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KTA
Challenge #14
Answer - Flame Cut Edge
Winner - John Bullard
Metallizing exhibits excellent adhesion when properly applied to thoroughly cleaned steel. A minimum SSPC-SP10, Near White degree of cleaning is required, with SSPC-SP5 often preferred. A dense, angular surface profile depth of 3.0 mils or greater is required. The specification in this case required an SSPC-SP10 degree of cleaning and a minimum surface profile of 3.5 mils.
The amperage of the application equipment must be properly adjusted, and adhesion and chisel tests conducted each day to assure that the process is controlled. In this case, adhesion tests were conducted each day while the work was in progress.
In addition, the applicators should apply the metallizing to coupons before beginning production work each day. The coupons are subjected to bend testing to verify that the surface cleanliness and surface profile are acceptable, the equipment is adjusted properly, and the application technique is sound. When all conditions are correct, the metallizing may crack slightly when the panels are bent, but will not lift or otherwise become detached from the steel. The inspection records indicate that the bend tests were performed and the results were acceptable.
Despite the tests and evaluations that were conducted while the work was in process, failure still occurred. The failure occurred because the contractor failed to pay attention to one key factor - the degree of surface roughening on the edges. The surface profile was measured and found to be acceptable on the flat surfaces. Because it is often difficult, if not impossible to easily measure the profile on the edges, the contractor assumed that since profile is primarily a function of abrasive type, size, and hardness, and because the profile was acceptable on the flat surfaces, it would be acceptable on the edges as well. Unfortunately this was not the case. An inspection of the edges in failing areas indicated that the profile was very sparse, and lacking in uniformity and depth. While some of this was due to inadequate attention during cleaning, the majority of the problem was associated with edges that had been flame cut.
When steel is thermally cut, a thin, very hard, martensitic layer is formed on the surface of the steel. This layer forms as a result of rapid resolidifcation after reaching the molten state during the cutting process. The martensitic layer remains relatively untouched by abrasive blasting because the surface is typically harder than the abrasive being used to clean it.
In order to achieve a profile on these surfaces, the flame cut edges
should be prepared by grinding to remove this surface layer prior to abrasive
blast cleaning. The correct preparation of thermal cut edges is discussed
in AASHTO/NSBA Steel Bridge Collaboration S 8.1-2002, "Guide Specification
for Coating Systems with Inorganic Zinc-Rich Primers." The AASHTO
standard was created by bridge engineers and steel fabricators to address
bridge structural steel fabrication and coating application problems.
The standard requires that all thermal cut edges be ground prior to abrasive
blasting to remove the hard layer. Once removed, the abrasive will generate
the same profile being achieved elsewhere.
KTA can assist you with your specifications or evaluations of metallizing
being applied in the shop or field, to help to avoid these types of problems.
Two thirds of the answers recognized that the edges had been flame cut
and did a good job explaining the problem that it creates. The remaining
answers concluded that the problem was related to surface profile, but
failed to acknowledge the contribution of the flame cut edges. The winner,
by random drawing from those who specifically called out the problem with
flame cut steel was John Bullard.
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