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October 14 - October 20, 2013

How do you prepare flame-cut edges for application of thermal spray metallic coatings? The problem is that these edges are often harder than the abrasive, so a very low profile is produced, resulting in lower-than-specified adhesion of the thermal spray to the steel.

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Selected Answers

From Ken Gray of TayMae Associates Inc. on October 17, 2013:
I would agree with David Shaw's response. Using a hard abrasive such as aluminum oxide  will aid in profiling the edges.

From Simon Hope of Bilfinger Salamis on October 16, 2013:
The normal option for this issue is to remove the heat-affected area, which is effectively a case-hardening due to the melting and flash cooling of the flame-burnt edge. Grinding is slow and laborious but is the most common method for this type of remedial action. As was previously mentioned by Dave Shaw, the need for 2 mm minimum radius is also imperative, not just for TSA but for any coating. Flame-cut edges have been a bane of the coating industry for years, and one or the easiest solutions is to move away from the use of oxy-propane gas cutters and go with a cold cut method such as UHP water with entrained fine abrasive, which removes the case hardening problem along with the need to remove it as well as taking away the known risk of crack nucleation from burnt edges due to the stresses, irregularities and brittle failure potential.  All a cold cut requires is edge radiusing. Expensive and more complex, maybe, but the end result is worth every penny if you can remove a potential failure risk!

From Giuseppe Santagata of NACE Certif. Coat. Insp. level III # 2737 on October 15, 2013:
Flame cut edges are very hard surfaces. Even if you try to have a certain blasting profile, you obtain a very poor anchor profile. TSA need a coarse profile between 80 and 100µm for a proper bonding. These surface will be ground properly until the hard layer is removed, and after, blast cleaned with a correct abrasive media and blasting technique.

From David Shaw of Barrier Ltd on October 15, 2013:
First, these edges must be suitably dressed by grinding to remove the surface and any remnants of 'flutes', and to provide the minimum radius of 2 mm on edges. Previous experience shows that these specific areas must be re-blasted with a very hard abrasive, such as aluminium silicate or aluminium oxide. My opinion is even chilled iron will not prepare these areas correctly because it's all due to the 'case hardening' process during the flame cut. Hope my contractor's perspective assists.

From Royce Gardner of CH2MHill on October 14, 2013:
Utilize a larger grit or steel shot to achieve the required anchor profile.

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Tagged categories: Galvanized/thermal spray coatings; Paint application

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