Accelerated low water corrosion

Citation

(2004), "Accelerated low water corrosion", Anti-Corrosion Methods and Materials, Vol. 51 No. 1. https://doi.org/10.1108/acmm.2004.12851aaf.004

Accelerated low water corrosion

Accelerated low water corrosion

Keywords: Corrosion, Steel, Water treatment

CAPCIS have recently taken a fresh look at an old problem – accelerated low water corrosion (ALWC). Corrosion of submerged or partly submerged steel structures has been observed for many years in ports and harbours. Only in the last decade or so, has it been recognised as a serious problem with very costly financial implications. The main damage is due to microbially-induced corrosion (MIC) which can badly damage submerged steel structures. When this phenomenon strikes berth and jetties, intense pitting corrosion, at about ten times the rate of normal marine corrosion, occurs.

A clear distinction needs to be drawn between accelerated low water Corrosion (ALWC), which occurs at or just below the low tide areas (and causes severe damage in these zones) and that due to MIC just below the mud line, which remains inaccessible and unseen but which can also be a serious and insidious problem. The cause of corrosion is the same in both cases – high sulfate reducing bacteria (SRB) colonies fed by organic pollutants such as sewage, food factory effluents etc., but the treatment may be different, except for cathodic protection (CP), which is effective for both. ALWC is characterised by an orange-brown surface layer underlain with black or grey/black sludge and a shiny pitted steel surface. Below the mud line, the orange-brown layer may be absent, but in all cases, freshly removed black sludge gives off a bad-egg smell of hydrogen sulfide (H₂S) when treated with acid.

Assessing the risk

CAPCIS believe that identification of high-risk areas with respect to MIC is a pre-requisite to any detailed surveys. This approach allows remedial measures to be prioritised in a highly cost-effective manner. An initial site survey and limited sampling are desirable to recommend the most appropriate sampling procedures and spread for optimal results.

CAPCIS has recently carried out surveys in the UK and Hong Kong. These surveys involved measuring SRB “counts” in mud and seawater by specialised culture techniques. Samples were taken from representative locations to determine where the potential “hot spots” are. CAPCIS have extensive experience of MIC and are able to both formulate suitable sampling programmes for new developments and existing structures and interpret the results.

Stopping the problem

The measures available to mitigate corrosion are:

1. 1.

   the application of CP systems,

2. 2.

   the application of protective coatings,

3. 3.

   dredging of infected sludge, and

4. 4.

   removal of the pollution source.

Structural measures, such as welding in patch plates at the low water line or concrete infilling of piles, may be used to replace the structural strength of the corroded steel rather than prevent it corroding.