ALWC on steel structures in marine environments

Accelerated low water corrosion (ALWC)
The most common form of AWLC 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 character istics, revealing clean cor roding 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 bacter ia (SOB), which converts it to sulphur ic 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.

Mar itime 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 …..

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Ashok Kumar & Larry D. Stephenson, U.S. Army Corps of Engineers, Engineer Research and Development Center, Construction Engineering Research Laboratory and NACE International, Houston, Tx, USA
Edition: Edition 32

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