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The
KTA Challenge #17
What Caused This Failure?
Win a $500 KTA gift certificate
redeemable
for KTA products or services
 |
Problem: Two fan motor hubs coated with epoxy were installed in an odor scrubbing tower of a waste processing plant. The coating on one motor hub failed within one year. The coating on the other performed well.
Background and clues:
Two fan motor hubs coated with epoxy were installed in an odor scrubbing tower of a waste processing plant. The coating on one motor hub failed within one year. The coating on the other performed well.
Background and clues:
- A rehabilitation project at a waste
water treatment facility included work in an odor scrubbing tower. As part of
the project, an existing carbon steel pipe run in one duct was cleaned and repainted
with two coats of epoxy. A second run in a parallel duct was completely replaced
with uncoated stainless steel.
- New fan motor hubs were bolted
onto both pipe runs. The fan motor hubs were fabricated from cast steel and were
coated by the supplier with 6 to 8 mils of epoxy.
- The service
environment in both ducts consisted of a moist gas stream containing a high concentration
of corrosive organic acids.
- Within one year, the carbon steel
piping in the first duct exhibited coating failure. The coating on the fan motor
hub (Hub #1) attached to the carbon steel piping was in good condition. In contrast,
the stainless steel piping and mounting bracket in the second duct showed no evidence
of attack or failure, but the coating on the fan motor hub (Hub #2) displayed
widespread corrosion.
5. The field investigation revealed:- The
coating applied to Hub #1 was within the specified thickness range. Little coating
remained on Hub #2, but the thickness of detached chips measured 6 to 8 mils.
-
The adhesion of the coating on Hub #1 was very good (ASTM D3359 ratings of 4A
and 5A). Little coating remained on Hub #2, but the coating that remained was
easily removed by light scraping, revealing underlying corrosion.
- The stainless steel piping showed no evidence of pitting or other attack. The coating on the carbon steel piping exhibited failure, but numerous pinholes were present throughout the film and the coating was very thin (as low as 1 to 2 mils). The failure on the piping was clearly application-related.
- The
coating applied to Hub #1 was within the specified thickness range. Little coating
remained on Hub #2, but the thickness of detached chips measured 6 to 8 mils.
The
question is why widespread failure of the coating occurred on one motor hub and
not the other, since both came from the same supplier and were exposed to the
same corrosive gas stream.
Click here to e-mail us and tell us why you think the coating failed. Send your answer to us before 5pm EST on Thursday, June 12, 2008. Don't forget to include your name and phone number. The winner will be selected at random from all of those who provide the correct answer. The answer and winner will be posted on the web site Friday, June 13, 2008.
A new challenge will be posted the beginning of each month.
*The $500.00 gift certificate is redeemable for any KTA product or service.
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