Radiation-induced segregation (RIS) to grain boundaries and interfacial microchemistry effects on intergranular stress corrosion cracking (IGSCC) have been investigated in 304 and 316 stainless steels (SS). Irradiation dose effects on RIS to grain boundaries were studied using nickel ion bombardment of fine-grained, sputter-deposited SS. Damage levels from 1 to 20 dpa were produced in near-surface regions encompassing several grain diameters in depth. The resultant "bulk" damage promoted RIS of silicon and nickel, as well as depletion of chromium at grain interfaces. Maximum enrichment/depletion ratios of about 4.0, 1.1 and 0.7 were measured for silicon, nickel and chromium, respectively. Composition changes were localized to within 20 nm of the boundary as measured by analytical transmission electron microscopy. No grain boundary enrichment of phosphorus was observed in these heats even though heats contained high levels of phosphorus in the bulk.

Grain boundary chemistry effects on the intergranular (IG) corrosion and stress corrosion cracking (SCC) were examined by varying phosphorus segregation and chromium depletion through controlled thermal treatments. Grain boundary enrichment of phosphorus was found to promote IG corrosion, but not SCC, of 316 SS in low-temperature, sulfuric acid solutions at transpassive potentials. Results from slow-strain-rate, straining-electrode tests at various anodic (active to transpassive) potentials show no evidence that grain boundary phosphorus enrichment (without chromium depletion) promotes IGSCC. Phosphorus segregation did promote IG hydrogen cracking at cathodic potentials. The fracture mode changed from transgranular (TG) to IG as the phosphorus at the boundary reached about 8% of a monolayer. This fracture mode transition corresponds to a drop in ductility from approximately 70% strain-to-failure (<5% of a monolayer phosphorus) to 15% strain-to-failure (>8% of a monolayer phosphorus).

Chromium depletion is shown to promote IG attack at active/passive potentials in a low-temperature sulfuric acid solution and IGSCC in high-temperature water. The minimum chromium concentration at the grain boundary controlled both corrosion and cracking susceptibility for 304 SS. Grain boundary chromium minimums below about 13 wt% induced

IG attack during electrochemical potentiokinetic reactivation (EPR) experiments, while minimums up to 14 wt% prompted IGSCC in slow-strain-rate tests in 300 C oxygenated water. The transition from TG to IG fracture was mapped as a function of minimum chromium level on specimens where the depletion width was constant (~50 nm). Ductility was found to drop consistently with the decrease in chromium minimum and the increase in IG fracture. The implications of these results on current understanding of irradiation-assisted SCC are discussed.

Subject
Materials, Intergranular stress corrosion cracking, Chromium, Irradiation, Irradiation assisted stress corrosion cracking, Phosphorus, Corrosion attacks, Silicon, Heat, Nickel, Grain boundary, Corrosion potential, Damage
© 1990 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
1990
Association for Materials Protection and Performance (AMPP)
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