Cathodic Protection

The first practical use of cathodic protection is generally credited to Sir Humphrey Davy in the 1820s. Davy’s advice was sought by the Royal Navy in investigating the corrosion of copper sheeting used for cladding the hulls of naval vessels. Davy found that he could preserve copper in sea water by the attachment of small quantities of iron or zinc; the copper became, as Davy put it, “cathodically protected”.

The most rapid development of cathodic-protection systems was made in the United States of America to meet the requirements of the rapidly expanding oil and natural gas industry which wanted to benefit from the advantages of using thin-walled steel pipes for underground transmission. For that purpose the method was well established in the United States in 1945. In the United Kingdom, where low-pressure thicker-walled cast-iron pipes were extensively used, very little cathodic protection was applied until the early 1950s. The increasing use of cathodic protection has arisen from the success of the method used from 1952 onwards to protect about 1000 miles of wartime fuel-line network that had been laid between 1940 and 1944. The method is now well established.

Cathodic protection can, in principle, be applied to any metallic structure in contact with a bulk electrolyte. In practice its main use is to protect steel structures buried in soil or immersed in water. It cannot be used to prevent atmospheric corrosion. Structures commonly protected are the exterior surfaces of pipelines, ships’ hulls, jetties,foundation piling, steel sheet-piling, and offshore platforms. 

Cathodic protection is also used on the interior surfaces of water-storage tanks and water-circulating systems. However, since an external anode will seldom spread the protection for a distance of more than two or three pipe-diameters, the method is not suitable for the protection of small-bore pipework. 

Cathodic protection has also been applied to steel embedded in concrete, to copper-based alloys in water systems, and, exceptionally, to lead-sheathed cables and to aluminium alloys, where cathodic potentials have to be very carefully controlled.

Source:http://www.npl.co.uk/upload/pdf/cathodic_protection_in_practise.pdf

CUI Affected Units / Areas in LNG Equipment:

Corrosion Under Insulation (CUI) occurs when water enters external insulation through holes or gaps in the insulation covering, or when moisture in the air condenses on the metal surface below the insulation (known as sweating). The water tends to collect at low spots, insulation support rings and other external appendages, or simply beneath the insulation. If an adequate protective coating is not present, significant external corrosion can occur. The often random and localized nature of the attack, and the fact that it is hidden under the insulation, makes CUI difficult to detect.

Critical Factors

• Location Issues 

Common areas of concern in process units are higher moisture areas such as those areas down-wind from cooling towers, near steam vents, deluge systems, acid vapors, or near supplemental cooling with water spray.

• Design Issues

CUI can be found on equipment with damaged insulation, vapor barriers, weather proofing or mastic, or protrusions through the insulation or at insulation termination points such as flanges.

• Equipment designed with insulation support rings welded directly to the vessel wall (no standoff ); particularly around ladder and platform clips, and lifting lugs, nozzles and stiffener rings. 

Prevention / Mitigation

• Since the majority of construction materials used in plants is susceptible to CUI degradation, mitigation is best achieved by using appropriate paints/coatings and maintaining the insulation/sealing/vapor barriers to prevent moisture ingress. 
• High quality coatings, properly applied, can provide long term protection.
• Careful selection of insulating materials is important. Closed-cell foam glass materials will hold less water against the vessel/pipe wall than mineral wool and potentially be less corrosive. 
• Low chloride insulation should be used on 300 Series SS to minimize the potential for pitting and chloride SCC. 
• CUI can be found on equipment with damaged insulation, vapor barriers, weatherproofing or mastic, or protrusions through the insulation or at insulation termination points such as flanges. 
• Equipment designed with insulation support rings welded directly to the vessel wall (no standoff ); particularly around ladder and platform clips, and li fting lugs, nozzles and stiffener rings.

Source:http://www.adgas.com/En/Publication/Corrosion%20Booklet.pdf

Radio frequency identification technology (RFID)

Radio frequency identification technology was originally created to identify friendly aircraft from enemy aircraft during the Second World War. It eventually found its way  into civilian applications ranging from door access control to livestock tracking.

It is a wireless technology which is recently being used as a replacement for the traditional barcode system. An RFID system consists of two hardware components: the reader and the tag/transponder. The reader itself typically consists of a microcontroller alongside RF circuitry such as envelope detectors and filters which are required to transmit and receive RF energy.

The reader includes a coil or antenna which is used to transmit and receive. The functions of the reader include:
(1) Transfer enough power to the tag to energise it;
(2) receive the data stored in the tag’s memory via response signal from the tag;
(3) write data to tags memory.

The hardware of the tag comprises a microchip with memory which stores the tags unique identification code. The RF portion of the tag is made up of a wound or printed coil connected to a capacitance to form a tuned LC circuit.

One of the biggest challenges posed by CUI is the inaccessibility due to the large standoff distance introduced by thick insulation. Moisture in the insulation and high temperatures create a microclimate which can accelerate the development of corrosion.

The hidden nature of CUI may result in it going unnoticed for long periods of time leading to potentially catastrophic failures. A review of the literature on the most common NDT techniques for corrosion detection showed that the majority of techniques are limited when it comes to online in-situ monitoring of CUI primarily due to the thick insulation layer.

Solutions to overcome this problem typically involves either applying much higher input power using bulky,pensive equipment or the use of inspection holes in the insulation layer to send waves along the length of the pipe to another inspection hole. The passive RFID sensors identified in the literature demonstrate the potential of cheap, battery free wireless sensing. RFID tags have been adapted to sense a wide range of diverse phenomena.

Due to their battery-free operation, RFID tags have shown the potential to be embedded into structures such as concrete for long term condition monitoring. However, the sensing tags used in majority of the studies examined make use of either thin films or sacrificial elements added to the tag. The changes that occur to the thin films or the sacrificial elements may not represent the true condition of the structure on which the tag is embedded.

Furthermore, long term monitoring using RFID tags is limited if the sensing elements of a tag degrades faster than the structure it’s monitoring. Another challenge not addressed by existing studies is the resulting performance degradation when tags are placed onto metallic structures. To tackle the particular problem of thick insulation, the use of passive LF RFID tags as corrosion sensors is proposed in this study.

The developed system aims to address the following issues:

• Obtain corrosion measurements on steel through thick layers of insulation/ large standoff distances.
• Since the system is passive, a method needs to be developed to address the issue of varying displacement between the reader and the tag, i.e. obtain position independent corrosion measurements. 
• The system much be very low cost, and use off-the-shelf components. To minimise costs further, the tag should be unmodified. The interaction between the tag coil and the metal being the sensing mechanism. 
• Suitability for long term monitoring by not using any sacrificial elements. 

Source: https://theses.ncl.ac.uk/dspace/bitstream/10443/2348/1/Alamin%2014.pdf

Detection of Corrosion under Insulation Using Real Time Digital Radiography

Corrosion under insulation is a major industry problem because it is difficult to detect due mainly to the insulation cover masking potential flaws and/or flawed areas. 

These flaws can cause failures in areas that are not normally a primary focus of an inspection program. Failures can be costly at best and disastrous at their worst. With the real time digital radiography (DRT) scanner, both internal and external corrosion defects on stretches of pipe can be detected while the pipe remains in service without removing any insulation. DRT system consists of a radiation source and a linear array of radiation detectors, which are designed to examine insulated piping systems ranging from 4 to 24 inches (8 to 36 inches with insulation) in diameter and up to 3 inches thick.

During the detection process, a narrow beam of radiation is projected through the pipe walls and onto the detector array, positioned on the opposite side of the pipe. As the scanner moves along the pipe, data are acquired, and a color-coded image that depicts the relative thickness of the pipe wall is generated and displayed on the monitor in real time. Rail mounted DRT scanner has the ability to scan straight piping (in the vertical or horizontal position), elbows, and tees. A circumferential scanner can also be used to examine 100% of the piping in localized areas near elbows, tees, and other suspect areas. The system is sensitive enough to detect corrosion defects as small as 0.25 inch in diameter and 0.05 inch deep

BENEFITS:

• No removal of insulation required
• Thorough and quick scan of long sections of insulated pipe via rail mounted system
• Pipes can be examined while in service
• 100% circumferential volume examination possible
• Detects corrosion defects as small as 0.25 inch and 0.05 inch deep
• Real time detection
• Capable of scanning vertical and horizontal straight pipe runs, elbows and T-s
• Capable of scanning from 4” to 24” diameter pipe (excluding insulation)
• System is compact and easy to install

Source:http://www.ihiswt.com/sites/default/files/DRT%2009092011_0.pdf

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