Gas Turbine Testing - Corrosion Coating Evaluation

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VEXTEC Laboratory Group's small gas turbine testing rig provides an excellent way to evaluate corrosion coatings. In a program with the Air Force, we produced approximately the same field service corrosion damage that accumulated in 5,000 hours of usage, with 300 hours of testing. The photos below show field service & rig damage.

Actual Field Damage

Test Rig Produced Damage

Since the corrosion was due to sulfidation, sulfur was added to the exhaust stream. Agricultural grade sulfur was mixed with water to obtain a concentration of 1730 parts per million. The mixture was constantly stirred to keep it uniform as it was pumped into the exhaust stream at a rate of 6.8 gallons per hour. Since the desired temperature was 300F, it was necessary to bring room temperature air into the exhaust stream, which was done by adapting a leaf blower. The blades were rotated at a speed of 5500 RPM. The purpose of the program was to evaluate 3 different coatings. 10 blades were tested:

4 were uncoated, to be used as control specimens
2 had coating A
2 had coating B
2 had coating C

The following mission profile was repeatedly applied to the blades:

Mission Profile for Corrosion Testing

After 200 hours of testing, all specimens were examined carefully. Their conditions are shown below in photographs.

Coating A
- Coating had crazed
- Coating had come off of leading edge & suction side
- Possible corrosion pits in regions where coating spalled off
Coating B
- No apparent coating damage
- No apparent corrosion
Coating C
- No apparent coating damage
- No apparent corrosion

Coating A After 200 Hours of Testing

Coating B After 200 Hours of Testing

Coating C After 200 Hours of Testing

After evaluating the specimens, it was decided there was no point in testing coating A further. The test was continued for another 100 hours with coatings B & C. Those coatings were evaluated again after a total of 300 hours.

Coating B
- Coating defeat on blade tip,along entire leading edge & on suction side
- Base metal corrosion was found where coating was defeated.
Coating C
- No apparent coating erosion
- No apparent corrosion

Coating B After 300 Hours of Testing

Coating C After 300 Hours of Testing

Summary

VEXTEC was able to simulate the full scale engine environment for corrosion
The failure mechanisms (corrosion & erosion) occuring in the field were reproduced
3 Different coatings were differentiated in 300 hours of testing

	VEXTEC Laboratoy Group 1033 South Hunt Street Terre Haute, IN 47803	America's Most Promising Company Forbes Magazine - October, 2009
	E-mail epope@vextec.com
URL http://gas-turbine-testing.com/corrosion-coating-evaulation.htm	Phone 1-317-750-3414 Ed Pope	Copyright © 2012 VEXTEC