Sea water systems on many offshore platforms are treated with small amounts of chlorine to prevent marine growth inside piping and equipment. It is well known that small additions of chlorine increase the oxidation strength (corrosion potential) of Ti and stainless steels in sea water. Regarding Al, CuNi and carbon steel, it is difficult to find the effect of chlorine on these materials described in the literature. Therefore, during laboratory tests the corrosion potentials of engineering materials under influence of additions of chlorine and chlorite were measured. The investigated engineering materials were an Al-alloy, 316L stainless steel, 6Mo stainless steel, 3 grades of duplex stainless steels, CuNi, Ti and carbon steel. A systematic test program for these engineering materials was carried out with chlorine and chlorite rest levels of 0.5 ppm, 5 ppm, 50 ppm and 500 ppm.

The motivation for the tests with chlorite, originated from a project on an offshore platform containing produced water with high amounts of H2S. The H2S was decided to be removed by adding sodium-chlorite to the water. The sodium-chlorite dissolves to sodium and chlorite prior to the H2S removal reaction. The H2S removal reaction is based on chlorite and water reacting with H2S and forming sulfate and H-ions. After treating the water with sodium-chlorite, the water was found to be free of H2S. However, it turned out that the water became very corrosive, probably due to high rest levels of chlorite. Pumps and equipment suffered from heavy corrosion, especially an Al-alloy. The literature is very limited with respect to data on corrosivity due to chlorite additions in sea water.

Based on available data on critical pitting and crevice potentials for the passive materials, and overvoltage curves for the active materials, it is possible to predict how the “chlorination” will influence on the resistance against corrosion.

Subject
Water, Materials, Circuit potentials, Fresh water, Seawater, Open circuits, Sodium chloride solution, Pitting, Chlorine, Carbon steel, Stainless steel, Critical pitting potential, Corrosion potential
Keywords:
fresh water, seawater, 3.5% NaCl, engineering materials, chlorite, chlorine, electrochemical potentials
© 2007 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).
2007
Association for Materials Protection and Performance (AMPP)
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