The first ends of two electrical probes extend through and are spatially separated on the interior of a steam turbine's crossover conduit expansion joint. The external, second ends of the electrical probes are connected to an electrical potential source in such manner as to cause the probes to be electrically opposite in charge. A current measuring device is electrically connected to the electrical probes and indicates electrical current flowing therethrough, which, in turn, provides a measure of the quantity of electrically conductive molten caustic which accumulates between the probes' first ends. The probes' first ends are placed in the expansion joint region where molten caustic is most likely to collect, allowing early detection of corrosive caustic during steam turbine operation. A non-corrosive baffle, disposed upstream from the probes' first ends, provides a shield for the first ends from steam born solid particles of low electrical conductivity.