We have a 15,000bbl tank (50ft Dia.) external floating roof tank. The tank bottom (with 2ft high shell) is coated. The tank material should be ASTM 36. It is welded tank. We are planning to store caustic (15%wt. ) in it. It is under ambient temperature (125oF max). After reading some documents in this Web site, I found that carbon steel itself holds pretty well in 15% wt caustics. So do we really need to coat the interior shell of the tank (above 2ft) and the interior side of floating roof? The tank bottom and lower 2ft shell have been covered by very good coating (80-mil thick). I am thinking about 3 choices: (1) Do not coat anything more than it is now. (2) Fully coat the rest wall and roof. (3) Only coat the weld joints of the steel plates since welding is easier to be corroded by caustic. Does anybody have some suggestion with good reasons? Thank you in advance.

Some years back PPG's chlor-alkali & derivatives business published guidlines on storing handling and transporting caustic soda As I recall, there were tables with corrosion rates of carbon steel for variuos concentrations of NaOH. Try this link and ask the people who have been making this stuff for sixty years..... www.ppg.com/chm_chloralk/chlor_home.asp?cms_link=Product_Info.asp?doctype=Bull Good Luck MJC

We have in our chlor alkaline plant two caustic storage tank which have not any interior coating and work well for at least twelve years operation of the plant.