Flow Accelerated Corrosion in Nuclear Power Plants Testing

Flow accelerated corrosion (FAC) testing measures wall thinning in carbon steel piping and vessels exposed to high-velocity water or steam–water mixtures in nuclear power plants. Metal loss concentrates at elbow extrados, reducers, points downstream of orifices, and two-phase drain lines. A complete program combines material verification, laboratory loop coupon exposure, and in-plant ultrasonic thickness surveys.

Testing: Principle & Mechanism

Flow accelerated corrosion is a coupled process of electrochemical dissolution and mass transport. The magnetite layer on carbon steel dissolves into the coolant. High-velocity flow thins the boundary layer and carries dissolved iron away. The oxide keeps dissolving, and the wall recedes. The rate rises with velocity and turbulence. It reaches a maximum at an intermediate temperature near 150 °C in single-phase water. Two-phase flow accelerates the loss further, because droplet impingement damages the oxide mechanically. Water chemistry modifies the oxide equilibrium. Higher pH and trace dissolved oxygen suppress dissolution, while reducing conditions favor it. Chromium at about 0.1 % or above suppresses FAC strongly. Laboratory testing therefore reproduces velocity, temperature, and chemistry together, since no single parameter governs the loss.

Sample Types & Coupon Preparation

Testing uses two sample families: coupons exposed in laboratory loops and pipe sections removed from the plant. Coupon stock should match the piping grade in chemistry and processing. The mill certificate is verified first. The composition is checked by optical emission spectrometry, with attention to chromium, molybdenum, copper, and carbon. Coupons are machined as flat plates or short tube segments. They are ground to a defined surface finish, degreased, dried, and weighed to record the initial mass. Dimensions are taken at marked positions so local loss can be resolved later. Edge effects are limited by masking or by keeping the exposed area away from cut faces. Removed pipe sections are cut along the flow axis. Half sections serve thickness mapping and metallography of the oxide layer.

Wall

Thickness Measurement — Ultrasonic Testing
Ultrasonic thickness measurement is the standard field method for detecting wall loss. A pulse-echo thickness gauge with a dual-element transducer reads the remaining wall from one side. The outer surface is ground first, and a coupling medium is applied. Calibration uses step wedges of the same material and thickness range. Acoustic velocity is confirmed on a known section. Readings are taken on a grid over suspect areas. These include the extrados of elbows, reducers, and points downstream of orifices. Grid pitch is tight enough to catch localized thinning. Probe selection follows the pipe diameter and curvature, and small-diameter transducers serve tight geometries. Automated scanners give denser coverage and archived scans for later comparison. Each location is tagged so successive outages measure the same points. This turns isolated readings into a thickness history.

Corrosion

Rate Calculation & Data Evaluation
Two data paths feed the evaluation. Coupon mass loss divided by exposed area and exposure time yields an average penetration rate. Successive ultrasonic surveys at fixed points yield a field thinning rate. This comes from the thickness difference over the elapsed operating time. Grid readings are sorted to report the minimum, the mean, and the spread. FAC loss is local, so the minimum governs remaining life. Measured rates are compared with values from FAC prediction codes and with earlier survey cycles. A rate that rises between cycles signals a change in chemistry or flow conditions. Wall thickness projected to the next inspection, together with the minimum required thickness, determines whether the component can remain in service. Uncertainty from surface roughness and instrument scatter is stated with the results so that margins are judged realistically.

Water Chemistry & Co-Test Parameters

Because chemistry drives oxide dissolution, loop tests run under controlled and logged water conditions. Treatment follows the all-volatile practice used in plant secondary systems. Ammonia sets the pH, and hydrazine scavenges oxygen. Target ranges for pH, conductivity, and dissolved oxygen are fixed before the run. Online monitors record conductivity, dissolved oxygen, and temperature continuously. Periodic grab samples are analyzed for dissolved iron and ammonia by spectrophotometry or ion chromatography. Flow velocity is measured with a calibrated flowmeter. In two-phase tests, the steam quality is derived from a heat balance. Dissolved iron in the recirculating stream also serves as a direct indicator of ongoing metal loss. Any chemistry excursion is time-stamped. Coupon results are therefore interpreted against the actual exposure history rather than nominal setpoints.

Acceptance

Criteria & ASME Section XI Standards
Acceptance links measured wall thickness to the minimum required by design. ASME Boiler and Pressure Vessel Code Section XI governs inservice inspection and the evaluation of degraded nuclear components. If measured thickness falls below the minimum required for design pressure, an evaluation under the applicable subsection applies. Its outcome determines fitness for continued service, repair, or a revised inspection interval. Components whose projected thickness stays above the minimum at the next inspection are accepted for that interval. Those below enter repair or replacement planning. Plant FAC management programs combine Section XI requirements with industry inspection guidance. Lines are ranked by material, temperature, velocity, and chemistry history. Documentation records measurement locations, instrument calibration, chemistry logs, and the evaluation basis for each disposition. Consistent records across outages make rate trends defensible when margins narrow.