Spot-Test Procedure 
1—Area of aluminum to be tested is 
cleaned, if necessary using soap and 
water, alcohol, or acetone 
2—Sample to be tested is made anode of 
galvanic cell by connecting it to 
positive terminal of 6-volt dry cell. 
Adequate connection is made by 
alligator clip or by resting specimen 
on plate connected to dry cell 
3—Prepare cathode by connecting nega- 
tive terminal to stainless-steel or 
aluminum electrode of proper size 
and shape to fit area to be tested 
4—Prepare pad of 2-4 thicknesses of 
Whatman No. 50 filter paper 
5—Moisten pad with electrolyte, which 
should penetrate all layers of pad 
and cover an area at least as large 
as that to be tested 
6—Place moistened pad on test area 
and cathode on top of it. Hold 
cathode without movement so that 
it exerts steady pressure against pad 
and specimen. Current flow through 
pad causes metal ions to leave 
specimen and be deposited in pad 
7—After 30-60 sec, remove cathode and 
pad; pad now contains ‘negative’ 
of test area 
8—Develop negative by spotting ex- 
posed surface of pad with proper 
indicator to form characteristic col- 
ored precipitate or complex with 
metal ions removed from specimen 
surface 
Precautions and 
Suggestions 
1—So that the tests be satisfactory, 
it is necessary that: (a) all electrical 
contacts be good; (b) moistened 
filter pad be in good contact with 
test area; (c) cathode be in contact 
with pad over test area; (d) sufficient 
time be allowed for ion transfer to 
take place; (e) in case of pits, in- 
dentations, and pinholes, electrolyte 
fill cavity before test is started 
2—In some instances, good results are 
obtained by mixing electrolyte and 
indicator before passing current 
through cell 
3—Increased current flow usually makes 
test more positive by causing more 
metal ions to collect in filter pad; 
optimum time depends on individual 
case and is affected by size and dis- 
tribution of inclusions 
4—Amount of pressure applied to 
cathode and pad has some effect; 
moderately heavy finger pressure is 
best when using filter pad of type 
described; pressure should be con- 
stant with no movement of cathode 
5—Cross check on results, using another 
test or indicator if available, is 
desirable 
Contaminant 
and electrolyte 
Indicator and color 
for positive test 
Specific Tests for Contaminants 
Comments 
Tron inclusions 
(K280,) 
Lead (10% acetic 
acid or 0.1N 
HNO;) 
Magnesium 
(K2SO,)—test 1 
Mg (K2S0O.)— 
test 2 
Mercury (K:2SO,4 
and HNO;) 
Molybdenum in 
“Molykote””— 
molybdenum 
disulfide (0.1N 
HNO; or 
K.80,) 
Nickel plating 
(K2S80,4 or 10% 
acetic acid) 
Tin (5% acetic 
acid) 
Copper flash on 
stainless steel 
(K2S0,) 
Copper inclusions 
(K2SO,) 
Uranium in U-Al 
alloys (KNO3) 
Uranium exposed 
in pits and pin- 
holes in Al 
sheath (10% 
acetic acid) 
Potassium ferro- 
cyanide (light to 
dark blue) or 
o-tolidine solution 
(Cu—blue; Fe— 
red brown) 
Potassium iodide 
(yellow) 
Quinalizarin solu- 
tion (red-violet or 
red indicates Al; 
blue, Mg) 
Quinalizarin solu- 
tion 
Diphenylcarbazide 
(deep blue to pur- 
ple) or p-dimethyl- 
aminobenzyl- 
idenerhodanine 
(red-violet) 
Hydrochloric acid, 
potassium thiocya- 
nate, and sodium 
thiosulfate (pink 
to deep red or 
blue); or diphenyl- 
carbazide (laven- 
der to deep purple) 
Dimethylglyoxime 
(bright red) 
Cacotheline (lav- 
ender to purple) 
Rubeanic acid (di- 
thiooxamide) 
(olive-green to 
black) 
o-Tolidine solu- 
tion (light to dark 
blue) or rubeanic 
acid (olive-green 
to black) 
Potassium ferro- 
cyanide (brown) 
Potassium ferro- 
cyanide (brown 
stain) 
Successful in identifying iron inclusions in 
tubing, bar stock, and vessels. With 
ferrocyanide, depends on amount of iron 
present and time of current flow; with o- 
tolidine, Cu and Fe can be detected simul- 
taneously if they occur discretely some 
distance from each other 
Useful in testing for removal of powder 
used as lubricant on extruded stock and in 
detecting inclusions in tubes 
After spotting filter pad with indicator, 
pad is held over mouth of bottle of concen- 
trated ammonium hydroxide until indi- 
cator turns blue; then over glacial acetic 
acid until indicator turns pink-lavender 
Follows same general procedure except 
that afterspotting with indicator, sufficient 
sodium hydroxide is added to dissolve alu- 
minum hydroxide and form aluminate ion; 
solution has faint blue tint; insoluble mag- 
nesium hydroxide remains in place and re- 
tains darker-blue color indicating Mg. 
Both tests useful on tubes and other ex- 
truded stock on which Mg was lubricant 
and on Mg-Al alloys 
Strongly recommended that this test, be- 
cause of severe attack on specimen, be last 
applied. Try electrolytesin order: K:SO,, 
1N HNO,, and 8N HNO;. Test is only 
moderately successful since it is difficult to 
dissolve Hg (6, 7) without seriously at- 
tacking specimen or cathode 
Successful with Molykote used as lubri- 
cant on hot-pressed Al pieces. First in- 
dicator consists of reagents added in order 
listed; color depends on time intervals be- 
tween addition of each reagent 
Satisfactory for determining if corrosion 
pit or indentation has penetrated through 
Al sheath to nickel plating on underside 
Detects remnants of tin solders left on Al 
sheeting after corrosion testing 
Ideal for identifying very thin copper plat- 
ing on stainless steel. Rubeanic acid 
must be used since Fe, Cr, and Ni inter- 
fere with other indicators 
Identifies copper inclusions in Al tubing, 
bar stock, and fabricated vessels 
Not satisfactory if alloy has been treated 
to produce protective layer of aluminum 
oxide on surface 
For determining if clad U is exposed 
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