Gulf Construction Online - Halcrow sheds light on ALWC problem

Corrosion Technology

Figure 1.

Halcrow sheds light on ALWC problem

The hot climatic conditions prevalent in the Gulf promote rapid biological activity and are conducive to accelerated low water corrosion (ALWC). Dr GAVIN CHADBOURN and DAVID POCOCK of Halcrow International suggest approaches for repair and remediation of ALWC-damaged structures.

01 June 2005

Microbial-induced corrosion of marine structures predominately affects steel structures, whereby multi-species colonies of bacteria present in seawater metabolise sulphates under anaerobic conditions generate hydrogen sulphide, a corrosive compound that reacts with steel to form iron sulphide.

The situation is exacerbated as iron sulphide is strongly cathodic to steel, thereby producing a very intense localised corrosion cell. Although sulphate is plentiful in seawater, the reaction is normally restrained by the oxidation reaction that requires dissolved organic carbon compounds, which are usually not available in sufficiently high quantities to sustain large bacteria colonies. Hence, under normal conditions, sulphate-reducing bacteria (SRB) are unable to form in sufficiently large colonies to present a significant corrosion risk.
Accelerated Low Water Corrosion (ALWC) is a form of microbial-induced corrosion (MIC) that is commonly observed on steel structures around the low tide mark. However, corrosion can also occur at the mud line, where anaerobic conditions prevail and high concentrations of sulphates and organic matter are deposited. Therefore, the conditions that are favourable for MIC occur outside the tidal and splash zones, where there is a high risk of atmospherically induced corrosion of steel.
While all marine structures are at some risk of corrosion due to atmospheric oxidation of the steel, microbial-induced corrosion is often much more rapid when it does occur. Several millimetres of section loss per year can occur, as compared with atmospheric corrosion of steel which, under typical exposure, would be expected to occur at the rate of 0.1 mm per year for a typical marine environment. Thus corrosion allowances used in the design of steel piles would be rapidly consumed by microbial-induced corrosion.

Figure 2.

Inspection & identification
The rapid section loss associated with this form of corrosive attack and its location below the water line or at the mud line means that cursory inspection of structure may not reveal its presence. The first indication of MIC may be the settlement of a quay deck, associated with loss of fill from behind a sheet-piled wharf.
Tubular steel piles can also fail prematurely as thinning and section loss can promote eccentric loading and ultimately bucking. It is, therefore, prudent to undertake frequent inspections of marine structures, where the potential for MIC is considered high.
The inspection should at least be carried out from a boat at low tide to enable close inspection of the structure at the water line, and ideally should include an underwater survey to inspect the mud line. Samples of suspected bacteria colonies should then be collected and tested for SRB.
Definitive identification of SRB can only be done through the collection of samples and examination in a laboratory. Bacteria collected from samples of mud or slime must be kept cool and moist at all times, to enable them to be cultured in a laboratory, prior to microscopic examination and identification. However, simple indicator tests are available such as Echa’s Sig sulphide test, which identify the presence of SRB through colour change of an indicator gel. The rapidity of any colour change is an indication of the severity of the infection. A combination of physical evidence and indicator tests is usually sufficient to confirm or negate the presence of MIC.
ALWC is characterised by a red or orange slime, located at low water level. Underneath the orange slime, black sludge – the corrosion product iron sulphide – is often observed. The slime is also characterised by a rotten egg smell, indicative of hydrogen sulphide, the reaction product of the metabolism of sulphides and organic matter by bacteria. If the sludge is removed, the steel will often appear bright or etched, and may have localised deeper pits. Other biological marine growth in the region of the slime is often absent.

Figure 3.

MIC damage in the UAE
The creeks and harbours of the Arabian Gulf present ideal conditions for MIC. Low tidal movements in creeks, lagoons and ports inhibit the dispersion of pollution and promote anaerobic conditions. In addition, the warm water temperatures accelerate bacteria activity, with a 10 deg C rise in temperature causing a doubling in reaction rate, up to a peak of 50 deg C. Corrosion rates of 4 mm per year for MIC has been quoted for the temperate conditions of Northern Europe, where average sea temperatures are around 10 deg C. This figure is expected to be higher in the warmer conditions of the Gulf.

Figure 4.

Dubai Creek
Inspection of disused tubular steel piles in the creek was undertaken as part of a study into upgrading of facilities. Visual inspection of the piles initially indicated that the piles were in acceptable condition. However, closer examination from a boat at low tide revealed the presence of an orange slime that was later identified as sulphate-reducing bacteria.
Figure 1 shows a tubular steel pile at low water level. The orange slime characteristic of ALWC was located in an area less than 500 mm either side of the low water mark. Removal of the orange surface slime revealed the black, iron sulphide sludge underneath as shown in figure 2. Cleaning of the area showed the underlying steel to be etched, with localised pits approximately 2 mm deep (figure 3). This localised corrosion was far more severe than corrosion observed in the tidal and splash zone where the steel was still largely protected by a coating. It appeared that ALWC had accelerated the localised deterioration of the coating.

Figure 5.

Sheet-pile quay wall
As part a study into the feasibility of repairing and upgrading a 20-year-old quay, a survey was undertaken of the sheet piled wall. The quay comprised a sheet-pile wall, with reinforced concrete capping beam and block paving forming the deck of the quay. Inspection of the quay showed signs of settlement of the deck, which had periodically been filled to maintain level and ensure the deck surface remained serviceable for port traffic.
Visual inspection of the sheet piled wall above low water mark revealed evidence of oxidation corrosion.
This was uniformly distributed along the length of the quay. In localised areas, corrosion had penetrated to full depth creating holes in the pile wall, through which fill material had escaped causing the settlement of the deck (figure 4).
An underwater survey of the sheet-piled wall revealed severe damage in localised regions, where corrosion had resulted in the puncture of the sheet piled wall below the low water level and at the mud line (figure 5, 6 and 7). Samples of deposits around the corrosion holes were taken and analysed using the Echa Sig Sulphide test, which confirmed the presence of SRB (figure 8).
Ultrasonic thickness measure-ments were taken at various levels on the sheet-piled wall above and below the water line. Section loss was typically noted to be less than 3 mm in areas not affected by ALWC, consistent with the expected rate of 0.1 mm per year associated with atmospheric corrosion. In locations where sulphate-reducing bacteria had been identified, full section loss of 15 mm had occurred. As the period of colonisation on the steel by the bacteria was unknown, the deterioration rate could not be estimated, although it would be far in excess of any corrosion allowances made in the original design.

Figure 6.

Repair & remediation
The ease and cost of repair of corrosion damage associated with SRB will largely depend on the severity of damage. Minor localised damage can be cleaned and steel patches welded in place to restore structural capacity. However, this method of repair does not control the underlying cause and corrosion can reoccur.
A number of methods exist that can be used to address the problem of controlling MIC and can equally be applied to new-build structures.
• Excavation: The removal of the source of pollution in conjunction with removal of mud contaminated with bacteria is possible. However, this method is expensive for large areas and would only be effective if the pollution source was removed. In addition, since the disposal of the dredged material can pose a potential risk of contamination of other sites, it is not a very practical solution.
• Coating: Application of appropriate protective coatings can provide a good solution, particularly for new-build structures. However, consideration should be given to repair of damage to the coating that may occur during piling operations. A coating is equally applicable for repairing corroded structure, where in order to ensure the quality of application the steel will need to be prepared and the coating applied in a dry environment. This will typically require cofferdams to coat the areas below low water level, which makes this a costly solution.

Figure 7.

• Cathodic protection: There is substantial evidence that MIC and in particular ALWC can be prevented on new structures, and stopped on existing structures where bacteria colonies are already established, through the use of cathodic protection (Wyatt 2002). Protection is afforded by passing a small current between surface-mounted anodes and the steel structure. The current can be supplied either by an external source (impressed current) or by the sacrificial corrosion of anodes.
The application of a cathodic protection system does not require the widespread removal of bio-fouling or corrosion products where ALWC has already established, eliminating the need for extensive surface preparation. It is good practice to ensure electrical continuity of the piles by fillet welding between sheet piles. Galvanic anodes can be welded onto the steel structure underwater or, by using an extended metal core, above the water line. The design of the system must ensure uniform distribution of current to provide protection. Sufficient current to give -0.9 volts steel/seawater with respect to Ag/AgCl/seawater reference electrode will normally provide adequate corrosion protection from SRB.

Figure 8.

Summary
MIC – evidence of which has been found in the UAE – can cause severe damage to marine structures if not addressed in a timely manner. Given the favourable climatic conditions, and rapid development of ports and other facilities along the coastline in the region, the incidents of MIC on marine structures are likely to increase in the near future.
With appropriate inspection regimes to facilitate early identification of MIC and timely intervention through maintenance programmes costly repair work can be avoided.
Regular periodic inspections are recommended that include visual inspection and sampling and testing of suspected cases of ALWC, including underwater inspection at mud line level.
Cathodic protection offers a reliable means of treatment of MIC, and tackles the underlying causes, as opposed to only treating the consequential effects.
References
Wyatt B S – Cathodic Protection: A solution for accelerated low water corrosion. Millennium Conferences International 2002.

• This paper was presented at the Annual Concrete Technology and Corrosion Protection Conference held during the Gulf Construction Conference Week in Dubai last year.