Major hot dip galvanizers in both the USA and Europe performed trials with EPS-processed coils last year and recently provided us with their results. The USA galvanizer reported ‘excellent results’ but wishes to keep their identity and data confidential and we respect that. The European trial was sponsored by SMS-SIEMAG, and is fully documented in Section K of the EPS End Use And Application Test Results Report, Revision 9.
The test coil was CQ low carbon steel of European Grade EN S355, which corresponds to a minimum yield strength of 355 MPa (51,500 psi). The strip size was 1106mm x 1.99mm (43.5″ x 0.0783″). It was EPS processed at Stripco in April 2013, packaged dry, and shipped to the European galvanizer, where it was processed in August 2013.
Because the normal EPS surface is ‘rougher’ (higher Ra) than the acid pickled material the galvanizer is accustomed to processing, they elected to slit the EPS coil into five mults and ‘skin pass’ roll four of these mults to achieve lower Ra values. They felt this afforded more of an ‘apples-to-apples’ comparison to the acid pickled material they typically galvanize. The average measured Ra values of the four mults that were skin passed were 1.88, 1.53, 1.09, 1.04 μm (74, 60, 43, and 41 μin). The remaining mult retained the original EPS surface roughness, having an Ra value of 2.45 μm (96 μin).
|All five mults were then were then run through the galvanizing line with zinc coating thickness set to Z275, which corresponds to 137.5 g/m2 per side (equivalent to US spec G90). The average line speed was 62 m/min (200 ft/min.). Afterward, visual inspection of all five galvanized mults was performed, followed by the standard test of zinc coating adhesion, the punch ball impact test.
In this test, an apparatus is used to drop a large bearing ball onto the sample in a controlled manner. The impact makes a pronounced indentation in the sample as shown in the closeup photo below ( photo is from the EPS galvanizing test conducted in the USA). The test determines how well the zinc coating adheres to the surface of the indentation where the material has been stretch-deformed by the impact, and some ‘flaking’ of the zinc coating is likely.
In all cases (all five mults) the galvanized coating was described as having a very uniform ‘tight spangle’ appearance, with the higher Ra (non-smoothed) sample having a slightly duller finish.
The punch ball adhesion test produced a result of “Excellent” (no observable flaking) for the four smoothed mults and “Good” (very minor flaking) for the non-smoothed sample.
(Note: of the several galvanizing test performed on EPS samples, this is the first where there were any observable differences between the normal (higher Ra) EPS surface texture and lower Ra surfaces).
After reviewing all test results, the SMS-SIEMAG galvanizing expert offered the following conclusions:
- EPS-processed steel offers very good performance for hot dip galvanizing.
- It presents a very uniform, “tight” finish with excellent adhesion.
- There is no indication that EPS-processed steel is susceptible to Iron-Zinc-alloying (a “Galvannealed” surface).
- The EPS descaling process can be combined with a hot strip
galvanizing line in place of an acid pickling section.
Again, all test details can be found in Section K of the EPS End Use And Application Test Results Report, Revision 9.