FTIR study on the amorphous corrosion products of galvanized steel in carbonated concrete

The composition and the structure of the passivating products of galvanized steel after concrete carbonation and wet-dry cycles in tap water were studied. The research was performed on mild steel sheets galvanized in a pure zinc bath and then embedded in cubic concrete specimens. After 39 days of air curing, the specimens were placed in a carbonation chamber and then were submitted to wet-dry cycles in tap water. After different periods of time, some concrete specimens were broken and the sheets were removed and submitted to scanning electron microscopy, energy dispersive X-ray analysis and Fourier transform infrared spectroscopy. The results confirm that concrete carbonation leads to the destruction of the passivating layer of calcium hydroxyzincate with formation of amorphous compounds. FTIR spectra indicate that these compounds are ZnCO3 and hydrozincite, with a prevailing presence of the former. SEM observations of the coating cross-sections confirm the results of previous corrosion rate measurements, which indicate that zinc remains passive both during concrete carbonation and during the subsequent wet-dry cycles in tap water. After 19 wet-dry cycles only to a slight decrease of the amount of the corrosion products on the coating surface was found by means of SEM observation and FTIR spectroscopy.