Figure 7.1: Polarization Diagram Illustrating Anodic Inhibition . . . 5 Figure 7.2: Polarization Diagram Illustrating Cathodic Inhibition . . . 6 Figure 7.3: Corrosion Control Expenditures by Means of Control
1. . . 12 Figure 7.4: Protective Coatings, Paint, And Internal Linings on a Large
Above-Ground Storage Tank . . . 13 Figure 7.5: Breakdown of Costs of Applying a Protective Coating to an Existing
Structure . . . 13 Figure 7.6: Marine Piling with Aging Coating . . . 24 Figure 7.7: Osmotic Blistering on a Pipeline Riser . . . 24 Figure 7.8: Blisters on Fusion-Bonded Epoxy . . . 24 Figure 7.9: Cracking Due to Structural Motion on the Exterior Wall of a Ship . . 24 Figure 7.10: Abrasion from the Floating Roof Scraping on the Inside Wall of
Aboveground Storage Tank Caused this Corrosion . . . 25 Figure 7.11: Impact Damage on the Front Hood of an Automobile Caused this Corrosion and Coating Blisters . . . 25 Figure 7.12: Disbonded Pipeline Coating . . . 25 Figure 7.13: Corroded I-Beam Flange Where Debris Accumulated and Promoted
Corrosion . . . 25
Figure 7.14: Corrosion Cells on Unprotected Structure . . . 26
Figure 7.15: Illustration of How Cathodic Protection Makes the Entire Surface
Cathodic; i.e., Corrosion Cells on the Structure . . . 27 Figure 7.16: Principle of Galvanic (Sacrificial) Anode Cathodic Protection
System . . . 28 Figure 7.17: Examples of Galvanic (Sacrificial) Anodes; Magnesium (Left) and Zinc (Right) . . . 29 Figure 7.18: Principle of Impressed Current Cathodic Protection (ICCP)
System . . . 30 Figure 7.19: Examples of ICCP Anodes; Silicon-Iron (Left) and Platinized
Niobium (Right) . . . 31 Figure 7.20: Example of ICCP Transformer-Rectifier (T/R) . . . 32 Figure 7.21: Principle of Anodic Protection System . . . 36 Figure 7.22: Polarization Diagram Showing Anodic Protection Principle . . . 36
Chapter 8: Inspection, Monitoring, and Testing
Figure 8.1: Areas of increased corrosion susceptibility in a horizontal piping system
1. . . 2 Figure 8.2: Manual Pit Gauge Measures the Depth of External Pitting on a
Pipeline . . . 3 Figure 8.3: Schematic of Film Radiography of a Metal With a Corrosion Pit, an Internal Crack, and Internal Porosity Defects . . . 4 Figure 8.4: Radiograph Showing Erosion Corrosion at a Piping Bend Where Fluid Flows From Right to Left . . . 4 Figure 8.5: Ultrasonic Inspection . . . 5 Figure 8.6: Inspection Port for Ultrasonic Equipment to Determine if Corrosion Has Occurred on a Piping System . . . 5 Figure 8.7: Ultrasonic Inspection of the Top (12 O’clock Position) of a Crude Oil Pipeline . . . 6 Figure 8.8: Eddy Current Inspection Of Heat Exchanger Tubes . . . 7 Figure 8.9: Dye Penetrant Inspection for Surface Cracks on Non-Magnetic
Piping . . . 7 Figure 8.10: Magnetic Particle Crack Indications on the Exterior of a Pipeline . . . 8 Figure 8.11: Portable X-Ray Fluorescent Spectrometer Being Used for Positive Materials Identification
3. . . 8 Figure 8.12: Thermographic Image Showing Location Where Insulation down Leads to CUI . . . 9 Figure 8.13: Slumping Acid Storage Tank Due to Excessive Wall Thinning
4. . . 10 Figure 8.14: Intrusive and Flush-Mounted Corrosion Probes Inserted into a
Three-Phase Oilfield Production System
2. . . 10
Figure 8.15: Mass Loss Coupons and Probes Used for Corrosion Monitoring . . . 11
Figure 8.16: Corrosion Rate Change vs. Time . . . 11
Figure 8.17: Typical ER Probes
2. . . 12
Figure 8.18: Voltage vs. Potential Plot at Potentials Near the Corrosion
Potential . . . 13 Figure 8.19: Applied Current Cathodic Polarization Curve of a Corroding
Metal Showing Tafel Extrapolation . . . 14
Figure 8.20: Schematic of Hydrogen Pressure Probe . . . 15
Figure 8.21: Measurement of Pipe-to-Soil Potential
4. . . 17
Figure 8.22: Typical At-Grade Test Station
4. . . 18
Basic Corrosion List of Tables Chapter 1: Introduction to Basic Corrosion
Chapter 2: Basics of Corrosion Electrochemistry
Table 2.1: Characteristics of Ions . . . 2 Table 2.2: Examples of Ions . . . 3 Table 2.3: Characteristics of Oxidation and Reduction . . . 3 Table 2.4: Examples of Oxidation and Reduction Reactions . . . 3 Table 2.5: Characteristics of Anodic and Cathodic Reactions . . . 4 Table 2.6: Examples of Anodic and Cathodic Reactions . . . 5 Table 2.7: Reference Electrodes . . . 9 Table 2.8: Galvanic Series for Metals in Seawater. . . 10
Chapter 3: Corrosive Environments
Chapter 4: Materials
Table 4.1: The Unified Numbering System for Alloys . . . 7 Table 4.2: Selected Martensitic Stainless Steels. . . 12 Table 4.3: Selected Ferritic Stainless Steels . . . 12 Table 4.4: Selected Austenitic Stainless Steels* . . . 13 Table 4.5: Nominal Composition of Selected Highly-Alloyed Austenitic
Stainless Steels . . . 14 Table 4.6: Nominal Composition of Selected Precipitation-Hardening Stainless
Steels. . . 14 Table 4.7: Nominal Composition of Selected Duplex and Super Duplex Stainless
Steels. . . 15 Table 4.8: Nominal Composition of Some Nickel Alloys . . . 16 Table 4.9: Common Copper Alloys . . . 17 Table 4.10: Nominal Composition and Mechanical Properties of Selected
Titanium Alloys . . . 19 Table 4.11: Aluminum Alloys . . . 21
Chapter 5: Forms of Corrosion
Chapter 6: . . . Designing for Corrosion Control
Chapter 7: Corrosion Control Methods
Table 7.1: Surface Preparation Methods and Standards. . . 20 Table 7.2: Current Requirements for Steel in Various Environments . . . 34
Chapter 8: . . . Inspection, Monitoring, and Testing
Chapter 1: Introduction to Basic Corrosion
Upon completion of this chapter, students will be able to:
• Define corrosion
• Describe the economic, environmental, and safety significance of corrosion
1.1 Definition of Corrosion
Corrosion is the deterioration of a material, usually a metal, or its properties because of a reaction with its environment. Figure 1.1 shows an example of corrosion of structural steel members in the splash and atmospheric zones of an offshore platform.
1.2 Importance of Corrosion
Corrosion is significant to the economy, in terms of financial costs for corrosion prevention and structural inspection and maintenance, and to society in terms of its environmental impact and potential safety issues.
Figure 1.1 Corrosion of Steel on an Offshore Platform
1.2.1 Direct Costs of Corrosion
comparative summary of corrosion costs of common economic sectors.
If corrosion control technology could completely eliminate corrosion, the cost of the control measures themselves may not be economically feasible. It may be easier to control corrosion to a reasonable limit than to eliminate it completely.
1.2.2 Excessive Maintenance, Repair, and Replacement
If corrosion is not properly considered and prepared for in the initial design of a system, it can lead to frequent breakdowns, and the need for excessive maintenance, repair, and replacement. The maintenance cost is more expensive than the cost to avoid corrosion at the design stage.
Preparation during the design stage includes the substitution of more corrosion-resistant materials, changing the operating conditions of the system, or applying other corrosion control measures.
1.2.2.1 Lost Production and Downtime
When corrosion damage creates the need for maintenance or repair, it usually interrupts production. These interruptions result in reduced income for the plant creating an economic impact. In addition, system shutdown and startup costs can be substantial.
1.2.2.2 Product Contamination
In many industries, contamination of a product caused by corroded material entering the product stream can be harmful. This is particularly true for the food processing and pharmaceutical industries, but it also impacts other systems. The direct cost of such contamination is the product’s loss of value, but there may be other indirect costs too including public trust and perception.
Figure 1.2 Costs of Corrosion1
1.2.2.3 Loss of Product
Losing a product due to leaks can have significant direct and indirect costs. The direct costs include the value of the product itself, the cost of repairs, the associated costs of downtime, including shutdown and startup, and the disposal costs of the contaminated products. However, corrosion leaks can have other implications and costs. For example, leaks in the plumbing of public and residential buildings often result in other water damage many times greater than the cost to repair or prevent the leak.
1.2.2.4 Loss of Efficiency: Oversizing and Excess Energy Costs In many cases, when designers expect substantial
corrosion, they may enlarge the system to accommodate the corrosion. In addition to the direct cost of excess material, oversizing can have other direct economic effects. For example, if heat exchanger tubes are made thicker than necessary, the extra thickness of the tube wall will reduce the efficiency of the heat exchanger, which can increase fuel costs or reduce output. Fouling of heat exchangers with corrosion products can have similar effects on fuel costs and productivity. Figure 1.3 shows an oversized offshore platform leg intended to provide a corrosion allowance for the portions of this leg that are exposed to tidal and splash-zone conditions.
1.2.2.5 Accidents
Corrosion can and has caused severe accidents, resulting in personal injury or loss of life.
Corrosion-related accidents have direct economic effects including medical bills, employee or plant downtime, investigations, and lawsuits. They also have other indirect economic and social implications. For example, if a plant or industry has a bad safety record because of corrosion, the cost of insurance will be higher than if they had maintained a good safety record.
1.2.2.6 Increased Capital Costs
The addition of extra material to a system for corrosion control can increase the capital cost for construction and maintenance. Capital costs include the initial costs of other corrosion control measures, such as protective coatings, cathodic protection systems, and equipment for the injection of corrosion inhibitors into the system.
1.2.2.7 Fines
The cost of environmental cleanup for product spills has increased greatly due to intensified awareness of the potential short- and long-term effects these spills can have on the environment.
Laws now require the cleanup of most spills. Even if the company responsible for the spill is no longer the owner of the offending system, the company that owned the system at the time of the spill can be liable for the cost of cleanup. If the incident resulted from negligence, the fines can be imposed on the system owners or system operators. Recent spill fines have exceeded $1,000,000.
1.2.3 Indirect Consequences of Corrosion
1.2.3.1 Safety Risks
Corrosion can and has caused many accidents. Most accidents could have been avoided by the proper application of corrosion control measures. Others could have been predicted and corrected before injury or loss of life.
Figure 1.3 Corrosion Allowance on an Offshore Platform Leg in Splash and Tidal Zone 2
1.2.3.2 Structural Collapse
Although complete structural collapse due to corrosion occurs infrequently, it does occur.
Corrosion can also reduce the resistance of structures to natural forces, such as earthquakes. Figure 1.4 and Figure 1.5 show examples of structural collapse due to corrosion.
1.2.3.3 Leaks
Leaks in systems carrying flammable or toxic materials are an obvious safety hazard. Fire and explosions from corrosion leaks in underground natural gas pipes are frequent, but avoidable, losses caused by corrosion. Although, in many cases, third-party damage, not corrosion, causes the leaks that result in fire and explosion.
1.2.3.4 Product Contamination
Product contamination can also affect safety, particularly if the contamination is not detected until after a product has been consumed. Corrosion can contaminate foods during both production and storage. Corrosion also frequently contaminates drinking water through the distribution lines and other plumbing-system components. The contamination may simply be unsightly, as in the case of
“red water” where relatively harmless levels of iron from iron pipe corrosion causes unsightly staining in the water and on the visible surfaces of plumbing fixtures. Corrosion from lead solders on food cans and copper pipes and corrosion in lead pipes caused many illnesses and deaths before anyone identified the toxic properties of lead. In addition to the elimination of the lead through the replacement of the lead-containing materials, water treatment to reduce the corrosion of the lead and the subsequent release of lead into the water successfully reduced this problem in some cases.
Pharmaceutical contamination can cause not only product loss during manufacture, but also premature deterioration and loss of potency during storage.
1.2.3.5 Consumer Confidence
Corrosion can impact the marketability of a product. Recent improvements in the corrosion resistance of automobiles, particularly with long-term guarantees against rust-through, are now a major selling feature. Even if the corrosion only results in unsightliness, a product that has a reputation of resisting corrosion will have greater sales appeal.
Figure 1.4 Structural Collapse, a Fatal Highway Bridge Collapse into the Ohio River
Figure 1.5 Parking Garage Collapse Due to Deicing Salt-Accelerated Corrosion of
Reinforcing Steel3
1.2.3.6 Loss of Redundancy
When an organization requires continuous processing or product supply, redundant systems ensure continuous operation. These systems either operate in parallel or use one system as a spare. In the first case, corrosion in one of the parallel systems reduces production until repairs can be completed. In the second case, corrosion in the primary system causes a loss of redundancy until the primary system is repaired. The worst-case scenario is failure of the backup system before the first can be repaired and placed back in service.
1.2.3.7 Appearance
Corrosion, particularly the all-too-familiar red rust from corroding iron and steel, is unsightly even if it does not interfere with system operation (see Figure 1.6). Industry spends a significant effort trying to eliminate such unsightly corrosion simply for the aesthetic benefits. This is particularly true when the appearance of a plant may be of considerable value to stockholders.
1.2.3.8 Increased Regulation Many aspects of corrosion control are now regulated. For example, legislation regulates the corrosion-related aspects for safe operation of pipelines carrying hazardous liquids or flammable gases. Figure 1.7 shows images from the 1965
pipeline compressor station explosion that killed seventeen and led to the creation of the U.S.
Office of Pipeline Safety, and, starting in 1975, Federal requirements for corrosion control on interstate pipelines. Due to recent failures, the Pipeline Hazardous Material Safety Administration (PHMSA), previously known as the “Office of Pipeline Safety,” has instituted new and more stringent regulations for all regulated pipeline systems.
Figure 1.6 Unsightly Corrosion on a Ship4
Figure 1.7 1965 Pipeline Explosion in Natchitoches, Louisiana2
1.2.4 Environment
Environmental pollution is an increasing concern worldwide. Figure 1.8 shows an oil containment boom limiting the spread of oil leaking from a corroded pipeline. During the 1990s the United States and Canada required the replacement of all underground storage tanks (USTs) at filling stations and similar operations with double-hulled USTs due to repeated problems with ground water contamination from corroded leaking USTs.
1.2.5 Changes in Engineering Practice
Sometimes corrosion changes engineering practices. The internal corrosion shown in Figure 1.9 and Figure 1.10 led to industrial efforts to control internal corrosion including the development of Internal Corrosion Direct Assessment (ICDA) efforts.
Figure 1.8 Oil Containment Boom and Oil-Absorbing Papers on the Surface of a River to Minimize the Spread of Crude Oil from a Corroded Pipeline2
Figure 1.9 Ruptured Pipeline Resulting in 12 Fatalities2
Figure 1.10 Internal Surface of Corroded Pipeline
Modern electronics are exposed to a wide variety of environments and are made from increasingly complex microelectronic components and circuit boards. The corrosion shown in Figure 1.11 could be controlled by keeping the equipment in atmospherically-controlled environments, but the manufacturing facility cannot guarantee that the item would not encounter corrosion during shipping and storage prior to use. The most common approach to limiting corrosion is to coat the circuits.
1.3 Forms of Corrosion
Corrosion is commonly classified in the following categories:
• General Corrosion
• Localized Corrosion – Pitting – Crevice – Filiform
• Galvanic Corrosion
• Environmental Cracking
• Flow-Assisted Corrosion
• Intergranular Corrosion
• Dealloying
• Fretting Corrosion
• High-Temperature Oxidation/Corrosion
1.4 List of Organizations Involved in Corrosion
The list below only emphasizes sources likely to be used by North American organizations that publish in English. Many other worldwide organizations also publish useful corrosion information and guidelines. The Internet can be a resource, but some of the information may be incorrect.
• NACE International
• American Gas Association
• American National Standards Institute
• American Petroleum Institute
• American Society of Mechanical Engineers
• American Society for Testing and Materials
• ASM International
• Materials Technology Institute
Figure 1.11 Short Circuit in Microelectronics due to Corrosion and Dendritic Growth5
• SSPC-The Society for Protective Coatings
• Steel Tank Institute
References:
1. Corrosion Cost and Preventative Strategies in the United States, September 2001, Federal Highway Administration Report, FHWA-RD-01-156.
2. R. Heidersbach, Metallurgy and Corrosion Control in Oil and Gas Production, John Wiley & Sons, New York, 2011.
3. NASA Kennedy Space Center, Fundamentals of Corrosion, http://corrosion.ksc.nasa.gov/
corr_fundamentals.htm, accessed March 2, 2012.
4. D. Ramirez, (2008), licensed as http://creativecommons.org/licenses/by/2.0/deed.en from http://
www.flickr.com/photos/danramarch/2873459569/, accessed April 30, 2012.
5. Matco, Inc., http://www.matcoinc.com/materials-engineering/environmental-testing-of-electronics.
Chapter 2: Basics of Corrosion Electrochemistry
Upon completion of this chapter, students should have an understanding of the:
• Terms and definitions of basic electrochemistry
• Basic electrochemical processes and concepts
2.1 Corrosion Occurs Through Electrochemical Reactions
Electrochemical reactions occur:
• In electrolytes, which are liquids that can carry an electrical current
• Through the exchange of electrons
The exchange of electrons in electrochemical reactions occurs at separate sites. The electrons flow through the metal from one of these separate sites to another.
2.2 Terms Used in Corrosion and Electrochemistry 2.2.1 Matter
Matter is anything that occupies space. Matter may be in the form of a solid, liquid, or gas. Matter may be formed from either elements, molecules, chemical compounds, or mixtures.
2.2.2 Element
An element is a substance that cannot be broken down through chemical reactions. Elements are the basic building blocks of all matter. There are 92 naturally occurring elements, ranging from the lightest, hydrogen, to the heaviest, uranium. Iron, oxygen, and gold are also elements.
2.2.3 Compound
A compound is a combination of two or more elements. A compound is a pure substance and has a fixed composition. Examples of chemical compounds and their chemical formulas are:
• Carbon Dioxide – CO2
– one atom of carbon (C), two atoms of oxygen (O)
• Salt – NaCl
– one atom of sodium (Na), one atom of chlorine (Cl)
• Water – H2O
– two atoms of hydrogen (H), one atom of oxygen (O)
• Ferric Oxide – Fe2O3
– two atoms of iron (Fe), three atoms of oxygen (O)
2.2.4 Mixture
A mixture is a combination of elements, compounds, or both held together by physical (rather than chemical) forces. A mixture does not have a fixed composition. Air, for example, is roughly 20%
oxygen and 78% nitrogen. It also contains other substances, such as argon (about 1%) and varying amounts of carbon dioxide and water vapor. Soil is a mixture of minerals formed from elements and compounds and contains varying amounts of these minerals and water. Most rocks contain one or more minerals that are either chemical compounds, elements, or mixtures.
2.2.5 Atom
An atom is the smallest chemical unit of an element. An atom consists of a nucleus surrounded by electrons. The nucleus contains positively charged particles called protons and all but the lightest element (hydrogen) contain neutrally charged particles called neutrons as well. The electrons surround and orbit around the nucleus. The number of electrons in an atom always equals the number of protons in the nucleus. Thus, atoms have a net electrical charge of zero, and are therefore electrically neutral.
2.2.6 Molecule
A molecule is the smallest particle of an element or compound that retains all the chemical properties of that compound or element.
2.2.7 Ion
An ion is a charged atom or molecule. An ion may either be an anion (negatively charged) or a cation (positively charged). Some examples of ions are shown in Table 2.1.
2.2.8 Electrolyte
An electrolyte is a liquid that contains ions. It can conduct electricity through the flow of ions.
Anions flow toward the anode and cations flow toward the cathode. An electrolyte contains equal amounts of charge on the ions contained in it. An electrolyte may be highly conductive because of its high content of ions (seawater) or only mildly conductive because of its very low content of ions (pure water).
2.3 Oxidation/Reduction Reactions
Most corrosion reactions are electrochemical reactions, called oxidation/reduction reactions (Table 2.3, Table 2.4). Oxidation/reduction reactions occur through an exchange of electrons. In corrosion reactions, these exchanges occur at specific sites. Oxidation occurs at sites called anodes and reduction occurs at sites called cathodes.
The electrons given off at the anodes travel through the metal to the cathode, where they are consumed in a reduction reaction. Corrosion occurs in electrolytes that supply the reactants for these reactions.
Table 2.1: Characteristics of Ions
Anion Cation
Negative ion Positive ion
Net negative charge Net positive charge
Formed by addition of electrons Formed by loss of electrons Attracted to anode Attracted to cathode
Table 2.2: Examples of Ions
Anion Cation
Sulfate ion (SO42-) Ferrous ion (Fe++) Chloride ion (Cl–) Ferric ion (Fe+++) Hydroxyl ion (OH–) Hydrogen ion (H+)
Sulfate ion (SO42-) Ferrous ion (Fe++) Chloride ion (Cl–) Ferric ion (Fe+++) Hydroxyl ion (OH–) Hydrogen ion (H+)