September 3 - September 9, 2012
What is the proper surface preparation for stainless steel before coating to resist a salt-laden atmosphere?
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Derek Righinni of Certified Coating Inspection Ltd on
September 13, 2012:
There is no clear consensus on a cleanliness standard that stainless steel is required to meet before the application of coatings to protect it from chloride-induced pitting or stress corrosion cracking. The question is, is the chromium oxide passive layer that forms rapidly on the surface of SS tightly adherent and suitable for painting? The answer to the question dictates the time period allowable between the start of blasting to the maximum time the SS substrate can be left before coating. NACE, SSPC, ISO, ICORR: pull your finger out and give the industry some clear guidance on the preparation of stainless steel for painting with protective coatings and linings.
William Slama of International Paint/Ceilcote Products on
September 10, 2012:
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Over the years, there have been many of these applications required where the original alloy was found to be inadequate for the operating conditions (usually chloride fluoride chemistry) of the vessel or equipment. In some cases, this was identified even before being placed in service; other times, when the alloy didfn't perform as expected. A recurring recent case is the many FGD vessels (absorbers and other vessels) constructed of Alloy 2205. Apparently, buildup of scale creates a concentration of chlorides/fluordes under scale buildup, which resusts in extreme pitting of the alloy.
The surface preparation, not suprisingly, is not much different than that needed for lining carbon steel. Basically, it requires a clean surface with a sharp profile with the required blast profile (depth) for the system being applied. For most high-build linings or coatings, a 3 mil profile is needed.
For most alloys, adjacent unlined surfaces should be protected during abrasive blasting to ensure that it is not contaminated by iron. That is because the most commonly used slag abrasives contain iron.
In some cases, that possibility is avoided by using a blast abrasive that does not contain iron. Garnet or aluminum oxide are suitable, providing they are able to produce the required sharp profile.
As with carbon steel, the surface must be tested for ion contamination and cleaned if necessary to the lining manufacturer's requirements.
One differing feature is that the surface must be coated or primed soon after blasting, unless DH control holds less than 40-50% RH. That is because the surface will not show discoloration of surface oxidation as carbon steel would.
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