Accelerated low water corrosion on steel structures in marine environments
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Accelerated low water corrosion (ALWC)
The International Navigation Association (PIANC) Working Group 44 (WG44), of which one of the authors (Ashok Kumar) is chairman, has recently submitted the final report on their findings on ALWC. The most common form is limited to a horizontal band around low water, although it can be found occasionally in patches, and extends down to bed level. It generally has a recognisable appearance and characteristics, revealing clean corroding steel under lightly adherent orange and black corrosion products, as shown in Figure 1. ALWC is a particular form of microbiologically induced corrosion (MIC), which occurs in marine environments due to the presence of sulphates. The sulphates are converted by sulphate reducing bacteria (SRB) into hydrogen sulphide (H2S) thereby causing direct anaerobic corrosion of the steel surface. The resulting H2S is a food (energy) source for the sulphide oxidising bacteria (SOB), which converts it to sulphuric acid. The oxidation of the hydrogen generated at the steel surface means that an equilibrium state is never reached in the electrolysis process and the corrosion is therefore accelerated by the action of a symbiotic colony of SRB and SOB participating in a microbial sulphur cycle.
Maritime structure design has traditionally considered corrosion conditions in distinct vertical zones in relation to the sea. These zones, and their typical corrosion conditions and rates are described below, including ALWC, and shown in Figure 2.
Atmospheric zone (in the dry)
This area is between the top of the structure and the splash zone. This area may be exposed to a salt laden atmosphere and rusting will occur. Where steel is capped by concrete, crevice corrosion may occur at the point of encapsulation.
Splash zone (above MHWS)
This area is alternately wet and dry, and is most susceptible to atmospheric corrosion, which can proceed as rapidly as in the low water zone when SRB is not a factor. Where the steel pile is capped by a concrete structure, differential aeration can occur where the concrete becomes saturated by seawater to act as part of the electrolyte.
Tidal zone (between MLWS and MHWS)
Corrosion is usually relatively slow and uniform but concentrated corrosion caused by dissimilar metals may occur on fixings (such as ladder brackets) located within this zone. Corrosion rates can be expected between 0.04 and 0.1 mm/side/year. However, if ALWC is present in the low water zone, there is a possibility that it will cathodically protect the rest of the tidal zone, which can lead to a false perception of the structure condition.
Low water zone (0.5 m below MLWS to LAT)
Corrosion is relatively severe due to differential aeration at the upper-most point of continual immersion of the steel (where electrolyte is permanent and oxygen levels peak). Typical corrosion rates of 0.08 to 0.17mm/side/year are normal or very severe corrosion (concentrated) due to microbially influenced corrosion by SRB and/or metal reducing bacteria (MRB). With ALWC, typical corrosion rates of 0.5mm/side/year can be expected and rates in excess of 1mm/side/year have been reported. The characteristic appearance of bright orange rust on piles can be overlain by marine fouling and therefore sometimes hidden.
Corrosion is relatively slow and uniform. In many cases piles are naturally passivated by corrosion by-products and/or marine growth. With the exception of MIC, corrosion rates of 0.04 to 0.13mm/side/year can be expected. There is generally sufficient oxygen and conductivity to support a corrosion rate.
In coarse granular materials where oxygen traces are present, corrosion is slow and uniform. In anaerobic conditions (clay, polluted mud) corrosion can only proceed if the soils are acidic or contain SRB but such conditions are seldom encountered; otherwise the corrosion can be generally regarded as negligible.
ALWC can cause rapid perforation of sheet pile, and result in surface instability. An accelerated corrosion rate will remove steel and value from a structure such that an expected life of 90 years may essentially be reduced to 45 years. If a large structure has a steel thickness of 30 mm, then at rates of 0.5 mm/year to 1 mm/ yr, half of the structural strength will be gone in 30 or even 15 years (and holed in 60 or 30 years) with a consequent loss in value. For small quays, the thickness of flange and web could be 12 mm and 8 mm, giving a service life of as little as 8 years, while the design could have been similarly for 60 or 90 years. As corrosion is independent of steel thickness, protection for small quays is likely to be a relatively large proportion of original construction cost, and a considerably smaller proportion for a large structure (although the size of the exposed area could be a complicating factor).
ALWC on H-Piles
The U. S. Army Engineer Research & Development Center Construction Engineering Research Laboratory (ERDC-CERL) installed steel H-piles, 20.3 cm x 20.3 cm and 12.2 metres long at LaCosta Island, FL in January 1971 (see Figure 3). The pilings were removed after 33 years. Glass flake epoxy and coal tar coatings were shown to be efficacious for protecting the piles against corrosion as shown in Table 1 and in Figure 4. The 10-year measurements of flange thickness profiles for bare carbon steel without sacrificial anodes indicated a corrosion rate of 0.32 mm/side/yr in the immersed zone. This indicates that some form of concentrated corrosion, such as ALWC, may have occurred. Corrosion rates were reduced to 0 in the immersed zone when the pilings were outfitted with sacrificial zinc anodes for providing cathodic protection (CP).
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