Corrosion R&D at the National Physical Laboratory

Citation

(2001), "Corrosion R&D at the National Physical Laboratory", Anti-Corrosion Methods and Materials, Vol. 48 No. 6. https://doi.org/10.1108/acmm.2001.12848faf.005

Corrosion R&D at the National Physical Laboratory

Corrosion R&D at the National Physical Laboratory

Keyword: Corrosion prevention

The UK's national standards laboratory, the National Physical Laboratory (NPL) reports on new developments in its Materials Centre's work on corrosion inhibition and the prevention of stress corrosion cracking.

New test method for corrosion inhibitors

Transients in water chemistry or temporary loss of inhibitor may induce localised pitting corrosion in water-cooling systems and oil production tubing. Work on a simulated cooling water system, says the NPL, has demonstrated that re-establishment of the control chemistry can stop the propagation of any pits formed during the excursion period but that an increased dosage may be required initially. Tests in simulated oil production formation waters have reportedly demonstrated that addition of an inhibitor can prevent pit growth but the effectiveness of the inhibitor depends strongly on the composition of the test environment.

This test method has been developed in the recently completed Degradation of Materials in Aggressive Environments programme, supported by the UK's Department of Trade and Industry (DTI).

Combating stress corrosion cracking

Another project within the Degradation of Materials in Aggressive Environments programme focused on stress corrosion cracking, a significant concern in the chemical processing industry. This can to a large extent be overcome by correct selection of materials. Austenitic stainless steels – especially 316L – have been widely used in chemical processes and cracking is not expected under normal service conditions. Cracking may, however, be induced during an excursion, such as an outage: e.g. a transition in chloride concentration or temperature.

Test methodologies have reportedly been developed by NPL Materials Centre to assess the impact of chloride excursions on the stress corrosion cracking of 316L stainless steel in concentrated acetic acid with 1,500ppm bromide and 200ppm sodium ions at 90°C. The impact of chloride excursions on the thin oxide film formed on the 316L specimens has been characterised using XPS and the threshold chloride concentrations for stress corrosion cracking of 316L in 70 per cent, 80 per cent and 90 per cent are said to have been established using proof ring and slow strain rate tests.

The Centre has also announced the development of software to enable the prediction of general corrosion and sulphide stress corrosion cracking of duplex stainless steels. The predictive algorithms have been developed using neural networks trained with the datasets from the NPL Corrosion Database of Duplex Stainless Steels. Said to be user-friendly, the software enables users to generate their own predictions using the validated algorithms. Where appropriate, confidence indicators are presented alongside these predictions.

Future developments

A new research programme on life performance of materials commenced in Summer 2001 at the NPL Materials Centre. Addressing industrial problems relating to in-service performance, degradation modelling and failure and lifetime prediction, the DTI-supported programme aims to measure and predict the life performance of materials and coatings through development of new knowledge-based products and evidence for enhanced maintenance strategies for products, process plants and structures and the improved design of components, finished products and process plant design. The work also aims to reduce the risk of failure due to degradation of materials.

Within this programme, a project on environment assisted cracking of aerospace materials has been designed to help the UK aerospace industry cope more effectively with prediction and prevention of environment-assisted cracking of aircraft landing gear. The aim is to establish a protocol for laboratory testing. A companion project on environment assisted cracking of steam turbine steels focuses on the risk of corrosion fatigue and stress corrosion cracking initiated from pits in steam turbine blades, discs and rotors. The risk increases with changes in operating practice or excursions in water chemistry; this project aims to identify conditions under which cracking is likely and to generate crack growth data to improve inspection and life prediction.

Other projects said to be underway include evaluation of test methods for characterising weldable martensitic stainless steels, principles of steam oxidation, life prediction of thermal barrier and corrosion resistant coatings for gas turbine operations and metal wastage under complex atmospheres or thermal cycling.

For further information on any of the activities mentioned above, and to find out how your company can participate, please contact: Materials Enquiry Point, National Physical Laboratory, Queens Road, Teddington, Middlesex, TW11 0LW. Tel: +44 (0)20 8943 6701; Fax: +44 (0)20 8943 7160; E-mail: materials@npl.co.uk; Web site: www.npl.co.uk/materials