Field studies of surface cleaning and salt retention on openly exposed metal plates

The paper describes a number of studies relating to the retention of marine salts on atmospheric corrosion test plates and the cleaning of such salts in field exposures. Data from three separate field studies are presented. In each study, instrumented zinc plates were openly exposed to the atmosphere. Plates were exposed in tropical, subtropical and temperate regions of Australia in severe marine, marine, urban and inland exposure conditions. Plates were washed at various times during exposure and, in some cases, they were ultrasonically cleaned after exposure. Ion chromatographic analysis was undertaken to determine the ion contents of the wash-off and ultrasonic cleaning solutions, and thus the salts present on the exposed plates. Sites and plates were also instrumented with rain gauges, salt candles, relative humidity sensors, surface temperature sensors and surface wetness sensors. Changes in the relative humidity at which the surfaces became wet were determined and used as a guide to infer the presence of salts on the surfaces. The effect of rainfall on the presence of salt was then determined. The factors controlling salt retention on surfaces are discussed, including the possibility that the ability of a surface to retain salt changes with time, and that sources of chloride other than crystalline salt may exist. The relevance of the study to atmospheric corrosion is discussed.