High concentrations of chlorides in reinforced concretes usually are held responsible for reinforcement corrosion. However, this conflicts with many practical examples of actual concrete structures that show little reinforcement corrosion even with very high chloride concentrations. Herein a series of tests is described for reinforced concrete specimens made with fresh water, seawater, and with seawater with additional sodium chloride and exposed for 10 years in a moisture‐rich environment. All concretes retained concrete pH values well above 9, and particularly for dense high strength concretes, pH around 12. The overall amount of reinforcement corrosion was zero or almost negligible, irrespective of concrete mix proportions or concrete chloride content. These results are in strong contrast with significant corrosion reported in 1959 after only 2–3 years exposure in a parallel experiment. The difference is attributed to poor compaction of those concretes inhibiting their inability to maintain sufficiently alkaline conditions and thus protection of the reinforcing bars. Overall the present controlled experiments support practical experience that reinforced concrete structures, properly made, can survive for long periods of time in chloride‐rich environments without the occurrence of serious reinforcement corrosion.
The adequate long-term performance of reinforced concrete infrastructure depends much on being able to resist deterioration mechanisms such as reinforcement corrosion. The main mechanisms for this are described and critically reviewed in the light of properly analyzed experience for and behavior of actual reinforced concrete structures. Recent research has shown that for structures in the marine environment, the role of chlorides lies mainly in the reduction of the alkalinity of the protective high-pH concrete rather than in the direct attack on the reinforcement steel.
Aluminium alloys have excellent corrosion resistance to a wide variety of exposure conditions. Usually they corrode by pitting rather than by uniform corrosion. For infrastructure applications long-term corrosion behaviour is of interest. The relatively limited long-term pitting data that is available shows that maximum and average pit depths do not follow the power law function as conventionally assumed but tend to follow a bimodal trend with exposure time. This is consistent with the bimodal trends observed previously for corrosion mass loss of aluminium alloys. Most likely it is the result of the accumulation of corrosion products over the pit mouths, leading to the gradual development of localised anoxic conditions within pits. In turn this permits the development within the pits of anoxic autocatalytic conditions, consistent with established theory for pitting corrosion of aluminium. It also is consistent with observations of hydrogen evolution from pits. The implications of this for practical applications are discussed.
Models for the prediction of corrosion often give the misleading impression that corrosion of steel in seawater environments is a linear function of time (the "corrosion rate"). More recently non-linear functions for corrosion loss have been proposed calibrated to aggregated data from a wide variety of sources. Unfortunately, this use of the data produces models with wide ranges of uncertainty and makes it difficult to predict future corrosion and to assess the effect of steel composition and environmental influences. Recent research has produced a model based on the fundamental characteristics of steel corrosion, including the effect of biological influences. Detailed investigations show that the process controlling the (instantaneous) rate of corrosion changes as corrosion progresses. This is represented as a sequence of phases for which fundamental theoretical justifications and mathematical relationships have been derived. To ensure the model has practical validity, it has been calibrated to carefully selected actual field observations. This has also allowed the effect of water temperature, dissolved oxygen levels, nutrient pollution, depth, water velocity, water salinity, and steel composition to be isolated, as reviewed briefly herein. Some observations about applications and research directions are given.
Soil electrical resistivity, polarisation resistance and similar techniques sometimes are used for predicting both the short and the long-term corrosion of metals such as steels in soils. Herein new field data for pipes after 63 years exposure are presented that show poor correlations to electrical resistivity. One of the reasons is that electrical current flow and hence resistivity cannot be a surrogate for the electrolytic ionic diffusion of metal ions in the corrosion process. Also, corrosion in soils largely is governed by differential aeration, facilitated by air-voids at the soil/metal interface. This phenomenon is not considered in conventional electrochemical experiments. The practical implications are discussed.
Reliability theory for the assessment of existing structures for remaining safe life must consider the probabilistic deterioration of structural strength as well as multiple applied loadings. This requires good-quality models of deterioration processes. Models for corrosion, which have been applied in the literature for marine applications, are reviewed. It is shown that these usually are statistical only, using data pooled from many sources. They also use little or no theoretical insight. As a result, they provide poor-quality mean-value information and very high statistical uncertainties. The conclusion is that better modeling is required and that this involves much better understanding of the factors influencing marine corrosion. These factors are reviewed briefly, with particular emphasis on marine immersion corrosion of mild steel.
The progression of corrosion loss and maximum pit depth, once initiated, is of much interest for predicting the future or remaining long-term life of infrastructure systems. Their failure can have substantial economic and environmental consequences. The model most widely used for the prediction of long-term corrosion is a strictly concave function such as the power law c = A t B where c is corrosion loss or maximum it depth, t is time and A and B are constants obtained from fitting the function to data. However, as now shown repeatedly and for immersion, tidal, coastal atmospheric and for different steel, including chromium steels and also for freshwater and brackish water immersion, this model is inconsistent with longer-term empirical observations. These mostly exhibit a bi-modal trend, even for sterile seawaters and triply distilled freshwaters. Herein it is proposed that the bi-modal behaviour arises from changes in the nature of the corrosion reactions as the rust layers deposited on the exterior surface of the steel become increasingly less permeable, particularly to oxygen. The mechanisms and chemistry involved are described. It is shown that the bi-modal characteristic primarily is the result of a change from corrosion rate controlled by the rate of oxygen diffusion with oxygen as the electron acceptor in the corrosion process, to one involving hydrogen reduction and associated diffusion for a short period followed by other possible rate-controlling mechanisms.
No abstract is provided for this article.
Pitting corrosion of aluminium alloy AA5005-H34 immersed in natural sea water for up to 2 years is presented, together with new interpretations of the development of the pitting process over extended exposure periods. Trenching initiated at surface-located Fe-rich intermetallic (IM) particles. It occurred more extensively in sea water compared to atmospheric exposures. With increasing exposures, the pitting morphologies became more complex. Three types, i.e. hemispherical pits (Type 1), crystallographic pits (Type 2) and characteristic 'petal'-like pits (Type 3), were observed after 12 months. It is suggested that the formation of Type 2 and Type 3 pits occurs at an advanced corrosion stage, and are associated with large Fe-rich IM particles near the metal surface. The influence of marine growth on pitting was negligible.
This paper reviews recent developments in the prediction of the likely future corrosion losses and of the maximum pit depth for steels exposed to marine environments. A robust mathematical model based on corrosion science principles and calibrated for immersion conditions to an extensive range of literature and new data is described. The model has provided explanations for the effects of steel composition, water velocity, depth of immersion and seawater salinity and also has facilitated new interpretations of data for long-term pitting corrosion. This paper briefly overviews these developments and refers to some typical applications, including marine corrosion of ship ballast tanks, corrosion of sheet piling in harbours and corrosion of offshore platform mooring chains.
This chapter provides an overview of the approaches that can be used to determine structural reliability based analytic safety assessment for steel structures corroding in marine environments. Corrosion and pitting of structural steels used in marine pipelines and for off-shore mooring chain are important factors in continued integrity and service performance and are best considered in a reliability framework. For steel structures fatigue, corrosion and various combinations are likely to be very aggressive. The actual corrosion losses and maximum pit depths develop in a strongly non-linear fashion with increasing exposure period. In corrosion engineering it is usual to think in terms of a 'corrosion rate'. This assumes that corrosion continues at a constant rate with time. Steel composition has been found to have little effect on the development of pitting corrosion in marine immersion conditions. Importantly, marine corrosion can be influenced by the involvement of microorganisms particularly in nutrient polluted waters.
The short-term immersion corrosion of mild and low alloy steels in seawaters is known to be proportional to the concentration of dissolved oxygen (DO) in the bulk water. Longer-term corrosion is a function of the activity of sulphate-reducing bacteria and is influenced by the concentration of nutrients in the bulk water. These influences are examined in more detail for the corrosion of steels in the brackish waters of the River Thames and for several immersion corrosion sites on the Eastern Australian seaboard and in the North Sea. The published data sources were supplemented with plausible assumptions about environmental conditions. New interpretations of the data are provided based on the previously published model for immersion corrosion. For waters with negligible salinity and sulphate levels early corrosion loss was shown to depend on the dissolved oxygen content of the waters, and later corrosion loss was a direct function of nitrogenous nutrient (pollution) levels. This also applies to longer-term corrosion.
Keywords MINIMUM, REINFORCEMENT, SOLUTIONS, FLAT, SLABS, METHODS, BENDING, COLUMNS, IDEALISED, IDEALISATION, POINTS, SUPPORTS, FINITE, STRIP... Show All
Interest in the use of polymer based composites, particularly polyester composites reinforced with E-glass fibres, has grown tremendously in recent years as a result of their attractive strength-toweight ratio and relatively low corrosion rates. Industry acceptance of advanced composite materials for marine use however is currently awaiting resolution of some major technological issues. Principal amongst these is the issue of reliability and in particular the current inability to predict the long-term degradation of FRP materials which are subjected to prolonged contact with moisture (Weitsman and Elahi (2000), Karbhari, et al. (2003)). Many factors are responsible for the current absence of durability models. Environmental aging mechanisms are complex and poorly understood. Durability studies by their nature require long term commitments of time which are often not practical. As a result durability data is sparse and findings from different studies are not easily compared thereby making the development of general models of the breakdown process difficult. Despite the paucity of available data some preliminary material has been published presenting methods by which the different aspects of the degradation process can be modeled. However there has been no attempt to pull the different strands together to generate a more general description of the breakdown process. The following paper outlines a method by which the moisture induced decline in mechanical properties of simple composite systems can be estimated using a combination of methods already available in the general literature. The hybrid model incorporates the effects of reversible damage to the matrix, debonding and stress corrosion of the reinforcing fibres. The aim of this paper is not so much to present a definitive model of the breakdown process but rather to present a preliminary modelling framework which allows the direction for future simulation and experimental studies to be assessed. The model comprises two parts; the evolution of the moisture distribution throughout the composite is first determined then the resultant changes to the mechanical properties of the composite are evaluated. For the purposes of this discussion the durability of a glass reinforced polyester composite will be examined.
An electrode array probe has been specially designed and demonstrated for in situ monitoring and visualising multiple localised corrosion processes and mechanisms, including crevice, weldment and pitting corrosion, occurring simultaneously on marine steel structures. It enabled the probing of time-dependent development of multiple localised corrosion processes, mechanisms and kinetics, thereby facilitating more in-depth understanding of the initiation, propagation and kinetics of localised corrosion of steel marine structures.
For infrastructure applications in marine environments, the eventual initiation of corrosion (and pitting) of steels (and other metals and alloys) often is assumed an inescapable fact, and practical interest then centres on the rate at which corrosion damage is likely to occur in the future. This demands models with a reasonable degree of accuracy, preferably anchored in corrosion theory and calibrated to actual observations under realistic exposure conditions. Recent developments in the understanding of the development of corrosion loss and of maximum pit depth in particular are reviewed in light of modern techniques that permit much closer examination of pitted and corroded surfaces. From these observations, and from sometimes forgotten or ignored observations in the literature, it is proposed that pitting (and crevice corrosion) plays an important role in the overall corrosion process, but that longer term pitting behaviour is considerably more complex than usually considered. In turn, this explains much of the, often high, variability in maximum depths of pits observed at any point in time. The practical implications are outlined.
