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.
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
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. paul.woollin@twi.co.uk