CORROSION INHIBITING STEEL PAINT PRIMER
1.0 VCI TECHNOLOGY
Imparting corrosion preventing properties to protective coatings continues to be a problem particularly for coating rusted and chloride contaminated steel surfaces. The cost of steel surface preparation to a white steel condition has a high cost and is needed for coatings containing pigmented type of corrosion inhibitors to function.
The Corps of Engineers recognized these problems and commissioned a R&D project with Surtreat Holding, LLC to develop a protective coating containing a vapor phase migratory corrosion inhibitor, designated here as a VCI agent, which could migrate through layers of rust. The project led to development of a 2 part polyamide epoxy primer meeting all specifications of MIL-DTL-4441. The primer is being manufactured and marketed by VCI Coatings, LLC.
The primer contains an amount of the VCI agent in part A such that when combined with part B the resulting dry film contains 2.5 weight percent of the VCI agent. Performance test on abraded white and corroded steel (24 hours in salt fog) surfaces, covered with about 3 mils of primer dry film (100% adhesion per ASTM D3359), after exposure for 500 hours in a salt fog chamber ASTM B117 showed a difference in level of corrosion based on the degree of corrosion per ASTM D610 of 9 with the VCI agent added and 3 without. Performance was about similar for both white and corroded steel surface.
2.2 COMPETITIVE PERFORMANCE
VCI agent will migrate through rust to the white steel surface and inhibit further corrosion in comparison to the same coating not containing the VCI agent, which will only perform best when in direct contact with the white steel surface. Performance on all surface conditions will be superior with the VCI agent added in comparison to a basic primer performance.
2.3 ECONOMICS FOR USE
A 2 part polyamide type of epoxy primer with the VCI agent added will be about twice the cost per gallon of the basic primer, but the higher cost will be more than off set by reduction in surface preparation cost (removal of rust to white steel), and the extended life cycle of the paint job by a factor of 2 based on the 500 hour salt fog exposure test results. The following table presents a version of the economic advantage for using primer with VCI agent added. * It includes variables and approximations subject to change.
Price Per Gallon $ 60.00 $120.00
Cost/Ft @ 3 mils
Primer cost $ 0.25 $ 0.50
White steel $ 5.00
Pressure water wash $ 1.00
Estimated Labor $ 0.50 $ 0.50
Total Applied Cost $ 5.75 $ 2.00
Expected Life with
Urethane Top Coat 10 years 20 Years
Life Cycle Cost (Primer only) $ 0.57 $ 0.10
3.1 NEW STEEL STRUCTURES
New structures fabricated with fresh surface abraded steel will benefit when coated with a primer containing a VCI agent by inhibiting corrosion caused by final coating pin holes, future surface damage, flash rust and anodic hot spots in steel due to copper and aluminum inclusions from scrap recycle.
3.2 EXISTING STEEL STRUCTURES
3.2.1 Existing Structures Prepared For Total Surface Painting
Depending on the degree of prior corrosion the prepared steel surface can have corrosion hot spots due to residual contamination. Primer with VCI Agent will inhibit these sited and extend life of the paint.
3.2.2 Existing Structures Prepared For Spot Touch-up Painting
Spot painting is economic when done with minimum of surface preparation. Primer plus VCI Agent requires pressure washing to remove loose surface materials and has a lower cost than surface preparation to white steel.
3.2.3 Application on Joints and Load-Bearing Areas
Most serious corrosion damage occurs in recessed areas where protective coating cannot penetrate. Primer plus VCI agent can reach these areas by migrating from the primer into recessed areas such as joint and load-bearing areas.