Experimental determination of duration of wetness on metal surfaces

The present paper analyses the time that wetness sensors on Zn and galvanised steel plates stayed wet in four locations within Australia: two in inland zones, one in a marine zone, and the other in a severe marine zone. All locations are classified as having a temperate climate. At each location, wetness was determined from interdigitated gold grids in thermal contact with plates openly exposed to the atmosphere. Individual periods of wetness were analysed, and it was found that the duration of wetness fell into three modes: short wetness periods of 1-4 h, but centred on <2 h; part day wetness periods of 8-20 h, depending on environmental severity; and multiple day wetness periods of three to five days, but with odd occurrences out to 20 days. The relative occurrence of each mode depended on the severity of the sites. To illustrate the dependence of corrosion events on the duration of wetness, time factors regulating secondary spreading were examined. It was observed that secondary spreading on Zn and galvanised steel only occurred if the duration of a drop on a plate exceeded certain critical limits. Furthermore, the spread of secondary spreading was highly dependent on the time period for which the drop remained stable. The implication of these findings to developing accelerated tests and to modelling atmospheric corrosion are discussed.