National Physical Laboratory

Corrosion

Focus ion beam milling for 3D imaging of stress corrosion cracks
Focus ion beam milling for 3D imaging of stress
corrosion cracks

With the increased demand for sustainability and life extension, the avoidance and control of corrosion are of the utmost importance. Great progress has been made in controlling general corrosion through coating technology and inhibition but localised attack and, in particular, environment assisted cracking remain challenges and have been a primary focus for research because of the often catastrophic nature of failure and the difficulty in detection. That research has led to improved materials selection and design supported now by a suite of testing protocols, most notably the series of ISO standards on environment assisted cracking.

Such developments have eliminated the more obvious failure processes but analysis of failures has highlighted ongoing challenges in predicting the impact of transients (temperature, environment or stress), in dealing with condensing and evaporating conditions, and in accounting for the variability associated with welded structures. In addition, materials are being deployed in increasingly harsh environments with respect to elevated temperature, pressure and aggressivity, e.g. oil and gas, nuclear, power generation, and this is imposing further demands on reliable materials selection and lifetime prediction.

To address these issues we undertake leading edge research combining advanced measurement and modeling to characterise the evolution of corrosion damage from its earliest stages, to develop testing standards, and to establish predictive tools for lifetime evaluation.

Facilities for corrosion testing:

  • Proof rings, 4-pt bend, C-rings; slow strain rate, corrosion fatigue, HIC
  • Eight autoclaves for high pressure/high temperature testing with and without H2S
  • Small-scale flow loop
  • Multi-electrode underdeposit corrosion facility
  • Pitting and crevice corrosion testing facilities
  • Scanning electrochemical microscopy (SECM)
  • Electrochemical impedance spectroscopy (EIS)
  • Potentiodynamic test stations
  • Electrochemical potentiodynamic reactivation (EPR)


Active Corrosion Research

Underdeposit corrosion of carbon steel in oil and gas applications and its control by inhibition
Failure of carbon steel pipelines due to underdeposit corrosion is a major concern in the oil and gas industry. Accumulation of sludge-like deposits, typically comprised of sand and corrosion products such as iron carbonates and iron sulphides, will give rise to coupling between regions covered quite deeply (e.g. 1 cm) and adjacent regions with marginal deposits. Differences in access of corrosion inhibitor can then lead to enhanced localised corrosion through a combination of differential concentration cells, retention of aggressive species under the deposit and a large cathode to anode surface area ratio. We have a multi-electrode test method that has been developed to assess the performance of corrosion inhibitors under sand deposits.

Multi-electrode system for underdeposit corrosion inhibitor testing for hydrocarbon pipelines
Multi-electrode system for underdeposit corrosion inhibitor testing for hydrocarbon pipelines


Measurement and prediction of environment assisted cracking of steam turbine steels for conventional and advanced steels
Stress corrosion and corrosion fatigue measurements are being made of the performance of steam turbine discs and blade steels in simulated condensate conditions that are designed to provide confidence to industry in the introduction of new materials and new fabrication methods.

Measurement and prediction of environment assisted cracking of steam turbine steels for conventional and advanced steels


Characterising the evolution of cracks from corrosion pits
Finite element (FE) analysis has been undertaken to evaluate the stress and strain distribution associated with a single corrosion pit in a cylindrical steel specimen stressed remotely in tension. The primary goal was to rationalise the observation from unique X-ray tomography measurements that stress corrosion cracks in a steam turbine disc steel initiated near the mouth of the pit and not the base. The modelling provided an explanation based on localisation plastic strain but in a major advance introduced the wholly novel concept that a growing pit in a static stress field generated dynamic strain, a key feature in initiating stress corrosion cracking.

X-ray tomography and FE analysis of pit-to-crack transition
X-ray tomography and FE analysis of pit-to-crack transition


Evaluating the impact of surface condition on stress corrosion cracking of stainless steels for the nuclear and oil and gas industries
Stress corrosion cracking of stainless steel: effect of surface finish - EBSD imaging
Stress corrosion cracking of stainless steel:
effect of surface finish - EBSD imaging
An investigation has been undertaken to establish the effect of surface preparation method on the susceptibility of a 304L stainless steel to stress corrosion cracking under simulated atmospheric corrosion conditions reflecting external exposure of stainless steel components in industrial plant, including nuclear reactor components, situated in a coastal region, Four surface preparation methods were evaluated: transverse grinding, longitudinal grinding, transverse dressing using an abrasive flap wheel, and transverse milling. For each case, surface topography, surface defect mapping, near-surface microhardness mapping, residual stress and electron back-scattered diffraction (EBSD) measurements were undertaken and related to the propensity for stress corrosion cracking. A key observation was the presence of a surface layer that could not be indexed by EBSD, implying a severely distorted (heavily cold worked) or nanocrystalline structure.


Test methods for evaluating stress corrosion cracking of corrosion resistant alloys under evaporative brine conditions in oil and gas production
Stress corrosion cracking (SCC) of duplex stainless steel (DSS) in concentrated salt solutions formed by evaporation of seawater has resulted in failure incidents in topside pipework in offshore oil and gas production. 'Leak before break' occurred and safety was not compromised. Nevertheless it was essential to investigate and to review existing guidelines for application of coating to limit such failures. To this aim, the susceptibility of 22 Cr and 25 Cr duplex stainless steels to stress corrosion cracking under evaporative seawater conditions was evaluated using a modified drop evaporation test. At an applied stress of 90% σ0.2, a threshold temperature for cracking of around 70 °C was identified for both materials. The engineering implication is that existing recommendations for the critical temperature for coating of duplex stainless steel to avoid failures in service need to be revised downwards.

Drop evaporation testing of DSSs - recommended lower temperature for coating application
Drop evaporation testing of DSSs - recommended lower temperature for coating application


People


Recent publications

  • Residual stress relaxation in shot-peened high strength low alloy steel and its implications for hydrogen assisted cracking
    A Turnbull and S Zhou
    Mat. Sci. Tech., 26, pages 824-836 (2010)
  • New insight into the pit-to-crack transition from finite element analysis of the stress and strain distribution around a corrosion pit
    A. Turnbull, L. Wright and L. Crocker
    Corros. Sci., 52, pages 1492-1498 (2010)
  • Comparative evaluation of environment induced cracking of conventional and advanced steam turbine blade steels. Part 1: Stress corrosion cracking
    A. Turnbull and S. Zhou
    Corros. Sci., 52, pages 2936-2944 (2010)
  • Novel multi-electrode test method for evaluating inhibition of underdeposit corrosion – Part 1: Sweet conditions
    G. Hinds and A. Turnbull
    Corrosion, 66 046001 (2010)
  • Novel multi-electrode test method for evaluating inhibition of underdeposit corrosion – Part 2: Sour conditions
    G. Hinds and A. Turnbull
    Corrosion, 66 056002 (2010)
  • Local hydrogen generation and its impact on environment-assisted cracking and crevice corrosion
    A. Turnbull
    Corrosion, 66 055001 (2010)
  • Comparative evaluation of environment induced cracking of conventional and advanced steam turbine blade steels. Part 2: Corrosion fatigue
    A. Turnbull and S. Zhou
    Corros. Sci., 53, pages 503-512 (2011)
  • Novel images of the evolution of stress corrosion cracks from corrosion pits
    D. A. Horner, B. J. Connolly, S. Zhou, L. Crocker, and A. Turnbull
    Corros. Sci., 53, pages 3466–3485 (2011)
  • Sensitivity of stress corrosion cracking of stainless steel to surface machining and grinding procedure
    A. Turnbull, K. Mingard, J.D. Lord, B. Roebuck, D. Tice, K. Mottershead, N. Fairweather and A. Bradbury
    Corros. Sci., 53, pages 3398–3415 (2011)
Last Updated: 19 Nov 2014
Created: 5 Jan 2012