Technical Presentations at the April 2015 Meeting

1.1  ‘External Corrosion Control of Submerged and Buried Pipelines by means of Coatings and CP’, Trevor Osborne, Immediate Past President ICorr & Deepwater Corrosion Services

An overview of the current best practices for the corrosion control of submerged and buried pipelines is presented.

Modern pipeline coatings are of two basic types: Single and dual layer fusion bonded epoxies (FBE) or three layer coatings of FBE and an adhesive with an outer layer of either polyethylene or polypropylene. These coatings are applied in a pipe coating mill and pass through a number of processes which must be rigorously controlled to ensure a high quality product. It is best practice to pre-qualify the system and ensure quality is controlled in production by testing and inspection.

Generally the pipeline coatings are the same for both onshore and offshore. However offshore pipelines are often concrete weight coated to provide negative buoyancy, a process which is carried out after the corrosion coating has been applied. In order to ensure the concrete survives the lay process and in-service conditions it is steel cage or wire reinforced.

Following coating the pipe is transported from the mill to the point of construction where it is welded and a pipeline is constructed. Pipeline welds must be coated to the same standard as the rest of the pipeline, the coating being FBE, heat shrink material or liquid coating. Quality must be controlled in the field by inspection and it is imperative to ensure that all coating damage is repaired.

Several influences determine the type of cathodic protection (CP) system which is applied to the coated pipeline, the main factors being the environment in which the structure is operating and the required life. A correctly designed CP system will enable the asset to reach the design life without metal loss. Correctly executed external corrosion control using barrier coatings and CP is an investment in the future of the asset and, if done correctly, will pay dividends from the outset.

[A pdf of this presentation has kindly been supplied and is available to members from the Secretariat]

1.2 ‘Galvanic Anodes – Boring? Easy?’, Ross Fielding, Impalloy

Despite the title of this presentation, the speaker looked at various alloys used for galvanic anodes to show that the work involved is neither easy nor boring.

Required skills were discussed, including Casting (looking at composition, temperature, geometry and insert type), also Fabricating & Welding, Fettling and Coating. Manufacturing processes were then considered.

Various Factory Acceptance Tests were explained, covering Anode Mass, Spectrographic Testing, Electrochemical Testing, Destructive Test, Dimensional, "Fit Up", MPI, Anode To Core Resistance and Coating Tests.

Finally discussion focussed on what was happening, looking at Diode Control of Al/In/Zn alloy and Introduced Gallium based Aluminium anode alloys which are good for Reduced Electrochemical Noise and to help ensure Over Protection of CRA's does not occur.  

[[email protected]]

[A pdf of this presentation has kindly been supplied and is available to members from the Secretariat]

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1.3   ‘The Links between Subsurface Deformation and Tribocorrosion Performance of Stainless Steels in CO2 Environments', Michael Bryant, University of Leeds

The drivers for this research project were explained:

• Deeper oil exploration = more aggressive environments
• Enhanced oil recovery = injection of corrosive gasses
• Use in marine environment = high flow, high halide salinity
• Lean/Duplex alloys present both integrity and cost savings

At the time of starting this research there was:

• Little information relating to the tribocorrosion degradation of these alloys.
• Limited information regarding their behaviour in CO2 conditions.
• No studies on the interaction between subsurface recrystallization and subsequent tribocorrosion.

Methodology was discussed and the main terms explained. The results were then considered in detail, leading to the summary and points for discussion:

• TWL is less for lean duplex compared to austenitic alloys
• Corrosive contributions greater for UNS S30403
• Mechanical contributions (i.e. less corrosion) for lean duplex
• Re-crystallisation of the austenitic alloy and increased work hardening
• It is thought that the recrystallization effects both corrosion and wear.
• Dislocation generation through shear, creep and glide exposes fresh surfaces for dissolution increasing the corrosion rates.
• Pure erosion is higher in austenitic alloys due to the lower stacking energy of 304 resulting in fatigue and grain detachment.
• Higher Cr content in the lean alloys enhances passivation but also stabilises the ferrite (BCT) phase reducing erosion.

[A pdf of this presentation has kindly been supplied and is available to members from the Secretariat]

3.1    ‘Materials for Seawater Applications: PREN as a Selection Criteria’, Jozef Soltis, MACAW Engineering

Pitting resistance of stainless steels (and other corrosion resistant alloys) as a function of their composition can be expressed through the Pitting Equivalent Resistance (PRE) number, which can be generalised in the following form for stainless steels with an addition on nitrogen: PREN = a%Cr + b%Mo + c%N where a, b and c are constants. It is now well established that the higher the PREN the higher the resistance of alloys to pitting; however, note that PREN does not provide information on materials’ behaviour in real service. Careful review of published results and available literature suggest that there is a considerable variability in derived PREN, which is likely due to differences in test and metallurgical conditions.

Selection process involving stainless steels and PREN is on one hand traditionally based on standards, e.g. ISO 21456 or NORSOK M-001. Based on these standards, materials are arbitrarily defined as seawater resistant if PREN > 40 and in the case of materials with PREN < 40 there is a need for cathodic protection. On the other hand a scientific approach requires calculation of pitting probability and due to the stochastic nature of pitting a statistical approach is necessary. In the latter case, the good candidate is Markov process; more specifically, studies of meta-stable and metapassive transient states. Here, the probability for a given material transiting from a meta-stable to meta-passive state depends on a critical chloride concentration. However, since the critical chloride concentration depends also on a metallurgical factor, represented by PREN, a well-defined relationship for PREN is required.

The current work considers 16 commercial stainless steel alloys, with PREN ranging from 12 to 56. Values of free corrosion potential, re-passivation and pitting potentials for these materials were established through electrochemical measurements at ambient temperature and in artificial seawater. Materials with similar PREN show considerable differences between pitting potentials, and in the difference between re-passivation and free corrosion potential, whilst materials with different PREN show relatively comparable pitting potentials, and values for the difference between repassivation and free corrosion potential; this overall inconsistency would complicate any attempt to establish the critical chloride concentration and the resulting pitting probability. Also, a few stainless steel alloys with PREN < 40 show the difference between re-passivation and free corrosion potential above the value of free corrosion potential, which suggest that these materials should have adequate resistance to pitting in seawater; this contradict the traditional selection criteria. Hence, material selection for seawater applications should not be based on PREN. It is also important to realise that there is a high risk in using PREN as a fine-tuning aid in any selection process. [Macaw Engineering Ltd., Quorum Business Park, Newcastle upon Tyne, NE12 8BS]

3.2   ‘Destructive Interference in Cathodic Protection: Examples Identified through Simulation’, John Baynham, CM BEASY

The presentation looked at a number of different situations in which destructive interference might occur when multiple cathodic protection systems are installed. These included:

• interaction between an ICCP system and a sacrificial CP system, in which (counter to expectation) increase of ICCP output caused an increase of output of some of the sacrificial anodes

• interaction between a seabed ICCP system and a marine pipeline and associated platforms, in which reduction of sacrificial anode life is difficult to avoid

• interaction between two sacrificial CP systems, in which the combined systems produced more positive potentials at end of life than either of the CP systems acting on its own

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