Publications

This paper presents an interpretation of corrosion loss data for mild steel coupons exposed for up to 2.6years to freshwater under a range of dissolved oxygen levels and temperatures 50–60°C. The water total alkalinity was 0.5–0.6mmol/l and pH 8.7–9.2. It is shown herein that the data is consistent with the early stages of the corrosion loss model proposed earlier for steel exposed to seawater, brackish water or freshwaters in the usual environmental range of 0–30°C. The new data was found to be consistent with the effect of dissolved oxygen and the effect of water hardness on the model.

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.

This paper considers the physical processes of corrosion that occur in bulk carriers. Three main types of corrosive environments are identified within a bulk carrier, namely, immersion in seawater, exposure to an enclosed atmosphere, and exposure to porous media. Fundamental variables influencing corrosion in each environment of the cargo hold region, ballast tanks and void spaces are identified. These serve to identify operational parameters that affect bulk carrier corrosion. It is proposed that such parameters can be used to obtain corrosion rate databases representative of vessel operation.

Longer term exposure of mild steel in natural (biotic) waters progresses as a bimodal function of time, both for corrosion mass loss and for pit depth. Recent test results, however, found this also for immersion in clean fresh, almost pure and triply distilled waters. This shows chlorides or microbiological activity is not essential for the electrochemical processes producing bimodal behaviour. It is proposed that the first mode is aerobic corrosion that eventually produces a non-homogeneous corroded surface and rust coverage sufficient to allow formation of anoxic niches. Within these, aggressive autocatalytic reduction then occurs under anoxic abiotic conditions, caused by sulfide species originating from the MnS inclusions typical in steels. This is consistent with Wranglen's model for abiotic anoxic crevice and pitting corrosion without external aggressive ions. In biotic conditions, metabolites from anaerobic bacterial activity within and near the anoxic niches provides additional (sulfide) species to contribute to the severity of corrosion. Limited observational evidence that supports this hypothesis is given but further investigation is required to determine all contributor(s) to the cathodic current for the electrochemical reaction. The results are important for estimating the contribution of microbiological corrosion in infrastructure applications.

No abstract is provided for this article.

In the past decade, there have been numerous advances in probability-based structural codes and codified

Steel infrastructure in contact with particulate media is at risk of failure due to pitting corrosion. For reliability analysis, it is important to understand the progression of pit depth with time and the associated extreme value statistics. An analysis was conducted on an extensive dataset of pit depth observations, made on mild steel coupons buried in different particulate media and immersed in natural Pacific Ocean seawater. It showed that the trends for the deepest pits with time were consistent with the bimodal model, and that pitting was deepest under seawater and increased with particle size when under particulate media. Gumbel plots of the data showed that the deepest pits plotted as a straight line, indicating a good fit. The plots also showed small, regular deviations from this straight line, and these permitted interpretations for pit depths developing in incremental steps. These step sizes were largest for surfaces under seawater and increased with increasing particle size under particulate media. These findings allowed for new conceptual models for the process of pit depth growth under rust and particulate media. They also allow for new insights into metastable pitting for steels. Opportunities for further research are also presented.

The response of an initially curved cantilever strut with a lumped mass at the tip subject to arbitrary axial impact loads is described, as a model for the dynamic response of piles being driven into the ground. The differential equation of motion defining the lateral movement of impact loaded cantilever struts is derived. This equation includes the effect of inertial resistance to motion provided by both the strut self mass and the mass of a pile hammer. The probable shape of force pulses produced during pile driving were obtained from observations by others and approximated by an equivalent trapezoidal force pulse. The differential equation of motion was solved for this force pulse using the Runge Kutta technique and for physical parameters corresponding to those of a slender steel pile which was observed whilst being driven. The resulting solution was found to agree well with observed pile behaviour during driving in that the magnitude of lateral displacements were very small. It is also shown that the magnitude of lateral displacements reduces as the mass of the pile hammer increases, due to inertial effects. In principle, the theory presented can be used to determine the maximum slenderness ratio for a cantilever pile so as to keep the lateral deflections within a prescribed value for a given pulse shape. (Author/TRRL)

The variation of corrosion rate of mild steel as a function of the quantity of moisture of coal and iron ore is investigated. Two types of black coal sieved to three different size fractions up to 2360 μm particle diameter and one type of iron ore of 600–1180 μm particle diameter were tested. A pronounced increase in corrosion rate was observed at a moisture content between 60% and 80% of the maximum water holding capacity for all samples. The corrosion rate was also observed to increase with decreasing particle size distribution. Parameters influencing the corrosion rate of coal and iron ore, as identified in this study and others, are discussed. Corrosion rates were measured inside a bulk carrier (ship) cargo hold that carried coal and iron ore cargo on successive voyages and are shown to be similar to those measured in the laboratory. The relevance of laboratory experiments using fine-particle samples for simulating the corrosion of steel exposed to coal and iron ore cargo is discussed.

Keywords MONASH UNIVERSITY PREDICTION, ACCIDENTS, PROBABILISTIC, MATHEMATICS, INTERSECTIONS, PROBABILITY, FAILURES, DESIGN, STRUCTURAL, ANALYSIS, SAFETY, SOFT... Show All

No abstract is provided for this article.

The paper presents a method for reliability assessment of reinforced concrete (RC) slab bridges with corroded reinforcement. Both a reduction of cross-sectional area of the reinforcing steel and a decrease in bond strength are considered. A non-linear finite element structural model and probabilistic models for traffic loads, corrosion propagation, bond characteristics, material properties, element dimensions and reinforcement placement are used. Time-dependent bridge reliabilities are illustrated for deteriorating simple and three span RC slab bridges, for ultimate strength and serviceability limit states. (a) For the covering entry of this conference, please see IRRD abstract no. 895164.

This paper discusses various issues in developing a structural deterioration model for naval surface ships for use in a reliability based management method. Over time a naval ship structure will reduce in strength due to deterioration processes that will gradually increase the risk of failure. Increased maintenance is then required to ensure an acceptable level of structural integrity. Due to increasing economic pressures to reduce the cost of ownership, and to extend the life of a platform, there is a need to develop cost effective maintenance plans. Structural reliability techniques are currently being developed to achieve this. The various aspects and information needed to develop this approach are discussed. The dominant deterioration mode on an ageing structure is corrosion with paint coating breakdown. The operational profile of the ship is a dominant factor in the amount of deterioration that occurs. Naval operations will affect coating breakdown and corrosion. This information is discussed with regards to the probabilistic data required for the development of models of these processes. Typical survey data is also presented with a view to determine the critical areas for research focus. Finally, a way ahead is developed for the use of this modelling in developing maintenance planning of naval structures.

Carbon steel pipelines are used extensively in the oil and gas industry for sub-sea applications. Usually they are cathodically protected and have exterior surface protective coatings. Where these are ineffective or not used, the evolution of weld zone corrosion with time may be of interest. Experience has shown that generally the heat-affected weld zone is more severely corroded by pitting than either the weld zone itself or the parent metal. However, quantitative data are scarce, particularly for longer-term exposures. Herein observations are reported of the pitting corrosion of API X56 Spec 5 L grade pipeline steel exposed continuously to natural Pacific Ocean seawater for 3.5 years, and extended to much longer exposure periods by comparison to similar steels exposed for up to 30 years. It is shown that the corrosion mass loss, maximum pit depth and pit depth variability are not simple linear functions of exposure time as often assumed in practice but are more complex functions. These functions are consistent with those observed previously for mild steels in various marine exposure conditions. However, considerable differences were noted in severity. The reasons for this are discussed and further work is outlined.

Existing models for predicting the time to the commencement of serious reinforcement corrosion in chloride conditions provide results that do not compare well with long-term observations and practical experience for actual concrete structures. Many high-quality concrete structures show little or no evidence of serious corrosion, even after decades of exposure to chloride conditions. Earlier it was proposed that the mechanisms for corrosion initiation differ from those for long-term corrosion. This is reviewed. New research shows corrosion initiation is dominated by the presence of air voids adjacent to the reinforcement. For good concretes this is mainly a short-term transient effect. Long-term active corrosion is the direct result of the usually slow loss of concrete alkalinity, a process accelerated by chlorides. This, rather than direct chloride attack, is the cause of damaging reinforcement corrosion. It is argued also that the role of concrete cracking intersecting the reinforcement must be reconsidered. These new interpretations open up the way for improved modelling and prediction of 'chloride-induced' corrosion.

Structures seldom fail, so it is reasonable to conclude that they are ‘safe’ and that this state was attained in the processes of designing and building them. The process of achieving structural safety and the process of assessing whether structural safety has actually been achieved for a given structure are, however, two very different matters. This is the essence of this paper and the issues are discussed in terms of the four related topics of responsibility, failure, uncertainty and decision. The first deals with the domain of responsibility of structural engineers: what it is and what it should be and how this relates to structural failure. Four sources of failure are distinguished: technical, process errors, technical ignorance and various non-technical matters. The relative contributions of these are then discussed. Recent codes of practice have focused narrowly on technical uncertainty and a notional probability of failure, with little attempt to incorporate non-technical matters and the ontological uncertainty, which is shown to be the origin of most failures. Finally, the nature of decision making is discussed for both code development and structural design and it is concluded that in both cases the decision process is one of satisficing rather than optimising. The whole process is less rational than is generally supposed and requires a broader view in the outlook and training of structural engineers.