R. Jeffrey

Bacteriological influence in the development of iron sulphide species in marine immersion environments

Abstract Bacteria and microbiological consortia play an important role in the corrosion of steel in marine environments. There are laboratory based experimental findings and images verifying the presence of such consortia but there are few images in the literature of the interaction between bacteria and corrosion processes. The present paper considers the marine immersion corrosion of steel and presents a number of new images which appear to show steps in the metamorphosis of the iron in the steel to forms of iron oxide and to iron–sulphur compounds. These transformations appear to involve interaction with bacteria. Scanning electron microscope images were obtained for the specimens which were exposed to actual seawater conditions. Energy dispersive spectroscopy and X-ray diffraction were used to analyse the corrosion products. Inferences are made about the role of bacteria in the transformations from iron to iron–sulphur compounds.

Probabilistic models for steel corrosion loss and pitting of marine infrastructure

Abstract With the increasing emphasis on attempting to retain in service ageing infrastructure models for the description and prediction of corrosion losses and for maximum pit depth are of increasing interest. In most cases assessment and prediction will be done in a probabilistic risk assessment framework and this then requires probabilistic corrosion models. Recently, novel models for corrosion loss and maximum pit depth under marine immersion conditions have been developed. The models show that both corrosion loss and pit depth progress in a non-linear fashion with increased exposure time and do so in a non-monotonic manner as a result of the controlling corrosion process changing from oxidation to being influenced by bacterial action. For engineers the importance of this lies in the fact that conventional ‘corrosion rates’ have no validity, particularly for the long-term corrosion effects as relevant to deteriorated infrastructure. The models are consistent with corrosion science principles as well as current understanding of the considerable influence of bacterial processes on corrosion loss and pitting. The considerable practical implications of this are described.

Influence of Water Velocity on Marine Immersion Corrosion of Mild Steel

Abstract The rate of immersion corrosion is known to increase with water velocity. This has been demonstrated in laboratory conditions under short-term exposures. There are very few field data or l...

Surface roughness effect on marine immersion corrosion of mild steel

Abstract For materials such as stainless steel, it is well known that the surface condition (roughness) can have an effect on corrosion loss or pitting as a function of exposure time. Although some isolated observations exist of this effect for mild steel under marine immersion, there appears to have been no systematic investigation. The present paper describes results obtained in field tests carried out over a period of 13 months in temperate waters on the Eastern Australian seaboard. Surface conditions considered were mill scale, surface ground, grit blasted, polished, acid cleaned, and pre-rusted. The effect of surface roughness on corrosion was found to be approximately proportional to the surface roughness index. Surface roughness has the most effect in the first few weeks of exposure, with the net effect then remaining approximately constant with time. The acid-cleaned coupons show only a low value of roughness index, but on the basis of scanning electron microscope (SEM) images and high early insta...

Shape and size effects for marine immersion coupons

Abstract The conventional wisdom in corrosion testing is that the size of the coupon and its perimeter/area ratio (the surface area of the perimeter expressed as a percentage of the total surface area) are important parameters and that these will have an effect on the results obtained. In order to test this proposition, sets of mild steel coupons of the same exposed areas but having different perimeters were immersed in a marine environment. Corrosion mass loss was observed for area/perimeter ratios in the range 3·52-6·03%. Mass loss observations were taken at various times to 12 months. Sets of rectangular coupons of different sizes (50 × 100 mm and 100 × 200 mm) were also immersed. The results indicate that any variation caused by shape or size falls within the expected variability of corrosion at a test location.

Modeling of Long-Term Corrosion Loss and Pitting for Chromium-Bearing and Stainless Steels in Seawater

Abstract Data for long-term marine immersion and half-tide corrosion loss and pitting of chromium-bearing and stainless steels is systematically re-examined and re-interpreted to show that it is consistent with the model previously proposed for mild and low-alloy steel corrosion. Early corrosion of chromium-bearing steels is well known to be influenced by bacterial activity in the biofilm adjacent to the metal surface. However, the effect appears short-lived and typically the corrosion rate declines, presumably due to the buildup of corrosion products, either on the surface or within pits. The data considered herein show that after some two to four years of exposure there is usually a considerable increase in the instantaneous corrosion rate both for weight loss and for pitting. This is interpreted in terms of the previously proposed model for the corrosion of steels as the result of bacterial activity but now located primarily within pits and crevices. This interpretation implies that the early and the l...

Effect of Vertical Length on Corrosion of Steel in the Tidal Zone

Abstract The corrosion loss of mild steel in the marine tidal zone varies considerably with elevation relative to mean tide. Classical studies on steel piles and electrically isolated coupons show ...

Long-Term Corrosion of Mild Steel in Natural and UV-Treated Coastal Seawater

Highly polished coupons (25 by 25 by 1.5 mm) sourced from the same steel sheet were continuously immersion-exposed either to natural coastal seawater or to seawater from the same source subjected to filtration and UV irradiation to eliminate microbiologically influenced corrosion as much as possible. This was continued for 943 days (2.6 years). Dissolved oxygen levels were very similar in both environments. On average the UV-treated seawater was 2°C warmer, but all coupons exposed to it showed less localized corrosion than those exposed to natural seawater. The typical topographical difference was about 60% as measured by surface roughness parameter Sa. Mass losses in UV-treated seawater were about 10% higher than in natural seawater, but after temperature correction were similar to natural seawater for the first year and tended to be lower subsequently. At all exposure periods the rusts in UV-treated seawater were less voluminous than the rusts in natural seawater. Eventually they also contained a higher...

Technical Note: Rust Removal from Steel Coupons After Short-Term Marine Immersion

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 addit...

Accelerated low water corrosion of steel piling in harbours

Abstract Accelerated low water corrosion of steel piling in sea water harbours in the UK, Europe and elsewhere has been shown recently to be the result primarily of water pollution. Elevated levels of dissolved inorganic nitrogen in sea and brackish waters are responsible for microbiologically influenced corrosion of steel piling below the low water tide level. This is demonstrated in field data from 13 Australian experimental sites, nine US naval sites and some severe sites in Australia, Norway, Japan and the UK. Localised perforation of sheet piling, often associated only with the webs of U and Z profile piles, is shown to be the result of centreline segregation, porosity and composition differences in steel profiles. These stem from the steelmaking process and are likely to be less severe for modern steels. The results explain most of the observations for actual steel piling in various harbours, both vertically and horizontally (i.e. for U and Z and other pile profile types).