Proposed FAC Mechanism Using a Macro-Cell Corrosion Model

Wall thinning at unusually high rates (0.5∼1 mm y⁻¹) due to FAC (flow accelerated corrosion) may sometimes occur on the walls of carbon steel piping which transports pure, high-temperature water. FAC generally tends to occur in locations where the solution temperature is 140∼150°C, downstream of pipe fittings and flow-meters. It arises quite accidentally: sometimes occurs and sometimes does not in spite of related parameters such as flow velocity and temperature being identical. In addition, the damage may be drastically reduced, when the material of the pipe tube in which unusual wall thinning had occurred is exchanged with low alloyed steel. The macro-cell corrosion mechanism, which is the principle mechanism for galvanic corrosion as well as for cathodic protection with a sacrificial anode, was modified by taking laboratory test results into consideration, and applied to incident cases to successfully explain the generation mechanism of this unusual pipe wall thinning. It explains the characteristic behavior of FAC described above and the anticorrosive effect of chromium. It was concluded that the unusual wall thinning of carbon steel pipe may be attributed to the formation an active/passive macro-cell on the surface. An active/passive macro-cell occurs due to gaps in the distribution of such passivation affecting parameters as the pipe wall temperature and the dissolved oxygen content, so that the basis of prevention amounts to avoidance to these gaps. Thus, it is recommended that stagnation watershed and localized lowering of temperature should be avoided with pipe lines.