Corrosion-selecting Materials for Sea Water Systems 001

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BMCI MONOGRA / J ., ". ' MARINE ENGINEERING PRACTICE Volume I Part 10 SELECTING MATERIALS FOR SEA WATER SYSTEMS by B. TODD, M.Eng. , C.Eng. , F.LMar.E., F.I.M . ' . .. P. A. LOVETT, A.LM. . i IN ) Z-..;U 5 -Tfno. r.. "'..J 10 THE INSTITUTE OF MARINE ENGINEERS 1 Published for The Institute of Marine Engineers by Marine Management (Holdings) Lt d. 76 Mark Lane, London EC3R 7JN ( England Reg. No. 1100(85) Reprinted 1978, 1987, 1989. Copyright Reserved This book is copyright under the Berne Convent ion. All rights reserved. Apart from any fair deali ng for the purpose of private study, resea rch, criticism or review- as permilled under the Copyright Act 1956-no part of this publication may be reproduced, stored in a retrieval system or trans-m i l l e ~ in any form or oy any means, electronic, electrical, chemIcal , mechanical, optical photocopying, recording or ot herwise, without t he prior permission of the copyright owners. Enquiries should be addressed to Mari ne Manage ment (Holdings) Ltd., 76 Mark Lane, London EC3 R 7JN. ISBN: 0900976 49 7 ""I 1111 '1, " """"' hv I he Chameleon Press Ltd., London Non-ferrous sea-water systems using copper-nickel alloys and cast bronzes. Information for marine engineers and deck officers, on design and operation of seawater systems for minimum maintenance. CONTENTS Part 1. The insidious enemy - corrosion Part 2. Materials for sea-water circulation systems - where and how to use them Part 3. National materials standards AUTHORS : B. TODD, M.Eng .. C.Eng .. F. I. Mar. E .. F. I. M. P. A. LOVETT, A. I. M . 9 27 52 3 Combating the effects of corrosion has been a feature of seafaring life for centuri es. Even 30 years ago a common complaint was 'condensentls' In steam ships but in time new materials were tried and tested to cure this problem and others of similar nature. It was often found that curing one problem produced another and so Improvements were brought about graduall y and piecemeal. With the advent of large modern high-powered steam ships It became economically Justifiable to Incorporate more sophi sti cated materials in salt water ci rcul ating systems In an eff ort to obtain increased reliability. Unfor-tunately many of the problems which have subsequent ly developed appear to be associated with errors I n the design of minor details and the methods of Installing the pipes, fittings and components which comprise a 'system' . Because of thiS the in-serVice reliability of two similar systems may be very different. ThiS booklet IS Intended to explain how and why thi S can come about so that the reader can recognise potential difficulties before they become problems and to enable him to apply the correct material s and methods to the repair of an eXisting problem. 5 6 8 POINTS TO REMEMBER 1. In pipelines, do not do anything which unnecessarily increases the water speed. 2. Avoid anything which creates turbulence. 3. If you have to create ' crevice conditions' make certain that the metal in the crevice is either resistant to crevice corrosion or is sealed and protected f rom the sea water. 4. Avoid accumulation of debris in any tank, pipeli ne, valve or pump. 5. Make repairs from matching materials. If you have to mix materials your repair may fail a year or so later and i nvolve another engineer, so record your use of the wrong material before it is disguised by a coat of paint and let the experts decide whether to replace it. 6. Regard stagnant water conditions in tanks, pumps, valves and pipelines with deep suspicion and drai n down rather than risk pitting corrosi on. 7. Maintain cathodic protection equipment carefully, check current settings. 8. Use the material specifications in Part 2 of this publication for guidance on materials for repair. Part 1 . Contents Electrochemical processes The driving force behind corrosion Two metals joined together in an electrol vte Some other sources of electrochemical potentials The effect of surface films Concentration corrosion cells Types of corrosion associated with static sea water General corrosion Pitting corrosion Types of corrosion associated with flowing sea water General corrosion Impingement attack Cavitation attack Galvanic coupling and anode and cathode size effects Selective corrosion Dezincification Graphitization Stress corrosion Cathodic protection Summary page 9 9 11 14 15 16 16 16 16 19 19 19 19 20 21 21 22 23 23 25 7 1. The insidious enemy-corrosion. Corrosion can be a baffling problem. even to the expert. and it IS therefore easy to dismiss corrosion prevention as being "too compli cated and difficult". This has led. in the past. to the design of sea-water systems that are satisfactory from the engineering POint of view but diHlcult and cost ly to operat e from the corrosion point of view. However. the expertise IS now readi ly avai lable to design a system that is not only good engineering but is highly resistant to corrosion. Basically it IS the job of the designer to aVOid the worst errors but. even so. ci rculation systems can stili be vulnerable to damage due to the use of unsuitable matenals as a result of in-service repairs carned out In good faith by the operating personnel. The purpose of this publication IS to Introduce the reader to the mechanism of corrosion and show how corrosion damage be aVOided when repairs are made. Whilst the subject matter IS slanted towards sea-water cooli ng systems. It will be of Interest to all ship-board personnel Involved In maintenance. Electrochemical processes The driving force behind corrosion. All metals eXist in tne earth's crust In a form which is more or less chemical ly stable. that IS. their reaction with their surroundings is minimal. Gold and platinum are stable In the metalli c form but most other metals are mined as OXide or sulphide mineral s and energy has to be applied to them to convert them fr om minerals to the metalli C form. see Fig 1 ThiS means that most of the metals used by engineers eXist In an unstable form and tend to react With their environment and revert to a more stable form. The reaCli on IS knowl1 as corrOSion and frequent ly the corrosion pmau t IS chemically Similar to the minerai from which thll motill was extracted. e.g. the rust formed on IrOll 1111