Problem Solving Forum
April 23 - May 5, 2010
A specification for a bridge to be built of high-performance weathering steel (HPS GR50/GR70W) requires an SSPC-SP 10, a profile of 3 to 5 mils, and metallizing with a final dft of 10-12 mils. What challenges, time, and costs are associated with obtaining the profile, primarily in the shop but also in the field? Are there any long-term performance issues with metallizing over weathering steel?
Chuck Pease of MMI Tank on
May 5, 2010:
It can be very difficult and costly to achieve a 3 ...read more It can be very difficult and costly to achieve a 3-5 mil blast profile on weathering steel because it has an inherent surface hardness greater than conventional carbon steel. It can therefore take 30-40 percent more hours of labor to achieve a 3-5 mil profile.
Weathering steel construction plays a part because a typical weathering steel bridge girder is unique in design to minimize the accumulation of debris and moisture.
We could not achieve a 3-5 mil profile using even using G-25 steel grit with a 40-50 Rockwell C hardness while meeting an SSPC-SP 10 degree of cleanliness. The best we could come up with was 1.75 mils. We were finally able to achieve an average of 4.2 mils using a mixture of G18/G16 steel grit.
We were able to convince the DOT to allow a more aggressive profile as a better anchor profile for the subsequent metallizing.
As for long-term performance issues with metallizing over weathering steel, I am not aware of any to date.
In specifying weathering steel for structures to be metallized, the specifier should take care not to include stitch welds; stiffener connections to bottom flanges should be specified to be continuous welds all along the stiffeners, and snipes should be eliminated or continously welded to the webs.
Other areas of concern for long-term metallizing performance would be to eliminate areas that will hold water or condensation for long periods of time, as crevice corrosion can develop and the oxide coating found on typical weathering steel will not develop on surfaces that are continually wet or covered with debris.
Zinc metallizing systems are porous until the zinc oxidizes, so care should be taken to design weathering or carbon steel structures where these micro-enviroments cannot develop.