| dc.description.abstract | The likelihood that a prestressed system fails as a result of corrosion attacks is significantly greater than that of conventional reinforced concrete systems. Steel strands used in prestressed concrete are continuously subjected to large tensile forces despite their small cross sections, and they are also made of a higher fraction of carbon compared with mild steel to raise the strength level of the steel strands. Therefore, the American Concrete Institute limits the chloride content in prestressed systems to 0.06 % by weight of cement, yet this threshold is often exceeded in marine structures and aggressive environments. A permanent solution to avoid degradation of the steel wires in concrete is cathodic protection that reduces the rate of redox reactions and prevents the initiation of corrosion pits. But, the use of an impressed current system can cause hydrogen evolution at extreme negative potentials leading to brittle cracks in high-strength steel wires.
Herein is documented a history of the utilization of cathodic protection in reinforced and prestressed concrete. The adverse effects of the impressed current that may cause the failure of those systems are addressed. A brief review on the imperfection of the current protection systems, the mechanisms that lead to corrosion in concrete, and those that cause the brittle failures of steel wires are discussed in detail. The past failures of non-building prestressed systems are also tabulated.
In this work, it is suggested that prestressing wires are coated with metallic alloys in order to prevent diffusion of hydrogen atoms in steel embedded in concrete. Zinc-nickel has been already applied to the pipelines used to transport oil and gas to reduce risks associated with hydrogen embrittlement, and nickel-cobalt coating has been proven to protect steel from aggressive environments in aerospace applications. However, the efficacy of these coatings to protect ASTM A 416 steel wires from corrosion and brittle failures in the concrete environment has not been studied in-depth yet. To this end, ASTM A 416 steel wires were coated with zinc-nickel and nickel-cobalt alloys; then, the corrosion properties of plated wires were identified with the performance of cyclic potentiodynamic polarization and electrochemical impedance spectroscopy. A galvanostatic chloride acceleration test was also performed to determine the efficacy of impressed current to protect steel wires in an aggressive environment. All coated specimens were exposed to atomic hydrogen during the embrittlement test to assess the durability of coatings in response to hydrogen diffusion. Finally, the wear resistance of the coated films was examined, and the results were subsequently reported.
Based on the findings from the experimental evaluations, it was concluded that the corrosion properties of nickel-cobalt alloy plated on steel were similar to the kinetic behavior of the zinc-phosphate layer, the current coating system. However, the nickel-cobalt film limited the diffusion of atomic hydrogen in the metal substrate. Zinc-nickel also acted as a hydrogen barrier, yet the electrodeposition of the alloy on high-strength steel wires is not recommended since the corrosion rate of the alloy was found relatively high in alkaline environments. That said, a multilayer coat of nickel prior to applying the current coating system on steel wires is also suggested as an alternative approach to nickel-cobalt coating. It was found that the wear rate of all the coatings studied in this work were higher when they were examined in the abrasive test with a metallic bar instead of a plastic sheet, while the nickel-cobalt coating showed a slightly greater wear resistance. | |
