Surface Oxidation - Thermo Scientific XL2 User Manual

5
Common Operations
Metal Sample Prep

Surface Oxidation

Table 1. Comparative test results with and without grinding
Sample
Reported Chemistry
48 Niton XL2 Analyzer User's Guide
• A Sample Grinder for removing deeper surface contamination. Choice of grinding
wheel media also may be important, depending on what you are testing for. Never
re-use grinding media, as contaminants can be transferred from sample to sample on
the media itself.
For light contamination on hard metal reference standards, remove the oxidation by
scrubbing the dry sample lightly with the diamond paper square, using the fingers to maintain
pressure. If the diamond paper begins to load up with material, discard it and use a fresh
square. When the oxidation is removed, wipe the sample with lint-free paper soaked with
isopropyl alcohol to remove any oils or dust. Let the sample dry before attempting analysis.
For soft metal reference standards, wipe the sample with lint-free paper soaked with isopropyl
alcohol, then remove the oxidation by scrubbing the wet sample lightly with the diamond
paper square, using the fingers to maintain pressure. If the diamond paper begins to load up
with material, discard it and use a fresh square. When the oxidation is removed, wipe the
sample again with lint-free paper soaked with isopropyl alcohol to remove any oils or dust. Let
the sample dry before attempting analysis.
Oils, lint and dust can be removed by wiping the sample with lint-free paper soaked with
isopropyl alcohol. Let the sample dry before attempting analysis.
With the exception of a limited number of metal types, most metal alloys form an oxide
covering on the surface when exposed to oxygen or air. This oxide covering is visible in carbon
and low alloy steel as a red colored substance called rust. Other metal alloys form oxidation
which is not always visible, but that does not mean that it is not present. If the test results for
low concentration elements are higher or lower than expected, remove the oxide coating by
grinding and retest. Follow proper safety procedures when changing discs or grinding
materials.
During a recent case study the effects of sample preparation became apparent. A customer
asked for low detection limits of nickel, chromium and copper in carbon steel pipe. The
reported chemistry of the purchased material is listed on the first line in the chart below. The
test results of a hand held Niton XL2t 900S GOLDD instrument appears in the second line
of the chart. The results from a test on the unground surface appear in the bottom line of the
chart. Note the values for nickel and copper in this carbon steel alloy in the chart below. The
oxidation on the surface of this pipe was not visibly egregious. We need to always be wary of
the presence of even low levels of oxidation and their possible effects on analytic accuracy.
% Mn
0.650
% Ni % Cr
0.090 0.070 0.030
% Mo % Cu
0.040
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