Technical Presentations at the January 2006 Meeting

2.1  Identification of, & protection against, ALWC in UK ports’, Ian Spring & Jim Preston (Corrosion Control Services Ltd) 

The phenomenon of Accelerated Low Water Corrosion (ALWC) has, in recent years, caused widespread problems in the UK Ports & Harbours Industry. 

ALWC is a form of microbially induced corrosion (MIC) which affects steel piled structures (tubular or sheet piles). The corrosion occurs in a band upward from lowest astronomical tide and quoted corrosion rates are 0.2-4.0 mm/side/year. 

This is considerably in excess of the general design corrosion rate for steel harbour structures, which, until recently were not generally afforded any type of corrosion protection. Some failures of piled facilities have been reported, and surveys have shown the problem to be nationwide. 

The reasons for the development of ALWC are not fully understood, neither are the precise mechanism or rate of propogation. However, cathodic protection has been proven to be an effective solution to the problem. 

Both galvanic anode or impressed current systems are used in the UK. Both systems have their own advantages, but UK practice has tended to use galvanic anodes where possible, to reduce the maintenance burden on operators.

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2.2   ‘Copper nickel chromium alloy as a replacement for nickel aluminium bronze in sea water system application’, Jagath Mawella (Sea Technology Group, DPA/MoD)

Resumé: A copper nickel chromium alloy is now available which has been shown to be much more corrosion resistant in seawater than the more traditional nickel aluminium bronze materials used on submarines.  Nickel aluminium bronze has a tendency for selective phase corrosion, which is not found in the copper nickel chromium alloy. 

Defence standards NES 824 parts I & II have been written to describe the production methods and properties of castings in the alloy.  Forgings are not yet available, although these have been produced.  The major difficultly in production is the formation of linear internal defects caused by oxides of titanium and zirconium.  These are not visible through the use of standard NDE techniques, but can be detected using Eddy Current examination.  Fatigue and fracture toughness properties are good.  Cast components which have been examined after ten years service in submarines have shown very low corrosion (0.15mm loss of thickness) and no biofouling.  It is envisaged that copper-nickel-chrome will gradually replace nickel aluminium bronze to allow the refit frequency of submarines to be reduced.  CT

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4.1      Avoiding hydrogen embrittlement failures of duplex and superduplex stainless steel in subsea service’, Paul Woollin (TWI) 

The paper presents the results of a programme designed to define the material, stress and environmental factors controlling sensitivity of ferritic-austenitic stainless steels to hydrogen embrittlement stress cracking when exposed to cathodic protection.  Factors examined in small and large-scale tests include microstructural coarseness, phase balance and hardness of a range of parent steels and welds. 

The results are presented in terms of threshold strain and normalised stress to develop hydrogen embrittlement stress cracks.  The effects of microstructure and applied potential on crack initiation and propagation are described.  Preliminary recommendations are made with respect to the limiting applied stress and strain levels for ferritic-austenitic steels under cathodic protection.  [email protected]

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