Technical Presentations at the April 2005 Meeting

1.1         ‘Development of Very High Strength Copper Alloys with Resistance to Hydrogen Embrittlement and Stress Corrosion Cracking’, Clive Tuck (Langley Alloys)  

Several types of very high strength copper nickel alloys are tested with respect to environment sensitive mechanical properties which include hydrogen embrittlement and exposure to sulphide and ammonium compounds.  It is found that Cu-Ni-Sn and Cu-Ni-Al-Mn-Nb alloys with nickel content up to 25wt% are resistant to hydrogen embrittlement.  The Cu-Ni-Al-Mn-Nb alloys tested are also found to be resistant to sulphide stress corrosion and stress corrosion in ammonium environments, whereas Cu-Ni-Sn materials demonstrate susceptibility to stress corrosion in these environments.  A study of factors controlling stress corrosion susceptibility of Cu-Ni-Al and Cu-Ni-Al-Mn-Nb alloys shows the principal influences to be the degree of age hardening, the grain size and the iron content.  Thus, with necessary controls of the composition and manufacturing processing of Cu-Ni-Al-Mn-Nb being undertaken, MARINEL 220 has been developed.  In this material, high mechanical strengths are achieved with the material’s possessing a fine grain size and having resistance to stress corrosion.  The use of the NACE TM-01-98-98 slow strain rate tensile test is advocated as a production test method for very high strength copper alloys to verify resistance to stress corrosion cracking susceptibility.

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1.2   ‘Oil and Gas Corrosion Related Work of LR EMEA’, Graham Gibb (LR EMEA Aberdeen)  

Lloyd’s Register EMEA’s corrosion related activities encompass Asset Integrity Management, Verification, Validation & Certification (Design Appraisal) and Ad-Hoc Corrosion Consultancy.  To show the type of work undertaken, several case studies were described in detail, including:

  • Pipeline Integrity Management: corrosion risk assessment of a new build deepwater gas export pipeline
  • Pipeline Integrity Management:  implementation, considering the pipeline H2S integrity limit
  • Supporting Corrosion Aspects of Chemical Management: cooling medium corrosion control
  • Trouble Shooting / Failure Investigation: causes of coating blisters which appeared on a new jetty before construction was finished
  • Process Plant Risk Assessment: RBI system required for a coastal gas & LPG process plant / terminal
  • Acoustic Monitoring of Stress Corrosion Cracking: chloride stress corrosion cracking on an offshore platform

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3.1  
‘Cathodic Protection of reinforced concrete in the marine environment’, Jim Preston (Corrosion Control  Services Ltd)  

Reinforced concrete is a material used universally and in the majority of situations it is a very durable material. Two main mechanisms can result of the corrosion of steel in concrete, these are carbonation (reduction in the alkalinity of concrete caused by acid rain) or chloride ingress through the concrete cover.

The main cause of deterioration is chloride ingress, and so marine structures are particularly at risk. Other common causes of chloride ingress are de-icing salts on bridges, wind blown chlorides or saline ground water. Many factors (such as concrete cover depth or porosity) may affect the time to corrosion. A case study was presented to demonstrate the extent and type of testing necessary when a structure is known to be at risk from corrosion.

CP is now commonly used by civil engineers to protect structures, either as a retro-fit as part of a repair project or sometimes (particularly in overseas markets) at time of construction. Different types of anode systems can be employed as part of an impressed current system and examples were shown using a MMO coated titanium mesh anode encapsulated in sprayed concrete, a MMO coated titanium ribbon anode installed both in slots in old concrete structures and cast into new concrete and a conductive coating used for inland systems. Latterly the technique has been adapted further, and an example of a steel framed masonry clad building was shown where CP can be used to protect the steel frame using discrete anodes.

CP of reinforced concrete is now a mature engineering solution with supporting international codes.

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3.2   Keynote lecture:   ‘Cathodic Protection of Offshore Systems Based on the New ISO Standard 15889-2.  Some Practical Applications’, Svein Eliassen (Statoil) 

1.    Hydrogen induced stress cracking (HISC) issues

HISC of weldable supermartensitic stainless steels has been experienced by offshore operators in environments of H2S and under cathodic protection.  The conclusions from a joint SINTEF/DNV/TWI study of this subject were: 

·      Reduce polarisation to -0.800 V  (Ag/AgCl)  - this gives no HISC for SMSS materials but the system could not be qualified within the time limits available in the study 

·      Standard polarising to -1.050 V  (Ag/AgCl) was qualified for butt welds for SMSS with:

o       No fillet welds (eg those used for anode attachments) to SMSS

o       The maximum allowable stress induced tensile strain to be 0.40 %.

o       Only grades with 2 % Mo or above to be used.

o       Post weld heat treatment of  5 minutes at 630ºC to be used  

2.   Cathodic protection to new ISO Standard 15889-2 – Specific design details 

·      New design criteria have been developed.  The following are the most important parameters that have been changed from previous standard:

o       No set maximum distance between anodes if adequate calculations are presented

o       Significantly reduced coating breakdown factors given for the new advanced coating systems (multilayer PP)

o       More positive protection levels allowed for CRA materials (eg – 500 mV for SMSS) 

·      The new concept for CP to insulated flowlines has been developed which includes the following:

o       Different techniques for attachment of anodes to pipe

o       Trailing wire

o       Remote reference electrodes

o       Calculated voltage profile along the length of pipeline using finite element analysis

·       CP for the largest offshore pipeline project ever (Langeled and Ormen Lange).  Additional CP requirements which have been agreed for these projects are:

o       For the current densities - the upper current density curve as given in ISO standard to be used

o       For critical and strategic pipelines such as major trunk lines the total current demand shall be multiplied with 1.5 as a safety factor

o       For these pipelines the safety factor within the 1000m for pipelines connected to subsea installations, platforms and landfalls shall be 3. 

3.  Pipeline heating issues

Further considerations for pipeline CP system requirements would arise if hating of the pipe is necessary for hydrate control.  If direct electrical heating of the pipe is used, there would be a need for AC corrosion control.  Design recommendations for this system would be as follows:

·        The maximum allowable distance between anodes to avoid AC corrosion outside the current transfer zone may be 4000 m for carbon steel and 5800 m for SMSS.

·        If the distance exceeds 300m, the anodes shall be directly exposed to the seawater.

·        All transition points between materials groups of different magnetic permeability shall be defined and effects should be evaluated

·        All transition points where diameter or wall thickness is changed shall be defined and effects should be evaluated

·        All locations for components shall be identified and anodes shall be installed so as to maximise CP effectiveness and minimise AC corrosion

·        Risk for AC corrosion in the current transfer zone shall be evaluated.

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