PURPOSE: To discriminate the degree of deterioration of a low alloy steel with high accuracy and good reproducibility by using the max. valve of the natural potential of the low alloy steel when the low alloy steel is brought into contact with an aq. soln. of an arom. compd. having either of hydroxyl group or carboxyl group and nitro group in a molecule.

CONSTITUTION: An acidic soln. of 2, 4, 6-trinitrophenol is prepd. as the arom. compd. having the hydroxyl group and nitro group in the molecule and the acidic soln. of 2, 4-dinitrobenzoic acid is prepd. as the arom. compd. having the carboxyl group and nitro group. The pH values of each soln. are adjusted to about 2.4 and 2.2 respectively. Several kinds of Cr-Mo-V steels are immersed into such solns. and the max. value of the natural voltage is measured. The max. value of the natural potential changes linearly with the degree of embrittleness and since the potential migrates to the nobler potential as the embrittleness progresses and therefore the non-destructive detection of the deterioration of the mechanical characteristics of the low alloy steel with good accuracy is made possible.