Corrosion in the Petrochemical Industry

263 Pages·1994·20.093 MB·English

by  Garverick, Linda (Eds.)

Corrosion in the
Petrochemical Industry
Edited by
Linda Garverick
Essential Research
Scott D. Henry, Manager of Handbook Development
William W. Scott, Jr., Director of Technical Publications
The Materials
lntormatlbn Soclety
Copyright 0 1994
by
ASM International@
All rights reserved
No part of this book 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 written permission of the copyright owner.
First printing, July 1994
Second printing, May 1995
Third printing, December 1999
Digital printing, June 201 1
This book is a collective effort involving hundreds of technical specialists. It brings together a wealth of information
from worldwide sources to help scientists, engineers, and technicians solve current and longrange problems.
Great care is taken in the compilation and production of this book, but it should be made clear that NO
WARRANTIES, EXPRESS OR IMPLIED, INCLUDING, WITHOUT LIMITATION, WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, ARE GIVEN W CONNECTION WITH
THIS PUBLICATION. Although this information is believed to be accurate by ASM, ASM cannot guarantee that
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Comments, criticisms, and suggestions are invited, and should be forwarded to ASM International
Library of Congress Cataloging-in-Publication Data
Corrosion in the Petrochemical Industryledited by Linda Garverick
p. cm.-Includes bibliographical references and index.
1. Petroleum chemicals industry-Equipment and supplies-Corrosion
I. Garverick, Linda
TP690.8.C68 1994
665.5'028-dc20
ISBN-1 3: 978-0-8717 0-505- 1
ISBN-10: 0-87170-505-2
SAN: 204-7586
ASM International@
Materials Park, OH 44073-0002
www.a sminternational. org
Printed in the United States of America
Preface
Corrosion in the Pehochemical Industry collects together authoritative, peer-reviewed data and informationo n corrosion
in the petroleum, petrochemical, and chemical processing industries from a number of ASM Internationalp ublications. The
principal sources are Corrosion, Volume 13, and Failurn Analysis andPrevention, Volume 11, of AS~~andbouOkt.h er
SOWS include the Handbook of Currosion Data; Handbook of Care HWories in Failure Analysis, Volumes 1 and 2; and
Metadex, the Materials Information database of materials abstracts. This information has been carefully selected and edited
into one cohesive, comprehensive volume on corrosion phenomena in these industries.
This project was initiated because survey results and other input from ASM members and book buyers indicated a strong
interest in collections of ASM data and infomation repackaged into logical, convenient new configurations. Based on this
inkrest, a team of ASM staff members was assembled to gather additional input and then use it to crafi a publication that is
highly responsive to the demands of its intended audience. This team effort was led by Grace Davidson, with team members
Steve Chang, Karen Germany, Scott Henry, Rich Majoros, Robert Pavlik, Dawn Summerlin, and Reva Zaretsky; Robert Uhl,
Dmctor of ASMs Education Department, sponsored the project.
The team chose the subject of Corrosion because of the great amount ofhigh-quality ASM published infomation available
on this subject. The team conducted over 100 in-depth phone interviewsw ith ASM book buyers who work or have an interest
in the area of corrosion. The results of these phone interviews were then used as input for detailed quality hction
deployment matrices. (Quality function deployment is a systematic method for organizing and analyzing customer demands
in order to develop products that are responsive to those demands.) Out of the quality function deployment process, the team
developed three potential outlines, each with a sepamte area of focus in the field of corrosion. The team then went back to a
small group of respondents to the original survey and asked them to review and comment on the outlines, and to rate them
according to their apparent usefulness to a clearly defined audience.
The outliie selected by this group was the one used to develop this voIume. The outline and survey data were turned over
to Linda Garverick, who has done a mastea job of compiling and editing the information to turn it into a cohesive,
well-rounded publication. ASM International owes a debt of gratitude to the survey respondents who gave so generously of
their time, opinions, and expertise to help develop this handbook. Of course, this book would not have been possible without
the efforts of the dozens of experts who wrote and reviewed the information it contains.
iii
Contents
Forms ofC orrosion in the Petrochemical Industry Case Histories in Failure Adysis ............................................... 225
General Corrosion ............................................................................ 3 Literatme Sources. ....................................................................... 247
Localized Corrosion ....................................................................... 14
Metallurgically Influenced Corrosion ............................................ 25 Corrosion in Petroleum Production Operations
Mechanically Assisted Degradation ............................................... 34 Causes of Corrosion ..................................................................... 259
Environmentally Induced Cracking ............................................... 39 Corrosion Control Methods ......................................................... 266
Corrosion Control in Field Operations ......................................... 278
Corrosion Proteetion Methods for the Petrochemical Industry Failures in Field Enviro~e.n... ................................................. 291
Fundamentalso f Corrosion Protection in Aqueous Industry Standards ....................................................................... 292
Solutions ...................................................................................... 77 Case Histories in Failure Analysis ............................................... 297
Anodic and Cathodic Protection .................................................... 80 Literature Sources ........................................................................ 313
Use of Inhibitors. ............................................................................ 88
Control of EnvironmentalV ariables in Water Corrosion in Petroleum Retiniig and Petrochemical Operations
Recirculating Systems. ................................................................. 96 Materials Selection ...................................................................... 321
Low- and High-Temperature Corrosion ...................................... 325
Designing to Minimize Corrosion SCC and Embrittlement ............................................................... 334
Materials Selection. ...................................................................... 113 Erosion-Corrosion ....................................................................... 342
Corrosion of Weldments ............................................................... 126 Corrosion Control ........................................................................ 343
Design Details to Minimize Corrosion ......................................... 151 Case Histories in Failure Analysis ............................................... 349
Literature Sources. ....................................................................... 363
Corrosion in the Chemical Processing Industry
Corrosive EfTects of Process and Enviro~entaVl ariables ......... 164 Corrosion of Petrochemical Pipelines
Cleaning Process Equipment ....................................................... 166 Causes of Pipeline Corrosion. ...................................................... 371
Corrosion Under Thermal Insulation ........................................... 173 Corrosion Control and Prevention ............................................... 372
Corrosion by Sulfuric Acid ........................................................... 178 Corrosion of Specific Types of Pipelines ..................................... 375
Corrosion by Nittic Acid .............................................................. 184 Case Histories in Failure Analysis ............................................... 379
Corrosion by Organic Acids ......................................................... I 86 Literature Sources. ....................................................................... 389
Corrosion by Hydrogen Chloride and Hydrochloric Acid ........... 19 1
Corrosion by Hydmgen Fluoride and Hydrofluoric Acid ............ 196 Corrosion Data ................................................................................ 397
..................................................................
Corrosion by C ~ ~ o ~ e 201
Corrosion by Alkalies and Hypochlorite ...................................... 204 Glossary of Terms ........................................................................... 453
Corrosion by Ammonia ................................................................ 212
Corrosion Failures of Pressure Vessels ......................................... 2 14 Index ................................................................................................ 461
Forms of Corrosion in the
Petrochemical Industry
General Corrosion ............................................................................................................................................................ 3
.......................................................................................................................................................
Localized Comsion 14
Metallurgically Influenced Corrosion ............................................................................................................................ 25
Mechanically Assisted Degradation ............................................................................................................................... 34
Environmentally Induced Cracking ............................................................................................................................... 39
Forms of Corrosion in the
Petrochemical Industry
OVER THE YEARS, corrosion scientists and system, these categories are not distinct or all-in- Many authors classify atmospheric corro-
eng~eersha ve recognized that corrosion mani- clusive and do not necessarily represent the only sion under categories of dry,d amp, and wet, thus
fests itself in forms that have certain similarities mode of attack that may be observed. emphasizing the different mechanisms of attack
and therefore can be categorized into specific under increasing humidity or moisture.
groups. However, many of these forms are not
unique but involve mechanisms that have over- General Corrosion Types of Atmospheric Corrosion
lapping characteristics that may influence or
control initiation or propagation of a specific Dry Corrosion. In the absence of moisture,
type of corrosion. General corrosion, as described in this article, most metals corrode very slowly at ambient tem-
The most familiar and often used categoriza- refers to corrosion dominated by uniform thin- peratures. Accelerated corrosion under dry con-
tion of corrosion is probably the eight forms pre- ning that proceeds without appreciable localized ditions at elevated temperatures is covered in the
sented by Fontana and Greene (Ref I): uniform attack. Weathering steels and copper alloys are section High-Temperature Corrosion” in this
“
attack, crevice corrosion, pitting, intergranular good examples of materials that typically exhibit article. Dry corrosion at ambient temperature oc-
corrosion, selective leaching, erosion corrosion, general attack, while passive materials, such as curs on metals that have a negative free energy of
stress corrosion, and hydrogen damage. This stainless steels or nickel-chromium alloys, are oxide formation and thus form a rapid thermody-
classification of corrosion was based on visual generally subject to localized attack. Under spe- namically stable film in the presence of oxygen.
characteristics of the morphology of attack. Fon- cific conditions, however, each material may Qpically, these films are desirable because they
tana and Oreene’s introductory remarks in their vary from its normal mode of corrosion. Exam- are defect free, nonporous, and self-healing and
chapter on forms of corrosion indicate that this ples describing the environmental conditions act as a protective barrier to further corrosive at-
classification is arbitrary and that many of the that promote uniform attack will be discussed tack of the base metal. Metals such as stainless
forms are interrelated, making exact distinction throughout this article. steels, titanium, and chromium develop this type
impossible. Other prominent corrosion authors In this section, the four specific types of gen- of protective film. Porous and nonadhering films
such as Uhlig (Ref 2) and Evans (Ref 3) have eral corrosion most relevant to the petrochemical that form spontaneously on nonpassive metals as
avoided a classification format and have simply industry are discussed. Atmospheric corrosion is unalloyed steel are normally not desirable.
discussed the classical types of corrosion (for ex- probably the most common form of corrosion Tarnishing of copper and silver in dry air with
ample, pitting and crevice corrosion) as they re- and may well be the most costly. Galvanic corro- traces of hydrogen sulfide (H2S) is an example of
late to specific metals and alloys. sion is an electrochemical form of corrosion that a nondesirable film formation at ambient tem-
Substantial advances in the field of corrosion protects cathodic areas at the expense of anodic peratures caused by lattice diffusion. For tarnish-
science have begun to define the mechanisms of areas. Stray-current corrosion is similar to gal- ing to occur, sulfur impurities must he present,
many forms of corrosion more clearly. However, vanic corrosion, but does not rely on electro- The sulfides increase the likelihood of defects in
rather than placing the mechanisms into distinct chemically induced driving forces to cause rapid the oxide-lattice and thus destroy the protective
categories, the overlap between many of the attack. High-temperature (gaseous) corrosion is nature of the natural film, which leads to a tar-
forms has become greater. For example, there is an area of great concern, particularly for the in- nished surface. Surface moisture is not necessary
evidence that hydrogen may dominate the crack dustrial sector. for tarnishing to occur, and in some cases, such as
initiation or crack propagation portion of fracture As noted in the introduction, some of the cate- copper in the presence of trace amounts of H2S,
in some metal/solution systems where stress-cor- gories of general corrosion described in this arti- moisture can actually retard the process of tar-
rosion cracking occurs. Additionally, in some cle also manifest themselves as other forms of nishing. In general, dry corrosion plays an insig-
metal systems where dealloying (selective leach- corrosive attack, such as stress-corrosion crack- nificant part in atmospheric corrosion as a whole.
ing) occurs, this form of corrosion may be a pre- ing, dealloying, or pitting. However, because Damp corrosion requires moisture in the at-
cursor to stress-corrosion cracking. uniform thinning plays an important role in all of mosphere and increases in aggressiveness with the
In a similar vein, the magnitude of contribu- the categories described, each can and will be moisture content. When the humidity exceeds a
tion of stress or corrosion to stress-corrosion discussed under general corrosion. critical value, which is around 70% relative humid-
cracking, hydrogen damage, or liquid metal em- ity, an invisible thin film of moisture will form on
brittlement is not currently understood and can Atmospheric Corrosion the surface of the metal, providing an electrolyte for
affect whether just pitting or crevice attack oc- current transfer, The critical value depends on sur-
curs or environmental cracking results. The tran- Atmospheric corrosion is defined as the face conditions such as clediess,c orrosion prod-
sition from uniform corrosion to highly localized corrosion or degradation of material exposed uct buildup, or the presence of salts or other
attack is not clearly understood, and there are to the air and its pollutants rather than im- contaminants that are hygroscopic and can absorb
conditions where a distinction cannot be drawn. mersed in a liquid. This has been identified as water at lower relative humidities.
The forms of corrosion presented in this article one of the oldest forms of corrosion and has Wet corrosion occurs when water pockets or
were categorized to represent the mechanisms of at- been reported to account for more failures in visible water layers are formed on the metal sur-
tack involved rather than to emphasize the visual terms of cost and tonnage than any other single faces because of sea spray, rain, or drops of dew.
characteristics. However, as with any classification environment. Crevices or condensation traps also promote the
4 / Corrosion in the Petrochemical Industry
rig. 3 Corroded weathering steel gutter. Courtesyof R.H.
Heidersbach, California Polytechnic State University
rig.1 Corroded steel formwork on the ceiling of a park-
ing garage. The seams in this corrugated structure
act as condensation traps and lead to wet atmospheric cor-
rosion. Courtesy of R.H. Heidersbach, California Polytech-
nic State University
rig.5 Corroded regions on a painted highway bridge.
Courtesy of R.H. Heidersbach, California Poly-
technic State University
rig.2 Corroded weathering steel I-beam. Note how cor-
rosion has thinned the bottom of the vertical web
where corrosion products have fallen and formed a moist
corrosive deposit. Courtesy of R.H. Heidersbach, California
Polytechnic State University
pooling of water and lead to wet atmospheric cor-
rosion even when the flat surfaces of a metal ~ i6 D~elam.ina tion of plain carbon steel due to SO, +
component appear to be dry (Fig. I). ash deposit outside the boiler area of a coal-fired
During wet corrosion, the solubility ofcorro- Fig. 4 Corroded weathering steel highway bridge girder. power plant. Courtesy of D.M. Berger, CilbedCommon-
sion product can affect the corrosion rate. Typi- Courtesyof D. Manning, Ontario Ministry of High- wealth
waysand Communications
cally, when the corrosion product is soluble, the
corrosion rate will increase. This occurs because Small additions of copper (0.1%) will in-
the dissolved ions normally increase the conduc- ronment. For example, steel pillars 25 m (80 ft) crease the resistance of steel to a sulfur polluted
tivity of the electrolyte and thus decrease the in- from the seacoast will corrode 12 times faster environment by enhancing the formation of a
ternal resistance to current flow, which will lead than the same steel pillars 250 m (800 ft) further tighter, more protective rust film. Additions of
to an increased corrosion rate. Under alternating inland. The level of marine salts found at the two nickel and chromium will accomplish the same
wet and dry conditions, the formation of an insol- locations can explain the difference in the ob- end. Nickel and copper alloys form insoluble sul-
uble corrosion product on the surface may in- served corrosion rates. More detailed informa- fates that help to protect the base metal and are
crease the corrosion rate during the dry cycle by tion on marine atmospheres and their effect on therefore used extensively in industrial environ-
absorbing moisture and continually wetting the the corrosivity of metals and alloys can be found ments. The remarkable longevity of ancient iron
surface of the metal. in the article Marine Corrosion" in Volume 13 is probably due to a S02-free atmosphere rather
"
The rusting of iron and steel and the forma- of the ASM Handbook. than a high degree of resistance to general corro-
tion of patina on copper are examples of metals Industrial atmospheres are more corrosive than sive attack.
experiencing either damp or wet atmospheric rural atmospheres, primarily because of the sulfur Other major contaminants that promote at-
corrosion. Figures I to 5 show examples of the compounds produced during the burning of fuels. mospheric corrosion are nitrogen compounds,
damp/wet atmospheric corrosion of weathering Sulfur dioxide (SO2)i s selectively absorbed on H2S, and dust particles. Nitrogen compounds oc-
steel components. metal surfaces, and under humid conditions the cur naturally during thunderstorms and are added
metal oxide surfaces catalyze the SO2 to sulfur tri- to the environment by the use of ammonia (NH3)
Atmospheric Contaminants oxide (SO3) and promote the formation of sulfuric base fertilizers. Hydrogen sulfide can be gener-
acid (H2S04) according to the reaction H20 + ated naturally by the decomposition of organic
Wet atmospheric corrosion is often controlled SO3 + H2S0,. An example of SOz-induced cor- sulfur compounds or by sulfate-reducing bacte-
by the level of contaminants found in the envi- rosion of plain carbon steel is shown in Fig. 6. ria (SRB) in polluted rivers. Detailed informa-
Forms of Corrosion in the Petrochemical Industry / 5
tion on SRB and their effect on alloy corrosion this temperature, the corrosion rate will de- relative humidity for copper, nickel, and zinc also
behavior can be found in the section “Localized crease quickly. appears to be between SO and 70$, depending on
Corrosion” in this article {see the discussion Climatic Conditions. Metal surfaces located surface conditions.
“Microbiological Corrosion” ). in areas where they become wet and retain mois- The nature of the corrosion product can
Dust particles can be very detrimental to cor- ture generally corrode more rapidly than surfaces greatly affect the time of wetness. If the corro-
rosion-resistant metals by adhering to the surface exposed to rain. The rain has a tendency to wash sion product film is microporous in nature, capil-
and absorbing water or H2S04, and trapping the the surface and remove particles of dust that can lary condensation can cause the condensation of
solution against the surface. Dust particles may lead to differential aeration corrosion. Excep- moisture well below the critical relative humidity
also contain contaminants, such as chlorides, that tions would be in areas that are subject to acid value. This occurs because of the differences in
can break down protective surface films and thus rain. Exposure of metals in different months of vapor pressure as measured over a curved surface
initiate corrosion. the year can have a pronounced effect on the cor- as compared to a flat surface. For example, a 1. S-
Oxygen is not considered a contaminant, but rosion rate. Winter exposure is usually the most nm capillary will condense moisture at 50% rela-
is an essential element of the corrosion process. severe because of increased combustion products tive humidity; a 36-nm capillary will condense
The normal cathodic reaction is the reduction of in the air. The presence of SO2 and other sulfur moisture at 98% relative humidity. This phe-
oxygen. In polluted areas with high concentra- pollutants leads to an aggressive environment nomenon accounts for the formation of electro-
tions of SO2, the pH of the surface electrolyte and the formation of a less protective corrosion lyte in microcracks and in contact angles
may be low enough so that hydrogen reduction is product film on normally passive metals. One between dust particles and metal surfaces. The
the principal cathodic reaction. Once a suitable should be cautious regarding the month in which condensation of moisture on a metal surface can
surface electrolyte has been formed by water va- outdoor corrosion tests are performed. In loca- also be enhanced by the formation of a saturated
por, oxygen will dissolve in the electrolyte solu- tions in which sulfur-containing fuel is not burned solution, which will lower the equilibrium vapor
tion and promote the cathodic reaction. Because during the winter months, the summer months pressure and allow condensation below 100%
the water layer on the surface of the metal is ex- may sometimes lead to higher rates of attack be- relative humidity.
tremely thin, the diffusion of oxygen to the sur- cause of the increased surface temperatures. Dew formation on metal surfaces can lead to
face of the metal occurs very rapidly and does not Time of Wetness/Relative Humidity. Time accelerated corrosion because of the tendency of
slow the corrosion rate. of wetness is a critical variable with respect tothe the dew to be acidic as a result of high SO2 values
Carbon dioxide (C02) does not play a signifi- extent of corrosion experienced. The time of wet- near the ground. The dew can form on open or
cant role in atmospheric corrosion, and in some ness determines the duration of the electrochemi- sheltered surfaces and leads to a corrosive attack
cases, it will actually decrease corrosion attack. cal process. The thickness and the chemical of galvanized sheet called white rusting.
composition of the water film are both important. The thickness of the electrolyte layer is also
Atmospheric Variables The critical relative humidity is the humidity an important factor in the corrosion process.
below which water will not form on a clean metal Water begins to adhere to a polished metal sur-
Atmospheric variables such as temperature, surface and thus electrochemical or wet com- face at an estimated 55% relative humidity and
climatic conditions, and relative humidity, as sion will not occur. The actual relative humidity will form a thin film, which will increase in thick-
well as surface shape and surface conditions that will change depending on the surface condition ness as the relative humidity increases. The thin
affect the time of wetness, are important factors of the metal. water layers can support an e1ectrochemical reac-
that influence the rate of corrosive attack. Addi- For iron, the critical relative humidity appears tion, but polarization of the cathodic and anodic
tional information concerning variables is avail- to be about 60%; at this level, rust slowly begins sites slows the process as the film thickness de-
able in the article “Effects of Environmental to form. At 75 to 80%r elative humidity, there is a creases and virtually stops at about 60%, the
Variables on Aqueous Corrosion’’ in Volume 13 sharp increase in corrosion rate that is speculated critical relative humidity value. The corrosion
of the ASM Handbook. to occur because of the capillary condensation of rate on a surface reaches a maximum when the
Surface temperature is a critical variable. As moisture within the rust corrosion product layer. water film thickness is above 150 pm. Therefore,
the surface temperature increases, the corro- At 90% relative humidity, there is another in- not only is the time of wetness an important pa-
sion rate will rise sharply to the point at which crease in the corrosion rate corresponding to the rameter but the thickness and conductivity of the
evaporation of the electrolyte takes place. A1 vapor pressure of ferrous sulfate. The critical surface electrolyte must also be known.
Atmospheric Corrosion of Specific
Metal Systems
Table 1 Average atmospheric-corrasion rates of various metals for 10- and 20-year exposure
times
Irons and steel, zinc, copper, nickel, and alu-
Corrosion rates are given in miis/yr (1 mil/yr= 0.025 mdyr).V alues cited are one-half reduction of specimen thickness.
minum are the metal systems of major economic
importance when dealing with atmospheric cor-
Amsphere rosion. Table 1 provides an overview of atmos-
New York, NY la Job, CA State College, PA pheric corrosion rates of various metals and
(ur~n.indus~~l) (marine) (rural) alloys. Metals that are not particularly resistant to
Metal 10 20 10 20 10 20 dilute H2S04 snch as copper, cadmium, nickel,
~luminum 0.032 0.029 0.028 0.025 0.001 0.003 and iron, show more rapid attack in industrial en-
Copper 0,047 0.054 0.0.52 0.050 0.023 0.017 vironments. Metals and alloys that are more re-
Lead 0.017 0.01.5 0.016 0.021 0.019 0.013 sistant to H2S04, such as lead, aluminum, and
Tin 0.047 0.0.52 0.09 1 0.112 0.018 ,.* stainless steels, are less affected in the industrial
Nickel 0.12% 0.144 0.004 0.006 0.006 0‘009
65%Ki,32%Cu,2%Fe,l%Mn 0.053 0.062 0.007 0.006 0.005 0.m environments. Copper forms a protective sulfate
(Monel) patina and is therefore more resistant than nickel.
zinc (99.9%) 0,202 0.226 0.063 0.069 0.034 0.044 Copper also forms a basic copper chloride in sea-
Z0.i2nc% (C9 9 S.0te%el)( a) (0.02%P ,0 .05%S , 00..4189 3 0..2.1.8 0..0..6 9 0..0..6 8 0..0.4. 2 0..0..4 3 coast environments. Nickel is very important in
marine atmospheres, but is sensitive to the
0.05%C u. 0.02% Ni, 0.02%C t)
Low-alloy steelja) (0.1% C, 0.2% P, 0.09 ... ... ... H2SO4 found in the industrial env~onments.
.I. 1..
0.04~~,0.03%1.N1%~C r,0.4%Cu) Low-alloy steels that resist atmospheric cor-
rosion are called weathering steels, These al-
(a) kamey, NJ (near New York City). Source: Ref 4
loyed steels form a protective rust film in
6 / Corrosion in the Petrochemical Industry
alternating wet and dry environments. The conducting nonmetal in the same electrolyte. The force series.Therefore, tabulationssuchas Table
weathering steels do not perform well under con- three essential components are: 2 can be very useful.
ditions of burial or total immersion, The atmos- Materials possessing different surface potential
8
pheric attack of wrought iron can sometimes A common electrolyte Polarization
progress along the internal planes formed during A common electrical path
rolling and cause swelling of the material. For As stated above, electron flow occurs be-
this reason, it is best not to cut across the grain A mixed metaJ system in a common elecirolyte that tween metals or alloys in a galvanic couple. This
boundaries and leave the face exposed to the en- is electrically isolated will not experience galvanic current flow between the more active and more
vironment. Stainless steels and aluminum alloys corrosion, regardless of the proximity of the metals noble members causes shifts in potential due to
are normally very resistant to atmospheric condi- or their relative potential or size. polarization, because the potentials of the metals
tions and will resist tarnishing in industrial, ur- During galvanic coupling, corrosion of the or alloys tend to approach each other.
ban, and rural environments. less corrosion-resistant metal increases and the The magnitude of the shift depends on the en-
Lead, aluminum, and copper corrode initially, surface becomes anodic, while corrosion of the vironment, as does the initial potential. If the
but form a protective film. In an urban atmos- more corrosion-resistant metal decreases and the more noble metal or alloy is more easily polar-
phere, nickel does not form a completely protec- surface becomes anodic. The driving force for ized, its potential is shifted more toward the more
tive film and will experience a paraholic corrosion corrosion or current flow is the potential devel- active metal or alloy potential. The shift in poten-
rate. Zinc attack appears to be linear after an initial oped between the dissimilar metals. The extent of tial of the more active metal or alloy in the direc-
period of decreasing corrosion rate. The corrosion accelerated corrosion resulting from galvanic tion of the cathode is therefore minimized so that
rate of steel depends on the alloying elements coupling is affected by the following factors: accelerated galvanic corrosion is not as great as
typically attributed to the compact nature of the would otherwise be expected. On the other hand,
rust formed because of the alloying elements. The potential difference between the metals or when the more noble metal or alloy is not readily
8
Copper, lead, and nickel form sulfates on the alloys polarized, the potential of the more active metal
surface when attacked by dilute H2S04.T he lead The nature of the environment shifts further toward the cathode (that is, in the
forms a protective film, but the copper and The polarization behavior of themetals or alloys direction of anodic polarization) such that appre-
nickel will slough off after a period of time. The geometric relationship of the component ciable accelerated galvanic corrosion occurs.
I
The protective carbonate film on zinc and cad- metals or alloys
mium is dissolved, and the metal is readily at- Area, Distance, and Geometric Mects
tacked. The oxide film on iron is formed by the The differences in potential between dissimi-
hydrolysis of the ferrous sulfate. More de- lar metals or alloys cause electron flow between Factors such as area ratios, distance between
tailed information on each of the metal sys- them when they are electrically coupled in a con- electrically connected materials, and geometric
tems mentioned above can be found in the ductive solution. The direction of flow, and shapes also affect galvanic-corrosion behavior.
Section “Specific Alloy Systems” in Volume 13 therefore the galvanic behavior, depends on Area effects in galvanic corrosion involve
of the ASM Handbook. which metal or alloy is more active. Thus, the the ratio of the surface area of the more noble to
more active metal or alloy becomes anodic, and the more active member(s). When the surface
Prevention of Atmospheric Corrosion the more noble metal or alloy becomes cathodic area of the more noble metal or alloy is large in
in the couple. The driving force for galvanic cor- comparison to the more active member. an unfa-
Two approaches can be taken to prevent the rosion is the difference in potential between the vorable area ratio exists for the prevailing situ-
onset of atmospheric corrosion. The first is a component metals or alloys. ation in which acouple is under cathodic control,
temporary fix that can be used during trans- The anodic current density on the more active
port or storage. This consists of lowering the Galvanic Series metal or alloy is extremely large; therefore, the
atmospheric humidity by using a desiccant, resulting polarization leads to more pronounced
heating devices, or by treating the surface A galvanic series of metals and alloys is use- galvanic corrosion. The opposite area ratio-
with a vapor phase or surface inhibitor. Per- ful for predicting galvanic relationships. Such a large active member surface, smaller noble mem-
manent solutions to atmospheric corrosion series is an arrangement of metals and alloys ac- ber surface-produces only slightly accelerated
can be accomplished by either changing the cording to their potentials as measured in a spe- galvanic effects because of the predominant po-
material or by applying a coating. Organic, in- cific electrolyte. The galvanic series allows one larization of the more noble material.
organic, and metallic coatings have been ef- to determine which metal or alloy in a galvanic Effect of Distance. Dissimilar metals in a
fectively employed. couple is more active. In some cases, the separa- galvanic couple that are in close physical prox-
When using an alloy steel, the addition of tion between the two metals or alloys in the gal- imity usually suffer greater galvanic effects than
small amounts of copper, phosphorus, nickel, and vanic series gives an indication of the probable those that are further apart. The distance effect is
chromium are particularly effective in reducing magnitude of corrosive effect. dependent on solution conductivity because the
atmospheric corrosion. It has been reported that The potential of a metal or alloy is affected path of current flow is the primary consideration.
copper additives are more effective in temperate by environmental factors. Corrosion product Thus, if dissimilar pipes are butt welded with the
climates than in tropical marineregions. The com- films and other changes in surface composi- electrolyte flowing through them, the most se-
bination of minor elements, such as the addition tion can occur in some environments; there- vere corrosion will occur adjacent to the weld on
of chromium and nickel with copper and phos- fore, no one value can be given for aparticular the anodic member.
phorus, appears to be very effective for all loca- metal or alloy, This requires a galvanic series Effect of Geometry. The geometry of the
tions. The effects of alloying additions in ferrous to be measured in each environment of inter- circuit also enters into the effect to the extent
alloys (wrought carbon, alloy, and stainless steels, est. Most commonly, however, the galvanic that current will not readily flow around comers.
and cast irons and steels) are detailed in the first series has been constructed from measure- This is simply an extension of the principle de-
five articles of the Section “Specific Alloy Sys- ments in seawater, as shown in Table 2. With scribed above, in which the current takes the
tems” in Volume 13 of the ASM Handbook. certain exceptions, this series is broadly ap- path of least resistance.
plicable in other natural waters and in uncon-
taminated atmospheres. Modes of Attack
Galvanic Corrosion Because most engineering materials are al-
loys, the measurement of galvanic corrosion em- Galvanic corrosion of the anodic member(s)
Galvanic corrosion occurs when a metal or al- ploying actual material is much more useful than of a couple may take the form of general or local-
loy is electrically coupled to another metal or predicting current flow from the electromotive ized corrosion, depending on the con~guration
Forms of Corrosion in the Petrochemical Industry / 7
Table 2 Galvanic series in seawater at 25 “C Potential measurementsa re made to con-
(77 “F) struct a galvanic series of metals and alloys, as
described above. As a first approximation, the
galvanic series is a useful tool. However, it has
Corroded end (anodic, or least noble) serious shortcomings. Metals and alloys that
Magnesium form passive films will exhibit varying potentials
(broken mill scale)
Magnesium alloys with time and are therefore difficult to position in
Zinc
the series with certainty. Also, the galvanic series
Galvanized steel or galvanized wrought iron
Alum~umal loys 5052,3co4,3o(E3, I100,6053, in this order ~ i7 S~chem.ati c showing how breaks in mill scale does not provide information on the polarization
Cadmium (Fe,O,) can lead to galvanic Corrosion of steel characteristics of the materials and so is not help-
Aluminum alloys 21 17,2017,2024, in this order ful in predicting the probable magnitude of gal-
Low-carbon steel vanic effects.
Wrought iron
Another example is the behavior of conduc- Measurement of galvanic currents between
Cast iron
Ni-Resist (hi~h-nickelc ast iron) tive films, such as mill scale (magnetite, Fe30,) coupled metals or alloys is based on the use of a
Qpe 410 stainless steel (active) or iron sulfides on steel, or of lead sulfate on lead. zero-resistance milliammeter. Zero-resistance
50-50 lead-tin solder Such films can be cathodic to the base metal ex- electrical continuity between the members of the
Type 304 stainless steel (active) posed at breaks or pores in the scale (Fig. 7) or galvanic couple is maintained electronically,
ryPe 316 stainless steel (active)
even to such extraneous items as valves or pumps while the resulting current is measured with the
Lead
Tin in a piping system. ammeter. Use of this technique shou~dta ke into
Copper alloy C28000 (Munu metal, 60% Cu) Metallic Coatings. Two types of metallic account certain li~i~tionFsir.s t, when localized
Copper alloy C67W( manganese bronze A) coatings are used in engineering design: noble corrosion such as pitting or crevice corrosion is
Copper alloys C W ,C4 6500, C46600,C 46700 (naval metal coatings and sacrificial metal coatings. possible in the galvanic couple, long induction
brass)
Nickel 200 (active) Noble metal coatings are used as barrier coatings periods may be required before these effects are
Inconel alloy 600 (active) over a more reactive metal. Galvanic corrosion of observed. Test periods must be of sufficientd ura-
Hastelloy B the substrate can occur at pores, damage sites, tion to take this effect into account. Also the
C h l ~2 ~ ! and edges in the noble metal coating. Sacrificial measured galvanic current is not always a true
Copper alloy C27000 (yellow brass, 65%C u)
metal coatings provide cathodic protection of the measure of the actual corrosion current, because
Copper alloys €44300, C44400,C W00 (admiralty brass)
Copper alloys C60800, C61400 {aluminum bronze) more noble base metal, as in the case of galva- it is the algebraic sum of the currents due to an-
Copper alloy C23W (red brass, 85% Cu) nized steel or Alclad aluminum. odic and cathodic reactions. When cathodic cur-
Copper CI loo0 (ETP copper) Cathodic Protection. Magnesium, zinc, and rents are appreciable at the mixed potential of the
Copper alloys C65i00, C655500 (silicon bronze) aluminum galvanic (sacrificial) anodes are used galvanic couple, the measured galvanic current
Copper alloy C7 1500 (copper nickel, 30470 Ni)
Copper alloy (592300, cast {leaded tin bronze G) in a wide range of cathodic protection applica- will be significantly lower than the true current.
Copper alloy C92200, cast (leaded tin bronze Mj tions. The galvanic couple of the more active Therefore, large differences between the true
Nickel 200 (passive) metal and a more noble structure (usually steel, corrosion rate calculated by weight loss and that
Inconel alloy 600 (passive) but sometimes aluminum, as in underground pip obtained by galvanic current measurements have
Monel alloy 400
ing) provides galvanic (cathodic) protection, while been observed.
Type 410 stainlcss steel (passive)
Type 304 stainless steel (passive) accelerated corrosion of the sacrificial metal (an- Polarizationm easurements on the mem-
Type 316 stainless steel (passive) ode) occurs. The section “Anodic and Cathodic bers of a galvanic couple can provide precise in-
Incoloy alloy 825 Protection” in the article “Comsion Protection formation concerning their behavior. The
Inconel alloy 625 Methods for the Pehrochemical Industry” in this polarization curves and the mixed potential for
Hastelloy t
Volume contains inf~ationon the principles and the galvan~callyc oupled metals in a particular
Chlorimet 3
Silver apptications of this method of corrosion prevention environment can be used to determine the magni-
Ti~ium and the selection of anode materials. tude of the galvanic-corrosion effects as well as
Graphite Metal Ion Deposition. Ions of a more noble the type of corrosion.
Gold
metal may be reduced on the surface of a more An important application in the use of polari-
Platinum
active metal-for example, copper on aluminum zation measurements in galvanic corrosion is the
Protected end (cathodic, or most noble) or steel, silver on copper. This process is also prediction of localized corrosion. Polarization
known as cementation, especially with regard to techniques and critical potentials are used to
aluminum alloys. The resulting metallic deposit measure the susceptibility to pitting and crevice
provides cathodic sites for further galvanic cor- corrosion of metals and alloys coupled in chlo-
of the couple, the nature of the films induced, and rosion of the more active metal. ride solution. In addition, this technique is valu-
the nature of the metals or alloys involved. Gen- able in predicting galvanic corrosion among
erally, there are five major categories. Predicting Galvanic Corrosion three or more coupled metals or alloys.
Dissimilar Metals. The combination of dis-
similar metals in engineering design by mechani- The most common method of predicting gal- Performance of Alloy Groupings
cal or other means is quite common-for vanic corrosion is by immersion testing of the
example, in heating or cooling coils in vessels. galvanic couple in the environment of interest. Light Metals. Magnesium occupies an ex-
heat exchangers or machinery. Such combina- Although very time consuming, this is the most tremely active position in most galvanic series
tions often lead to galvanic corrosion. desirable method of investigating galvanic corro- and is therefore highly susceptible to galvanic
Nonmetallic Conductors. Less frequently sion. Initially, screening tests are conducted to corrosion. It is widely used as a sacrificial anode
recognized is the inthence of nonmetallic con- eliminate as many candidate materials as possi- in cathodic protection.
ductors as cathodes in galvanic couples. Carbon ble. These screening tests consist of one or more Aluminum and its alloys also occupy active
brick in vessels is strongly cathodic to the com- of the following three electrochemical tech- positions in the galvanic series and are subject to
mon structural metals and alloys. Impervious niques: potential measurements, current meas- failure by galvanic attack (Fig. 8 and 9). In chlo-
graphite, especially in heat-exchanger applica- urements, and polarization measurements. ride-bearing solutions, aluminum alloys are sus-
tions, is cathodic to the less noble metals and al- Additional information can be found in the arti- ceptible to galvanically induced localized
loys. Carbon-filled polymers can act as noble cle “Evaluation of Galvanic Corrosion” in Vol- corrosion, especially in dissimilar-metal crev-
metals in a galvanic couple. ume 13 of the ASM H ~ ~ d ~ ~ ~ k . ices. In this type of environment, severe galvanic