2.2     ‘Corrosion behaviour of carbon steel, low alloy steel and CRAs in partially deaerated seawater and comingled produced water’, M. J. Schofield, CAPCIS Ltd 

A test programme was conducted to study the corrosion behaviour of a range of steels and CRAs in low oxygen content (20 & 200 ppb) seawater. The test materials ranged from carbon and low alloy steels through austenitic, martensitic and duplex stainless steels to nickel-based Alloy 718. Seawater injection conditions were simulated in tests conducted in the above conditions at 30ºC. Commingled water (a mixture of produced water and injected seawater) was simulated by adding carbon dioxide to these test environments and testing at 60ºC. High oxygen levels were injected periodically to simulate the effects of poorly controlled seawater deaeration. Tests were conducted in static and flowing conditions. 

The results show the sensitivity of these materials to the dissolved oxygen content of injection and commingled waters. Pitting, crevice and under-deposit corrosion occurred to varying degrees enabling guidelines to be developed for material selection in this area. 

This work was sponsored by T. N. Evans (BP) & P. I. Nice (Statoil).

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4.1   ‘X-ray Tomography Techniques for Localised Corrosion and Environmentally Assisted Cracking Studies’, Brian Connolly (Birmingham University) 

In situ experiments in aqueous salt solution utilising high-resolution (0.7 mm) synchrotron X-ray tomography have been performed to investigate the propagation of intergranular corrosion (IGC) within an aluminium airframe alloy.  These observations will be used to quantify growth rates for this three dimensional phenomenon as a function of the differing metallurgy found in the various regions of a AA2024-T351 friction stir weldment as well as a function of applied remote stress in a AA2024-T8 temper used to simulate the electrochemical reactivity of a weld heat affected zone.   

The evolution of IGC in aluminium alloys is a problem of fundamental interest to the corrosion community, and is poorly understood.  Accurate IGC rates are difficult if not impossible to quantify using conventional 2D measuring methods.  Of particular interest is the determination of how IGC propagates from localised corrosion sites such as pits associated with large (3-10 mm) intermetallic constituent particles.  Furthermore, it is important to assess how the presence of a remote stress affects the propagation of intergranular attack. 

In situ tomography scans have been carried out on individual specimens at incremental corrosion exposure times to determine the time-dependence of the 3D propagation of IGC through the microstructure.  These experiments show the progressive development of localised corrosion sites within the bulk of the sample and quantify the mode and rate of attack for different microstructural regions and applied stress state.  

On-going analysis of this work will make a significant contribution to the effort to elucidate the controlling electrochemical/physical mechanisms dictating the development of localized corrosion sites, specifically, the propagation of IGC and possible transitions to stress corrosion cracking. Clearly defining the evolution of the localized corrosion morphology as well as quantifying its growth rate will provide valuable underpinning scientific knowledge that will significantly contribute to the on-going modelling effort for life prediction of aircraft components. 

[B.J. Connolly,a, B.J.Connolly@bham.ac.uk, S.J. Fox,a, D.A. Horner,a, S. Ghosh,a, A.J. Davenport,a, C. Padovani,a, M. Preuss,b, N.P. Stevens,b, T.J. Marrow,b, J. -Y. Buffiere,c, E. Boller,d, M. Stampanoni,e, and A. Grosoe 

a. Metallurgy and Materials, University of Birmingham, United Kingdom, b. School of Materials, University of Manchester, United Kingdom, c. GEMPPM INSA, Lyon, France, d. European Synchrotron Radiation Facility, Grenoble, France, e. Swiss Light Source, Paul Scherrer Institut, Switzerland] 

4.2    ‘Vistar - The super austenite corrosion solution’, Trevor Machin (Vistar Stainless Limited) 

Vistar was developed following a request by the European oil industry for a weldable, high strength, durable, corrosion-resistant castable steel alloy. The target was a material with a proof strength of greater than 415N/mm² along with a resistance over a wide range of temperatures to various complex media such as sour/sweet gases, acids, alkalines, aerated sea water and their combinations. Vistar exceeds these criteria and has passed every test made of it to date by a considerable margin. In 2001 Vistar was accepted in the industry by being awarded a UNS grade number (J95370) and has been approved by NACE. It is the only castable high strength super austenitic stainless steel on both the UNS & NACE accredited lists and is also included in ISO 15156 part 3. 

Vistar, unusually, includes a high proportion of low atomic mass elements. Together with the finely balanced ratios of each of its other component elements these elements create an extremely dense austenitic structure and prevent the formation of intermetallic phases. The low atomic mass elements create an electron relationship of full orbitals within the face centred cubic lattice. This closely packed structure prevents “shear” and “slip”, and minimises dislocation mobility, achieving extreme levels of corrosion resistance and exceptional strength for an austenitic. 

Long term testing to determine the level of Vistar’s corrosion resistance was performed by Bodycote Material Testing under the close observation of Dr Chris Fowler. Following the testing Dr Fowler confirmed that no signs of pitting, crevicing or cracking had occurred. The material demonstrates high resistance to sour chloride environments and has passed all the tests to NACE MRO175. 

In conclusion the testing performed to date demonstrates that Vistar has performed better than all the austenitics that it has been tested against, as well as matching, and in some places out-performing, the titanium products used.

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