International Paint, a worldwide leader in protective paints and coatings, has launched a new coating to tackle the problem of corrosion on the externals of equipment operating in cyclic temperature conditions.

Called Intertherm 751 CSA, the new coating has undergone successful testing and field trials and offers a solution to a previously unsolved corrosion issue.

CORROSION UNDER INSULATION
In order to provide increased thermal efficiency, increased control of process temperatures and/or for personal safety, equipment operating at elevated temperatures is often insulated. 
Moisture entering the insulation can collect and the corrosion this moisture causes to the steelwork under the insulation (the external surfaces of pipes, vessels and equipment) is commonly called corrosion under insulation (CUI).
CUI is a major industrial problem which causes billions of dollars of damage to equipment. The effects of CUI are often hidden under the insulation and therefore difficult to inspect and are sometimes only discovered when catastrophic failure occurs.
According to the European Federation of Corrosion, CUI can occur at up to operating temperatures of 175 deg C, with the most aggressive attack occurring between 80 deg C and 120 deg C when the warm, wet conditions are ideal for corrosion.
Water can enter the insulation from external water sources or form as condensation. External water sources will only make contact with the insulation if the outer layer is not water-tight. Upon installation the outer layer should be water-tight, commonly utilising aluminium cladding, but experience has shown that in time mechanical damage and degradation of the sealing compounds lead to water penetration into the insulation. 
It is difficult to create an outer layer which is impermeable to vapour (if this does occur there is also the possible disadvantage of sealing moisture into the insulation) so formation of condensation will always be a possibility if the surface temperature drops below the dew point temperature.
It has been reported that CUI risk rises significantly after five to 10 years of service in a plant and an average of 60 per cent of all insulation in service over 10 years will contain corrosion inducing moisture. 
Once water has entered the insulation, the higher temperatures inside the insulation create an ideal environment for accelerated corrosion, which can be made more aggressive by contaminants leaching out of the insulation material.

PROTECTIVE COATINGS FOR CUI
NACE (document RPA0198) recommends protecting the metal surface of equipment under insulation with a high performance coating as it is very difficult to ensure that the insulation will remain dry for the duration of the service life of the equipment. NACE does not recommend the use of zinc silicates under insulation between the temperatures of 50 deg C and 150 deg C as the zinc can be consumed rapidly if the insulation becomes wet.
If the equipment remains at a constant temperature below 200 deg C, epoxy phenolics have been shown to provide good long term protection. The limitation of epoxy phenolics is that they can start to degrade above 230 deg C. If the equipment experiences temperatures above 230 deg C at any time during its service life, an epoxy phenolic coating will not be suitable. 
Until recently, there was a gap in the protective coatings market, where there were no polymeric coatings to offer corrosion protections for vessels experiencing high temperatures, whether during service or steam clean-out procedures. If these vessels were to also experience lower temperatures (below 150 deg C) where CUI can occur, either during cyclic service, intermittent use or shutdown, due to the prior higher temperatures experienced any conventional protective coating would no longer be capable of providing corrosion protection.

INTERTHERM DEVELOPMENT
The development of the new generation of high temperature coating has been a 10-year process and the coating developed has been named Intertherm 751 CSA. Along with development of the coating system, new laboratory test methods were developed in order to replicate the high temperature, corrosion inducing environments experienced in industry.
Current international testing standards do not test the combination of heat resistance and corrosion resistance and therefore do not truly represent what happens in the actual industrial environment where warm and wet insulation creates high rates of CUI and equipment is also likely to experience thermal cycling processes.
It was decided to test the Intertherm 751 CSA under the new test methods – the International Paint Cyclic Pipe Test – and compare these results against coatings which are commonly used against on external surfaces, those being aluminium silicone and epoxy phenolic.

SUITABILITY FOR INSULATED SUBSTRATES
 The International Paint Cyclic Pipe Test consisted of an external coating of a 60-mm diameter, 600-mm-long carbon steel pipe which is allowed to cure at ambient temperatures.  Insulation (calcium silicate) was then placed on the pipe and the insulated pipe stood upright on a heating plate. 
One litre of water – including one per cent NaCl (sodium chloride) – is poured into the open end of the insulation to stimulate corrosion. After adding the water the test plate is heated to 450 deg C producing a thermal gradient of 450 deg C at the base of the pipe (in contact with the heater) to 60 deg C at the top of the pipe (exposed to the ambient laboratory temperature).
After eight hours of heating another litre of one per cent NaCl water was added to the insulation, the ambient temperature of the water causes the hot coating to experience a thermal shock as it comes in contact with the cooler water. The heater turned off and the pipe is allowed to cool for 16 hours. This cycle is repeated 30 times.
This test shows how the coating performs at various temperatures under insulation. The addition of water and cycling of the temperature allows all areas to become wet and allow corrosion to occur even at areas of the coating heated to higher temperatures where moisture would not be present.
Performance varied between the systems.  There was rusting over the full length of the pipe coated with aluminium silicone showing that in warm or hot corrosion environments aluminium silicones will not provide long term corrosion protection.  As expected, below 230 deg C the epoxy phenolic showed good corrosion protection with no rust being present, but degraded at temperatures above 230 deg C.  Intertherm 751 CSA was in excellent condition with no degradation to the coating displaying excellent corrosion resistance at all temperatures indicating a combination of excellent corrosion and heat resistance.
Following success in the laboratory, trials were carried out in the field. A propane treater tower in Australia, which had previously been coated with zinc silicate, was showing extensive corrosion under insulation (figure 1). The tower required corrosion protection before the metal loss exceeded the corrosion allowance requiring replacement of the tower. 
The tower operated a 168-hour temperature cycle, starting at the ambient temperature of 40 deg C and increasing rapidly to 260 deg C which was maintained for three hours before cooling over a further three hours to 80 deg C.  It was then reheated rapidly to 260 deg C where it remained for 21 hours before cooling naturally to the ambient temperature of 40 deg C over 140 hours. 
The corroded carbon steel surface was grit blasted to SA 2.5 and Intertherm 751 CSA applied by airless spray in one coat at a thickness of 200µm (figure 2). After cure, the insulation was replaced and the vessel entered service. A year later an inspection hole was cut into the insulation at the site of an external support ring on the vessel (this ring often trapped water and was an area of high corrosion potential). Intertherm 751 CSA was found to be in good condition with no sign of corrosion (figure 3).

CONCLUSION
Experience has shown that test methods must be carefully selected to reflect the true operating conditions experienced by the coating.  If the testing does not truly reflect the actual service environment it is difficult to select the correct coating to protect the equipment.
Field trials have shown that the new International Paint laboratory test methods offer a fair representation of the environment experienced in the field.
Testing and field trials have shown that Intertherm 751 CSA provides corrosion protection on external surfaces whether under insulation or exposed to the environment up to temperatures of 400 deg C and down to -196 deg C.  The corrosion protection offered is unaffected by exposure to temperatures up to 400 deg C and/or rapid thermal cycling including thermal shock.
At temperatures below 200 deg C, Intertherm 751 CSA shows similar corrosion protection properties to an epoxy phenolic.
Intertherm 751 CSA therefore brings a new, unique offering to the range of protective coatings available in the market.  It provides answers to until now unsolved corrosion issues. A further feature to the coating is the possibility to use it to reduce the coating selection complexity in a plant.  Instead of having different coating systems for different temperature ranges, Intertherm 751 CSA can be specified for the externals of all vessels, pipes and equipment operating up to a maximum temperature of 400 deg C.
* Matthew Fletcher is International Paint’s market development manager for downstream petrochemical coatings, EMEA.