Long-term corrosion of cast irons and steel in marine and atmospheric environments
Highlights
► Longer term corrosion of various cast irons shows a bi-modal trend with exposure time. ► This applies for marine immersion, tidal and freshwater and for atmospheric corrosion. ► Proposed as resulting from pitting under anoxic conditions under corrosion products.
Introduction
Grey cast iron is the most widely used of the various cast irons. It consists predominantly of iron with 2.5–4% C, 1–3% Si and minimal other alloys. The carbon is mainly in the form of graphite, giving grey cast iron its typical grey appearance when fractured [1]. At one time grey cast iron was used widely and extensively for major infrastructure applications such as bridge piers and for water supply pipelines and a considerable legacy of such structures remains. Many of these are in apparently satisfactory condition but structural safety may have been compromised by material loss due to corrosion and by cracking due to high tensile stresses and fatigue. Replacement of major infrastructure usually is expensive, and there is increasing interest in keeping existing structure in-service for longer periods, provided safety and serviceability requirements can be met. Often life extension will be beyond the service life originally intended. If it was predicted at all, expected service life was estimated at the time of the original design only very poorly, or based on past experience, and based perhaps more on wishful expectation than on sound theoretical bases [2].
For many infrastructure systems interest lies in structural strength. For this the ultimate governing criterion is the probability of structural failure and this is influenced, strongly in most cases, by the remaining average thickness of materials, that is, the material remaining after ‘uniform’ corrosion [3]. Grey cast iron has favourable long-term corrosion characteristics, particularly in the atmosphere, as witnessed by the existence of many railway bridges in the UK, USA and elsewhere that are over 100 years old. However, grey cast iron can suffer from substantial corrosion in the earlier stages of exposure particularly in seawater, fresh water and in-ground environments [4].
Compared with structural and other steels the quantitative data and information about the corrosion of cast iron is relatively scarce [5], [6]. Further, for structural steels, models that describe (and predict) the short- and the longer-term corrosion loss and the maximum pit depth as functions exposure time are available. These include the influence of environmental parameters such as water temperature, water velocity and water quality. This is the case for immersion conditions and to a lesser extent for atmospheric corrosion [7], [8].
Since both steels and cast irons consist predominantly of ferric iron with relatively small amounts of alloying, they can be expected to display generally similar corrosion behaviour and trends, even though corrosion losses or pit depths are likely to be different [4]. The analysis below is based on this proposition. Data available in the literature for the corrosion of grey (and some other) cast irons are examined and assessed for trends, using for comparison the bi-modal model earlier shown to be relevant for mild and low alloy steels and for chromium steels under various exposure conditions [9], [10]. Both corrosion loss and maximum pit depth are considered. Although the form and the sequential phases of the model have been described previously (e.g. [10]) for reference the key features are shown in Fig. 1.
The division between the two modes is denoted with the parameter ta. The first mode consists of a very short phase (0) during which corrosion initiates and also, for seawater, the metal surface is colonised by biofilm and then by microorganisms. In phase 1 the rate of corrosion is controlled by the rate of diffusion of oxygen from the water or moisture immediately adjacent to the metal surface (‘concentration control’) while in phase 2 the corrosion rate is controlled by the rate of oxygen diffusion through the increasing thickness of corrosion products on the metal surface. This produces the characteristic attenuation of the rate of corrosion. Eventually, at around ta, the rusts have built up sufficiently to develop anoxic conditions over much of the corroding surface. This then provides the conditions under which aggressive autocatalytic pitting corrosion can occur under the rust products and under which anaerobic microbiological activity also can occur within anoxic niches in the rust layers adjacent to the metal surface [10]. This is phase 3. Phase 4 represents the long-term, probably steady state, corrosion condition.
For infrastructure life extension purposes the long-term rate rs is of primary interest and also the intercept cs at t = 0 on the corrosion loss axis of Fig. 1 [11]. Estimates of rs can be made by inspection or one or more of a variety of specialised techniques to ascertain corrosion loss or pit depth. Let the outcome of such an inspection be denoted c(ti) where 0 − ti is the time interval between when the structure was first placed into service and the time of inspection or observation (Fig. 2a). If there had been an earlier time of inspection or observation te, say (Fig. 2b) it is possible to use the respective observations of corrosion loss (or pit depth) to make an estimate of the long-term corrosion rate rs as rs ≈ (c(ti) − c(te))/(ti − te). However, since in many practical situations concerns about corrosion tend to arise relatively suddenly, c(te) is seldom measured in practice. Typically only one or some estimates of c(ti) are available at time ti and these are insufficient by themselves to estimate rs and cs. More information is required.
More information can be brought to the problem if a theoretical model, preferably calibrated to actual field data, is available to describe the expected trend of the corrosion loss (or opt depth) as a function of exposure time. In this ways the underlying corrosion loss trend expected for the particular application can be predicted from past experience and, ideally, relevant theoretical concepts (Fig. 2c). Herein the possibility is explored that the underlying corrosion loss trend (and the trend for maximum pit depth) for grey cast iron is consistent with the bi-modal model (Fig. 1). The approach throughout is to consider whether it is possible for the data reported in the literature to have been generated by an underlying bi-modal distribution of the form of Fig. 1. Conversely, there is no a priori assumption that the data fit a power law or any other similar continuously smooth concave function.
The next section outlines the study approach and reviews all the quantitative data found, after extensive searches, to be available in the literature. This is followed by an examination of the data for grey cast iron and examines possible consistency with the bi-modal model. Data for some other cast irons and cast steel are then considered. In all cases the data either clearly follows a bi-modal trend or can be interpreted as following such a trend. The possible reasons for this behaviour are then discussed and some practical implications outlined.
Section snippets
Study methodology and data
The most extensive quantitative observations for the corrosion loss of grey cast iron were obtained in the Panama Canal Zone (PCZ) for marine immersion, mid-tide and two different marine atmospheric exposures and also for fresh water immersion corrosion. The seawater exposures were carried out at Naos Island and the fresh water immersion exposures in Gutan Lake [12], [13]. At these two sites the water temperatures averaged 27 and 28 °C respectively. Full details are given in the original papers
Grey cast iron
Under marine immersion and tidal conditions Fig. 5 shows the corrosion losses for machined and for ‘as-received’ surfaces for coupons. The ‘as-received’ will be taken throughout as equivalent to ‘as-cast’ surface finish. It is clear that the data needed only a small amount of interpretation to obtain the trends shown through the data points. The corrosion losses for the grey cast iron with ‘as-received’ surface finish are much lower than those for the machined surfaces, at least for the 4 years
Some other cast irons and a cast steel
Some longer-term corrosion data are available in the literature for cast steel and for austenitic cast iron (Table 1), although again it is limited. Fig. 10 shows data for cast steel as reported by Southwell and Alexander [12] but with the reported data at 16 years interchanged between immersion and mid-tide corrosion, based on the expectation of consistency with all the other data reported by these authors for immersion and tidal exposure corrosion losses. Their data shows that the trends for
Discussion
In the approach used herein the data available from all sources quoted are viewed in the first instance as sets of samples generated by underlying (physical–chemical) processes. These samples provide information about the outcome of the state of the underlying processes at the time the sampling was done but say nothing about the state(s) any other time or about the transition from one state to the next. Thus, for the PCZ experiments, all sampling was done at 1, 2, 4, 8 and 16 years and the
Conclusions
- 1.
Literature data for the long-term corrosion of grey cast iron, austenitic cast iron and cast steel exposed to marine, fresh-water and atmospheric environments as a function of time is consistent with the bi-modal relationship previously observed for mild and low alloy steels. Compared to immersion and tidal exposures the bi-modal characteristic is less pronounced for atmospheric exposure conditions. The corrosion trends are not consistent with the classical power law typically used for
Acknowledgement
The financial support of the Australian Research Council is acknowledged.
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