[updated:LAST EDITED ON Jul-31-04 AT 12:41 PM (CDT)]We are providing our client with Horizontal Cylindrical tanks to be used to transport 98% sulphuric acid. To test the tanks, our client left the 98% acid in the tank for 3-4 months., stationary. Each tank had 2 manhole covers at the top. It was discovered after 3-4 months that there was greenish deposits on the underneath of the manhole cover i.e the side of the manhole cover is within the tank. The manhole cover is made of Stainless Steel, the tank itself is made of Mild Steel. What is the deposit accumulating under the manhole cover? Is it the oxidation of the Fe or Cr or some other alloy element with the air layer in the tank or with the sulphuric acid? Is it a sign of corrosion of the steel? THe tanks are stored near the sea and in environmental temperature of 34 celcius. Please help - thanks!!

A few notes/observations: 1) I do not understand the "test" that was performed....Was this a pressure test or corrosion test ?...This is not clear 2) Carbon steel tanks, with a generous corrosion allowance (.125") is the preferred material choice for 98% sulfuric acid. I have personally inspected the inside/outside of tanks with 30+ years of sulfuric acid service and find this to be suitable. This material has been used succesfully for 75+ years. Carbon steel is not a good choice for tanks storing more dilute sulfuric acid or where some pick-up of iron cannot be tolerated by the process. 3) In order to get a long service life from a carbon steel tank in 98% acid service, some method must exist to keep water from forming a dilute upper layer of acid within the tank. This dilute layer can cause internal "grooving" (corrosion) of the tank shell. Some tank owners use protective internal coatings to mitigate this effect. Others install a "dessicant dryer" to remove the moisture that causes the dilute layer. 4) NACE has published a "Recommended Practice" about concentrated sulfuric acid tank design. Specific tank design features must be incorporated where the acid velocity exceeds %7E3 ft/second. (inlets and outlets, for example) 5) There are several guidelines available on the internet regarding sulfuric acid tank designs. hope that ths helps.... MJCronin