Letter 35094

Carbon steel corrosion in aqueous ammonia high temperature system [Bangladesh] 

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We have a carbon steel tower, packed with 0.1 mm thick SS 304 sheet metal packing, acting as a stripper to recover dissolved ammonia from ~ 30% ammonia solution; operating at 210 deg C and 20 Kg pressure. Dissolved CO2 of the order of 600 to 1000 ppm is also analyzed in the aqueous ammonia solution. The residual hydrazine is 100 ppb, which is indicative of absence of Oxygen. Feed water is SBA O/L having silica less than 0.2 ppm, which shows absence of halides. Prior to operation tower internal surface was passivated by forming magnetite layer. After operation of one year we have opened the tower and found corrosion on tower's internal surface and SS packing as well. The SS packing is coated with black layer. Corrosion product collected from the inside wall of top dome is soft brown material analyzed for iron and carbonate. During operation liquid samples collected after passing tower top vapors through water cooler were analyzed for soluble iron and found to be ~ 250 ppm which indicates that iron was getting dissolved in the aqueous ammonia solution.

It is requested to give the possible mechanism of corrosion and measures to prevent it in future.

Hassan Ali
Ammonia recovery from spent liquid - Chittgaon, Bangladesh

I did not fully understand the arrangement you described. By packing do you mean that the tower has an internal liner or jacket made of SS 0.1mm thick? Or how is the SS packing disposed inside? G. Marrufo-Mexico

Guillermo Marrufo
Monterrey, NL, Mexico

By packing I mean that tower is packed/filled with 0.1 mm thick SS sieve trays of the size of tower internal dia. Hope I am clear now.

Hassan Ali
- Chittgaon, Bangladesh

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