The effects of alloying elements and precipitated phases on the corrosion rate of high alloy OCTG in the ferric chloride solution have been evaluated. The corrosion rate of Fe-Cr-Ni-Mo alloys without precipitated phases e.g. carbides and sigma phase, can be estimated from the alloy composition using the following equation:
where, C.R. is the corrosion rate in g/m2/hr; PRE is Cr+3Mo+16N in percent and T is the test temperature in °C.

The activation energies of the ferric chloride test for Fe-Cr-Ni-Mo alloys are almost the same regardless of PRE or the Ni content when no detrimental phase precipitates. When carbides or the sigma phase precipitate, the corrosion rate is higher than that of specimens free from precipitates and the activation energy of the ferric chloride test is lowered. This fact suggests that secondary phases give preferential sites for initiation of pitting corrosion.

Subject
Sigma phase, Pits, Corrosion resistant alloys, Stainless alloys, Corrosion rate, Precipitates, Nickel based alloys, Ferric chloride, Chloride solutions, Pitting, Corrosion resistance, Alloys, Heat treatment
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1986
Association for Materials Protection and Performance (AMPP)
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