Electrolytic etching

Electrolytic etching can only be used with electrically conductive ceramic materials. Etching guidelines have been provided for materials based on NiO, TiC, TaC, SiC, 

The conditions for color etching and subsequent grain boundary etching of silicon carbide and grain boundary etching of boron carbide are given below as examples. 

SiC Color etching, dependent on grain orientation with 10% oxalic add at IS V Grain boundary etching after removal of colored surface layer with 10-20% hydrofluoric add 30-40 s 

10% hydrofluoric acid solution. This removes the surface layers and clearly accentuates the grain boundaries (Fig. 35). The result is a high degree of brightness contrast between the SiC grains and the pores filled with epoxy resin. 

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Electrolytic corrosion (EC) test, 9 790 Electrolytic etching, of silicon,... 

Tips are most easily prepared by electrolytic etching. The following represents a few recipes which the author has found serviceable. 

ELECTROLYTIC ETCHING OF metals produces various results intergranular attack, attack of crystalline surfaces which is orientation dependent formation of etch pits, and anodic oxide films. The behavior of a metal or alloy depends on composition, temperature of the electrolyte, and above all on the electrode potential which varies with the metal. Applications to Al, Fe, stainless steel, Ti, 2r, U, and their alloys will be discussed. 

Etching of stainless steel represents the most typical case of the application of electrolytic etching. This technique has been used on die 18/8 stainless steels in order to obtain a reliable and rapid measure of their sensitization to intergranular corrosion after a thermal treatment at 600 to 900°C. Several organic electrolytes such as oxalic acid (1) and tartaric acid (2) as well as inorganic electrolytes such as chromic acid (3) and sodium cyanide (4) have been employed. All these electrolytes attack preferentially the grain boundaries in which a second phase has precipitated. The composition of this phase responsible for the etching is not always known. 

The sensitivity of die grain boundaries to electrolytic etching is most conclusively demonstrated in the case of high purity metals such as A1 where the intergranular etching by hydrochloric acid can only be explained by the concentration of residual impurities in the grain boundaries (5). 

Electrolytic etching reagents are especially suited for the detection of the distribution of dislocations in a heterogeneous strained metal. It is also possible to distinguish au even slightly work-hardened metal from a perfectly recrystallized one. Electroetching is so sensitive that it is possible to distinguish a crystal of secondary recrystallization from a crystal of primary recrystallization by differences in etch- pit den si ties (10). 

Electrolytes Employed for Electrolytic Etching of Ti and Zr and Their Alloys ... 

The problem of electrolytic etching cannot be divorced from that of the prior preparation of the metallic surface. Electropolishing is an absolute prerequisite, not only to avoid any surface cold-work but also to realize a surface as free as possible of insoluble films (oxides or others). The case of uranium bears out the importance of this factor. 

M. A. STREICHER (DuPont) A number of detailed investigations provide information on several points raised in this paper in connection with the use of electrolytic etching to detect susceptibility of stainless steels to intergranular corrosion. A brief summary of pertinent aspects is given below. 

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