Internal Coating For Sour Water Tanks

[Inspector Gadget]

Hey folks,
Are there any seasoned coating Inspectors or some API 653s that may have some experience with coating failures on Sour Water Storage? I have a 10 year old 67 foot dia. external floater that had an epoxy phenolic applied upon new installation. This coating has failed in numerous areas and also has a thick buildup (unanalyzed as yet) but appears to be a hard calcium type deposit on shell. This is damaging seals, so we must remove. Management has asked do we need to reinstall coating? Costly for blast prep prime and coat. I suggested as this is sour water that in atmospheric wetting and drying that sulfide oxidization would likely be accelerated on bare metal and that an inorganic zinc primer with a TNEMEC 61 Epoxy Phenolic Shell coating would be best to inhibit Shell corrosion while a TNEMEC series 300 bottom liner may lengthen service life. Simple question: Do any of you guys/gals have Sour Water Service Storage Tanks with no internal coating and is corrosion rate accelerated if so?

[MBazan]

Hey Larry I sent your question to a NACE III Inspector and this was his reply.(First off, what is the chemical analysis of the sour water? If failures are present, the system would definitely need to be replaced. The only way to find the coating system to do the best job and have the longest service life is to find out what chemicals it could see and at what concentrations. Did the failures have a pattern? Were they just in the normal wet level of the tank or were they throughout the entire tank shell and floor? The biggest problem with using Zinc is that you have to be extremely careful with the millage applied in a Zinc application. Zinc applied too heavy will break down over time and you will not know of this problem until sheets start to float in the tank. If a Zinc primer were used, I would definitely make sure there are no mill readings outside of the manufacturers recommended thickness. Coating contractors tend to use PA-2 which is taking an average on the thickness. The problem with that is that a reading of 1 mil and a reading of 10 mils will give you an average of 5.5 mils which is acceptable. The 10 mil area will definitely break down and cause a breach in the system which will accelerate the corrosion rate. Over here we would get the chemical analysis and pass it on to specialty engineering for a recommended coating for the particular service.) I hope that this is of some help.

[Inspector Gadget]

Thanks MBazan,
 Yes the coating failures were random and I have not made entry as of yet for floor evaluation. I definately will ask the lab for chemical analysis but I have info from Tank Farm Operators that indicate this could contain most all Sourwater and various by products from the plant, most hazadous of course being H2S. I am leaning toward the improper initial application theory for the variable coating failures. This tank of course sees liquid and atmospheric drying of shell as levels rise and fall. I have just seen histories of most gasoline storage tanks in other facilities requiring an Epoxy Phenolic internal coating. Unfortunately we do not have specialty engineering i.e; coating specialists in this plant and a lot is relied upon inspection for this; hence I have to seek outside assistance. That's why I am seeking coating specialists knowledge from external sources. I'm still investigating the best application for service. Your assistance is well appreciated.
Thanks again.

Larry