Is it safe to use CRAs in oxygenated saline solutions?

TWI has developed a unique and robust facility allowing close monitoring and control of dissolved oxygen (DO) at the ppb level.

It is well established that the corrosion rate of carbon/low alloy steels in aqueous solutions (e.g. seawater) is affected by the concentration of DO, along with other environmental factors including temperature, chloride ion concentration and pH. Several models based on the theory proposed by Oldfield et al (1981) have been developed and they have been used to predict the corrosion rates of carbon steels at different DO levels. However, carbon/low alloy steels cannot offer sufficient corrosion resistance in certain conditions, e.g. produced water systems in oil fields which may experience high temperatures, leading to high corrosion rates. There is a lack of defined DO limits for CRA applications for which the materials are considered to be ‘safe’ (free of corrosion).

Corrosion resistant alloys (CRAs) are often used in water injection systems due to their better corrosion resistance when compared with low alloy steels. Especially in high temperature applications, use of CRAs may be considered to be more cost effective as a long-term solution. However, CRAs can suffer from localised corrosion in oxygen-containing saline solutions as oxygen ingress into the system is likely during service. Intermittent breaches of the design limit (e.g. 10ppb dissolved oxygen) are more likely to occur, for example, the inhibitor injection process could introduce oxygen/air into the system. Higher levels of DO (e.g. 50 to 500ppb) can be found in the mixed water injection system (i.e. a mixture of produced water and de-aerated seawater). Depending on the grade of CRAs employed, the DO tolerance level may vary.

Localised corrosion resistance (including pitting and crevice corrosion) is commonly assessed as part of materials’ qualification procedures, using the ASTM G48 and G150 tests. The aggressive test solution employed for the ASTM G48 test, i.e. containing 6% ferric chloride (FeCl₃) and 1% hydrochloric acid (HCl), has an oxidising power equivalent to a potential of +700mV vs a saturated calomel reference (SCE) for stainless steels. This potential is employed in the ASTM G150 procedure. None of the commonly used CRAs has such a high free corrosion potential (E_corr) value in a saline environment, even when in their passive state. This means that the environment employed for G48 and G150 testing does not represent the actual conditions commonly experienced in oil and gas production. Nonetheless, these standards provide a means of ranking localised corrosion resistance of CRAs.

An exploratory project was funded by TWI to examine the effect of DO on the performance of 316L (UNS S31603) austenitic stainless steel (ASS) and 25%Cr (UNS S32750) super duplex stainless steel (SDSS) in saline solutions. In this project, pitting corrosion tests were carried out in solutions containing different levels of DO, chloride ion concentration, temperature, etc. A gas mixture, containing the required oxygen content, balanced with nitrogen, was used, and a bespoke test set-up was developed (see Figure 1). The DO was measured and recorded using an oxygen sensor during testing.

The experiments showed a clear influence of DO and temperature on localised corrosion. The surface finish and chloride content also had an effect. The increase in surface roughness resulted in pitting of the material (see Figure 2). The outcomes from this research investigation are documented in NACE Paper No. 9062.

Further work is needed to establish the DO limits for SDSS and other CRAs (e.g. 22%Cr duplex stainless steels and 13%Cr martensitic stainless steels) in order to use these CRAs safely in saline solutions. The work can be conducted as single client projects (SCP) at TWI, to examine any specific combination of environment and alloys or to cover more parameters to examine the effects of DO on localised corrosion and stress corrosion cracking (SCC) in oxygen-containing environments.

Figure 2a: Effect of surface finish on pitting corrosion resistance of 316L in a salt solution (61g/L chloride ions) with a 100ppb DO: effect of surface finish

Figure 2b: Effect of surface finish on pitting corrosion resistance of 316L in a salt solution (61g/L chloride ions) with a 100ppb DO: corrosion pit profiles obtained from the test specimen surface using surface profilometry method after pitting corrosion test

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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