Technical Presentations at the January 2008 Meeting

2.1   Corrosion of Welds – Underlying Problems and Practical Solutions’, Mike Robinson, Cranfield University 

The talk focussed on two main topics; the behaviour of oilfield inhibitors on preferential weld corrosion in pipelines carrying an oil and gas stream and the effects of cathodic overprotection on welded structural steels used for the latest generation of platforms and jack-ups. 

Galvanic current and self-corrosion rate measurements have been performed on the weld metal, parent plate and heat affected zones of welds sectioned from X65 pipeline steel and tested in flowing brines saturated with carbon dioxide.  The effectiveness of inhibitors in controlling the corrosion rate of each region of the weld has been investigated over a range of shear stresses.  The inhibitor performance has been shown to differ in each region of the weldment and in some circumstance the direction of the galvanic current has been found to reverse so that accelerated localised corrosion occurred in the weld metal.  The roles of corrosion product and inhibitor films were discussed. 

Welds in high strength steels are known to have an increased risk of failure by hydrogen embrittlement, particularly when hydrogen sulphide is present and when the welding process produces susceptible phases with high hardness.  Hydrogen embrittlement has been studied in high strength steels immersed in both natural and artificial seawater and held at a range of cathodic protection potentials.  The effect of potential on the safe threshold stress intensity was discussed. 

Methods of corrosion control were considered.  Experimental corrosion rate data from seawater trials was presented and used to establish an optimum potential to protect welded steel, while reducing the risks of embrittlement.  Finally, the behaviour of protective organic coatings in deep water was described, together with the effect on coating properties of water absorption and overprotection.  Data was shown from seawater trials at pressures of 250 bar.

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2.2  Welding, Joining and Corrosion’, Charlie Barraclough, Commtech Associates Ltd 

Charlie Barraclough gave a presentation on 'Welding, Joining and Corrosion'. He gave examples of places where welding defects, shape, microstructure and residual stresses cause corrosion, pitting, corrosion fatigue, stress corrosion, erosion corrosion, bacterial corrosion and galvanic corrosion.  

Mechanical joints also suffered from corrosion for the same reasons.  However welding is often used to prevent corrosion by cladding flange faces, valve seats and other vulnerable areas, and in wholly weld overlaid construction.  Cathodic protection may itself cause failures, especially at welds and stress concentrations in susceptible materials such as duplex and martensitic and other high strength steels.  Coating and cathodic protection themselves required good bonding in order to avoid disbondment due to Hydrogen and to avoid galvanic effects.   CP involves welded joints which themselves may adversely affect performance; brazed anode and earthing attachments are recommended since these do not involve as much local hardening,  Hydrogen entrapment and stress concentration.  Hydrogen is a common factor in welding, joining and corrosion which needs to be considered by all project disciplines when design and constructing marine systems.

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4.1     ‘What Reliability Engineers need to know about Corrosion’, John Strutt, Atkins Boreas onsulting

 

Corrosion is a very complex process. It is often difficult to forecast rates of deterioration and impact on equipment performance and life.

 

Of all the mechanisms that can cause equipment to fail Corrosion is one of the probable but also the most preventable in Marine and Offshore industry

 

Despite the corrosion controls put in place failures still occur because the control systems themselves are vulnerable and susceptible to failure

 

In many cases the root causes of corrosion control failures are human and organisational rather than technical: Mistakes are often made for practical reasons without understanding the corrosion and reliability consequences

 

Reliability Engineers need to engage with Materials and Corrosion engineers to ensure that the right information gets to the right people at the right time 

 

Employing experienced corrosion engineers in the organisation is necessary but not sufficient to prevent corrosion failures.

 

Key factors include:

  • The need to understand how design, manufacture, installation and operation affect materials performance and their impact on reliability

  • Use engineers who are familiar with problems and give them quality thinking and preparation time

  • Get information to the right people at the right time

  • Understand human and organisational weaknesses and develop good management practices throughout the whole organisation:

  • Defining your requirements (what you need to do)

  • Analysing and Planning (understanding what to do to get what you want)

  • Implementation (Do it in accordance with the plan so that goals are met)

  • Feedback and Assurance (provide assurance that what has been done meets the requirements

and last but not least

  • Investing in organisational learning (capture – disseminate – exploit)

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