Overview
Corrosion is the destructive attack of a material by reaction with its environment. The serious consequences of the corrosion process have become a problem of worldwide significance. In addition to our everyday encounters with this form of degradation, corrosion causes plant shutdowns, waste of valuable resources, loss or contamination of product, reduction in efficiency, costly maintenance, and expensive overdesign; it also jeopardizes safety and inhibits technological progress. The multidisciplinary aspect of corrosion problems combined with the distributed responsibilities associated with such problems only increase the complexity of the subject. Corrosion control is achieved by recognizing and understanding corrosion mechanisms, by using corrosion- resistant materials and designs, and by using protective systems, devices, and treatments. Major corporations, industries, and government agencies have established groups and committees to look after corrosion-related issues, but in many cases the responsibilities are spread between the manufacturers or producers of systems and their users. Such a situation can easily breed negligence and be quite costly in terms of dollars and human lives.
 
The Cost of Corrosion
Although the costs attributed to corrosion damages of all kinds have been estimated to be of the order of 3 to 5 percent of industrialized countries’ gross national product (GNP), the responsibilities associated with these problems are sometimes quite diffuse. Since the first significant report by Uhlig1 in 1949 that the cost of corrosion to nations is indeed great, the conclusion of all subsequent studies has been that corrosion represents a constant charge to a nation’s GNP.2 One conclusion of the 1971 UK government-sponsored report chaired by Hoar3 was that a good fraction of corrosion failures were avoidable and that improved education was a good way of tackling corrosion avoidance.

Corrosion of metals cost the U.S. economy almost $300 billion per year at 1995 prices.4 Broader application of corrosion-resistant materials and the application of the best corrosion-related technical practices could reduce approximately one-third of these costs. These estimates result from a recent update by Battelle scientists of an earlier study reported in 1978.5 The initial work, based upon an elaborate model of more than 130 economic sectors, had revealed that metallic corrosion cost the United States $82 billion in 1975, or 4.9 percent of its GNP. It was also found that 60 percent of that cost was unavoidable. The remaining $33 billion (40 percent) was said to be “avoidable” and incurred by failure to use the best practices then known. In the original Battelle study, almost 40 percent of 1975 metallic corrosion costs were attributed to the production, use, and maintenance of motor vehicles. No other sector accounted for as much as 4 percent of the total, and most sectors contributed less than 1 percent. The 1995 Battelle study indicated that the motor vehicles sector probably had made the greatest anticorrosion effort of any single industry. Advances have been made in the use of stainless steels, coated metals, and more protective finishes. Moreover, several substitutions of materials made primarily for reasons of weight reduction have also reduced corrosion. Also, the panel estimated that 15 percent of previously unavoidable corrosion costs can be reclassified as avoidable. The industry is estimated to have eliminated some 35 percent of its “avoidable” corrosion by its improved practices. Table I.1 summarizes the costs attributed to metallic corrosion in the United States in these two studies.
 
Costs Attributed to Metallic Corrosion in the United States
  1975 1995
All industries    
Total (billions of 1995 dollars) $82.5 $296.0
Avoidable $33.0 $104.0
Avoidable 40% 35%
Motor vehicles    
Total $31.4 $94.0
Avoidable $23.1 $65.0
Avoidable 73% 69%
Aircraft    
Total $3.0 $13.0
Avoidable $0.6 $3.0
Avoidable 20% 23%
Other industries    
Total $47.6 $189.0
Avoidable $9.3 $36.0
Avoidable 19% 19%
Examples of Catastrophic Corrosion Damage
Sewer explosion, Mexico
An example of corrosion damages with shared responsibilities was the sewer explosion that killed over 200 people in Guadalajara, Mexico, in April 1992.6 Besides the fatalities, the series of blasts damaged 1600 buildings and injured 1500 people. Damage costs were estimated at 75 million U.S. dollars. The sewer explosion was traced to the installation of a water pipe by a contractor several years before the explosion that leaked water on a gasoline line laying underneath. The subsequent corrosion of the gasoline pipeline, in turn, caused leakage of gasoline into the sewers. The Mexican attorney general sought negligent homicide charges against four officials of Pemex, the government-owned oil company. Also cited were three representatives of the regional sewer system and the city’s mayor.
 
Loss of USAF F16 fighter aircraft
This example illustrates a case that has recently created problems in the fleet of USAF F16 fighter aircraft. Graphite-containing grease is a very common lubricant because graphite is readily available from steel industries. The alternative, a formulation containing molybdenum disulphide, is much more expensive. Unfortunately, graphite grease is well known to cause galvanically induced corrosion in bimetallic couples. In a fleet of over 3000 F16 USAF single-engine fighter aircraft, graphite grease was used by a contractor despite a general order from the Air Force banning its use in aircraft.7 As the flaps were operated, lubricant was extruded into a part of the aircraft where control of the fuel line shutoff valve was by means of electrical connectors made from a combination of gold- and tin-plated steel pins. In many instances corrosion occurred between these metals and caused loss of control of the valve, which shut off fuel to the engine in midflight. At least seven aircraft are believed to have been lost in this way, besides a multitude of other near accidents and enormous additional maintenance.
 
The Aloha aircraft incident
The structural failure on April 28, 1988, of a 19-year-old Boeing 737, operated by Aloha airlines, was a defining event in creating awareness of aging aircraft in both the public domain and in the aviation community. This aircraft lost a major portion of the upper fuselage near the front of the plane in full flight at 24,000 ft.8 Miraculously, the pilot managed to land the plane on the island of Maui, Hawaii. One flight attendant was swept to her death. Multiple fatigue cracks were detected in the remaining aircraft structure, in the holes of the upper row of rivets in several fuselage skin lap joints. Lap joints join large panels of skin together and run longitudinally along the fuselage. Fatigue cracking was not anticipated to be a problem, provided the overlapping panels remained strongly bonded together. Inspection of other similar aircraft revealed disbonding, corrosion, and cracking problems in the lap joints. Corrosion processes and the subsequent buildup of voluminous corrosion products inside the lap joints, lead to “pillowing,” whereby the faying surfaces are separated. Special instrumentation has been developed to detect this dangerous condition. The aging aircraft problem will not go away, even if airlines were to order unprecedented numbers of new aircraft. Older planes are seldom scrapped, and the older planes that are replaced by some operators will probably end up in service with another operator. Therefore, safety issues regarding aging aircraft need to be well understood, and safety programs need to be applied on a consistent and rigorous basis.
 
The MV KIRKI
Another example of major losses to corrosion that could have been prevented and that was brought to public attention on numerous occasions since the 1960s is related to the design, construction, and operating practices of bulk carriers. In 1991 over 44 large bulk carriers were either lost or critically damaged and over 120 seamen lost their lives.9 A highly visible case was the MV KIRKI, built in Spain in 1969 to Danish designs. In 1990, while operating off the coast of Australia, the complete bow section became detached from the vessel. Miraculously, no lives were lost, there was little pollution, and the vessel was salvaged. Throughout this period it seems to have been common practice to use neither coatings nor cathodic protection inside ballast tanks. Not surprisingly therefore, evidence was produced that serious corrosion had greatly reduced the thickness of the plate and that this, combined with poor design to fatigue loading, were the primary cause of the failure. The case led to an Australian Government report called “Ships of Shame.” MV KIRKI is not an isolated case. There have been many others involving large catastrophic failures, although in many cases there is little or no hard evidence when the ships go to the bottom.
 
Corrosion of the infrastructure
One of the most evident modern corrosion disasters is the present state of degradation of the North American infrastructure, particularly in the snow belt where the use of road deicing salts rose from 0.6M ton in 1950 to 10.5M tons in 1988. The structural integrity of thousands of 4 Introduction bridges, roadbeds, overpasses, and other concrete structures has been impaired by corrosion, urgently requiring expensive repairs to ensure public safety. A report by the New York Department of Transport has stated that, by 2010, 95 percent of all New York bridges would be deficient if maintenance remained at the same level as it was in 1981. Rehabilitation of such bridges has become an important engineering practice.10 But the problems of corroding reinforced concrete extend much beyond the transportation infrastructure. A survey of collapsed buildings during the 1974 to 1978 period in England showed that the immediate cause of failure of at least eight structures, which were 12 to 40 years old, was corrosion of reinforcing or prestressing steel. Deterioration of parking garages has become a major concern in Canada. Of the 215 garages surveyed recently, almost all suffered varying degrees of deterioration due to reinforcement corrosion, which was a result of design and construction practices that fell short of those required by the environment. It is also stated that almost all garages in Canada built until very recently by conventional methods will require rehabilitation at a cost to exceed $3 billion. The problem surely extends to the northern United States. In New York, for example, the seriousness of the corrosion problem of parking garages was revealed dramatically during the investigation that followed the bomb attack on the underground parking garage of the World Trade Center.11
 
The Influence of People
The effects of corrosion failures on the performance maintenance of materials would often be minimized if life monitoring and control of the environmental and human factors supplemented efficient designs. When an engineering system functions according to specification, a three-way interaction is established with complex and variable inputs from people (p), materials (m), and environments (e).12 An attempt to translate this concept into a fault tree has produced the simple tree presented in Fig. I.1 where the consequence, or top event, a corrosion failure, can be represented by combining the three previous contributing elements. In this representation, the top event probability (Psf) can be evaluated with boolean algebra, which leads to Eq. (I.1) where Pm and Pe are, respectively, the probability of failure caused by materials and by the environment, and Factorp describes the influence of people on the lifetime of a system. In Eq. (I.1), Factorp can be either inhibiting (Factorp <1) or aggravating (Factorp >1):
Psf = Pm PeFactorp
 
 
The justification for including the people element as an inhibit gate or conditional event in the corrosion tree should be obvious (i.e., corrosion Introduction 5 is a natural process that does not need human intervention to occur). What might be defined as purely mechanical failures occur when Pm is high and Pe is low. Most well-designed engineering systems in which Pe is approximately 0 achieve good levels of reliability. The most successful systems are usually those in which the environmental influence is very small and continues to be so throughout the service lifetime. When Pe becomes a significant influence on an increasing Psf, the incidence of corrosion failures normally also increases. Minimizing Psf only through design is difficult to achieve in practice because of the number of ways in which Pm, Pe , and Factorp can vary during the system lifetime. The types of people that can affect the life and performance of engineering systems have been regrouped in six categories (Table I.2).13 Table I.2 also contains a brief description of the main contributions that each category of people can make to the success or premature failure of a system. Table I.3 gives an outline of methods of corrosion control14 with an indication of the associated responsibility. However, the influence of people in a failure is extremely difficult to predict, being subject to the high variability level in human decision making. Most well-designed engineering systems perform according to specification, largely because the interactions of people with these systems are tightly controlled and managed throughout the life of the systems. Figure I.2 breaks down the causes responsible for failures
 
Positions and Their Relative Responsibilities in System Management
Designer
What is the main system being specified?
What is the function of the main system?
Did the budget introduce compromise into the design?
How was a subsystem embodied into the main system?
Does the envelope of the subsystems fit that for the main system?
Designer
What is the subsystem being specified for?
What is the function of the subsystem?
What is the optimum materials selection?
Has the correct definition of the operating environment been applied?
By what means will the component be manufactured?
What is the best geometrical design?
Have finishing operations, protective coatings, or corrosion control techniques been specified?
Have the correct operating conditions been specified?
Has the best maintenance schedule been specified?
Does the design embody features that enable the correct maintenance procedures to be followed?
Manufacturer
Were the same materials used as were originally specified?
Did the purchased starting materials conform to the specification in the order?
Has the manufacturing process been carried out correctly?
Has the design been reproduced accurately and has the materials specification been
precisely followed?
Have the correct techniques been used?
Have the most suitable joining techniques been employed?
Have the specified conditions/coatings necessary for optimum performance been
implemented?
Did the component conform to the appropriate quality control standards?
Was the scheme for correct assembly of the subsystem implemented correctly so that
the installation can be made correctly?
Installer
Has the correct setting-to-work procedure been followed?
Have any new features in the environment been identified that are likely to exert an influence and were not foreseen by the design process?
Maintainer
Has the correct maintenance schedule been followed?
Have the correct spares been used in repairs?
Have the correct maintenance procedures been carried out?
Has the condition of the system been correctly monitored?
User
Has the system been used within the specified conditions?
Is there a history of similar failures or is this an isolated occurrence?
Do aggravating conditions exist when the system is not in use?
Is there any evidence that the system has been abused by unauthorized personnel?
 
Outline of Methods of Corrosion Control
Method Responsibility
Selection of Materials Direct Managerial
Select metal or alloy (on nonmetallic material) for the particular environmental conditions prevailing (composition, temperature, velocity, etc.), taking into account mechanical and physical properties, availability, method of fabrication and overall cost of structure Designer Procurer (for user)
Decide whether or not an expensive corrosion resistant alloy is more economical than a cheaper metal that requires protection and periodic maintenance Designer Procurer (for user)
Design  
If the metal has to be protected, make provision in the design for applying metallic or nonmetallic coatings or applying anodic or cathodic protection Designer Designer
Avoid geometrical configurations that facilitate corrosive conditions such as Features that trap dust, moisture, and water Crevices (or else fill them in) and situations where deposits can form on the metal surface Designs that lead to erosion corrosion or to cavitation damage Designs that result in inaccessible areas that cannot be reprotected (e.g., by maintenance painting) Designs that lead to heterogeneities in the metal (differences in thermal treatment) or in the environment (differences in temperature, velocity) Designer Designer
Contact with other materials  
Avoid metal-metal or metal-nonmetallic contacting materials that facilitate corrosion such as Bimetallic couples in which a large area of a more positive metal (e.g., Cu) is in contact with a small area of a less noble metal (e.g., Fe, Zn, or Al) Metals in contact with absorbent materials that maintain constantly wet conditions or, in the case of passive metals, that exclude oxygen Contact (or enclosure in a confined space) with substances that give off corrosive vapors (e.g., certain woods and plastics) Designer, user Designer, user
Mechanical factors  
Avoid stresses (magnitude and type) and environmental conditions that lead to stresscorrosion cracking, corrosion fatigue, or fretting corrosion: Designer, user Designer, user
 
Outline of Methods of Corrosion Control (Continued)
Method Responsibility
Selection of Materials Direct Managerial
For stress corrosion cracking, avoid the use of alloys that are susceptible in the environment under consideration, or if this is not possible, ensure that the external and internal stresses are kept to a minimum. For a metal subjected to fatigue conditions in a corrosive environment ensure that the metal is adequately protected by a corrosion-resistant coating. Processes that induce compressive stresses into the surface of the metal such as shotpeening, carburizing, and nitriding are frequently beneficial in preventing corrosion fatigue and fretting corrosion.    
Coatings  
If the metal has a poor resistance to corrosion in the environment under consideration, make provision in the design for applying an appropriate protective coating such as Metal reaction products (e.g., anodic oxide films on Al), phosphate coatings on steel (for subsequent painting or impregnation with grease), chromate films on light metals and alloys (Zn, Al, cd, Mg) Metallic coatings that form protective barriers (Ni, Cr) and also protect the substrate by sacrificial action (Zn, Al, or cd on steel) Inorganic coatings (e.g., enamels, glasses, ceramics) Organic coatings (e.g., paints, plastics, greases) Designer Designer
Environment  
Make environment less aggressive by removing constituents that facilitate corrosion; decrease temperatures decrease velocity; where possible prevent access of water and moisture. For atmospheric corrosion dehumidify the air, remove solid particles, add volatile corrosion inhibitors (for steel). For aqueous corrosion remove dissolved O2, increase the pH (for steels), add inhibitors. Designer, user Designer, user
Interfacial potential  
Protect metal cathodically by making the
interfacial potential sufficiently negative by
(1) sacrificial anodes or (2) impressed current.
Protect metal by making the interfacial
potential sufficiently positive to cause
passivation (confined to metals that passivate
in the environment under consideration).
   
 
Outline of Methods of Corrosion Control (Continued)
Method Responsibility
Selection of Materials Direct Managerial
When there is no information on the behavior of a metal or alloy or a fabrication under specific environmental conditions (a newly formulated alloy and/or a new environment), it is essential to carry out corrosion testing. Designer Designer, user
Monitor composition of environment, corrosion rate of metal, interfacial potential, and so forth, to ensure that control is effective. Designer Designer, user
Supervision and inspection  
Ensure that the application of a protective coating (applied in situ or in a factory) is adequately supervised and inspected in accordance with the specification or code of practice. Designer, user User
 
 
Pie chart attribution of responsibility for corrosion failures investigated by a large chemical company.
 
investigated by a large process industry.15 But the battle against such an insidious foe has been raging for a long time and sometimes with success. Table I.4 presents some historical landmarks of discoveries related to the understanding and management of corrosion. Although the future successes will still relate to improvements in materials and their performance, it can be expected that the main progress in corrosion prevention will be associated with the development of better information- processing strategies and the production of more efficient monitoring tools in support of corrosion control programs.
 
Landmarks of Discoveries Related to the Understanding and Management of Corrosion
Date Landmark Source
1675 Mechanical origin of corrosiveness and corrodibility Boyle
1763 Bimetallic corrosion HMS Alarm report
1788 Water becomes alkaline during corrosion of iron Austin
1791 Copper-iron electrolytic galvanic coupling Galvani
1819 Insight into electrochemical nature of corrosion Thenard
1824 Cathodic protection of Cu by Zn or Fe Sir Humphrey Davy
1830 Microstructural aspect of corrosion (Zn) De la Rive
1834–1840 Relations between chemical action and generation of electric currents Faraday
1836 Passivity of iron Faraday, Schoenbein
1904 Hydrogen overvoltage as a function of current Tafel
1905 Carbonic and other acids are not essential for the corrosion of iron Dunstan, Jowett, Goulding, Tilden
1907 Oxygen action as cathodic stimulator Walker, Cederholm
1908–1910 Compilation of corrosion rates in different
media
Heyn, Bauer
1910 Inhibitive paint Cushman, Gardner
1913 Study of high-temperature oxidation kinetics of tungsten Langmuir
1916 Differential aeration currents Aston
1920–1923 Season-cracking of brass = intergranular corrosion Moore, Beckinsale
1923 High-temperature formation of oxides Pilling, Bedworth
1924 Galvanic corrosion Whitman, Russell
1930–1931 Subscaling of “internal corrosion” Smith
1931–1939 Quantitative electrochemical nature of corrosion Evans
1938 Anodic and cathodic inhibitors Chyzewski, Evans
1938 E-pH thermodynamic diagrams Pourbaix
1950 Autocatalytic nature of pitting Uhlig
1956 Tafel extrapolation for measurement of kinetic parameters Stern, Geary
1968 Electrochemical noise signature of corrosion Iverson
1970 Study of corrosion processes with electrochemical impedance spectroscopy (EIS) Epelboin