Technical Presentations at the July 2016 Meeting

1.1   ‘Susceptibility of Low-temperature Plasma Nitrided 17-4 PH to Sulphide Stress Cracking in H2S-containing Environments’, Mario Coseglio, University of Birmingham

The selection of corrosion resistant alloys (CRA) for oilfield components has been a challenge for the oil and gas industry due to its complex interactions with the environments containing hydrogen sulphide (H2S).

Field failures have been frequently attributed to Sulphide Stress Cracking (SSC). It can occur due to the applied stress in the presence of H2S and it is associated with the entry of atomic hydrogen into the metal substrate, although its mechanisms still unclear. Precipitation-hardening stainless steels (PHSS) are a cost-effective choice for some oilfield components due to its favourable combination of properties such as high strength, relatively high toughness, and good corrosion resistance. However, recent reports of field failures at stress levels lower than the accepted limits poses a challenge for its applications. Some surface engineering techniques, such as nitriding, carburizing, and nitrocarburizing have been successfully used to improve the surface strength of different CRAs and recent studies indicate that it can increase the resistance to SSC.

The purpose of this work, which is part of a PhD programme from the University of Birmingham in partnership with Exova Corrosion Centre in Dudley, is to determine the susceptibility of the surface-modified PHSS to SSC in H2S-containing environments. This will contribute to the better understanding of the mechanisms of failure in order to help defining the limits of application so that this material can be safely and economically selected for oilfield components. The SSC resistance will be evaluated in accordance with the NACE TM0177. Preliminary results indicate that the surface modification can be effective in reducing the SSC while the corrosion performance is yet to be confirmed.

1.2  ‘Microbially Influenced Corrosion of Copper-Nickel Tubed Marine Coolers: investigations and solutions’, Robin Oakley & John Galsworthy, QinetiQ Ltd

While copper-nickel alloy tubes have been successfully used in marine heat exchangers for many years, occasional incidences of accelerated pitting corrosion have been attributed to the adverse effects of polluted seawater on the normally protective surface films formed in service. Previous research in the US and elsewhere has identified the presence of sulphides in seawater, deriving from microbial activity, as a primary cause of these failures.

Previous corrosion problems in copper-30% nickel heat exchanger tubes in Royal Navy submarines have been identified as being due to Microbially Induced Corrosion (MIC), associated with changes in cooler management practices, especially during initial commissioning. To mitigate further occurrences of these problems, various tube conditioning practices using differing water sources or synthetic chemical pre-treatments were examined. A series of laboratory screening tests using linear polarisation resistance and gravimetric methods was undertaken, with the more promising options identified for use. However, these tests also indicated that full control of the MIC problem would require a reduction in the microbial activity in the heat exchanger systems. Therefore, a temporary chlorination system was developed to permit anti-microbial treatment of inlet seawater during vulnerable periods after copper-nickel tube conditioning. The results of a boat trial of this chlorination equipment, and lessons learned on the microbiological sampling of in-service seawater heat exchangers, are summarised.

[A pdf of this presentation has kindly been offered to members and will be available in the autumn. To register interest please contact the Secretariat].

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4.1  ‘The Selection of Gaskets for Stainless Steel Seawater Piping Systems’, Roger Francis, RF Materials

The presentation describes laboratory and service experience with a wide range of gasket materials for low pressure (10bar) seawater systems with stainless steel piping. The presentation also includes experiences with high pressure (10 to 100bar) seawater systems, such as occur in SWRO desalination plants. This includes both metal wound gaskets and Victaulic-type couplings. The presentation concludes with recommendations for gasket materials for both low and high pressure systems to minimize the risk of crevice corrosion.

Further details can be seen in a pdf version of this presentation which has kindly been provided and can be obtained by staff of member companies from the Secretariat.

 

4.2    ‘Developments in ‘Fitness for Purpose’ Sour Service Corrosion Testing’, Phil Dent, Exova

Traditionally the corrosion evaluation of materials for sour service applications (i.e. H2S-containing) has predominately involved the use of small-scale specimens in ‘standardised’ test environments for test durations of 4-days for hydrogen induced cracking (HIC) evaluation and 30-days for sulphide stress cracking (SSC) / stress corrosion cracking evaluation.

Using the ‘standardised’ approach can lead to laboratory tests being over conservative, with regard to the intended service conditions, resulting in the rejection of materials that would perform satisfactorily in service. Conversely materials ‘qualified’ using small-scale standardised test specimens may fail in manufactured components, when used in service.

For sour service corrosion applications there has been an increasing demand to conduct laboratory tests under ‘fitness for purpose’ conditions, often using full scale components, due to the use of higher strength steels, an increase in the residual / applied s tress and more focus cost reduction.

The presentation introduced the three primary sour service cracking mechanisms for carbon steels (i.e. HIC, SSC and stress orientated hydrogen induced cracking (SOHIC)).

The evolution of the HIC test method, from the 1st publication of the failure mechanism in the late 1940s through to the latest development in the NACE TM0284 test method (due to be published in 2016) was presented, which included the recent inclusion of a fitness for purpose procedure into the document.

The presentation detailed the development of full scale component testing and in particular the full ring test method, for the evaluation of SSC, HIC and SOHIC in steels.

The evolution of the full ring test has resulted in the recent publication of the British Standard (BS 8701:2016), “Full ring ovalisation test for determining the susceptibility to cracking of linepipe steels in sour service – Test method”.

The use of the full ring test, for the evaluation of SSC, HIC and SOHIC, compared to small-scale tests was discussed and the development / modification of the full ring test for the evaluation of external cracking, cracking under high axial loads and cracking under low partial pressure environments for sour service applications was presented.

 

 

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