Observation of Precipitates and Metallographic Grain Orientation in Steel by a Non-aqueous Electrolyte-Potentiostatic Etching Method

Abstract

A new potentiostatic etching method using non-aqueous solutions as an electrolyte (Selective Potentiostatic Etching by Electrolytic Dissolution: abridged as SPEED Method) has been developed and applied to the observation and analysis of precipitates and metallographic grain orientation in steel samples. By the proposed SPEED Method, samples are etched at the predetermined constant electrolytical potential in non-aqueous solutions such as 10% acetylacetone-1% tetramethylammonium chloride-methyl alcohol and 3% nitric acid-2% perchloric acid-methyl alcohol. Precipitates and micro facet pits are observed and analyzed by analytical instruments such as OM, SEM and XMA, after the etched surface of the sample were washed with methyl alcohol.
The advantages of the SPEED Method are as follows.
(1) In situ observation of unstable precipitates in good contrast in the microstructure of the steel sample.
(2) Easy estimation of the relationship between precipitates and metallographic grain orientation by a simultaneous observation of precipitates and micro facet pits. Size of facet pits are easily controlled in the range of 10 to 0.5 μm by controlled electrolytical potential.
(3) Three dimentional observation of precipitates with various etching depths, controlled by the quantity of electricity.
(4) Easy identification and accurate analysis of precipitates with no contamination of sample surface, easily judged by red-color of the iron chelate compound.