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Corrosion Testing
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ASTM G36 - STRESS CORROSION
CRACKING in a BOILING MAGNESIUM CHLORIDE SOLUTION
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Presented in this report are the results of stress
corrosion cracking testing per ASTM G36 (Standard Practice for Performing
Stress-Corrosion Cracking Tests in a Boiling Magnesium Chloride Solution) of
welded bare and finned tubes fabricated from steel.
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TUBE SAMPLES |
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Two (2) 1-foot long pieces of 0.750” O.D. tube
of each of the bare and finned tube were supplied for the testing. Accompanying
paperwork states that both types of tube were fabricated from the same heat lot
of material.
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TEST SPECIMENS |
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One (1) 4” long full tube section test specimen
was prepared from each type of tube. No preparation other than deburring and
degreasing was performed on the test specimens prior to testing.
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TEST PROCEDURE |
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Test procedures conformed to the referenced ASTM
test method. As specified, the concentration of the boiling magnesium chloride
solution was adjusted to achieve a boiling point of 155
±
1°C.
The test specimens were immersed in the boiling solution and supported using the
suggested ladder-back cradles. Regular examination periods for test specimen
cracking were scheduled.
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RESULTS |
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Circumferential cracking of the bare tube test
specimen was apparent within the first 8 hours of testing even while the test
specimen was on test. Both test specimens were taken off test at 8 hours for
examination at low magnification. Cracking of both test specimens was observed,
and the tests were terminated. Numerous circumferential cracks with occasional
connecting longitudinal cracks were observed on the base metal of the bare test
specimen; however, cracking in the weld/HAZ was mostly longitudinally-oriented.
On the finned test specimen, cracks extended down the fins from the outer edge
to the tube wall in a direction roughly normal to the tube wall; at the tube
wall the cracks extended in a short arc both longitudinally and
circumferentially, the arcs stopping before the next fin was encountered. The
weld and HAZ of the finned tube appeared to have more cracks than the base
metal, and the cracks appear to nearly connect from fin to fin. Photographs of
typical appearance of the cracks on both tubes are shown in Figures 1 and 2.
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METALLOGRAPHY |
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Crack characteristics were investigated by
preparing metallographic sections of the test specimens. It was decided that
cracks in the bare tube could be best examined using a longitudinal
cross-section, while the cracks in the finned tube could be best examined using
a transverse cross-section. Microphotographs of the deepest cracks found on the
mounted sections are shown in Figures 3 and 4. The microphotographs show that
the deepest crack on the bare tube extends through 80% of the tube wall, while
the deepest cracks on the finned tube extend through 12% of the tube wall.
Etching both sections did not reveal any additional crack depth as sometimes
occurs in metallographic studies of stress-corrosion cracking.
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