Physical infrastructure in coastal and offshore locations often is constructed from mild and low alloy steels. These are prone to marine corrosion even with the application of protective coatings and cathodic protection. Particularly for high‐value assets such as energy facilities (wind farms, oil and gas production and subsea pipelines), the prediction of their safe and economic life is of much interest. Herein, a review is given of the modern development of models for the prediction of long‐term marine corrosion of steel exposed to various marine environments. These include immersion, tidal, splash and atmospheric zones and contact with sands and soils. The effects of temperature and water pollution are especially important, as is the ability of the steels or irons to form protective corrosion products. Comments are made also about other alloys such as copper‐nickels and aluminium. All show that corrosion, including pitting corrosion, develops with increased exposure time in a bimodal manner. The reasons for such behaviour are considered, together with the practical implications, important for containment structures such as pipelines, tanks and floating offshore energy platforms.
Field data for the corrosion of structural steel in the tidal and near-shore marine atmospheric zones taken from a variety of sources are reexamined and then interpreted in the light of a ...
Unexpectedly high levels of corrosion of sheet and other steel piling were observed in UK harbours during the 1980s, and a phenomenon termed ‘accelerated low water corrosion’ (ALWC) caused considerable concern about the structural safety of harbour quays and facilities. This paper shows that the severity of corrosion of steel piling both in the tidal zone and in the immersion zone is correlated with the concentration of dissolved inorganic nitrogen (DIN) in the local seawater. In the lower tidal zone this is consistent with the thinning of piles associated with ALWC. Previously it has been shown that in the horizontal direction the ALWC phenomenon is associated with material variability. Models are developed to allow the prediction of expected average corrosion loss both in the immersion zone and for the lower tidal zone (the ALWC effect) as a function of DIN, water temperature and exposure period. Data are presented to show the relatively low beneficial effect of changes in steel composition. It is concluded that for long-term protection against corrosion reliance must be placed on protective and preventive measures.
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Rejoinder to separate Discussions by David Blockley, D.G. Elms, and Colin B. Brown of ‘Structural reliability theory in the context of structural safety’ by R.E. Melchers, Civil Engineering and Environmental Systems, Volume 24, Number 1, March 2007, pp. 55–69.
Finely ground limestone, perhaps as part cement replacement, has been shown to reduce concrete permeability and enhance strength. Whether this holds also for coarse limestone aggregates is of both practical and environmental interest, particularly the effect on corrosion of steel reinforcement. This is confirmed herein with exposure tests over 14 years for a range of concretes exposed to a high chloride environment. Importantly, these concretes also showed delayed initiation of reinforcement corrosion and lower long-term corrosion losses compared with similar concretes made with coarse igneous or siliceous aggregates. The observations are attributed to enhanced bonding between cement-hydration products and the limestone aggregate surfaces. These findings are important both for reducing environmental impacts and greenhouse gasses and for extending the durability of reinforced concrete structures.
The reliability analysis of offshore structures under wave and wind actions is considered using second order random wave theory. To represent non-Gaussian properties of the resulting wave kinematics, the Hermite moment transformation is used. Further, the so-called sample-specific linearization method developed already (to be used in conjunction with the directional simulation method and the linear wave theory) will be extended to take into account both (1) non-Gaussianity of wave/wind load due to nonlinear load processes and also (2) the non-Gaussianity of wave kinematics due to the nonlinear wave theory. This allows an out-crossing approach to be used to assess the structural probability of failure and the involving out-crossing rate (which is not generally available for non-Gaussian processes) is required to be estimated. Using the proposed procedure, simple structures are analyzed in one- and multi-dimensional cases and the results for structural probability of failure are compared with those obtained using simple linear wave theory. Outcomes show that the use of nonlinear wave theory may affect the results considerably.
This paper reports pitting corrosion loss data of AA5005-H34 aluminium alloy immersed in natural seawater for up to 2 years. It is shown that the data for mass loss, maximum pit depth and the average value of 15 deepest pits as a function of exposure time are not closely consistent with the classical power-law function. Instead, the data show a greater affinity to the early part of a bi-modal trend. The uncertainty of the pit depth data was analysed using extreme value theory. The results are that scatter in the data sets increases with exposure time. This is considered to be the result of the pit depth data population being non-homogeneous, characterised by a mixture of deep pits with differing pitting morphologies. The results of this study suggest that longer-term data and homogeneous data population are likely to be more reliable for future corrosion loss prediction purposes.
This paper presents several aspects of a method for reliability assessment of reinforced concrete (RC) slab bridges with corroded reinforcement. It is assumed that corrosion will lead to a reduction in cross-sectional area of the reinforcing steel and/or a reduction in bond strength. Two types of corrosion are considered: general and localized corrosion. The method includes a non-linear finite element structural model and probabilistic models for traffic loads, corrosion propagation, bond characteristics, material properties, element dimensions and reinforcement placement. Reliability is estimated in terms of the reliability index using the first-order reliability method (FORM). For illustrative purposes, bridge reliabilities are calculated for a deteriorating, simple-span RC slab bridge, for ultimate strength and serviceability limit states.
The assessment of the remaining safe and serviceable life of deteriorating structures using structural reliability theory requires realistic models for corrosion. Using existing data it is shown that corrosion is a nonlinear function of time. A probabilistic model for "at-sea" immersion corrosion of mild and low alloy steels based on fundamental physiochemical corrosion mechanics is presented. The mean-value part of the model is made operational by calibrating its parameters to existing and new field data. A simple model for corrosion uncertainty is presented and its initial calibration discussed. More refined calibration awaits field data over practically significant seawater temperature ranges but particularly for lower temperature regimes. Extensions to the model and its application to other exposure regimes are outlined briefly.
Even in harsh marine environments, concrete structures reinforced with steel can show excellent long-term durability, with little or no reinforcement corrosion. Very few actual reinforced concrete (RC) structures have been closely scrutinized over many years and subject to interpretation using recent state-of-the-art understanding gained from detailed laboratory observations. Such a case is described for an 80-year-old RC structure observed annually over about 30 years in what is essentially an extraordinary long experiment. Despite very high chloride concentrations, field excavation evidence showed that reinforcement corrosion overall remains minimal, except where insufficient concrete compaction permitted air-voids to initiate quite severe, very localized corrosion even with still high concrete pH. It is possible that the use of blast furnace slag as aggregate may have assisted the observed durability. The case study supports other studies that show that it is possible to achieve long-term durable and therefore sustainable RC structures without additives and using only conventional reinforcement steels and conventional cements and aggregates. However, the potential dangers of deep narrow cracking extending to the reinforcement and the potentially deleterious effects of alkali–aggregate reactivity of some aggregates needs to be considered.
The sustainability of reinforced concrete is critical, particularly for structures exposed to marine environments. Chlorides are implicated in causing or accelerating reinforcement corrosion and potentially earlier expensive repairs, yet there are many older reinforced concrete structures in good condition for many decades despite very high chloride levels at the reinforcement. The reasons for this are reviewed briefly, together with recent experimental work that better defines the role of chlorides. One is initiation of reinforcement corrosion but only through localized pitting at air-voids in concrete at the interface with the steel reinforcement. These tend to be small or negligible for high quality well-compacted concretes. The other role for chlorides has been shown, in experimental work, to accelerate the long-term loss of concrete alkali material. On the other hand, a review of practical experience shows that what has been termed chloride-induced reinforcement corrosion often is not that at all, but is the end-product of factors that impair the protective nature of the concrete. As reviewed herein, these include poor compaction, physical damage to concrete cover, concrete shrinkage, and alkali-aggregate reactions. The various observations presented are important for the proper understanding, analysis, and design of durable reinforced concrete structures exposed to chloride-rich environments.
Microbial induced corrosion (MIC) of reinforced concrete sewers and manholes is a significant problem in sewage systems with global repair costs estimated to be in the order of billions of dollars per year. When the corrosion is sufficiently advanced it can lead to structural failures with potentially serious longer-term operational consequences. While the fundamentals involved in concrete sewer deterioration are reasonably well-understood, the prediction of the rate of deterioration and the quantitative effects of various influencing factors are not well developed. This paper outlines the basic mechanisms involved in deterioration of concrete sewers and the likely effects of various influencing factors, based on currently available information. A brief description of the current knowledge gaps is also provided. A new ARC and industry funded research project is discussed that combines industry and multi-disciplinary university research with the aim of determining the relationship between sewer environmental parameters such as H2S level, temperature and humidity and the instantaneous corrosion rate of concrete using in situ field observations and extensive laboratory testing. Investigations into the molecular, chemical and material dynamics of the corrosion process will enable an accurate picture of the various underlying processes to be constructed. This data along with historical records and the experience of the industry participants will enable a phenomenological model of the corrosion process to be constructed. It will enable industry to estimate the reliability and the expected remaining physical life of concrete sewers.
Quantification of long term integrity for steel marine pipelines and containing structures is important for predicting future remaining service life. Often that is limited by the occurrence of wall perforation. The present paper outlines a statistical extreme value analysis that considers the variability of the maximum pit depth and its changing behaviour with long-term exposure. Pitting in the weld heat affected zone is considered critical in the analysis. To assess the effect of the level of variability of the random variables on the failure probability a sensitivity study is carried out for a typical example problem. The results show that pit diameter has a high influence, followed by the pitting corrosion parameters, on the probability of failure.
The shrink–swell action of expansive soils is one source of structure foundation movement responsible for widespread problems with respect to the serviceability performance of lightweight masonry structures. The most obvious problem is that of cracking in masonry walls. Numerical models for soil movement and structural response have been combined to develop a probabilistic model for crack prediction. The model is capable of predicting the likelihood of cracking and the expected crack widths based on variability in both external effects and structural response. The ultimate aim is to develop rational design criteria for the serviceability design for masonry structures.
The external corrosion of buried cast iron water mains is a significant problem for the Australian Water Industry. To improve maintenance and replacement strategies for these mains the water industry requires a method to estimate their remaining service lives. Predictions of long-term corrosion losses, pit depths and pitting extents are therefore required. A conceptual model for the prediction of long-term corrosion has been developed and the external soil conditions influencing corrosion were identified in a previous study. To calibrate the model, long-term data of corrosion (average losses and maximum penetration) and associated soil parameters for pipes in service is required. Field data has been collected at 18 condition assessment sites within Hunter Water Corporations network. This paper presents the field observations from the first 11 sites, the model calibration procedure and results of the calibration. The evidence gathered in this work suggests that the long-term maximum corrosion penetration increases with increasing soil wetness (measured using degree of saturation) for the sites investigated. The initial model calibration produced model parameters for cast iron pipes buried at standard depths in relatively homogeneous, low-permeability, soils with degrees of saturation equal to 0.66 and 0.76.
Natural deterioration of mild steel exposed to marine environment compromises the long-term integrity, serviceability and safety of new and existing infrastructure and increases the risk of structural failure. Welded structures are known to be prone to even higher risks as a result of adverse effects of pitting corrosion in weld-heated areas. A bi-modal model has been shown recently to be a better description for the long-term development of the maximum depth of pits. Also, the statistics of pit depth have been shown to be better represented, for long term exposures, by the Frechet extreme value distribution. Both new developments present challenges for structural reliability analysis. Herein a linearization is used to represent long-term development of pit depth. It is shown that data for maximum pit depths can be separated into those with Gumbel statistics and those for which a Frechet distribution is more appropriate. An example is given for the reliability analysis of a welded pipeline subjected to localized corrosion. The effect of random variable uncertainty is assessed using a sensitivity study. Results show the considerable influence on the probability of failure of pit diameter and the parameters describing the pitting corrosion model.
