Corrosion of alloy 800 in PHWR primary and secondary conditions

A hot leg section of a steam generator tubing was removed for destructive examination from one of the steam generators (SG) of the Embalse Nuclear Power Plant. The tube material is Alloy 800 and carbon steel is the tube support plate material. Samples of the deposits were taken at the first tube support plate and at the top, mid-height and bottom of the sludge pile. Transverse sections were taken at several locations along the tube length measuring the oxide thicknesses and studying the morphology of the oxide layer by scanning electron microscopy on the primary and secondary side at each location. Deposit layers on the outer tube surface revealed iron as major component and the presence of calcium, phosphorous, zinc and manganese. The oxide scale thickness at the secondary side in the open area was around 22 to 30 μm. The oxide thickness grown under isothermal conditions on the corrosion test samples installed in the autoclaves facilities of the primary circuit of the plant was measured and compared with that found on the inner surface of the examined tube section. The oxide thickness of the test samples was around 1-2 μm showing the influence of the deposition of corrosion products from the coolant. Deposition and precipitation of oxide was also found in the actual tube, where the common feature was the irregularity of the oxide layer on the primary side and thicknesses values in the range 4 to 10 μm were measured. The autoclave tests and SG tubing examination permit to compare the influence of materials and of operating (flow rate, isothermal vs non-isothermal) conditions on corrosion and deposition. (author)