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Corrosion resistance of CRA’s in oxygenated saline solutions

TWI has been conducting an exploratory project to examine the effect of dissolved oxygen (DO) on the performance of 18%Cr austenitic stainless steel (ASS) and 25%Cr super duplex stainless steel (SDSS) in saline solutions, to demonstrate comparable corrosion resistance and find the limit of DO that is safe before corrosion occurs.

In oil and gas fields corrosion resistant alloys (CRAs) are often used in saline water injection systems due to their better corrosion resistance compared to low alloy steels. In high temperature applications, low alloy steels suffer from significant metal loss due to high corrosion rates, so CRA’s can offer considerable savings, especially if the cost of scavenging and de-aeration chemicals are added in.

TWI identified the factors influencing local corrosion resistance as dissolved oxygen levels, chloride ion concentration, temperature and surface finish, with pitting corrosion tests selected to identify corrosion resistance.

The team calculated controlled DO levels and incorporated the effects of salt concentration and temperature on oxygen solubility at ambient pressure. The investigation used a gas mixture containing the required oxygen content balanced with nitrogen which could be measured using an oxygen sensor when under test.

Testing showed the influence of DO and temperature on localised corrosion with surface finish and chloride content also having an effect. The increase in surface roughness from surface finish resulted in pitting of the material. Testing showed that 25%Cr SDSS had little pitting and the most resistance to corrosion, with 18%Cr showing pitting and localised corrosion.

These findings provide valuable empirical data to support further research, as further work is needed to define the limit of DO on SDSS and other CRA’s eg 22% Cr duplex stainless steels and 13%Cr martensitic stainless steel. This provides an ideal opportunity for a single client project to expand the research into a combination of environment and alloys or a joint industrial project examining the effects of DO on localised corrosion and stress corrosion cracking in oxygen containing environments.

For more information, please email

Avatar Qing Lu Principal Project Leader, Metallurgy, Corrosion and Surfacing Technology Group

Qing Lu graduated with a BSc in Materials Science and Engineering from Harbin University of Science and Technology, China, in 1984. She obtained a Ph.D in Corrosion Science and Engineering at University of Manchester Institute of Science and Technology (UMIST) in Manchester in 1999. This was followed by working on a number of industrial post-doctorate projects at this institute, including corrosion in chemical processing plants and surface engineering studies for automotive applications, between May 2000 and April 2005.

Prior to re-joining TWI in August 2012, she worked for DNV as a senior engineer between November 2011 and August 2012, and for Wood Group Integrity Management as senior materials and corrosion engineer between February 2011 and October 2011 after leaving TWI in February 2011. Prior to joining TWI initially in October 2006, she was appointed as senior research scientist at Westmoreland Mechanical Testing and Research Ltd in UK where her main area of expertise was in failure investigations, primarily related to corrosion.

Her role at TWI carries a particular emphasis in Corrosion Resistant Alloys, which are studied through various types of projects addressing failures, corrosion testing and evaluation, mitigation of corrosion, especially in structures and components joined by welding or other joining techniques.

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