Electroless nickel-phosphorus coatings with PTFE (Ni/P-PTFE) have been reported to have excellent corrosion resistance1 . The corrosion behaviour of the Ni/P-PTFE varies depending on the P content in the coating system due to its microstructure. High P (HP) content has been reported to have higher corrosion resistance in acidic environment compared to low P (LP) content due to more amorphous structure2 .

This work demonstrates the integrity of Ni/P-PTFE top functional coating layer with low and high P content deposited on Ni/P adhesion layer without PTFE both on coupons and U-bend carbon steel samples exposed to geothermal conditions. The samples were tested for three months in-situ geothermal steam at Hellisheiồi power plant. the tested samples were taken out after 3 months and evaluated using SEM/EDX analysis. Characterization results revealed a reaction between sulphur (S) from geothermal environment to Ni, resulting in nickel sulphide (NiS) compound generated on the top functional layer. The results showed stress crack formation in coating possible due to hydrogen released from hydrogen sulphide (H2S) with reaction to the Ni in the film. In aerated environment where pin hole formed during exposure or production an under film galvanic corrosion was seen.

Subject
Nickel coatings, Geothermal energy, Coatings, Materials, Substrates, Vacancies, Wellheads, Coating adhesion, Scale formation, Environments, Carbon steel, Cracks, Samples
Keywords:
Geothermal corrosion, electroless plated nickel coatings, PTFE particles
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2021
Association for Materials Protection and Performance (AMPP)
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