Structural reliability theory based on probability provides a systematic, logical and consistent means to make estimates of the safety of complex structural systems. However, its application currently leaves it open to criticism that its predictions do not necessarily compare with reality and with accepted risk criteria in other contexts. Herein, it is proposed that this is because applications have not dealt satisfactorily with the issues of human error and particularly human intervention in the management of loads and resistances. Probability-based models for loads can be modified to include the influence of human intervention such as through including the effect of the degree of enforcement of regulatory requirements. Similarly, the standard of enforcement of minimum standards of material quality should be added to probability-based models for resistances. This is relevant particularly for the assessment and prediction of the future reliability and safety of existing structures.
This paper describes a probabilistic method for the assessment of the suitability of corroded pipelines under pressure loading. The method takes into account the uncertainties of the variables that influence suitability. The paper starts with a review of the various available techniques for the assessment of failure pressure of corroded pipelines and then develops a probabilistic limit state model from the available deterministic failure equations. The advanced first-order second moment method is then employed to estimate reliability. This approach is then applied to some typical examples. The results are presented in graphical form.
Maximum pit depth is a matter of considerable practical interest. In applied corrosion mechanics it is conventional to represent it by an extreme value distribution, usually Gumbel. Howeve...
High-quality models are required to describe the corrosion of structural steels in marine, brackish and fresh waters and these models should, preferably, be based on a proper understanding of the corrosion processes involved. The models should then be used to interpret field data, rather than attempting to derive models empirically from field data. On this basis a multi-phase corrosion–time model has been proposed previously for mild and low alloy steels subject to immersion in 'at-sea' conditions. This model is reviewed and extended to brackish and fresh waters in this paper. The scientific principles show that salinity is not the critical issue for the corrosiveness of water. Instead, the critical parameters are the temperature, pH and calcium carbonate solubility when corrosion is governed by oxidation. For later corrosion, governed by anaerobic bacterial activity, the available nutrient level and rate of supply of nutrients control the observed rate of corrosion. The data available at the present time for estimating all the parameters of the new model are somewhat preliminary and new corrosion monitoring programmes will be required to augment the information and to fully characterize the corrosivity of coastal and harbour waters. However, indicative trends for the parameters, based on currently available data, are given and permit the construction of corrosion loss curves for practical applications.
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Because field studies seldom recover coupons less than 6 months from immersion they provide no information about early corrosion behaviour. Linear or power law functional behaviour for corrosion loss with time is often assumed. New field studies performed on the Australian east coast and described herein using very closely spaced recoveries of coupons show that the corrosion loss–time behaviour is initially highly non-linear and then almost linear until corrosion product formation begins to control the rate of corrosion. The reasons for this behaviour are discussed in terms of the recently introduced multi-phase phenomenological model for marine corrosion. Conventional oxygen diffusion arguments are then used to provide a mathematical model to describe the main part of this behaviour. The possible influences of seawater temperature and oxygen concentration are discussed.
Long-term corrosion trends are important for assessing the remaining life of critical infrastructure. Herein the available literature data for grey cast iron and cast steel exposed to marine and to some atmospheric environments are shown to be consistent with the bi-modal model previously developed for steels. However, the development of the second mode takes longer. This is proposed as resulting from the graphitized zone slowing the diffusion of species controlling the longer term corrosion processes. A practical simplified model and its parameters are proposed.
The immersion corrosion of structural steels in seawater is not a simple linear function of time (a corrosion rate) as often assumed in engineering estimates of future losses of material. To predict the likely loss of steel with time field trials are expensive, take a long time and may be misleading. An alternative approach is to use the basics of corrosion mechanics to develop engineering models for the prediction of corrosion loss. This approach is outlined herein and the recently developed multi-phase model for corrosion loss with time of exposure is reviewed. It has been accepted in the corrosion literature and has been extended to consider dissolved oxygen levels, nutrient pollution, depth, water velocity and water salinity. These effects are described briefly.
Seabed 'parking' of steel pipelines is a common practice in the oil and gas industry. For 'wet parking' severe internal corrosion, often at the 6 o'clock position, remains an issue, despite the use of corrosion inhibitors and biocides ostensibly to control microbiologically influenced corrosion. Biocides may have undesirable environmental consequences if eventually or accidently released. Recent findings for internal corrosion inside water injection pipelines are reviewed as these have implications for managing the internal corrosion of parked pipelines. After initial oxygen controlled corrosion longer term corrosion can occur under anaerobic conditions. This has implications for corrosion during periods of stagnant conditions and for corrosion under deposits of rusts and other debris. Microbiologically influenced corrosion can increase internal corrosion but only if the nutrients necessary for bacterial metabolism are available. Acceptably low levels of corrosion can be achieved inside 'wet' parked pipelines with the use of seawater with low concentrations of particulate matter and low concentrations of microbiologically-critical nutrients.
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The deterioration of marine steel infrastructure caused by corrosion may be influenced by a changing climate and/or pollution level which may lead to its serviceability and structural failure. However, almost all corrosion research until recently assumed time-invariant environmental conditions. A structural reliability analysis is applied here to simulate steel sheet piles in sea water conditions under a changing environment. Corrosion of marine steel infrastructure is modelled as a spatial time-dependent process including sea water temperature and sea level rise due to global warming and dissolved inorganic nitrogen concentration increase caused by pollution. The steel sheet piles are divided vertically into 70 elements to consider the spatial variability of different corrosion zones and sea level rise effects. Two limit states are considered: (i) stress of steel sheet piles reaches their yield stress and (ii) pitting corrosion perforation to provide an alert to repair or maintenance. The results show that ignoring the effects of a changing environment can underestimate structural capacity failure risks, and pollution will have a more significant effect on capacity of steel sheet piles than sea water temperature and sea level rise caused by global warming.
SummaryRecent observations derived from actual structures show that compared to other concretes, concretes containing high levels of carbonates (such as limestones and dolomites) tend to have longer reinforcement initiation times and also much longer durations before the occurrence of active corrosion causing obvious damage. This is provided that the structures do not suffer from poor workmanship, alkali reactions or other failure mechanisms. These findings are reviewed and a small number of additional examples given. The observations are consistent with careful laboratory experiments available in corrosion science literature. However, they are not consistent with the conventional wisdom that associates reinforcement corrosion initiation with the formation of calcium carbonate within the concrete matrix, such as caused by the influx of carbon dioxide from the environment. To cause initiation other processes, such as alkali leaching, must occur to lower the pH to around 8.5 or lower. Only this level of pH is consistent with electrochemistry fundamentals for initiation of steel corrosion. These observations have implications for achieving durable concrete structures.
In 2002, the 45 year old Sorell Causeway Bridge in Tasmania’s south east was demolished due to fears surrounding the serviceability of the prestressed concrete beams, in relation to the corrosion of the post-tensioning strands. Attempts were made to determine the extent and severity of the corrosion prior to its demolition by employing a number of conventional non-destructive and diagnostic techniques, yet a firm conclusion could not be reached regarding the structure’s condition. To further investigate the degree of correlation between these conventional assessment techniques and the physical condition of the embedded steel in relation to corrosion risk guidelines recommended in the literature, a number of beams of varying condition were salvaged from the bridge demolition and subjected to further detailed investigations. The current paper focuses on the half-cell potential and chloride profile results obtained for two such beams in good and poor condition relating to the reinforcing and prestressing steel condition. In summary, the comparison of results yielded inconsistencies with the literature guidelines. Survey areas showing highly negative potentials and elevated chloride concentrations did not necessarily indicate corrosion activity or severity. In contrast, instances of severe corrosion were found on both reinforcing and prestressing steel where literature guidelines for these tests recommend that the risk of corrosion is low.
Reinforced concrete is an option for pontoons for offshore wind and other energy-related facilities. It has the potential for lower maintenance in situ and ease of construction but despite its long successful applications history, concern persists about the risk of reinforcement corrosion. Recent investigations have concluded that chloride-induced corrosion is unlikely, short-term or long-term, for well-made, well-compacted and low permeability concretes. This experience is briefly reviewed. Initiation of corrosion occurs as pitting corrosion, irrespective of chloride concentration, and for reinforced concrete only within wet air-voids in the concrete at the steel–concrete interface. Experimental evidence shows that in parallel to this, a gradual loss of concrete alkalis to the external environment occurs, thereby eventually lowering the pH of the porewater solution sufficient for general corrosion of the steel to become thermodynamically feasible, irrespective of chloride concentration. A model for the development of corrosion loss as a function of period of exposure and of concrete permeability is described. It is concluded that reinforcement corrosion in marine environments is not a significant risk for high-quality, well-compacted, low permeability concretes made with non-reactive aggregates. This has direct implications for the construction and inspection of new reinforced concrete structures in seawater and ocean waters.
