Yang Techniques for Corrosion Monitoring

1. Auflage 2008
ISBN: 978-1-84569-405-0
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, 711 Seiten

Techniques for Corrosion Monitoring

Autoren/Hrsg.

Yang, L

Leseproben

1 Introduction
Lietai Yang    Department of Earth, Material and Planetary Sciences, Southwest Research Institute, San Antonio, Texas, USA 1.1 Definition of corrosion
Corrosion is the deterioration process of a material due to reactions with its surroundings. As defined in the Random House Unabridged Dictionary: ‘Corrosion is the act or process of eating or wearing away gradually as if by gnawing, especially by a chemical action.’ The materials that are subject to corrosion include metals, ceramics, polymers, and even our own teeth. To most corrosion engineers, however, corrosion refers to the oxidation of metals by chemical and/or electrochemical processes. The most common example of metal corrosion relates to its reaction with oxygen or water. Rusting of steel due to exposure to water or humid air is a well-known example of electrochemical corrosion. In this process, the metal reacts with water or oxygen and forms iron oxides, eventually causing damage to the steel. The following sections describe the importance of corrosion monitoring and the scope of this book. 1.2 Corrosion cost
Corrosion is a costly worldwide problem. According to a recent systematic study commissioned by the United States Federal Highway Administration (FHWA), the annual direct cost of corrosion in the United States was $276 billion in 1998, or 3.1% of the gross domestic product (GDP).1 Corrosion cost studies were also conducted in other countries, such as the United Kingdom, Japan, Australia and Kuwait. Even though the level of effort varies greatly among these studies, all of them estimated the total annual cost of corrosion as ranging between 1% and 5% of each country’s gross national product (GNP).1 In addition to the huge cost in economic terms, corrosion is also blamed for many of the disasters that cause loss of life and devastating pollution to the environment. For instance, in April 1992, in Mexico, the Guadalajara Sewer Explosion took the lives of 215 people and caused injury to another 1500 people. The financial loss was estimated at $75 million. The accident was traced to the corrosion of a gas line that caused a leak of the gas into a nearby sewage main.2 Another example involved the sinking of a tanker, Erika, off the coast of Brittany in France, on 12 December 1999. In this accident, approximately 19 000 tons of heavy oil was spilled, equal to the total amount of oil spilled worldwide in 1998. Corrosion caused the sinking of the Erika.2 Because corrosion takes place in many different forms, some of them cannot be eliminated, but others are avoidable by simply applying appropriate corrosion prevention/mitigation technologies. In the report commissioned by the FHWA, it was estimated that 25% to 30% of the annual corrosion costs in the United States could be eliminated, if optimum corrosion management practices were employed.3 Other studies estimated that from 10% to 40% of the total corrosion cost could be avoided.3 Knowing this, it seems prudent for worldwide industries to use appropriate corrosion prevention and control methods; taking such pre-emptive measures will not only avert huge economic losses (potentially close to one hundred billion dollars annually in the US alone), but also protect the environment and public safety. 1.3 Corrosion monitoring and its importance in corrosion prevention and control
Corrosion monitoring is the practice of acquiring information on the progress of corrosion-induced damage to a material or on the corrosivity of the environment surrounding the material. Corrosion inspection is usually a survey of the material condition at any given time, while corrosion monitoring consists of a series of surveys in a given time period. While test coupons are one of the most widely used and most reliable methods, corrosion monitoring usually relies on the use of electronic corrosion sensors or probes that are exposed to an environment of interest, such as outdoor air or seawater, or inserted into the inner space of a containment system, such as a vessel or a pipe in which a liquid or a gas flows or is contained. On a continuous or semi-continuous basis, the electronic corrosion sensors or probes emit information relating to the corrosion of a metal system. The study commissioned by the FHWA further pointed out three preventive strategies in technical areas, to lessen or avoid unnecessary corrosion costs and to protect public safety and the environment. They are: • advance design practices for better corrosion management; • advance life prediction and performance assessment methods; and • advance corrosion technology through research, development and implementation. These strategies are inter-related. For example, advance design requires better life prediction and performance assessment methods, and better prediction and performance assessment methods require advancement in corrosion technology. Corrosion monitoring is part of corrosion technology. In today’s electronic age, many of the industrial process parameters – such as temperature, pH and flow – are controlled by automated feedback controllers. Only after the introduction of these controllers and the associated reliable sensors for these parameters was it possible to precisely manage them and to either improve the product quality or to produce original products. Unfortunately, the control of corrosion in many industries is still in its ‘stone age’ stage. According to the FHWA study, the annual use of corrosion inhibitors in the United States came to over a billion dollars; the annual cost may be over $4 billion worldwide, if the usage of inhibitors is assumed to be proportional to the GDP in each country. It is rather alarming to realize that nearly all of these inhibitors were added into the controlled systems based on parameters that are not the direct measures of corrosion (indirect parameters) or based on historical results acquired from the test coupons that were exposed in the controlled systems several months earlier. Examples of the indirect parameters include the concentration of the inhibitors and the concentration of dissolved oxygen in the controlled systems. Because of the complexity of corrosion, some concentrations of corrosion inhibitor that are shown to be effective under certain conditions may not be effective under others. In addition, the concentration of the inhibitor sampled from the bulk phase may not represent the actual concentration on the metal surface where corrosion takes place. With the advancement of corrosion monitoring techniques, corrosion sensors may be used in the feedback controllers – as in the control systems for pH, temperature and other process parameters – to automatically control the addition of corrosion inhibitors. When such a system is implemented, it will not only provide an adequate control of corrosion – which means a better performance and a longer life for the equipment – but it will also create tremendous savings in the inhibitor costs by avoiding over-dosing. This, in turn, produces a significant risk reduction for our environment, because many corrosion inhibitors are toxic. Corrosion monitoring also provides performance data and a basis for life prediction; corrosion monitoring is one of the most important components in corrosion prevention and corrosion control. 1.4 Organization of the book
This book is organized into 28 chapters. Chapter 2 presents an overview of corrosion fundamentals and evaluation techniques. It includes detailed discussions on the different forms of corrosion and their electrochemical characteristics. Chapter 2 is followed by in-depth discussions on the various methods that can be used for corrosion monitoring (i.e., to measure the corrosion damage to metals or the corrosivity of a surrounding environment with an adequate response time so that the measurements can be made at desired time intervals). The response time is relative to the purpose of monitoring. For example, if a corrosion monitor is interfaced with a corrosion inhibitor dosing-controller and the dosing-controller completes the addition of an inhibitor to a system in a few minutes, the response time of the corrosion monitor should be less than one minute. However, if the purpose is to monitor the long term corrosion damage to a metallic structure in the air near a pollution source, one month may be considered a sufficient response time. Chapters 3 through 8 discuss the electrochemical techniques for corrosion monitoring. These techniques include electrochemical polarization techniques (Chapter 3), electrochemical noise methods and harmonic analyses (Chapter 4), zero resistance ammetry and galvanic sensors (Chapter 5), differential flow through cell technique (Chapter 6), potentiometric methods for measuring localized corrosion (Chapter 7) and multielectrode systems (Chapter 8). Chapters 9 through 14 describe the gravimetric, electrical, and other physical or chemical methods. These methods include gravimetric techniques (Chapter 9), radioactivity methods (Chapter 10), electrical resistance techniques (Chapter 11), nondestructive evaluation methods for corrosion monitoring (Chapter 12), hydrogen permeation methods (Chapter 13) and rotating cage and jet impingement techniques (Chapter 14). Although some of the methods discussed above in Chapter 12 are primarily...