The effect of σ-phase precipitation at 800°c on the corrosion resistance in sea-water of a high alloyed duplex stainless steel

Corrosion Science, Vol. 36, No. 5, pp. 871-881. 1994

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Pergamon

Elsevier Science Ltd Printed in Great Britain 0011)-938X/94 $6.00+ 11.00

0010-938X(93)E0020-R

THE EFFECT OF o-PHASE PRECIPITATION A T 800°C O N THE CORROSION R E S I S T A N C E IN S E A - W A T E R O F A HIGH ALLOYED DUPLEX STAINLESS STEEL
M . E . WILMS,* V . J. GADGIL,t J. M . KROUGMAN* a n d F. P. IJSSELING* *Marine Materials Engineering Laboratory of the Royal Netherlands Naval College, P. O. Box 10000, 1780 CA Den Helder, The Netherlands tCentre for Materials Research (CMO), University of Twente, EL-TN Gebouw, P. O. Box 217, 7500 AE, The Netherlands

Abstract--Super-duplex stainless steels are recently developed high alloyed stainless steels that combine good mechanical properties with excellent corrosion resistance. Because of a high content of chromium and molybdenum, these alloys are susceptible to o-phase precipitation during short exposure to temperatures between 650 and 950°C. The effect of 800°C aging on o-phase formation and on the mechanical properties of a super-duplex stainless steel have been reported previously by the authorsJ This investigation concerns the effect of o-phase precipitation at 800°C on the corrosion behaviour in seawater and comprises anodic polarization scans and potentiostatic crevice corrosion tests. A serious deterioration of the corrosion resistance is found after aging times longer than 7 min, resulting in a drop of both the critical crevice corrosion temperature (CCT) and the breakdown potential (Ebd). The initiation of localized corrosion takes place next to the o-phase, in the newly formed secondary austenite (Y2).

INTRODUCTION

DUPLEX stainless steels have a microstructure which consists of about equal amounts of ferrite and austenite. These materials are attractive for numerous applications because of high strength, 2~3 good corrosion resistance 4 and ease in fabrication. 2 Applications in more corrosive environments have created a