Natural sea water is known as a very agressive environment which generates pitting and crevice corrosion on stainless steels. High chromium grades with sufficient molybdenum and nitrogen additions (PREN >40) are generally recognized as resistant materials in natural sea water but the material selection criteria must be improved to take into account the effect of climatic conditions and of biocide treatments which are widely used as anti-fouling agents in sea water circuits.

The paper deals with the localized corrosion properties of conventionnal stainless steels (SS), duplex and super-austenitic alloys. The results of laboratory investigations conducted in more or less oxydizing chloride containing media are discuused. Then, immersion tests carried out in natural sea waters in different climatic conditions are presented and discussed. Finally, the effect of biocide addition on fouling and its consequences on corrosion is investigated.

The results are interpreted taking into account the chemical composition of the stainless steels and biofilm criteria. The results showed the Mediterranean Sea to be slightly more agressive than other european seas but a PREN value higher than 40 is sufficient for stainless steels to withstand localized corrosion in European natural sea waters. A residual chlorine level around 0.3 - 0.4 ppm was found to be very effective to limit the fouling and to avoid localized corrosion on SS. Nevertheless, due to difficulties in monitoring chlorine addition, PREN values higher than 50 are recommanded to withstand localized corrosion in treated sea waters. As an example, the new super-austenitic grade 25Cr-22Ni-5.8Mo-1.5Cu-2W-0.45N with a PRENW value of 54 was found to be perfectly resistant to crevice corrosion with 0.5 ppm free chlorine at ambiant temperature.

Subject
Water, Materials, Molybdenum alloys, Crevices, Crevice corrosion, Pitting, Chlorine, Corrosion resistance, Localized corrosion, Stainless steel, Pitting resistance equivalent number, Fouling, Rest potential
Keywords:
Stainless Steels, Pitting Corrosion, Crevice Corrosion, Alloying elements, Electrochemical tests, Sea Water, Biofilm, Fouling, Chlorination, Climate
© 1996 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).
1996
Association for Materials Protection and Performance (AMPP)
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