W. D. Ganther

Holistic model for atmospheric corrosion: Part 2 - Experimental measurement of deposition of marine salts in a number of long range studies

Abstract This paper is the second in a series looking at understanding the factors controlling and predicting marine aerosol concentration on land. It looks at results from three transects across the Australian continent. In each transect, the airborne salinity was measured, using the wet candle method at distances from 10 m to 40-300 km from the coast. The positions of the transects were selected to give a significant variation in the factors controlling salt production and transport. For example, one transect in South Australia was established where both high whitecap activity is likely to promote salt production and flat terrain and prevailing winds are likely to favour transport. Another, in Queensland, was established where calm seas will limit salt production and very seasonal winds and high relative humidity and rainfall will limit transport. On the basis of this experimental study, the general validity of the fundamental concepts put forward in Part 1 is assessed. Further, the feasibility of building a mathematical model to predict salinity is determined and the main factors causing variations in salinity on land are outlined. The results are then used to assist in the interpretation of previous work in the literature.

The rate of drying of moisture from a metal surface and its implication for time of wetness

Abstract A model of the evaporation of water and saline solutions from a metal surface is developed. The evaporation of a droplet of moisture from a flat metal plate over which a uniform laminar flow of air occurs is modelled by assuming an analogy between heat and mass transfer. The model is adjusted to predict the evaporation of a saline solution under the assumption that the presence of the salt affects only the vapour pressure. The predictions of the model are compared with observed experimental evaporation rates and are found to be of reasonable accuracy. Predicted drying times for moisture films under conditions typical of those associated with building microclimates are determined. The implications of these drying times to the traditional estimation of time of wetness are assessed.

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

Abstract 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.

Experimental studies on dependence of surface temperatures of exposed metal plates on environmental parameters

Abstract The present study reports on an investigation into the dependence of the surface temperature of metal plates on environmental parameters. Monitored plates were exposed at three locations (one coastal and two inland) in southern Australia. The following parameters were monitored: surface temperature, surface wetness, wind speed, ambient relative humidity, air temperature and lifting condensation level. (The lifting condensation level is the height at which a parcel of air rising from the ground will reach saturation.) An analysis was undertaken of the temperature difference between the exposed plate surfaces and ambient air. It was found that the maximum temperature difference during the day depended on season, wind speed and, to a lesser extent, the lifting condensation level. The maximum undercooling during the night showed a relatively limited range and a weak dependence on wind speed. The rate of rise in temperature difference after sunrise was found to be only slightly dependent on season, but highly dependent on whether the surface was wet or dry. If the surface was dry, the rise in temperature difference depended on the wind speed and lifting condensation level. The relevance of these findings to moisture formation and evaporation from surfaces, and thus to corrosion processes, is discussed.

Development of a sensor-based learning approach to prognostics in intelligent vehicle health monitoring

This paper reports the development and testing of an intelligent sensing network for monitoring corrosion of aircraft structures. The agent-based system uses sensed microclimate and corrosion data with an innovative analysis approach to diagnose corrosion and to infer the presence of corrosion in locations such as crevices and fasteners where it cannot be sensed directly. An important feature of the system is its ability to provide prognostic information to enable timely corrective maintenance scheduling.

Recent Progress in Intelligent Vehicle Health Monitoring

Boeing, CSIRO and DSTO are collaborating on research into Intelligent Health Monitoring for Aerospace Vehicles (IHMAV). The system has the ability both to predict the probable development of corrosion and to elucidate the environmental factors that are promoting corrosion. The paper discusses the system capabilities, data output including the data derived from previous monitoring programs on B-707. In particular this data established the connection between local microclimate and corrosion events and highlights variations in microclimate and corrosion that can occur for different spaces in an airframe and different airframes.

Influence of leaf litter on corrosion of gutters

Abstract Leaf litter is commonly deposited onto roofs, where it affects the local environmental conditions that control the degradation of roof sheeting and gutters. This study investigated the chemical nature and corrosivity of electrolytes resulting from the infusion of deionised water with leaf litter. Leaf litter specimens were collected from a variety of tree species commonly found in southern Australian suburbs, and from gutters in which the leaf litter had accumulated. After infusion for approximately three weeks, leaf litter suspensions were filtered and the electrolytes present in the filtrates were analysed to determine their chemical composition and pH levels. The filtrates from leaf litter obtained directly from trees tended to contain higher concentrations of salts and increased acidity compared to the suspensions from leaf litter sampled from gutters. The corrosivity of the electrolytes was explored electrochemically by potentiodynamic analysis of rolled zinc coupons immersed into the filtrates. Leaf litter was found to influence corrosion rates by increasing the acidity of solutions, and through the release of chloride ions during organic matter decomposition. Soluble products of leaf litter decomposition (i.e. fulvic acids) appeared to provide limited protection against corrosion. Over time, the aggressiveness of electrolytes in gutters should decrease as rain events leach salts from leaf litter, and pH should be buffered through the adsorption of carbon dioxide and possibly through the deposition of airborne dust such as mineralogical particles. In addition to the chemical impacts of leaf litter on corrosion, it can also result in gutters being exposed to longer periods of wetness. Data from wetness sensors installed in both clean and dirty gutters showed that, once wet, gutters containing leaf litter can take over twenty-five times longer to dry than clean gutters. Although there are many variables involved, it has been shown that leaf litter can influence long term zinc corrosion rates by a factor of over 2.