The corrosion resistance of coated steel dowels determined by impedance spectroscopy

The corrosion resistance of coated smooth steel dowels in simulated pore solution with and without 3.5% sodium chloride solution is reported. The dowels are coated with a double-layer structure composed of a 180 μm of Nickel-Chromium-Boron (referred to as NiCrB) that is overlaid by a 20 μm inorganic layer made of silicon powder, silica fume, blast furnace slag, and combinations thereof. The composition of the synthetic pore solution is based on the composition of pore fluids expressed from Type I portland cement that is characterized by pH of about 13.4 and an ionic strength of 0.4.Alternating current impedance spectroscopy measurements conducted in the range of 10 mHz to 10 kHz were found to be instrumental in detecting any change in the resistance of the coatings. It was found that the NiCrB sub-layer provides a good corrosion resistance that outperforms that of the inorganic coating. Of the five inorganic compositions tested, the combination of slag and silica fume has the best performance. The protection against corrosion is attributed mostly to particle packing rather than densification of the matrix due to pozzolanic reaction.