Formation of Corrosion Products on Open and Confined Zinc Surfaces Exposed to Periodic Wet/Dry Conditions

The formation of corrosion products was followed during the atmospheric corrosion of open and confined zinc surfaces exposed to periodic wet and dry conditions. The periodic wet and dry exposure consisted of the immersion of samples in sodium chloride (NaCl) solutions followed by exposure to laboratory air for different time periods. Phases of the corrosion products were identified by Fourier transform infrared spectroscopy (FTIR) and x-ray diffraction (XRD), while the relative proportions of the corrosion products were determined by FTIR. Three main components were identified in the corrosion products on the zinc surfaces, namely zinc oxide (ZnO), hydrozincite (Zn5OH6CO32), and simonkolleite (Zn5OH8Cl2·H2O). The wet/dry pattern of the exposure conditions was an important factor affecting the composition of the corrosion products for open zinc surfaces and zinc surfaces confined in crevices. Similar proportions of zinc oxide, simonkolleite, and hydrozincite were formed on both open and confined surfaces at shorter drying times. However, hydrozincite became the dominant compound on the confined surfaces for longer drying times, whereas simonkolleite was the dominant compound on the open surfaces. Results are explained in terms of difference in corrosion rate and extent of pitting corrosion attributable to different drying dynamics between open and confined conditions. The composition of the corrosion products depended heavily on the degree of confinement. Simonkolleite was the dominating compound in the corrosion products at large and small confinement widths, whereas hydrozincite dominated at intermediate widths. When the width of the crevice was small, oxygen diffusion was restricted, which may have favored the formation of simonkolleite. Conditions approached that of an open surface when the width of the crevice was large.