BEHAVIOR IN HOT AMMONIA ATMOSPHERE OF SIO2-COATED STAINLESS-STEELS PRODUCED BY A SOL-GEL PROCEDURE

AISI 304 stainless steel coated with silica films obtained by a sol-gel procedure was tested together with uncoated 304 and 316 samples in anhydrous ammonia at high temperature. The highest rate of attack was experienced by all uncoated samples; the ammonia attack develops a nitride scale that embrittles the metal. After 115 h of testing at 500 degrees C uncoated samples were completely degraded, while the coated 304 samples were lightly attacked. The SiO2 coating is a good barrier against ammonia corrosion. The multilayer coatings provide lower corrosion protection than single-layer coatings.