Intergranular attack of Alloy 600: evaluation of remedial actions in AJAX model boiler tests

The results of the previous four series of AJAX tests performed by CEA are reviewed. Summaries of these tests are included. The fifth series of experimental runs involved assessment of remedial actions: crevice flushing and boric acid additions. The experimental procedure and test matrix are given. The AJAX tube sheet was slightly modified using CE-type inserts. The same heat of mill annealed Alloy 600 tubing (heat No. NX 3332 from Valourec) was used in this series. The crevices were preloaded with a mixture of sodium hydroxide, sodium sulfate and magnetite. The composition of the mixtures preloaded in the different crevices is given. The model boilers were continuously fed with AVT faulted by sodium hydroxide (0.1 ppm OH) and sodium sulfate (molar ratio of 3:1). No sludges was simulated on the top of the tube sheet. Crevice flushing was performed after exposure under normal conditions. On line boric acid addition was initiated on one boiler after crevice flushing. The results of these tests showed that: a non homogeneous composition was found along the crevice length. Dilution occurred in the upper parts while solidification probably took place in the bottom; A 508 insert material suffered a denting type corrosion in some areas leading to occlusion of some crevices and tube deformation; crevice flushing efficiency was not high, but it was improved by increasing the depressurization rate; and boric acid flushing seemed to have stopped the IGA progression