Chris, LSI has been ...

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Chris,

LSI has been disproved as a useful method for predicting corrosion. Calcium carbonate won't protect steel. What forms is actually ferrous carbonate. As iron corrosion occurs, ferrous ions release into water and react with carbonate alkalinity to form a protective scale.  As with other carbonate scales, higher pH will create a more scaling environment.

You need to keep your pH above 8.5 and your carbonate alkalinity above 40 ppm. Calcium doesn't matter except for taste.

Using a phosphate based corrosion inhibitor will result in further improvement, but is no substitute for high carbonate alkalinity and optimal pH. A phosphate inhibitor won't work below pH 7.5.

A calcite contactor should help but may not reach the required carbonate alkalinity unless you reduce the pH ahead of it to about 6 using CO2 or sulfuric acid.  

A finished pH of 9.1 would be optimal for corrosion control, especially for iron and lead. What's the reason for the pH reduction, is it for the biocidal activity of the chlorine? Corrosion will deplete your chlorine as released ferrous ions oxidize to the ferric state.

 

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Hi Mohannad.

Thanks for your help.

Yes, my primary reason for pH control is for disinfection. We were instructed to keep our 2x 300,000 ltr concrete storage tanks at 0.30ppm with a pH of high 8-9.10. At 4 of the work sites i deliver water to, these sites have additional Chlorine dosing, however, the public of 10-15,000 people have no disinfection and the conditions of the infrastructure/vermin proofing is minimal at best. From this point of view to protect the public, i control pH. Would i be correct in doing this as corrosion in steel tanks is a distant second in regards to water quality control ??

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First implement what has been suggested, which is based on field experience, thereafter review the corrosion prevention procedure. . The tank external can be protected by protective coating like ( IOZ + HBE + AAPU ). LSI is more so relevant for circulating Cooling Water System.

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