1、 GPA TITLEM1975 * 3824b 00017b8 3 GPA CORROSION DETECTION REPORT SINGLE COPIES . . . . . . . . . . . . . . . . $10.00 10 OR MORE COPIES . . . . . . . . . . . 8.00 MEMBERS 7.50 10 OR MORE COPIES . . . . . . . . . . . 6.00 PRINTED SEPTEMBER 1967 REPRINTED DECEMBER 1967 REVISED SEPTEMBER 1975 REPRINTED
2、 AUGUST 1981 GPA TITLE*1975 * 3824699 00017b9 5 M INDEX CORROSION DETECTION REPORT CHAPTER I Introduction and Scope 1 CHAPTER II T5eory of Aqueous Corrosion A . Direct Corrosion B . Electromotive and Galvanic Series C . Rate Factors 1 . Films 2 . Aeration 3 . Velocity 4 . Temperature 5 . Pressure D
3、. Nature of the Electrolyte 2 2 3 4 4 4 4 4 5 6 CHAPTER III Occurrence of Corrosion 7 A . Gas Comppression System 7 B . Steam Strlpping Still System 7 1 . Failures in the Presence of Oxygen 7 2 . Failures in the Absence of Oxygen 7 C . Fractionation System 7 1 . Factors Minimizing Corrosion 9 2 . Co
4、rrosion in Fractionation Systems 7 D . Dehydrators 11 1 . Glycol Systems 11 2 . Solid Bed Desiccant System 11 E . Direct Fired Heaters 11 1 . Heater Failures 11 F . Chemical Treaters 13 1 . Regenerative Caustic Treater 13 2 . Copper Chloride Treater 13 3 . Amine Gas Treater . 13 . * . * . CHAPTER IV
5、 Chemical Tests 18 B . Tests for Water, Gas Products, and Scale 18 1 . Iron 18 2 Copper and Copper Compounds 18 3 pH, Alkalinity, and Mineral Acidity 17 4 . Conductance 19 5 . Oxygen 19 4 pH of Glycols 17 7 . Chlorides in Amine 19 8 Non-regenerative Salts in Amine 19 9 . Organic Acids 19 10 . Mercap
6、tan Sulfur 20 11 . Mercury Vapor 20 Metallurgical Examination 2 1 Metallurgical Examination of Equipment Failure 2 1 1 . Physical Location 21 2 . Service Condition 21 B . Types of Analyses 21 1 . Spectrograph 21 2 . X Ray Fluorescence 21 3 . Wet Chemistry 21 4 . Metallograph 21 5 . Hardness Testing
7、21 6 . Impact Tester 22 A . Sampling 18 . . . . CHAPTER V A . 7 . Tensile Testing 22 C. Miscellaneous Tests 22 CHAPTER VI Visual and Optical Methods 23 B . Mechanical Aids 23 1 . Steel Rule 23 2 . Caliper 23 3 . Vernier Caliper . 25 4 . Depth Gauge 25 5 . Micrometer . 25 6 . Inspectors Gauge 25 7 .
8、Dial Test Indicator 26 C . Remote Surface Inspection 27 1 . Borescope 27 D . Accessible Surface Inspection 28 1 . Magnetic-Particle Inspection . 28 2 . Black Light Inspection 28 3 . Dye-Penetrant Inspection . 30 E . Thermal Inspection 30 F . Corrosion Definitions and Illustrations 30 A . Visual Insp
9、ection 23 CHAPTER VI1 Electronic, Ultrasonic, and Magnetic Methods 46 A . Radiography 46 1 . Gamma ray 49 2 . ray -. 49 B . Ultrasonic Testing 50 1 . Pulse Echo 54 2 . Two Transducer Reflection Method, Pitch and Catch 57 3 . Through-transmission 57 4 . Resonance Type 57 C . Eddy Current Testing . 58
10、 Miscellaneous Methods of Corrosion Measurement - 60 A . Corrosion Coupons 60 1 . Type of Coupon 60 2 . Surface Preparation 60 3 . Mounting 60 4 . Preparation for Weighing 60 5 . Evaluation of Data 60 6 . Cost and Availability 61 CHAPTER VI11 * B . Electrical Resistance Probes 61 1 . Advantages of P
11、robe Technique 61 2 . Disadvantages of Probe Technique . 61 C . Polarization Resistance 61 1 . Pressure Type Hydrogen Probes 63 2 . Vacuum Type Hydrogen Probes 64 E . Galvanic Probes . 64 F AC Corrosion and Inhibitor Film Life Monitor 65 G . Embrittlement Detector 65 H . Test Nipple or Spool 65 1 .
12、Closed System 65 2 . Process Stream . 65 I . Sentinel Holes . 65 J . Hammer Tests 65 1 . Equipment Not Suitable For Inspection By Hammer Tests 65 Pressure Testing 65 D . Hydrogen Probes . 62 K . GPA TITLE*1975 * m 3824699 0001770 1 m GPA Corrosion Detection Report Introduction and Scope CHAPTER I Th
13、is Report was prepared originally in 1967 by Technical Section E of the Gas Processors Association and revised in 1975. It is intended as a practical and convenient guide in detecting, locating, and measuring common corrosion problems in gas processing plants and related equipment. It was developed
14、pri- marily for use by field operating and maintenance personnel whose responsibilities include the safe and economical operation of such facilities. All processing plants and their equipment components are subject to corrosion in some degree. Its control and mitigation are essential, not only for t
15、he protection of life and property, but for profitable operation of these processes. While corrosion may be less severe in later designed plants because of lack of moisture and lower operation temperatures, corrosion control may be even more cri tical since closer material tolerances and smaller cor
16、rosion allowances will be required. Moreover, plant capacities and in- vestments are becoming much larger. Therefore, plant downtime, for any reason, must be held to an absolute minimum. The corrosion control program should be founded on thor- ough, systematic, and frequent inspections in order to l
17、ocate and evaluate corrosion damage. Adequate data and records of these inspections will enable the operator to predict possible hazards before they become serious and will indicate plant components that need more critical examination. A suggested system for maintaining satisfactory corrosion record
18、s is shown in “Guide for Inspection of Refinery Equipment“ as published by the American Petroleum Institute. For specific systems refer to the chapter on the particular equipment in question. Even in very thorough inspection, only a small part of the equipment exposed to corrosion will be examined.
19、It is necessary, therefore, that the successful control program take into account other related information such as original plant design, specifi- cations, and conditions specific to the locality or the process. A thorough knowledge of the plant, the equipment, and the process, together with data g
20、athered from thorough inspections will provide the information needed to set up and maintain practical and effective control measures. To be practical as well as effective, control measures must use economical means to secure personnel and equipment safety. This report summarizes the experiences of
21、a cross section of the industry in the practical application of inspection devices and corrosion detection methods. It is for information purposes only and references to particular processes, tests, or manufacturers should not be construed as an endorsement by GPA. For more detailed and technical in
22、formation concerning corrosion, the reader is referred to other publications, some of which are listed in the references at the end of each chapter. The GPA takes no position as to whether any method, apparatus, or product mentioned herein is covered by an existing patent, nor as to the validity of
23、any patent alleged to cover any such method. Furthermore, nothing contained herein grants any right, by implication or otherwise, for manufacture, sale, or use in connection with any method, apparatus, or product covered by patents; nor does it insure anyone against liability for patent infringement
24、. This report may 6e used by anyone desiring to do so but the GPA shall not be held responsibile or liable in any way, either for loss or damage resulting therefrom or from violation of any federal, state, or municipal regulations with which it may con- flict. In all instances, applicable regulation
25、s or codes should be followed. -1- GPA TITLEu1975 * = 3824699 OOOL771 3 Theory of Aqueous Corrosion CHAPTER II Broadly speaking, corrosion is the deterioration of a metal or its properties because of a reaction with its environment. The effects of corrosion are obvious in most, but not all, cases. T
26、he readily obvious cases are thinned or perforated tubing or vessel walls. There may be large amounts. of colored rust or corrosion products adhering to the metal. If the metal is in contact with with swiftly moving streams, the corrosion products are usually swept away from the area of attack leavi
27、ng a “clean” surface. Again, the deterioration may only be evident by the metal hav- ing lost its strength, ductility, or “ring”. Sometimes when corrosion occurs, people ask what went wrong? Actually, corrosion is a natural tendency. Man finds metals in nature in the combined form, such as oxides, c
28、arion- ates, or sulfides. These are ores of industrial importance. To change from the combined form to purer and more useful refined metals, requires energy addition. Therefore, metals are at a higher energy level or an unnatural state as compared to the native ores. Thus, there is a normal tendency
29、 for the refined metal to revert back to the combined form as it occurred in nature. This occurs through reaction with moisture, carbon dioxide, hydrogen sulfide, oxygen and other components common- ly found in the atmosphere, soil, waters, and process streams. This reversion (corrosion) is likely t
30、o be accelerated in natural gas processing plants because of the higher concentrations, tempera- tures, and pressures of the reactants. There are, of course, many corrosive agents, but those mentioned above are the more com- mon and frequent sources of corrosion. ELECTROLYTE I A. DIRECT CORROSION Ir
31、on + Oxygen * FezOa-Iron Oxides Iron + Carbon Dioxide + FeCOa-Iron Carbonate Iron + Hydrochloric Acid FeCl:%-Iron Chloride Iron + Hydrogen Sulfide + FeSS-Iron Sulfide Aluminum + Oxygen -+ AleOs-Aluminum Oxide Aluminum + Caustic 4 Al (OH) :-Aluminum Copper + Caustic 4 Cu (OH) 2-Copper Hydroxide In su
32、bstantially all cases of corrosion in the presence of water, the driving force of the corrosion reaction between a metal and its environment is electrochemical. As the name implies, both electrical and chemical changes occur. An electrochemical cell or battery involves several com- ponents. On an ar
33、ea of the metal called an anode, the metal goes into solution by entering an adjacent electrolyte as tiny, positively-charged particles called ions, Figure i. For this dis- cussion, an electrolyte will be defined as any media containing water and some dissolved substance that imparts electrical con-
34、 ductivity. Included are natural, boiler, and cooling tower waters as well as aqueous solutions of amines, glycol, or similar solutions. The electrolyte can also be the soil with its moisture and salts, films condensed from the air, moisture in gasoline, porous gaskets, or insulation. The entry of t
35、hese positive ions into adjacent electrolytes displaces or forces other positive ions from the electrolyte and onto the metal to maintain an electrical balance in the electro- Hydroxide -+ ELECTROCHEMICAL CELL FLASHLIGHT BATTERY Figure 1 Electron Flow -2- GPA TITLE*1975 * lyte. The positive ion most
36、 frequently forced out of solution is the hydrogen ion (H) since it is always present to some degree in water. Present in water as positive hydrogen ion, it becomes an electrically neutral hydrogen atom upon contact with metal surface near the anode. The change of hydrogen ions to neutral atoms is c
37、alled chemical reduction and takes place at metal surfaces known as cathodes. An opposite type of chemical action, . axidation, is taking place simultaneously at the anodic areas, i.e., the change of neutral metal atoms to positive metal ions. These two chemical reactions, oxidation and reduction, i
38、nvolve an exchange of electrons. Electron movement through a metal constitutes electric current flow. In summary, on all metal surfaces there are some areas with a greater natural tendency to revert to a combined form than other areas. Therefore, when metal surfaces are in contact with an electrolyt
39、e, metal at the anodic areas “dissolves” in the elec- trolyte as positive ions. An amount of electrons equivalent to the quantity of metal corroded is released and these electrons move to the cathodes to reduce hydrogen ions to hydrogen atoms. The accumulation of atomic hydrogen on the cathodic surf
40、aces and positive metal ions near anodic surfaces reduces the voltage difference which causes corrosion current and hence begins to stifle the corrosion. This effect is known as polariza- tion and is very beneficial since it limits corrosion. Polarization of a corrosion cell occurs in other ways, bu
41、t it is sufficient to say that it reduces the voltage difference between the anodes and cathodes by affecting conditions at either or both areas. Some conditions and additives inhibit corrosion by aiding polarization. Other factors stimulate corrosion by de-polarizing effects. Oxygen from air as wel
42、l as other oxidizing agents pro- mote corrosion by destroying the polarizing films of hydrogen on the cathodic areas. Oxygen does this by combining with the hydrogen to form water. For this reason, the presence of air in natural waters and in many aqueous solutions is the main pro- moter or stimulat
43、or of corrosion. The film of atomic hydrogen on cathodic areas may also be destroyed by hydrogen atoms combining and leaving the metal surface as bubbles of hydrogen gas. This is most prevalent in acid environments. When active metals are placed in strong acids, the action is quite evident from the
44、vigorous evolution of hydrogen gas bubbles. Cathodic protection of metals involves the purposeful use of direct current to create and nzaintain a polarizing film of hydrogen over all the metal surface. B. ELECTROMOTIVE AND GALVANIC SERIES The various metals useful to man have inherently different te
45、ndencies to revert to the combined form. Expressed another way, they exhibit different degrees of chemical reactivity. A list of the metallic elements can thus be made up starting with the most active and continuing with a decreasing order of activity. Near the top of such a list would be found the
46、metal sodium which is so reactive it must be stored in kerosene to pre- vent reaction with the moisture and oxygen in air. Near the bottom of the list would be gold, so much less reactive that it may he found in nature uncombined. Such an activity list has been made up and is called the electromotiv
47、e series. In addition, the reactivities have been assigned values determined by actual measurement to show quantitatively the relative tendency to corrode. Since electro- chemical corrosion results in czwrent flow, it is not too surprising that the tendency or driving force for corrosion is expresse
48、d as volts. W 3824699 0001772 5 W The voltages in the Electromotive Force (EMF) Series are called solution potentials, Table I. These standard potentials were measured with individual metals in contact with a standard solution containing ions of the same metal and under accurately controlled conditi
49、ons, These solution potentials change with the type, concentration, and temperature of the solutions to which the metals are exposed. TABLE I Electromotive Series Metal (Active End) Calcium, Ca Sodium, Na Magnesium, Mg Aluminum, Al Zinc, Zn Chromium, Cr Iron, Fe Nickel, Ni Tin, Sn Lead, Pb Hydrogen, Hn Copper, Cu Silver, Ag Gold, Au (Noble end) Standard Oxldatlon Metal Ion Potential E (volts) 77OP - Ca+ Na+ Mg+ Al+ Zn+ Cr+ Fe+ Ni+ Sn+ Pb+ H+ CU+ Ag+ Au+ 2.87 2.71 2.37 1.66 0.763 0.74 0.44 0.250 0.136 0.126 Arbitr
