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July 27 - July 31, 2020

What can be done to prevent rust bleed in crevices when applying a zinc/epoxy/urethane system?


Selected Answers

From Wayne Senick of Termarust technologies Inc. on July 30, 2020:
The solution is simple! “Do not cover up an active corrosion cell” until you have cleaned it properly with the right chemistry (flushed out the non-visible contaminants and removed loose matter) and then applied the right chemically active low-viscosity, alkaline penetrant to chemically neutralize the corrosion cell. The corrosion cell will otherwise continue to persist. Refer to Purdue University study SPR-4121 • Report Number: FHWA/IN/JTRP-2018/16 • DOI: 10.5703/1288284316788. The corrosion-causing elements in the crevice-corroded or pack rust joint must be neutralized not covered up. (Otherwise they will only continue to bleed rust.) If you cover these corrosion cells up without cleaning, then chemically neutralizing them, you will only be recreating the same conditions which caused the crevice corrosion and pack rust to develop in the first place, thereby creating an area of negative oxygen where existing corrosion-causing chemistries continue to flourish. See Corrosion Doctors (NACE affiliate) website https://www.corrosion-doctors.org/Forms-crevice/Crevice.htm https://www.corrosion-doctors.org/Forms-crevice/crevice2.htm https://www.corrosion-doctors.org/Forms-crevice/crevice3.htm. Caulking these corrosion cells or covering them up with thick layers of high-performance state of the art barrier coatings does nothing to neutralize the acid, scavenge out the oxygen and displace the moisture inside the active corrosion cell. Testing has shown that covering them up before proper cleaning and chemically neutralizing the areas of corrosion actually accelerates the problem. Please refer to the JPCL article in the May 2017 issue, "Bridges Bedeviled." In this article the authors discuss a bridge which had hundreds of thousands of lineal feet of crevice-corroded and pack rust joints, most in the 1- to 2-inch-wide range that were treated with an alkaline, chemically active, low viscosity penetrant/sealer and a chemically compatible primer /topcoat after proper cleaning. The joints have been stable with no bleed except for 13% inside the splash zone, which could have been maintained after 15 years as was scheduled, but since the areas were so small they chose to forego the maintenance task. The rest of the 1,000,000 plus square foot structure is still stable after 22 years. The only way to stop rust bleed in joints and connections is the same way you would treat a cut on your finger. You would first clean it out with the right disinfectant to remove contaminants, then you would treat the wound with the right chemistry to kill any bacteria, and only then would you cover it up with a band aid. You would never cover up a dirty, bacteria laden wound with the world's best bandage and expect it to heal, yet we do that to crevice-corroded and pack-rusted joints and connections on structures every day and wonder why the joints bleed rust.

From Marco Fabio Ramenzoni of ZINGA METALL BRASIL on July 29, 2020:
If the crack is bleeding, seal it off with a sub-aquatic sealer, following the instructions of the sealer manufacturer. After that, prepare the surface and apply a flexible zinc primer in a duplex system combined with compatible epoxy plus polyurethane of choice, depending on the environment.  If we are talking about a crack between joint parts, and if it is possible, disassemble the parts, exposing the inner part of the crack. Prepare the surface, then apply a zinc rich flexible primer. Assemble the parts and proceed with the surface preparation and application of the preferred system. If the crack is not sealed, or somewhat open, seal it as suggested above, and proceed with the process.

From Jamie Armstrong of CBRE on July 29, 2020:
I would suggest using a wire wheel or hand tool to remove what rust you could, prime with a rust-inhibitive primer, then apply a two-part epoxy or urethane caulk to seal crack. Document area(s) well to show inspector(s) what you were dealing with, or get them involved early on.

From Erik Andreassen of Vivablast Vietnam on July 27, 2020:
If the areas are what is classed as inaccessible, seal off the areas with a gun grade sealer, prior to the application of any coating.

From Jaime Marcos of Profesional independiente on July 27, 2020:
If the crack is not repaired by welding, first mechanically repair the crack to leave it open, so that its interior can be cleaned well and facilitate being filled with putty. There are direct metal putties, and those that need to be applied over a primer. In this case, since it is a zinc-rich primer, which has to be applied directly to the blasted surface, it will be necessary to apply the putty to seal the cracks on the zinc-rich primer. Next, the rest of the layers are applied.

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Tagged categories: Asia Pacific; EMEA (Europe, Middle East and Africa); Latin America; Maintenance + Renovation; North America; Rust


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