The National Academies

NCHRP IDEA 20-30/IDEA 156 [Completed (IDEA)]

Novel Coating Technology for Improving the Corrosion Resistance and Mechanical Properties of Reinforcing Steel in Concrete
[ NCHRP 20-30 (NCHRP-IDEA) ]

  Project Data
Funds: $115,000
Staff Responsibility: Inam Jawed
Research Agency: Southwest Research Institute
Principal Investigator: Leonardo Caseres
Fiscal Year: 2011

The goal of this project was to develop a new solution-based diamond-like carbon (s-DLC) coating with improved corrosion and mechanical performance as compared with commercially available coatings for concrete reinforcing steel. The s-DLC synthesis process differs from traditional techniques in that it does not require high temperature or vacuum conditions. Work in the initial stage involved optimizing the coating performance prior to application to reinforcing steels. For coating optimization, scanning electron microscopy (SEM), Raman spectroscopy, and electrochemical techniques [such as electrochemical impedance spectroscopy (EIS) and linear polarization resistance] were used to characterize the coating applied to flat steel coupons. After coating optimization, several coated bars were embedded in mortar samples admixed with various amounts of chloride (from 0% to 5% wt. of mortar) and partially immersed in simulated seawater for up to 140 days. For comparison, additional reinforcing steel samples were vacuum deposited with DLC coatings such as standard DLC, multi-layer Si DLC, Si-F-O DLC, and thick DLC coatings and tested in mortar in a similar way as the s-DLC samples. SEM analysis revealed microcracks in the s-DLC films deposited on the rebar surface. To mitigate cracking, changes in pyrolysis process parameters such as the heating and cooling rates were investigated along with exploring alternative wet-coating techniques. Multiple layers of s-DLC films were also applied to the rebars in efforts to mitigate cracking. Both approaches appeared successful in mitigating cracks. The corrosion properties of the coated mortar samples were examined using linear polarization techniques. The results showed that for 0% chloride, corrosion rates were negligible/low (<0.1 mpy) for the s-DLC, Si-F-O DLC, and thick DLC coatings, but moderate (0.65–0.90 mpy) for standard DLC and multi-layer Si DLC coatings. The corrosion rates increased with an increase in chloride content, which was more notable for the s-DLC coating. By the end of the 140-day exposure, the corrosion rate for 0.5% chloride was high (1.9 mpy) for the s-DLC coating, moderate (0.55 mpy) for the standard DLC and multi-layer Si DLC coatings, and negligible (<0.07 mpy) for the Si-F-O DLC and thick DLC coatings, indicating that coating defects are likely to be present in the s-DLC, standard DLC, and multi-layer Si DLC coatings. Similar trends were recorded for the 1% and 3% chloride contents. For the 5% chloride content, all of the DLC coatings showed high corrosion rates (>1.4 mpy) except for the thick DLC coating, which exhibited a moderate corrosion rate (0.75 mpy). The linear polarization results were in agreement with the EIS measurements. Further research is needed to improve the corrosion protection of s-DLC coating if it is to be viable for use on reinforcing steel. The Southwest Research Institute is working on this issue and, once this is resolved, will formalize teaming arrangements through negotiated business agreements to support technology integration and transition. The Institute intends to involve small businesses for scaling up synthesis from the pilot synthesis to larger production (500–2,000 gallons) of s-DLC coatings.
The final report is available.

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