Marine and Offshore Corrosion

419 Pages·1985·27.065 MB·English

by  Kenneth A Chandler (Auth.)

Marine Engineering Series
MARINE AND OFFSHORE PUMPING AND PIPING SYSTEMS
J Crawford, CEng, FIMarE
MARINE AUXILIARY MACHINERY—5th edition
E Souchotte, CEng, FIMechE, MIMarE
David W Smith, CEng, MIMarE
MARINE DIESEL ENGINES—5th edition
Edited by C C Pounder
MARINE ELECTRICAL PRACTICE—5th edition
G O Watson, FIEE, FAIEE, MIMarE
MARINE STEAM BOILERS—4th edition
J H Milton, CEng, FIMarE, MNECIES
R M Leach, CEng, MIMechE, FIMarE
MARINE STEAM ENGINES AND TURBINES-4th edition
S C McBirnie, EEng, MIMechE
Marine and Offshore
Corrosion
Kenneth A Chandler
BSc, CEng, FIM, ARSM, FICorrT
BUTTERWORTHS
London—Boston—Durban—Singapore—Sydney—Toronto—Wellington
All rights reserved. No part of this publication may be reproduced or
transmitted in any form or by any means, including photocopying and
recording without the written permission of the copyright holder,
application for which should be addressed to the publishers. Such
written permission must also be obtained before any part of this
publication is stored in a retrieval system of any nature.
This book is sold subject to the Standard Conditions of Sale of Net
Books and may not be resold in the UK below the net price given by
the Publishers in their current price list.
First published 1985
© Butterwort h & Co (Publishers) Ltd, 1985
British Library Cataloguing in Publication Data
Chandler, Kenneth A.
Marine and offshore corrosion.
1. Sea-water corrosion.
I. Title
620'.4162 TA462
ISBN 0-408-01175-0
Library of Congress Cataloging in Publication Data
Chandler, Kenneth A.
Marine and offshore corrosion.
Bibliography :
Includes index.
1. Sea-water corrosion. 2. Offshore structures—
Corrosion. I. Title.
TA462.C462 1984 620.Γ1223 84-7009
1 ISBN 0-408-01175-0 (U.S.)
Filmset by Mid-County Press
Printed by Thetford Press Ltd.
Bound by Anchor Brendon Ltd.
Preface
'Thy needles—now rust
disus'd and shine no more'
W Cowper (17th Century)
This book was originally intended as a revision of Marine Corrosion written
by T Howard Rogers and published fifteen years ago. There have,
however, been many developments in the field of marine corrosion in the
intervening period. Corrosion control in marine situations has become a
matter of increasing importance with discoveries of offshore oil and the
greater demands being made on the utilisation of the sea's resources for
food, fresh water and for energy requirements, e.g. by wave motion.
The original book Marine Corrosion was particularly concerned with
shipbuilding and the marine engineering associated with it. To allow for
a more detailed treatment of general engineering requirements, particu­
larly of protective coatings for steelwork, the balance of content places
less emphasis on ships although a treatment of this topic is included.
Consequently, this is really a new book although elements from the
original book have been included. It also has a new title to reflect its
purpose.
Clearly when preparing a book, the author must have in mind his
readership. This particular book has been prepared for engineers and
designers who are not corrosion specialists but have to deal with marine
corrosion problems as part of their day-to-day professional activities.
Although the term 'engineer' covers a wide field, the book is not primarily
for those who describe themselves as 'corrosion engineers'. However,
because corrosion is becoming very specialised and many experts in, say,
cathodic protection do not necessarily have expertise in alloy selection, it
is hoped that much will be of interest to them. Because the topic—marine
and offshore corrosion—covers such a large field, it would be imprac­
ticable in a book of this size to provide a detailed treatment of every
aspect. The aim has been to cover the major aspects in reasonable detail
but where appropriate, having drawn a matter to the attention of the
reader, to indicate further sources of information.
Although general data is included in the book it is not intended as a
data handbook. The aim is to indicate the principles on which good
corrosion control treatments are based and to highlight some of the
problems that have to be overcome to achieve economic solutions to
corrosion.
Data is generally provided to illustrate a point or a principle and is not
intended for design purposes. Where corrosion data is used by designers it
is advisable to refer to the original publication because there may be
'caveats' regarding the use of such information in circumstances different
from those in which it was obtained. Materials are often being used to
near their engineering limit in marine situations and new materials will
be developed to meet the requirements which are bound to increase in the
next decade or so. Such materials may well be subject to localised and
stress-related forms of corrosion and considerable research efforts will be
required to overcome some of these difficulties and some of our present
problems.
The requirements for adequately protecting steel structures from
corrosion by painting will also lead to new developments in coatings,
cleaning and application techniques. Those involved in marine corrosion
must, therefore, maintain close contact with such developments. The
basic principles of corrosion and its control are largely established and
they are set out in this book but the solutions to many problems remain to
be resolved.
It should be borne in mind that the aim of designers is not usually to
prevent corrosion but to control it within acceptable economic limits, so
they are not necessarily concerned with the most corrosion-resistant
materials but the ones that have the overall properties of strength,
weldability, formability, corrosion resistance and cost that will provide
the most effective economic solutions to engineering problems. Corrosion
control is only a part of this overall requirement but often a very
important part, and so deserves the attention appropriate to the
economic consequences that will arise from its neglect.
Finally, in any book there will be some chapters where the reader
considers the level of treatment too low and others where it is at a level
unnecessarily high for his purposes. This book is essentially practical in
concept and, apart from the chapter concerned with principles of
corrosion, the treatment of theoretical matters is kept to a minimum,
although it is hoped that readers will—where appropriate—study
matters in more detail by referring to the publications noted at the end of
each chapter.
KAC
Acknowledgements
In this book reference is made to published work by other authors and
investigators but because of the practical nature of the book and because
it is intended for practising engineers, references have been limited to
those papers and books from which data has been extracted or where the
original work may, to advantage, be studied.
The knowledge of corrosion and its control is, of course, gained in
many ways other than by one's own reading, research and experience.
Discussions with colleagues, attendance at conferences and meetings are
all important means of developing views on matters of interest. Often the
original source of the information or concepts that one uses as a basis for
published work is no longer clear. I, therefore, take this opportunity to
thank all those with whom I have been involved on corrosion and related
matters and apologise for any omissions of specific acknowledgements for
contributions they may have made to my views on marine corrosion.
I wish to thank Dr Lionel Shreir, OBE, for useful comments and for his
permission to use material from his many published papers on the
principles and theories of corrosion. These and his work for the
Department of Industry (UK) form the basis of Chapter 2 'Principles of
Corrosion'.
I thank Mr Bryan Wyatt who supplied the material for the chapter on
cathodic protection and Mr Hector Campbell for his advice on parts of
the chapter on non-ferrous metals. Also thanks are due to the late Mr T
Howard Rogers who gave permission shortly before his death for the use
of material from his book Marine Corrosion (Butterworths, 1968) in the
chapters dealing with marine environments.
Mr. John Morley is thanked for data on piling.
Finally, I wish to express my thanks to my wife, Veronica, who has
painstakingly typed and checked the drafts of this book.
In tr odu ct ion :
The control of
corrosion in marine
1
environments
This book is concerned with the control of corrosion in marine
environments and in this chapter the general causes of corrosion and the
various means of controlling it will be considered in broad terms. Other
chapters deal with specific areas of corrosion and control in more detail.
Corrosion can be defined in a number of ways but ‘the chemical or
electrochemical reaction of a metal or an alloy with its environment’
provides a reasonable explanation of the term ‘corrosion’. It is one of the
two common causes of metal deterioration, the other being the
mechanical loss of the metal by erosion, abrasion or wear. Sometimes
there is a joint action of corrosion and erosion.
Although corrosion can sometimes be prevented, the aim is usually to
control it within economic limits. There are situations where no corrosion
at all is acceptable, but these are few. Generally, the choice ofmaterials is
based on regular maintenance during the life of the construction. This is a
sensible approach provided it is part of the design philosophy. Corrosion
can be a very serious problem when it occurs unexpectedly and
emergency measures have to be taken to deal with it.
Economic considerations are the essence of corrosion control
procedures, but not necessarily in a straightforward way. Designers and
engineers should be aware of the economics of corrosion but they cannot
necessarily make the final decision in these matters. Other personnel are
involved, not least accountants, and hopefully the corrosion require-
ments will properly be considered in relation to other factors. Never-
theless, the deciding factor may be the strength of a structure, time for
construction, availability of materials and, of course, capital cost.
Therefore corrosion control is a matter ofoptions to fit in with the many
other requirements to be taken into account by the design team. Often
corrosion problems arise because, due to changes in design that often take
place in the early stages of a project, the materials or coatings originally
selected may not be suitable at a later stage, where the conditions and
environments may have changed. Furthermore, on large structures and
1
2 INTRODUCTION: THE CONTROL OF CORROSION
plant, different parts may be designed by different teams. If there is not
close contact between them, corrosion and, incidentally, other problems
may well occur.
Generally, the overall corrosion requirements can be handled in a
fairly straightforward way. The solutions may be a compromise but if the
materials or coatings chosen are known to have limitations, this can be
taken into account in the maintenance or monitoring procedures. More
serious problems arise where changes are made, of which those concerned
or responsible for the corrosion elements in the design are not aware. A
change to a more corrosion-resistant alloy may be advantageous for a
particular section of the plant or structure. However, the new alloy may
cause problems, e.g. bi-metallic corrosion, in another part of the plant.
Designers must take such matters into account.
The word 'design' will appear many more times in this book. Corrosion
control in marine engineering is basically a design problem, using the
term in its widest sense. It is not just a question of choosing resistant
materials or very durable coatings, it is a matter of fitting together all the
different elements to reduce corrosion to an acceptable and economic
level. Additionally it concerns the choice and selection of materials and
coatings that will meet the requirements of both performance and cost.
The marine environment is a very aggressive one and new materials
will be developed to provide improved properties, including corrosion
resistance, for the constructions and plant that will be required as more
demands are made upon the sea's resources. It is, therefore, necessary to
maintain a close contact with such developments. The basic principles of
corrosion and its control, however, remain largely unchanged; it is their
application to new problems that will result in improvements both of an
economic and technical nature.
1.1 THE CORROSION PROCESS
Corrosion is an electrochemical process and is discussed in detail in
Chapter 2. To assist in the explanation of control methods, a summary of
the basic reactions will be considered. The process can be broken down
into anodic and cathodic reactions. If one of these reactions is controlled
then the overall rate of corrosion is affected. It is important to appreciate
the electrochemical nature of corrosion because the control methods are
based on altering or stifling one or other, or both, of the electrode
processes. If the corrosion of carbon steel is considered in very simple
terms it can be explained as follows. The steel is not homogeneous and at
the initiation of corrosion anodic and cathodic sites are formed on the
surface of the alloy. In the presence of an electrolyte, small corrosion cells
are set up on the surface and at the anodic areas iron goes into solution as
INTRODUCTION: THE CONTROL OF CORROSION 3
ferrous ions, i.e. the steel corrodes. Various reactions can occur at the
cathode. The cathodic reaction under ordinary atmospheric or immersed
conditions results in the production of hydroxyl ions and the two
reactions can be written as follows:
Anode: 2Fe -► 2Fe2+ + 4e~
Cathode: 0 + 2H 0 + 4e" -> 40H"
2 2
The two products Fe2+ and OH~ react together to form ferrous
hydroxide which eventually is oxidised further to rust :
Fe2+ + 20H- -+ Fe(OH) -+ FeOOH (rust)
2
Either the anodic or cathodic process can be controlled. If rust is formed
at the steel surface, it will stifle the process so, despite the availability of
moisture and oxygen, the corrosion rate may be reduced. In practice, on
carbon steels, the anodic process at one place is stifled and corrosion
begins at new sites—hence the general nature of the corrosion. With
other alloys, e.g. stainless steel, the anodic reaction may continue for some
time at the original site and result in localised pitting. However, under
most circumstances, there are many fewer anodic sites on stainless steel
than on carbon steel, so the overall corrosion is much less.
Most corrosion is of the general type and arises from the rusting of steel,
the most widely used constructional alloy. The general methods for
controlling it are considered below and, of these, coating the steelwork is
the most commonly used. To achieve reasonable standards of protection
by coatings does not generally require a very deep knowledge of
corrosion. However, particularly in marine environments, an under­
standing of corrosion processes does assist engineers to appreciate the
importance of matters such as surface preparation of steelwork and the
choice of suitable coatings.
When considering other forms of attack such as pitting and bimetallic
corrosion, an appreciation of corrosion processes is important. Many
simple general corrosion concepts may turn out to be either incorrect in
specific situations or not quite as simple as had been anticipated. For
example, it is generally considered that reinforcements in concrete do not
corrode because concrete provides an alkaline environment, which is
non-corrosive to steel. This is generally true, but if chloride ions—in
plentiful supply in marine conditions—reach the steel, they can in fact
affect its passivity, causing attack.
Again although in theory two metals joined together might be
expected to lead to serious bimetallic corrosion of one of them, this may
not necessarily happen in practice because of various effects such as the
polarisation of the anodic and cathodic sites, the nature of the
environment or the relative areas of the two metals.
4 INTRODUCTION: THE CONTROL OF CORROSION
1.2 CORROSION CONTROL
As corrosion results from a reaction between an alloy and its environ­
ment, the basic control methods are concerned with treating the
environment or selecting suitable alloys. The methods can be grouped
into four main categories, irrespective of which of the two factors—
environment or material—are of major importance.
(i) Use of coatings.
(ii) Selection of materials that will resist a particular environment.
(iii) Cathodic protection.
(iv) Control or treatment of the environment.
A fifth method really covers the proper application of the other
methods in practice but may be considered as a method of control, i.e.
design.
If corrosion control is considered to be more than a technical approach
to the problem, then other matters related to the maintenance of the
structural integrity or operational efficiency must be considered. This
would include requirements for access to carry out maintenance and the
overall design in relation to the successful application of the control
procedures.
Often more than one method of control may be employed and this may
be advantageous from the economic standpoint.
1.2.1 Coatings
Coatings used to protect carbon steel in marine environments are
considered in detail in other parts of the book. A few fundamental points
concerning them are, however, worth considering. Coatings are the most
common method by far, in terms of steel tonnage, for controlling
corrosion. The basic concept is not usually electrochemical in nature, it is
simply to insulate the alloy from the corrosive environment. In practice,
however, many problems arise and often insufficient attention is paid to
them because of the apparently simple nature of coating systems. A range
of different types of coating is available, the most common of which are
paints although metal coatings, plastics, waxes, greases and other
materials are also used.
Organic coatings are basically of two types : those that are applied
manually, e.g. by spraying or brushing, and those that are applied in
works, e.g. plastisols. All organic coatings are permeable to moisture and
oxygen to a greater or lesser extent and to overcome this, there has been a
growing tendency to specify thicker coatings for marine situations. This
has been made possible by the nature of coatings such as epoxies and