Metallography: Principles and Practices - ASM International

Metallography: Principles and Practice (#06785G)
Author(s): George F. Vander Voort
Copyright © 1984 ASM International ®
All rights reserved.
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MACROSTRUCTURE 57
addition of a wetting agent, such as Teepol, is recommended. Since the solution is
poisonous and contains a strong corrosive acid, it should be handled carefully.
Picard and Greene have described printing methods to detect contamination
of metallic surfaces by foreign metals that results from machining, brushing,
and/or accidental contact [54]. Using this method, these authors have detected
iron contamination on the surfaces of nickel, titanium, and zirconium and copper
contamination on stainless steel. For printing, they use either desilvered unexposed
or exposed single-weight Polycontrast photographic paper. The choice of
exposed or unexposed paper depends on the color of the precipitate formed. If a
light precipitate is formed, black exposed paper provides the best contrast.
Exposed paper was used to reveal porosity in a number of metallic coatings. The
test is performed as follows: The paper is saturated in the appropriate electrolyte
solution and placed on the test surface. After an appropriate time, it is placed in a
developing solution, rinsed in distilled water, and dried. For steels, including
stainless steel, they employed a 5 wt % Na2S04 solution as the electrolyte, and for
copper-clad aluminum, they used a 3 wt % NaCl solution as the electrolyte.
Hunter et al. have described electrographic print tests that employ procedures
used in colorimetric spot testing for the detection of a variety of metals [55].
In this method, gelatinized paper (e.g. desilvered photographic paper) is soaked
in an appropriate electrolyte and placed on an aluminum block. The specimen is
placed on the paper, and pressure is applied. Current from a 6-V battery is
applied, with the aluminum block as the cathode and the sample as the anode. An
ammeter is used to measure the current flow to ensure rapid dissolution. The
current required depends on the size of the sample, the element to be detected,
and the electrolyte chosen. In general, application of the current for 30 s was
adequate. These authors have used this procedure for detecting a wide variety of
inclusion types in many different metals, for alloy identification, and for detecting
porosity in metallic coatings.
1-7 SUMMARY
This chapter has presented information pertaining to the visualization and
interpretation of the macrostructure of metals. These techniques permit examination
of a large number of materials using either nondestructive or destructive
methods. When areas of questionable quality are detected, microscopic analysis
may be desired to more clearly identify the nature of the problem. Thus,
macrostructural examination is often the prelude to microscopic examination.
Macroscopic examination is clearly a fundamental step in both quality control and
failure analysis.
REFERENCES
1. Sorby, H. C: "On the Microscopical Structure of Iron and Steel,"/. Iron Steel Inst., pt. 1: 1887,
pp. 255-288.
2. Berglund, T.: Metallographers' Handbook of Etching, Sir Isaac Pitman & Sons, Ltd., London,
1931.