Selecting a strategy for dealing with accelerated low water corrosion (ALWC): Part 2

Authorship

Dr Jim Breakell, Corrosion Control and Materials Consultant, on behalf of CIRIA, London, UK

Publication

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Dr Jim Breakell is an independent corrosion control and materials consultant with a particular interest and expertise in marine corrosion and its control, including for ALWC.

He is the principal author of the recently published CIRIA guidance, Management of Accelerated Low Water Corrosion in steel maritime structures (C634).

In the second part of a two-part article for Port Technology International, Dr Breakell examines how to protect structures against ALWC, on-going monitoring methods and conclusions.

Protecting against ALWC

The protection strategies to treat existing structures, particularly those in the early to intermediate stage of ALWC attack or in an advanced stage after repair, are generally based on well established, conventional corrosion control methods which already have a track record of protecting steelwork from other forms of corrosion in marine environments.

These include:

Cathodic protection (CP) using galvanic anode and/or impressed current system. Either system can be retrofitted to an existing structure. CP is a proven technique for protecting steel against both microbiological and corrosion processes in seawater and can be expected to be effective on maritime structures up to about the mid tide zone area if designed, installed, commissioned and maintained properly. The provision of partial CP to locally protect repaired areas, by fixing a limited number of galvanic anodes adjacent to the repair, has also been used to extend the life of such repair areas including welds.

Coatings can protect maritime steel structures that are continuously or intermittently immersed in seawater. They are available in a variety of types (i.e. organic, inorganic and metallic) and forms (e.g. paint films, wrappings/tapes, claddings etc) and are most commonly applied by spraying, brushing or wrapping to suitably prepared surfaces. Coatings should have good resistance to abrasion, biodegradation, chemical and microbial attack and be compatible with CP.

Concrete jacketing/encasement; both precast concrete elements and concrete cast in-situ have been used for protecting solid and skeletal structures. Depending on the cover and quality of concrete used, corrosion problems on the substrate steel surface and/or reinforcement bars (if used) can occur over time due to chloride diffusion and/or carbonation. The use of corrosion inhibitors in the concrete mix, pre-coated reinforcement bars and CP have been used in the past to reduce the risk of such corrosion occurring. Build up of corrosion product may eventually lead to spalling of the concrete. Beware also of introducing corrosion concentration cells onto the structure if only partial jacketing/encasement is applied.

Combinations; most commonly CP in combination with organic coating applied in the tidal zone and above. The presence of a coating in these upper zones not only provides protection where the CP is ineffective but also acts to reduce the overall protection current required. This, in the case of a galvanic anode system, reduces the number and/or size of anodes required or can prolong anode life. Such coatings should be fully tested for compatibility with CP (i.e. cathodic disbondment testing). Overprotection with impressed current CP is to be avoided to minimise the risk of coating disbondment or hydrogen embrittlement of any high strength steels or at sensitive weld areas. Other combinations such as increased steel thickness with partial coating or CP in conjunction with partial concrete jacketing/encasement have also been used.

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