Saltwater corrosion of hollow aluminum tubing and cast iron bathtub construction materials was a concern after Hurricane Ivan. When the hurricane struck Grand Cayman in September, 2004 the storm surge covered a construction site. The effect of wind driven storm debris and sand on coated aluminum windows and patio doors was also evaluated. Sections of aluminum alloy tubing from balcony railings which had been partially submerged by the storm surge visually indicated the presence of saltwater residue. A scanning electron microscopic (SEM) photograph of dendritic salt crystal residue on an uncoated aluminum alloy surface is shown in Photograph 1.
Photograph 1 (SEM) of a dendritic salt crystal residue on uncoated aluminum surface.
A close-up view of the outside surface of an uncoated cast iron bathtub, exhibiting pitting corrosion , is shown in Photograph 2.
Photograph 2 Pitting corrosion on outside surface of cast iron bathtub.
Energy Dispersive Spectroscopy (EDS), a method of chemical fingerprinting, was performed on the dendritic residues. The presence of sodium chloride (salt) was confirmed. Residual chlorine is especially insidious in that removal by cleaning and neutralization is extremely difficult. Chlorine is a very mobile atom which will migrate to the base of a corrosion pit and/or any sharp corner or weld toe where cleaning and neutralization is difficult. Once present on an unprotected metal surface, if not neutralized from the effect of saltwater exposure, chlorine, will promote rapid pitting corrosion. The resistance of cast iron to chloride attack, especially hydrochloric acid is very poor. Complete chloride removal by neutralization was believed to be more expensive than replacement.
The client was advised to replace all bare, uncoated metal, items subjected to the hurricane storm surge.