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January 13 - January 17, 2020

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?

Selected Answers

From Grant Blohm of Metalizing Pro Inc on January 23, 2020:
Attaining the specified blast profile is basically a function of a number of different factors, including grit size, grit hardness, grit shape, impact energy, impact angle and embedment. Abrasive blasting for metallized coatings does require a bit more profile than conventional paint coatings; however, an SP  10 is adequate for all applications except immersion. When blasting, use a good, hard steel grit (G25) in a conventional pressure blaster and maintain a minimum of 110 psi at the nozzle and it will yield the desired 3-5 mil profile. I would stay away from using a centrifugal blast machine for steel girders. They just don't generate the velocity and impact angle that comes from pressure blasting. Although, typically, metallized coatings are applied only on the ends of weathering steel girders, there shouldn't be any long term issues any more than there would be with conventional steel girders.

From Joe McGreal of Ervin Industries. on January 22, 2020:
On the subject of surface preparation on weathering steel and any challenges associated with achieving the profile in a timely and efficient manner, the following information will be useful. Cast steel grit provides the fastest and most economical surface cleaning in a shop environment. Providing the lowest dust generation and minimal grit breakdown keeps the blasting production at its highest level. More importantly, high hardness steel grit (60 Hardness Rockwell C) has the highest impact value of all known abrasives, which speeds up blasting speeds to achieve a 3 -5 mil profile faster. Blasting can be performed in a centrifugal blast wheel machine or a compressed air system with a manual operator using a hose and nozzle set up. Regardless of the blasting system type, steel abrasives offer the ability to select a shot or grit size and hardness to precisely get the desired results. This makes the project economical and reliable.

From Julian Hay of JhayCon Inspection and Consulting on January 21, 2020:
The main challenge here appears to be stupidity. Why are they using (more costly) ACS type steel and then requiring it to be metalized? ACR steel is intended to be left exposed (and its very name, "weathering steel," should tell you this.) Secondly,  if it is in fact to be metallized, why is SP10 accepted when metallizing commonly requires ultra-clean steel with an SP 5 white metal blast (a commonly recognized fact). And lastly, why is there thought to be a problem obtaining a 3-5 mils blast profile. There isn't .Simply choose the correct grit size and the issue is solved (and don't use worn out, worn down, over-used, excessively recycled grit.  That goes without saying,

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Tagged categories: Asia Pacific; EMEA (Europe, Middle East and Africa); Latin America; North America; Quality Control; Weathering

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