I.S. Cole

The initiation mechanism of corrosion of zinc by sodium chloride particle deposition

Abstract This study details the mechanism of attack by sodium chloride particles on zinc. The dissolution and subsequent reaction of a single micro-particle of salt on zinc has shown explicitly the mechanism by which attack occurs. Following the initial formation of a hemispherical droplet when the particle wets in humid air, a secondary spreading xa0effect occurs. The rate and potential associated with this spreading process has been measured using a Kelvin probe, and the chemistry of the compounds formed as a result of the corrosion mechanism have been determined using spectrographic techniques.

Holistic model for atmospheric corrosion Part 1 - Theoretical framework for production, transportation and deposition of marine salts

Abstract This paper discusses the limitations of parametric modelling of corrosion and presents the case that accurate and flexible models of atmospheric corrosion require an holistic approach. In such an approach, the processes controlling corrosion across a wide range of physical scales are modelled. These models are based as much as possible on the processes controlling the phenomena under consideration. Being fundamentally based, these models can be extended outside the data sets from which they are derived. This is the first of a series of papers outlining the use of this approach to predict corrosion in marine environments. It will consider the theoretical formulations required to model the production, transport and deposition of marine salts. It will present some predictions from these formulations and it will discuss the implications to corrosion of this theoretical understanding. Later papers will present the incorporation of these models into an information system and the validation of these models against data.

A Study of the Wetting of Metal Surfaces in Order to Understand the Processes Controlling Atmospheric Corrosion

This paper explores the simple principle that a metal surface wets when the surface relative humidity (RH) exceeds the deliquescent RH (DRH) of any salts on the surface. Data from field exposures across 19 sites in China, the Philippines, Indonesia, and Australia is used to determine the conditions under which openly exposed surfaces wet. At each site, surface temperature (of a zinc plate), ambient RH, sensor wetness, airborne salinity, and gaseous SO x and NO x were determined over a one-year period. In conjunction with these microclimate measures, the chemistry of airborne and deposited aerosols, as well as rainwater, were measured. Complimentary data from an environmental scanning electron microscope are presented in which salts derived from the evaporation of sea salt are rewetted. Using all of this data, an assessment of the probable contaminants controlling sensor wetting at each site is made. It is found that sites with similar International Standard Organization, (ISO) 9223, classifications in terms of industrial and marine airborne pollutants show similar surface contaminants and wetting characteristics. It is proposed that dominant contaminates can be identified for each ISO classification that are consistent with the general principle that wetting occurs when surface RH exceeds the DRH of the salts making up the contaminates. These salts can change from day to day due to the continual change in the composition of the contaminates and the ongoing homogenization of previously deposited salts through chemical reaction between salts and with the surface. Rain events usually clean the surface and start the cycle over again. The application of these findings to process models of corrosion is discussed, while generalized rules for predicting surface wetting based on climate data are proposed. It is found that these generalized rules predict total time of wetness to a high degree of accuracy.

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.

Products Formed during the Interaction of Seawater Droplets with Zinc Surfaces II. Results from Short Exposures

This paper reports on a study in which fine seawater droplets were placed on zinc surfaces for periods of 15 min to 6 h. After each exposure, droplets were either extracted from the specimens or allowed to evaporate in laboratory conditions. Scanning electron microscopy (SEM)-energy-dispersive spectroscopy, X-ray microdiffraction, and SEM―focused ion beam studies were undertaken on the specimens after exposure, as was in situ Raman spectroscopy on zinc covered with a saline droplet. Approximately 30 min after droplets were placed on the surface, a thin moisture layer (termed the secondary spreading film) spread out from the central droplet. As a function of position and time, oxide development in the central region was initially slow; however, after ∼30 min, a significant local attack of the underlying microstructure occurred. Correspondingly, precipitate phases developed on top of the oxide, which produced an oxide consisting of three bands: an irregular zone with a localized metal attack and significant voids at the metal/oxide interface, overlaid by a relatively void-free layer, which in turn was overlaid by a porous zone. At the edge of the drop (before the secondary spreading), oxide growth was much faster, and the deposition of phases that precipitate in solutions appeared critical. We discuss the chemistry and identification of the various phases and the implications of oxide formation to the electrochemical processes occurring in drops.

Holistic model for atmospheric corrosion Part 6 – From wet aerosol to salt deposit

Abstract This paper is the sixth paper outlining a holistic model of atmospheric corrosion. Previous papers outline how airborne salinity could be estimated at any given location and how the deposition onto a surface could be modelled. This paper investigates the initial reactions of raindrops or deposited marine aerosols on metal surfaces (zinc, galvanised steel, aluminium and gold). It combines short term field exposures with a series of controlled laboratory experiments. Surface changes to the metal upon field exposure are characterised by the nature and distribution of corrosion nodules, oxide films and retained salts. Eleven distinct forms of surface change are observed. The conditions promoting each pattern of surface changes were estimated by comparison with laboratory tests with a variety of deposition modes. Laboratory tests included deposition of saline droplets in three size ranges. A strategy to use the results in a holistic model of corrosion is proposed.

Holistic model for atmospheric corrosion Part 5 – Factors controlling deposition of salt aerosol on candles, plates and buildings

Abstract This paper is the fifth in a series of papers outlining a holistic model of atmospheric corrosion. Previous papers have outlined how the airborne salinity could be estimated at any given location. This paper presents a theoretical analysis of aerosol/particulate deposition, in conjunction with computational fluid dynamics (CFD) studies. Theoretical and CFD studies address whether salt aerosol/particulate deposition is controlled by mean winds, wind turbulence or Brownian motion (deposition of salt via rain/fog etc. is not considered in this paper). Analysis indicates that deposition due to wind turbulence is the dominant mechanism for marine aerosol, and that other deposition modes can be effectively ignored. Given this, further studies are presented for salt candles, plates and buildings. From these studies, approximations are derived so that salt deposition on exposure plates and on buildings can be estimated from measurements from ISO salt candles.

Products Formed during the Interaction of Seawater Droplets with Zinc Surfaces: I. Results from 1- and 2.5-Day Exposures

This paper reports on a study in which fine seawater droplets were placed on zinc surfaces for periods up to 2.5 days. At the end of each exposure, the droplets were either extracted from the specimens or allowed to evaporate in laboratory conditions. Scanning electron microscope-energy-dispersive spectroscopy, X-ray diffraction studies (including mapping), and Fourier transform infrared and Raman spectroscopy were carried out on the specimens after exposure and on the dried extractions. It was observed that soon after the droplets were placed on the surface, a thin moisture layer (termed the secondary spreading film) spread out from the central droplet. X-ray analysis indicated the presence of gordaite and simonkolleite in the center of the drop when the droplet volume was maintained. However, during droplet evaporation a wider range of species was detected, including boyleite, zinc chlorate hydrate, and sodium zinc chloride hydrate. In the secondary spread region, an amorphous phase with a composition and morphology consistent with zinc hydroxy carbonate was present. The phase analysis is supported by electrochemical data and by analysis of changes in droplet chemistry with time. A synthesis of the data permits a description of the processes leading to the phase development. The implications to atmospheric corrosion are discussed.

Investigating the Effect of Water Content in Supercritical CO2 as Relevant to the Corrosion of Carbon Capture and Storage Pipelines

Steel coupons were exposed to a supercritical carbon dioxide (CO2) environment in which water contamination was deliberately added over the range from 100 ppmw to 50,000 ppmw. Exposure was carried ...

Holistic model for atmospheric corrosion Part 4 – Geographic information system for predicting airborne salinity

Abstract This paper is the fourth in a series of five papers addressing the factors controlling and predicting marine aerosol concentrations on land. It outlines the procedures used to develop a geographic information system (GIS) model of airborne salinity across Australia. The basic mathematical formulae have been presented previously, and this paper outlines how these formulae are applied in a GIS system. While the basic formulae are based on models of the processes controlling the formation and distribution of marine aerosols, some constants have been determined empirically. These constants are established by regression analysis using data derived and presented previously from a long range study of atmospheric salinity levels in a transect across the Australian State of South Australia. The model was then tested with databases of airborne salinity measurements throughout Australia and was shown to possess a high level of accuracy. It was then used to predict the salinity across Australia, with a salinity map of Australia being produced. A number of features of the map are discussed. Lastly, the model is used to illustrate the role of coastal forms in controlling salinity in adjacent regions.