Failure Analysis of Tubes in Heat Recovery System in a Phosphoric Acid Plant

A phosphoric acid unit in India was facing a lot of issues with the Heater and pre-heater tubes in the Heat Recovery System. The alloy 310s for failing prematurely due to corrosion in the tube and also at the tube to tube sheet welding. The tubes are exposed to very high concentrations of sulphuric acid at high temperatures. The tubes are failing within 2-3 years which leads to a reduction in the overall productivity of the plant.

The same was studied at the Sandvik R&D center in Pune. Various tests were performed on the failed sample. Severe pitting corrosion and oxidation was observed on the tube samples. Various methods like SEM, EDS, Chemical Analysis, etc were performed to understand the failure mechanism and its probable cause.

After a lot of field trials UNS S32906 an austenitic-ferritic alloy was chosen for this critical application due to its exceptional performance in the high concentration sulphuric acid.

UNS S32906 is characterized by –

  • Excellent resistance to caustic corrosion
  • Excellent resistance to seawater corrosion
  • Good resistance to sulphuric acid at higher concentrations
  • Better yield strength and tensile strength than austenitic stainless steel
  • High resistance to SCC
  • Good weldability


Vikram Pandit, Regional Technical Marketing Manager, APAC
Sudhindra Hathwar, Business Development Manager, India

Date and time

Recorded October 13 by Sales Region APAC