Field Corrosion Testing of Metals and Alloys
Author: Sheldon W. Dean, Jr.
Copyright © 2018 Materials Technology Institute, Inc. All Rights Reserved
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Chapter 1. Introduction
The Chemical Processing Industries (CPI) manufacture a large number of products that are important to modern society. The manufacturing process often use large quantities of compounds which are both dangerous to human health and corrosive to many construction materials. An uncontrolled release of such compounds into the environment from a failure resulting from corrosion could be catastrophic in terms of damage to the environment, risk to adjacent communities, fines, and lost production. An in depth understanding of how to use and deal with such materials, to prevent their release and damage to equipment, is essential for the successful operation of manufacturing operations.
To deal with this situation a number of approaches have been used to obtain reliable performance. These include the following:
- Laboratory corrosion testing
- Field corrosion testing
- Inspection of equipment used in the handling of compounds
Previous experience has also been an important source of information on how materials behave. The concept of laboratory corrosion testing involves exposing a specimen of a material to the environment for a known period of time, then evaluating the extent of corrosion or other damage. In cases where the environment can be simulated, in a laboratory setting, this has been a preferred approach.
Field corrosion testing is carried out by preparing specimens of materials, with known composition and properties, then placing them into an existing environment of interest. The specimens are retained in the environment for a period of time, then removed and evaluated. In the case of a chemical processing operation, the environment is created by the chemical operation and is not under the control of an investigator; as in a laboratory test. Field tests performed in other environments such as the atmosphere, waters, and soils are also not under the control of an investigator. In this regard field corrosion testing, in most cases, is the most reliable method of evaluating how a material will perform in a particular service.
The objective of this book is to present information on field corrosion testing which has been used to develop valuable results for minimizing the corrosion damage to materials of construction used in the CPI. In cases where standard methods or guides have been developed, documents are discussed and references provided. In addition, the process of planning field testing programs, designing racks and specimens for these tests is described, along with procedures for carrying out the test program. Related information on the inspection of equipment and corrosion monitoring is also provided. Although the emphasis is primarily on process equipment for chemical manufacturing, other types of field testing have been included, along with their evaluation procedures as these types of tests are occasionally necessary. To illustrate the value of field corrosion testing five case histories have been included.
This book does not provide detailed procedures for analytical methods. Although several approaches are discussed for evaluating specimens and evaluating their results, the procedures for any particular program must be decided by the investigators in order to meet the program’s objectives.
As this book’s focus is primarily on metallic materials, many of the procedures used for metallic materials could also be applied to non-metallics. However, detailed procedures are not provided. Composite materials are not covered in detail and studies that are intended to reveal corrosion mechanisms are also not discussed.
The scope of this book is to cover the following topics:
- Chapter 2 covers the development of field corrosion programs along with information on planning the program. Also, included are the steps required to formulate the program together with some details, on specific issues, that may arise during the program’s development.
- Chapter 3 covers the design of coupons for a number of types of tests used in the program along with the design of racks to hold the coupons during exposure. References are provided on these different types of specimens including welded coupons, bimetallic coupons, coated coupons, stressed coupons, non-metallic coupons, and crevice coupons.
- Chapter 4 discusses electrical resistance and polarization resistance methods for corrosion monitoring. The use of other types of electrochemical testing, including electrode potential measurement, polarization scans, and multi electrode arrays are also discussed. See Case History 1 in Chapter 11 for an example of the benefits of corrosion monitoring.
- Chapter 5 covers field tests for environmentally-assisted-cracking which also include specimen design and stressing methods.
- Chapter 6 covers field tests in process equipment. Issues involved in installing test racks in different types of equipment, including heat exchangers, piping systems, reactors, and distillation towers are also discussed.
- Chapter 7 covers the inspection of process equipment. Inspection results often provide key information for carrying out field tests in process equipment; inspection shut-downs are often used for installing and retrieving test racks.
- Chapter 8 covers other types of field tests such as atmospheric tests, high temperature tests, soil tests, tests in natural waters, soil tests, and tests in concrete.
- Chapter 9 covers evaluation of the specimens. A variety of techniques including mass loss, microscopic examination, and metallographic examination are discussed.
- Chapter 10 covers the analysis of results with particular emphasis on dealing with outliers. The procedure for determining corrosion rates from mass loss measurement and the variability of results from a variety of measurement methods are discussed.
- Chapter 11 provides five case histories describing field studies which developed information used to deal with corrosion issues from operating facilities.
1.4 Notes on Using This Book
In view of the extensive scope of subjects covered in this book, it may be helpful to review particular chapters at different times during the planning and execution of a field corrosion study. The organization of the subject matter has been developed so that each chapter could be reviewed for content without having to refer to previous or subsequent chapters. The information in Chapters 2 and 3 would be most useful when planning a program. Chapters 6 and 8 cover issues regarding tests in process equipment and other environments, respectively. These chapters should be the basis of any program.
Chapter 5 covers environmentally assisted cracking studies; this type of testing requires that the specimens be subjected to a controlled tensile stress during exposure. This requirement makes this type of testing more difficult than mass loss corrosion rate testing, but in cases where it is necessary the information in this Chapter may be helpful.
Corrosion monitoring by means of electrical or electrochemical techniques are covered in Chapter 4. Inspection is covered in Chapter 7. These chapters are related but not necessarily essential to a field corrosion testing program, however they can be important in cases where these techniques are relevant. For example, one of the case histories in Chapter 11 discusses when corrosion monitoring was important in the testing program. Inspection of equipment likewise can reveal areas where corrosion is occurring and can also identify possible sites for installing coupon racks or sampling probes.
Chapter 9 covers techniques that can be revealing for examining the specimens obtained after exposure. Although, in most cases mass loss measurements may be sufficient, additional information regarding localized corrosion may be an important issue that more sophisticated analysis can reveal. Review of this chapter may be important when deciding the evaluation portion of the test program.
Chapter 10 covers the final phase of the test program where the results are analyzed to determine the program’s conclusions. The issue of outlier results is highlighted; this may be helpful when dealing with outliers.
Note: Case histories discussed in Chapter 11 were chosen because they illustrate cases where test programs have provided valuable information. They are not intended to be a comprehensive listing of different types of cases, but are illustrative of several different types of studies that have been accomplished.
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