Modelling immersion corrosion of structural steels in natural fresh and brackish waters
Introduction
The corrosion of structural steels in fresh waters is generally acknowledged to be less severe than that in seawater. Nevertheless, in the research literature there is at least one case, widely reported, of a field observation over 5 years in (slightly) brackish tropical waters for which steel corrosion was very much greater than in seawater, essentially at the same location and at similar water temperatures [1]. This indicates the importance, particularly for engineering design considerations, of understanding of the factors that control immersion corrosion of steel as a function of water salinity.
The corrosion of steels in fresh and brackish waters is known to be a function not so much of salinity itself but rather of the calcium–calcium carbonate balance (alkalinity and hardness) of the water and how this is influenced by its pH [2]. It is also well-known from (relatively short-term) laboratory investigations that for a given bulk water, pH in the range 4–10 is not itself a significant factor in the rate of immersion corrosion.
In the longer term the aerobic corrosion process is overtaken by anaerobic corrosion resulting from the action of sulphate-reducing bacteria (SRB) and their metabolites, principally hydrogen sulphide. As in seawater, the rate of metabolism of SRB in fresh and brackish waters is temperature- and pH-dependent and therefore these factors will influence the rate of anaerobic corrosion [3].
Unfortunately, there appears at this time to be no unifying framework for relating these various observations to actual field data and hence no possibility of predicting the likely amount of corrosion under given field conditions. A central aim of the present paper is to begin to assemble such a framework and to point to where additional work is required to allow it to be properly calibrated. The approach to be presented below commences with a review of the salient features of corrosion in brackish and fresh waters, an examination of the appropriateness of adapting the corrosion-time model previously developed for marine immersion corrosion and an assessment of the literature data for the corrosion of structural steels in brackish and fresh water. The general approach is similar to that already demonstrated for immersion corrosion under nominally ‘at-sea’ conditions, both for general corrosion and for pitting [4], [5].
Only a small number of field observations of the corrosion of structural steels in natural fresh and brackish waters have been reported in the literature. Early observations of high rates of corrosion were largely anecdotal [6]. However, these early observations did lead to long-term (15 years) field trials being conducted for four different sites world-wide [7] and also to shorter-term, more detailed investigations in estuarine tropical waters [8]. Separately, these tests influenced also the famous 16-year exposure trials at the Panama Canal Zone [9]. Some of the data obtained in these experiments is applied herein, supplemented by additional information now known to be required to interpret and utilize the data. A complete list of data sources considered herein and relevant parameters is given in Table 1. The composition of the steels where known is summarized in Table 2.
Section snippets
The carbon dioxide–carbonates-pH-corrosion balance
For metals exposed to fresh waters calcium and magnesium carbonate tend to deposit within the corrosion product layer. These and the increasing thickness of the corrosion product layer tend to reduce the rate of supply of oxygen to the corrosion interface and hence reduce the corrosion rate. This and the influence of pH in such deposit formation was recognized already many years ago, as was the lower corrosion that occurs in ‘hard’ fresh waters [2].
Similarly, for seawater, the presence of
Marine growth and sulphate-reducing bacteria (SRB)
Even apparently quite clean waters contain bacteria and the larvae of macro-organisms that produce marine growth. For seawater, at least, calcareous deposits are known to be very suitable for colonization by all types of marine growth [3]. Photosynthetic activity of plant matter will consume carbon dioxide during daylight hours and can have a small effect on the pH of seawater. Similarly, water pollution involving sulphides may cause a slight reduction of water pH. These pH changes are thought
Corrosion loss model
It is useful at this stage to review briefly the model (Fig. 1) for general marine immersion corrosion [4]. As will be shown, it can be used as a basis for developing a coherent view of the above observations for general corrosion loss in fresh and brackish waters.
Before proceeding it is noted that Fig. 1 represent the expected or mean-value function f(t, E) for the corrosion loss c(t, E) as a function of duration of exposure t in a probabilistic setting given bywhere b(
Supporting field evidence
A variety of field observations for the general corrosion of structural steels for fresh and brackish waters will now be reviewed and interpreted in terms of the model of Fig. 1. In each case it will be assumed that the data is a sample drawn from a corrosion loss–exposure time plot having the same general shape and phases as Fig. 1. Thus simply ‘connecting the dots’ is the antithesis of the approach adopted here. This may be re-stated as attempting to establish whether the observed data is
Interpretation of data sources
Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11, Fig. 12, Fig. 13 allow the parameters ta, r0, ca, ra, cs, and rs that describe the mean-value model (Fig. 1) to be estimated. The values obtained are summarized in Table 1. There was insufficient information to estimate ra and only a limited number of observations for cs, and rs. For ta, r0 and ca the estimates have been plotted as a function of average water temperature and average pH in Fig. 14, Fig. 15, Fig. 16
Discussion
The above interpretation of the reported data in terms of the model of Fig. 1 has allowed estimation of the parameters of the model and these are seen in Fig. 14, Fig. 16 and to a lesser extent in Fig. 15, Fig. 17, Fig. 18, to be consistent with each other. They are also seen to be consistent with the previously developed trend for seawater immersion corrosion. Moreover the trends are in accord with expectations, particularly for the effect of pH on calcite deposition for soft and seawaters and
Conclusion
The field data available in the literature for the fresh and brackish water immersion corrosion of low carbon and low alloy structural steels was interpreted using the multi-phase mean-value model previously proposed for general corrosion under marine condition. In broad terms, the data exhibit characteristics consistent with the various phases of model, thereby lending support to its appropriateness for near or fully aerated brackish and fresh water.
As for seawater, corrosion in brackish and
Acknowledgements
The support of the Australian Research Council is gratefully acknowledged, as is the continued assistance of the interlibrary loan librarians at The University of Newcastle. The author is also indebted to a number of individuals who responded to his requests for detailed pieces of information. In addition to those acknowledged in the references, these include Murt Lyon and Jim Barkuloo, Baywatch Program, St Andrews Bay RMA and Alison Jones-Humphrey, Environmental Agency Wales. Finally, the
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