Technical Presentations at the October 2016 Meeting

1.1   'Asset Integrity Management and the Law’, Andy Duncan, Intertek Production and Integrity Assurance

This presentation looked at UK Health & Safety Law and the duties of employers to their workforce. It then considered Statutory Instruments (SI) or regulations, which add detail to the law. Examples of SIs relating to corrosion and asset integrity management were discussed, including Offshore Installations, Management of Health & Safety at Work, Provision & Use of Work Equipment, Pressure Systems Safety, and Pipelines Safety. The implications to corrosion and integrity management were outlined for each.

The conclusions were:
• Degradation of process plant and structures is a potentially serious risk to health and safety
• The UK H&S law is goal-setting
• UK H&S law requires risks to be ALARP (as low as reasonably practical)
• Risks can be mitigated by:
      o Corrosion Threats Assessments
      o Corrosion Risk Assessment studies
      o Risk based Inspection
      o Corrosion Management Strategies
      o Written Schemes of Examination
      o Maintenance

Note: The speaker sent this further information: “There seems to be interest by designers of bridges and buildings in using very high strength steels for bolting without being aware of the risks of hydrogen induced cracking. They should be made aware of these risks, and be advised that stronger is not always better. This is a matter which has also been raised at Energy Institute meetings”

[A pdf version of this presentation has kindly been provided and can be obtained by members from the Secretariat]

1.2  ‘The Effects of Microstructure and Material Hardness on the HAC and Corrosion Susceptibility of Martensitic Stainless Steels’, Michael Lowden, Rolls-Royce

Martensitic stainless steels are those which contain enough chromium (over about 11%) to be corrosion resistant, and with chemical compositions that make them hardenable by the martensitic transformation. Typical heat treatment of martensitic stainless steels: austenitised above approximately 1100oC and quenched to form martensitic structure (strong but quite brittle) and then tempered at intermediate temperatures (650 – 700oC) to reduce hardness and strength, and to provide improved toughness, ductility and corrosion resistance.

Typical Applications: Martensitic stainless steels are used where combinations of corrosion resistance, high strength, and/or high hardness are required. Also used widely in Light Water Reactors (LWRs), both in safety systems and non-safety systems. Although martensitic stainless steels generally perform well in variety of applications, they can become susceptible to Stress Corrosion Cracking (often attributed to a hydrogen embrittlement mechanism – known as hydrogen assisted cracking, HAC) if heat treated to high hardness values, loaded under tensile stress and exposed in environments that can generate hydrogen.

Several case studies of failures were discussed. An experimental programme was then outlined, and the tests conducted, and the results, were described in detail.

• Results obtained from this test programme demonstrate that susceptibility to HAC is dependent upon a complex interplay between microstructure (tempering temperature/ hardness), stress and environment (availability of hydrogen).
• Key differences in microstructure can result in increased corrosion susceptibility, which in turn affect material susceptibility to HAC. Specimens tempered at 600°C (363 HV) exhibited increased susceptibility to crevice corrosion compared to as-quenched specimens (488 HV).
• Failures appear to occur over a wide range of nominal stress, with hardness (strength level) being an important factor governing HAC. In general, as hardness increases the minimum nominal stress for failure, hydrogen content required for failure and time to produce failure decrease. Data from Incremental Step Load (ISL) tests support this premise.
• There appears to be a critical value of nominal stress below which failures do not occur and the material can support the stress indefinitely.
• From constant load tests it is clear that environment plays a key role in HAC and must support the necessary corrosion processes and hydrogen evolution required to drive the mechanism – 3 failures in atmospheric conditions; compared to 9 failures in immersed tests
• Tempering material at 650°C has shown to result in material that is relatively resistant to HAC with hardness values below 340 HV.

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4.1  ‘Hull Biofouling - Still an Area of Growth’, Stuart Downie, Lloyd’s Register EMEA

The reasons for the requirement for marine coatings which resist biofouling and the history of their development was reviewed. Fuel saving and environmental improvement are the major benefits of anti-fouling coatings - biofouling is estimated to cost the shipping industry an extra US$60bn in fuel annually. Other advantages are increased speed and range, reduced cleaning costs and control of invasive species.

The most successful antifouling agent, introduced in the 1960’s, was tri-butyl tin but this was found to have adverse effects on marine organisms. This led to its being banned by the IMO in 1990. Since then, recognition schemes have been developed, overseen by the Classification Societies, which review manufactured anti-fouling coating systems with a view to signing them off against the IMO requirements. The various regions of the world have separate biocidal regulations, with Europe and USA having most stringent legislative frameworks. Biocidal antifoulings remain the majority of products applied onto hulls but foul release coatings continue to grow in market share. Novel products of both of these types were reviewed - the best eventual solution is likely to be a combination of technologies.

4.2  ‘Role of Highly Concentrated Solutions in Atmospheric Corrosion of Austenitic Stainless Steels’, Steven Street, University of Birmingham.Intermediate-level nuclear waste (ILW) in the UK is held in containers made from 304L and 316L stainless steels. Permanent storage for these containers is planned to be in a geological disposal facility, but this facility has not yet been built. In the meantime, this waste is held in warehouses where the containers are exposed to salty aerosols that may cause atmospheric corrosion.

Experiments on the effect of cumulative deposition of aerosol salts were simulated using droplets of MgCl2 solutions. It was found that a threshold limit for chloride deposition density exists, below which pitting will not propagate. Pit morphology was observed to be influenced by relative humidity. Pitting and repassivation behaviour was also analysed electrochemically in solutions ranging from 1 M to 5 M (saturated) MgCl2. It was observed that in highly concentrated solutions, similar to those experienced during atmospheric corrosion, no sharp transition was observed between active and passive dissolution. This allows propagation to continue even under very low potentials. Pitting events in highly concentrated solutions showed evidence of multiple sites of attack during the initiation period.

[A pdf version of this presentation has kindly been provided and can be obtained by members from the Secretariat]

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