Associate Professor Igor Chaves

The critical role of manganese sulphide (MnS) inclusions for the initiation of the short-term growth of pitting or localized corrosion of low carbon steels has long been recognized. Classical results show that pitting probability and pitting severity increases with increased sulphide concentration for low carbon steels as a result of magnesium sulphides acting as local cathodes for initiating pitting corrosion. However, the iron carbides (cementite) in steels can also act as local cathodes for initiation of pitting corrosion. Herein it is proposed that there is competition between pits for cathodic area and that this will determine the severity of pitting and general corrosion observed in extended exposures. Preliminary experimental data for immersion exposures of up to 56 days in natural seawater of three low carbon steels show, contrary to conventional wisdom, greater pit depths for the steels with lower S content. However, the pit depth results are consistent with lower C/S ratios. This is considered to support the concept of cathodic competition between C and S. It is proposed that this offers explanations for a number of other phenomena, including the thus far unexplained apparently higher reactivity of some MnS inclusions.

The longer-term serviceability and structural safety of steel infrastructure exposed to seawater conditions may be affected by global warming and by seawater nutrient pollution. These may affect abiotic and biotic (microbial) corrosion. A model for long-term corrosion is developed from data obtained from steel piling exposed for 33 years in a seawater harbour. The effects on corrosion losses on the structural reliability of steel sheet piling as used in harbours world-wide were investigated as a function of seawater temperature rise from global warming and of seawater nutrient pollution. The results show that structural reliability is more sensitive to likely nutrient pollution than to predicted increases in seawater temperature, noting also that global warming also could increase nutrient pollution from anthropological sources.

The quantification of mass loss, surface topography, depth of pitting, and localized corrosion for steels subject to marine corrosion requires the removal of rusts, preferably without causing additional mass loss, pitting, or other damage. Conventional procedures using inhibited hydrochloric acid or Clarke's solution are shown to remove short-term marine rusts but also to cause new corrosion including pitting and related rust products. Both increase with longer exposure to the cleaning solutions. Water washing with soft water left calcareous materials on the metal surface. Ultrasonic removal did not cause additional corrosion but was effective only for very light rusts. Although relatively slow, electrolytic cleaning (electrolysis) produced clean surfaces without new corrosion.

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. © 2014 Elsevier Ltd.

Localised or pitting corrosion can be detrimental for steel pipes and containment structures, since wall perforation may cause system failure. Herein maximum pit depth quantification and its development with time are considered for samples taken from longitudinal welds on 33-year-old tubulars exposed in Newcastle Harbour. Relationships between pit depth and material metallurgy and corrosion properties were investigated by means of standard macro-etching, rest potential and zero resistance ammetry techniques. It is considered that the observed results are the result of the lack of homogeneity at the corrosion interface caused by differences in grain size, grain structure and the potential for pitting to occur preferentially along boundaries. The results are compared to measurements for longitudinal welds obtained previously on samples of API X56 Spec 5L pipe exposed in similar waters for up to 3?5 years, showing a reasonable degree of consistency between the two sets of data. The reasons for this are discussed. © 2013 Institute of Materials, Minerals and Mining.

This paper presents an analysis of design guidelines for steel-concrete composite beams, formed by concrete-filled cold-formed steel sections. The study is based on experimental results for connector resistance (push-out) and for four full-scale beam bending tests. The accuracy of analytical design equations is evaluated by comparing their predictions with experimental results. Model bias and model uncertainty of analytical design equations are evaluated. The uncertainty in design variables (steel and concrete resistance, dead and live loads, model errors) is taken into account, and reliability index of code-compliant beams is evaluated. Results show that the models for shear connector and for beam bending resistance are fairly accurate, and represent very little contribution to problem uncertainty and failure probabilities. Results show that for practical beam lengths, full material interaction is guaranteed, and failure is dominated by bending. Reliability indexes of the order of 2.2 to 2.8 are obtained, reflecting reliability of the design procedures studied. These values are low, in comparison to target reliability levels of 3.0 used in code calibration, and should be interpreted carefully in future code revisions. Copyright © 2010 by ABCM.

Severe localized corrosion has long been considered the main cause for containment loss of steel assets such as pipes and tanks, particularly as caused by pitting corrosion on or around weld zones. The weld and heat affected zones for welds in steel structures are known for their higher rate of localized corrosion but there is still some controversy about the specific mechanics causing this phenomenon. This paper overviews the weld corrosion literature that has considered the possible mechanisms involved and the associated material and environmental conditions. Experimental observations are then presented on the metallographic analysis of five separate grades of low-carbon steel. The possibility is examined whether there is a likelihood of greater pit depths being associated with the weld and heat affected zones and whether they can be considered the result of the presence and influence of macro-galvanic cells within the steel matrix. Also, the possibility of localized corrosion being the result of thermal microstructural gradients produced during the welding process is discussed.

Increasingly climate change, including the possible rise of seawater surface temperatures is causing some concern regarding the integrity of commercial and industrial structures in marine exposed environments. Structural integrity (serviceability and safety) may be compromised by the increased rates of corrosion reactions caused by increased average temperatures. Immersed structural systems also may be prone to microbiologically influenced corrosion. In this paper, three different extrapolation techniques are used to model expected corrosion loss. They are based on the same set of short-term empirical mass loss data for 1 year exposures. The results are compared with mass loss data obtained from steel piling exposed for 33 years. It is shown that the extrapolations from short term data can be very significantly in error compared with actual observations and with better modelling approaches. The effects of temperature increase and of increased microbiological influences are considered as part of the investigation.

The apparently premature corrosion of steel reinforcement in reinforced concrete structures exposed to marine environments continues to be a matter of concern in practice despite decades of research effort and much research funding expenditure and despite much practical experience. Herein results are reported of a long-term (10 year) study of the influence of alkalis on the corrosion initiation and progression of 6mm diam. reinforcing bars centrally embedded in 40x40x160mm concrete specimens. These were made with seawater as mixing water and with a variety of mix proportions including some with added alkalis. All were equally well compacted by vibration. After 10 years continuous exposure in a fog-room only a limited amount of corrosion initiation was observed. These results are in stark contrast with those reported in the famous paper by Shalon & Raphael (1959) who found serious corrosion of reinforcement after only 1-2 years exposure for all mixes. It is shown herein that the corrosion loss trends in those tests are similar to those of steel exposed directly to seawater. This indicates that in the 1959 tests the concrete provided little protection against reinforcement corrosion. The reasons for this are considered herein and the practical implications discussed.