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Ted Mooney, P.E. RET
Pine Beach, NJ
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for Metal Finishing since 1989
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pH of Deionized Water as related to Fusion Bonded Epoxy Coating
August 13, 2008
I am a coating inspector working on a project for a oil and gas company. Currently, I am working in a coating mill - which is applying FBE powder coating. Before the pipe gets coated it goes through the following surface preparations. 1. 1st blasting
2. Through an acid wash 3. Rinsing with deionized water 4. Then a second round of blasting.
My question involves a concern over the rinse water ( deionized ) supply. I am not exactly sure how to tell whether or not the water being supplied is actually deionized. Are PH tests even useful at all when trying to determine this. I have checked the pH levels of samples taken directly from an outlet on the reverse osmosis equipment and each time got a reading of 7.0 or very close. However, by the time the water makes its way to the end of the supply line, samples generally return readings that are higher - 8.0 to 8.5 - some times higher. I've had all the filters changed and new hoses installed and yet we are still getting a spike instead of a drop in the pH levels. I'd just like to know why this might be happening... is there a chance that the water is getting contaminated to the point that it is no longer deionized?
coating inspector - Crosby, Texas, United States
Hi, Anthony. Deionized water has had a substantial portion of its dissolved salt ions removed. Ions are what conduct electricity in water. So the way to measure deionization is by the conductivity of the water. You should be able to locate conductivity meters.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
August 13, 2008
Deionized water does not typically use pH as an indicator. Depending on the conductivity or resistivity, pH measurement is difficult. PLUS, after the water is deionized ... it is hungry for dissolved solids and will absorb anything. The pH can shift or change quickly, thus not being truly deionized very long.
Ken Burris- Nashville, TN
September 10, 2008
RO water typically is not as pure as DI water, but it will still leach out metals from metal lines/valves/faucets. It needs to be an appropriate plastic pipe/tubing. Also, keep it from being in long term contact with air as CO2 will be absorbed and will change the pH. As stated, use a medium priced conductivity meter and have standards for calibrating it. I like Myron-L.
James Watts- Navarre, Florida
September 11, 2008
It seems like the second round of blasting would erase the benefits of the other pretreatment steps. Why does the rinse water have to be so pure? Is a high failure rate of the coating(holiday test) prompting the question?
Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina
September 13, 2008
Antony,
Though pH testing is a good idea, it is of little value here. You should be using a conductivity meter. Rejection criteria for the DI water should be anything above 5 micromhos. After that, conductivity tends to increase quickly. An increase of conductivity of rinse water will be directly reflected as a decrease in adhesion results of your elevated temperature moisture permeation tests.
- Houston, Texas, USA
August 12, 2009
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