Corrosion of Types 316 and 317 Stainless Steel By 75 Percent and 85 Percent Phosphoric Acid

The corrosion of Types 316 and 317 stainless steel by 75 percent and 85 percent H₃PO₄ was studied in the 78-115 G range in laboratory, pilot and plant scale tests. Corrosion rates were found to be considerably affected by acid temperatures and by testing conditions. Although laboratory data indicated a maximum permissible temperature of over 115 C for 316 stainless steel in either 75 percent or 85 percent H₃PO₄, and pilot data indicated a maximum permissible temperature in excess of 105 C in 75 percent acid, extensive plant data showed 316 and 316L to be acceptable (< 5 mpy penetration) only at 100 C or less. Type 317 proved more resistant than 316L in both pilot and plant tests. Aerated and agitated samples showed lower corrosion rates than stagnant samples. Welded samples were as resistant as solid sheets in all tests. Cast 316 proved as resistant as rolled 316. 4.3.2