It is well known that stainless steels of the 300 series are not safe in stagnant, "living", sea water. The reason for this is as yet not fully understood. Their behaviour can be related to high values of the electrochemical potential at which these steels are brought when immersed in "quiet" conditions, as opposed to high velocity conditions. Indeed, electrochemical potentials as high as + 350 mV/AgCl have been observed to prevail. As most usual stainless steels present pitting potential in the region of + 100/ + 300 mV/AgCl, they are prone to pitting, or likewise to crevice corrosion. These high values of the electrochemical potential were observed to be related to the formation of a bio-fouling film which appears in quiescent water, and which can depolarize very markedly the cathodic process. Bio-chemical activity seems thus to be the driving force for the corrosion processes involving several species of microorganisms. As biological activity is much dependant upon local climatic conditions, it can be infered that corrosive conditions could vary in a significant manner depending upon the geographical location.

These observations highlight the importance of the initiation stage for corrosion, i.e. pitting corrosion and we are thus led to assign great importance to the pitting potential. This pitting potential depends primarily upon the composition of the steel, contents in Cr, Mo, Ti, N and eventually Ni, but also upon the contents in non-metallic inclusions, which are a preferential site for pit initiation. As pit initiation due to surface defects cannot be ruled out, a resistant alloy must withstand the corrosive conditions prevailing in a pit or a crevice, namely an acidic deaerated Cl- concentrated solution.

Laboratory tests carried out either in NaCl solutions or in FeCl3 medium, together with long term immersion tests in natural sea water demonstrate that only the high alloyed stainless steels can be considered as relevant for safe use in sea water. In particular, the super austenitic grade UR SB8 (Cr 25 - Ni 25 - Mo 4.8 - Cu 1.5 - N) performs quite well in such an environment and it is important to notice that GTA or SMA welded assemblies using Ni based type fillers are as resistant to sea water as the base material. But one must also underline that the high chromium containing duplex grades UR 47 N and UNS 32550 are quite resistant to cold sea water.

Subject
Pits, Materials, Repassivation potentials, Corrosion initiation, Crevices, Sodium chloride solution, Crevice corrosion, Steel, Pitting, Corrosion resistance, Stainless steel, Electrochemical potential, Electrode potential
© 1988 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).
1988
Association for Materials Protection and Performance (AMPP)
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