Flow accelerated corrosion

Flow Accelerated Corrosion (FAC) refers to a wear mechanism which results in severe wall thinning (up to 3mm/year) and metal loss of carbon steel piping and vessels exposed to flowing water and wet steam. FAC is often occurs in power generation units, including both conventional plants as well as nuclear facilities worldwide, but also occurs in other industrial plant. In heat recovery steam generators (HRSGs) it remains the leading cause of damage and failures. There has, however been substantial research and progress in the mitigation of this problem over the last forty five years, much of it on an international collaboration platform (EPRI, EDF, Kraftwerk and CEGB). However, the application of this science and understanding to generating plant has not been entirely satisfactory. Damage is not uncommon and has resulted in injury and in cases fatalities.

Part of the reason for this is that the process is complex, with several variations, including both single-phase (flowing liquid) and dual phase (a mixture of flowing liquid and steam) fluid conditions, as well as a variety of metallurgical variants.  FAC is an extension of the generalized corrosion of carbon steel caused by the dissolution of the protective magnetite layer (Fe3O4) in a stream of flowing water (or wet steam). This process removes the protective oxide layer, and leads to rapid removal of the base metal, until, in the worst case, the pipe ruptures.  FAC is a two part process in which i) soluble ferrous ions formed at the steel interface diffuse through the porous oxide to the oxide water interface where the Fe3O4 dissolves by reduction (that is promoted in the presence of hydrogen), and ii) a turbulence enhanced movement of the ferrous irons through the boundary layer on the inner surface of the pipe into the bulk flow.  This dissolution is controlled by the fluid’s oxide reducing potential (ORP), which is dependent on alkalinity and temperature (over a range of 340°C, but peaking at approximately 150°C).

There are some common trends that characterise FAC failures, which include:

  • There is a high tendency for FAC at feed-water access points that are often at high pressure.
  • Rapid degradation of stainless steel (there is reduced tendency for FAC at low chrome contents (less than 1%)).
  • Feed-water entry areas treated with hydrazine generally perform better.
  • Oxygen content needs to be low, (< 1ppb) to minimise FAC at header drain regions.
  • Heat recovery steam generators (HRSG) appear very susceptible to FAC.
  • For best performance to minimise FAC, the feed water should be added in a volatile form (i.e. added as steam rather than water).

Some features characterising FAC include a scalloped surface appearance, which in two-phase flow, results in regions of material characterised by thin black oxide staining, alongside bare uncorroded metal commonly referred to as ‘tigers-striping’.  Control of FAC can be achieved by optimising water chemistry and limiting the ORP of the fluid (and hence the decreasing the dissolution of the oxide layers).  Controlling the temperature and hydrodynamics (and degree of turbulence) is obviously also highly important.

Even though solutions are available, FAC remains a major problem for nuclear, fossil fuel and industrial plants.  To limit failure regular NDT inspection should be carried out and a full understanding of the mechanisms of the particular FAC type and the water chemistry is required.

Suggested essential reading: B Dooley and D Lister ‘Flow accelerated corrosion in steam generating plants’, Power Plant Chemistry, Vol 20, No 4 2018; pp 194 – 244.