand in sleeve design of the brake cable combined with the cyclical environment of heat and moisture creates a severe crevice corrosion environment. The technology has been used for over 30 years in this application, which has resulted in an increased service life and greater reliability.

The first non-automotive industrial application was with the United States Navy. Following successful laboratory, pier side, and shipboard demonstrations of the effectiveness of the gel in preventing crevice corrosion in anchor chain detachable link cavities, the US Navy in 1999 changed the Planned Maintenance System (PMS) to specify the use of a mineralising gel as the replacement for white lead and tallow in all surface ship anchor chain detachable links. Also in 1999, following extensive testing, the Navy issued MACHALT 526 which changed the design of the internals of weather deck watertight and airtight door dogging mechanisms. The basis for the change is the use of a mineralising lubricant inside the spindle sleeve in the door frame to stop the corrosion that had been the cause of dogging mechanism failure. The watertight door dogging mechanism corrosion problem was one of the top maintenance issues for the fleet. In May 2002 a second MACHALT, 544, was approved to apply the same technology to ballistic type dogs in three watertight doors in DDG-51 Class ships. These solutions represented a significant saving for the fleet.

The gel has years of history on CUI applications in the food and beverage industry. It has also been used as an anti-corrosion coating in well head casings, on pig doors, structural steel, tank chimes, ammonia systems, vessels, and as flange filler. Field trials are currently underway to further evaluate this technology in areas where it is cost prohibitive to achieve optimal surface preparation.

Source:http://pipeliner.com.au/news/methods_for_mitigation_of_corrosion_under_insulation_and_other_crevice_corr/80887