Measurement of Thickness of Magnetite Layers Through Alloy 600

It is well known that magnetite builds on the outside of boiler and steam generator tubes. Low-frequency (10 kiloHertz) eddy-current testing (ECT) has been used in attempts to quantify magnetite deposits on alloy 600 steam generator tubes. At this frequency, ECT has some sensitivity to the presence of magnetite on the outside of the tube with measurements made inside the tube. However, it has been historically impossible to resolve differences in deposit thickness, a quantity of some interest to power plant operators. ECT can be further inhibited by layers of copper which may be deposited on the outer surface of Inconel tubing. AC magnetics (ACM) may be conveniently operated at a few hundred Hertz where the small thicknesses of Inconel and the copper deposits are essentially transparent to the electromagnetic fields. The problem then becomes detection of magnetite at large lift off. Here again, ACM is a good technique to use. Preparatory to the design of an ACM probe to fit within steam generator tubing, a feasibility study was conducted which is described here. Flat copper-cladded Inconel plates were used to simulate the tubing, and magnetite of various thicknesses were placed on the side opposite a magnetic-circuit gap. The results indicate that the depth of magnetite through Inconel and copper can be quantified through the use of ACM to about 10 mm