51314-3958-The Influence of Thermally Sprayed Aluminium Coatings on Controlling the Hydrogen Embrittlement of Welded Superduplex and Supermartensitic Stainless Steels

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Product Number: 51314-3958-SG

ISBN: 3958 2014 CP

Author: Qing Lu

Publication Date: 2014

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The Influence of Thermally Sprayed Aluminium Coatings on Controlling the Hydrogen Embrittlement of Welded Superduplex and Supermartensitic Stainless SteelsAuthors: Q Lu P Woollin S Paul M F Gittos MDF Harvey1 AbstractThermally sprayed aluminium (TSA 99.5%Al min) is an effective coating that has been used increasingly for the protection of carbon steel offshore structures topside equipment and flowlines/pipelines exposed to both marine atmospheres and total immersion conditions. However there are limited data on its application to on corrosion resistant alloys such as duplex stainless steels and there are some concerns regarding hydrogen-induced stress cracking (HISC) on duplex stainless steels. The effect of TSA coatings on 25Cr superduplex stainless steel (SDSS) and 12Cr supermartensitic stainless steel (SMSS) substrates and weldments without insulation has been investigated for subsea operations. Four surface finishes were studied including the uncoated surface surfaces coated with TSA surfaces coated with modified TSA and surfaces coated with an epoxy paint.For the welded 25Cr SDSS HISC tests were carried out in agitated and aerated artificial seawater at 20 and 80°C respectively using the dead weight loading method. Initially the test specimens were stressed in seawater to 100% of the 0.2% proof stress of the parent metal for 30 days. Increments of 25MPa were made on a weekly basis until failure.A modified slow strain rate test (SSRT) method was used for the welded 12Cr SMSS. HISC tests were carried out in recirculated aerated artificial seawater and also in air at 4°C. The specimens were stressed in seawater to 20% of the 0.2% proof stress of the parent metal at the test temperature (4°C) prior to testing. The specimens were then pre-charged for two days at cathodic polarisation (CP) potentials of -800 or -1100mV at room temperature. They were tested at the same CP potential under a constant strain rate of 1x10-6s-1 to a total of 5.2% strain (elongation).The present work has established that both welded 12Cr SMSS and 25Cr SDSS when cathodically polarised to -800/-1100mV or polarised naturally by TSA are susceptible to HISC in the test environments. However a modified TSA with the composition of Al-0.5%Cu producing a corrosion potential of -730 to -830mV SCE provided the best resistance to HISC among the coatings examined.