Electroless nickel-phosphorus coatings with PTFE (Ni/P-PTFE) have been reported to have excellent
corrosion resistance ¹. 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 HellisheiOi 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.