December 20, 2010 - January 2, 2011
What is the Cold Wall Effect? Can it cause premature coating failure and, if so, how do I prevent or minimize it?
Simon Hope of BIS Salamis (M&I) on
December 29, 2010:
'Cold Wall Effect' is normally due to a temperature gradient being built up across a coating as described in the previous two answers. It is commonly encountered in ships' tanks where the contents are at a significantly higher temperature than the surrounding ambient, or it regularly occurs in process vessels where a process liquid is running significantly higher than the ambient. Classically with process vessels these have originally been insulated with heat conservation materials but have been selectively stripped (typically from 5 o'clock to 7 o'clock for belly inspection and corrosion monitoring as well as prevention of CUI (Corrosion Under Insulation). This lack of insulation creates the necessary thermal gradient that can lead to Cold Wall failures of the coating. Reinstatement of insulation using a syntactic material against the more common mineral wool insulations can help reduce Cold Wall failures due to lack of insulation and saturated areas of wool cooling the shell. The major drawback is the lack of ability to undertake shell inspections using conventional U/T methods externally, so removable panels need to be integrated into the design so that all parties are satisfied. Internal coatings need to be high density and low permeability with a coefficient of expansion closely in line with the substrate to avoid micro cracking when heat-cycled. Typically, solvent-free epoxy phenolic type coatings with carefully selected fillers (normally ceramics) will give the best resistance to this type of failure.
As can be seen, it is not normally attributable to a single source, but a combination of problems leading to a situation where failure may occur. Careful design and a full understanding of the operation of the item can help considerably with mitigation and avoidance.
David Grove of Shaw Nuclear Power Services on
December 28, 2010:
The cold wall effect is where there is a temperature difference between one side of a sheet structure to the other side, which generally can cause condensation or blisters under the coating. The best way to eliminate this is to ensure that the surface is washed and rinsed with clean fresh potable water before final surface preparation with sufficient profile. For heavily rusted, chemically contaminated surfaces, I used to specify a commercial (or rough) blast to expose the steel, then a final wash followed by a near-white (SSPC-SP 10) blast. Since all coatings are some- what permeable, the key is to ensure that any moisture intrusion does not have a surface contaminant to react to and cause a blister. Contact your coating technical service for specifics.
Craig Wallbank of W Abrasives on
December 21, 2010:
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Cold wall effect is a term commonly used when a structure, say a holding tank, has a lower atmospheric temperature on the outside than the product that is stored inside. This thermal difference can cause water vapour to build up between the substrate and the lining that was applied to the inside of the tank. As the water may hold soluble salts, this will have an effect of 'blistering' and therefore a failure. Although I am not a coating 'expert,' I understand that the lining system should have a low permabilty rating to help prevent this from happening.
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