1、THE CORROSION SOCIETY Stand a rd NACE Standard RP0497-2004 Item No. 21083 Recommended Practice Field Corrosion Evaluation Using Metallic Test Specimens This NACE International standard represents a consensus of those individual members who have reviewed this document, its scope, and provisions. Its
2、acceptance does not in any respect preclude anyone, whether he has adopted the standard or not, from manufacturing, marketing, purchasing, or using products, processes, or procedures not in conformance with this standard. Nothing contained in this NACE International standard is to be construed as gr
3、anting any right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or product covered by Letters Patent, or as indemnifying or protecting anyone against liability for infringement of Letters Patent. This standard represents minimum requirements and
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10、national standards may receive current information on all standards and other NACE International publications by contacting the NACE International Membership Services Department, 1440 South Creek Drive, Houston, Texas 77084-4906 (telephone +I 281 228-6200). Reaffirmed 2004-03-31 Approved 1997-12-22
11、NACE International 1440 South Creek Dr. Houston, Texas 77084-4906 +I (281)228-6200 ISBN 1-57590-048-3 02004, NACE International RP0497-2004 Foreword Corrosion test information is often needed for selecting metallic construction materials for chemical process and other equipment. Usually, the informa
12、tion most readily available is laboratory corrosion test results for a specific set of controlled conditions. However, these conditions frequently do not represent service environments of specific equipment. Varying levels of contaminants, aeration, flow, temperature variations, liquid and/or vapor
13、exposure, and other factors can make service conditions difficult or impossible to duplicate in the laboratory. Thus, in many instances, the more accurate method of evaluating metals for a particular application is by exposing metallic test specimens in the field or actual service environment. These
14、 methods can provide a much more accurate estimate of service performance than laboratory testing. As with any corrosion study, careful design of the field test program and accurate interpretation of the test results are needed to develop meaningful data that lead to a conclusive study and correct c
15、onclusions. To that end, this standard recommended practice aids the design, implementation, and evaluation of field corrosion testing programs using metallic test specimens. No other materials or testing are addressed in this standard. This standard is particularly helpful when used in conjunction
16、with ASTM“ G 4. This standard was originally prepared in 1997 by NACE Task Group T-5A-29, a component of Unit Committee T-5A on Corrosion in Chemical Processes. It was reaffirmed in 2004 by Specific Technology Group (STG) 36 on Process Industry-Chemicals. This standard is issued by NACE under the au
17、spices of STG 36. In NACE standards, the terms shall, must, should, and may are used in accordance with the definitions of these terms in the NACE Publications Style Manual, 4th ed., Paragraph 7.4.1.9. Shall and must are used to state mandatory requirements. The term should is used to state somethin
18、g considered good and is recommended but is not mandatory. The term may is used to state something considered optional. (I) ASTM International (ASTM), 100 Barr Harbor Dr., West Conshohocken, PA 19428-2959. NACE International I RP0497-2004 NACE International Stand a rd Recommended Practice Field Corr
19、osion Evaluation Using Metallic Test Specimens Contents 1 . General . 1 2 . Applications of Field Corrosion Testing 1 3 . Guidelines for Conducting a Field Corrosi 4 . Data Obtained by Field Corrosion Testing 5 . Test Rack Design and Exposure Location 6 . Test Specimens 7 . Test Specimen Cleaning an
20、d Scale Characterization After Exposure . 18 8 . Interpretation of Corrosion Types . 20 9 . Reporting the Results . 22 1 O . Reproducibility 22 References 23 . Figure 2: Checklist for Test Specimen Selection . 3 Figure 3: Making and Stressing a U-Bend Stress Corrosion Test Specimen 7 Figure 4: C-Rin
21、g Stress Corrosion Test Specimen 8 Figure 5: Double-Cantilever-Beam (DCB) Stress Corrosion Test Specimen 9 Figure 6A: Flat Bar Test Rack . 11 Figure 6B: Typical Spool Rack (Side View) . 11 12 12 Figure 6E: Pipe Insert Racks . 13 Figure 6F: Test Specimen Spacers (PFTE or Ceramic) 13 Figure 7: Retract
22、able “Slip-In” Test Specimen Holder . 14 Figure 8: Typical Welded Test Specimen 18 Figure 9: Determination of True Test Specimen Mass Loss During Cleaning . 19 Figure 6C: Typical Spool Rack (End View) Figure 6D: Welded Test Specimen Rack II NACE International RP0497-2004 Section 1: General 1 .I 2.1
23、2.2 Scope 1.2 Limitations 1.1.1 This standard describes how field corrosion test- ing using metallic test specimens is conducted, what types of corrosion information may be obtained, and how test racks and test specimens are designed. A summary of critical data that must be recorded is pro- vided. G
24、uidelines for interpreting and reporting test results are also discussed. 1.1.2 Corrosion fatigue can be a serious problem, but no field methods of testing are known. Therefore, cor- rosion-fatigue testing is outside the scope of this standard. The ability of the field test program to predict system
25、 com- ponent petformance accurately depends on the design of the program and control of factors affecting results. These factors include test rack design and location, test specimen preparation and cleaning, and interpretation and reproduc- ibility of results. The choice of test rack location, for i
26、nstance, is very important because corrosion often does not occur uniformly in a piece of equipment. Each process has its own problems for monitoring. One of the most ser- ious limitations of field test racks is the inability to duplicate hot-wall heat-transfer effects, velocity or flow effects, and
27、 liquid-vapor intetface effects accurately. Section 2: Applications of Field Corrosion Testing Using Corrosion Rate Data 2.1.1 Observations and data derived from metallic test specimens exposed in the field are used to determine the average rate of corrosion and the type and severity of localized at
28、tack that occurred during the exposure period. The data may be used as part of an evaluation of potential materials of construction for use in similar service or for replacement materials in existing facili- ties. 2.1.2 Test specimen data do not provide information on changes in the corrosiveness of
29、 the environment that occur during the exposure period unless specific- ally planned testing techniques, such as the planned interval test, are used. 2.1.3 Corrosion rate data derived from a single expo- sure generally do not provide information on corrosion rate change versus time. Corrosion rates
30、may in- crease, decrease, or remain constant depending on the nature of the corrosion products. The onset of pitting or crevice corrosion can affect the validity of corrosion rate determination. Process changes may also affect corrosion rates. Using Metallic Test Specimens to Collect Data 2.2.1 Meta
31、llic test specimens must be designed to detect the forms of corrosion expected on a specific metal in a specific environment as shown in the follow- ing examples. 2.2.1 .I Tensile-stressed test specimens are re- quired to evaluate the susceptibility of a metal to stress corrosion cracking (SCC). How
32、ever, even if a stressed test specimen does not fail, SCC may still be occurring. 2.2.1.2 Metallic specimens tested in the solution- annealed condition do not undergo localized corro- sion associated with changes in microstructure caused by welding. 2.2.1.3 Evaluating crevice corrosion susceptibilit
33、y can be influenced by such matters as method of test specimen attachment, sutface finish, crevice geometry (.e., tightness), and crevice-area-to- specimen-area ratio. 2.2.1.4 Galvanic corrosion resulting from coupling metallic test specimens of different compositions is not predicted from specimens
34、 that are electrically isolated while tested. 2.2.1.5 The effect of added cyclic stresses, which may be due to such causes as vibrations, cannot be evaluated by specimen testing. 2.2.2 It is important for the equipment user to be fam- iliar with how various components are fabricated from such metal
35、forms as castings, forgings, and bar stock. Most valve bodies, pump impellers, and pump casings are castings. Cast and wrought alloys often behave differently in identical service conditions. In particular, many of the wrought alloys are more corrosion-resist- ant than the cast alloys that are commo
36、nly considered as equivalents, e.g., S31600 and CF8M, S30400 and CF8, and NO8020 and CN7M. In nearly all cases, the cast compositions are altered from those of the wrought alloys to improve castability. In addition, there may be a number of cast alloys commonly used as equivalents for a single wroug
37、ht alloy. For example, CW12MW, CWGM, CW2M, and CX2MW are all cast equivalents for wrought nickel-based Alloy C. NACE International 1 RP0497-2004 3.1 3.2 2.2.2.1 Castings are produced by a number of dif- ferent molding processes, such as green sand, air- set sand, resin-bonded sand, rammed graphite,
38、and investment. The corrosion resistance of the as-cast surface is a function of the molding pro- cess. Carbon pickup and mold reactions are just two of the factors that influence corrosion resist- ance. The corrosion resistance of most machined sutfaces is independent of the molding process if 2.0
39、to 3.0 mm (0.080 to 0.12 in.) of material is removed. 2.2.3 The petformance of the test specimens is helpful in estimating the life expectancy of existing compon- ents and can be used to develop maintenance and repair schedules for general corrosion. 2.2.3.1 The differences between the test speci- m
40、ens and the equipment that the test specimens are meant to simulate should be considered when using test specimen data to estimate the life expectancy of existing equipment. For example, differences in product form, metal sutface finish, heat treatment, or stress state influence the corro- sion rate
41、s of test specimens of the same com- position. In addition, the fluid flow characteristics around a test specimen may differ significantly from the flow in the equipment. Immersed test specimens may not corrode at the same rate as the same metal that is also acting as a heat trans- fer medium. 2.2.3
42、.2 The effectiveness of process control pro- grams can be monitored by periodically evaluating corrosion test specimens exposed to the process. For example, test specimens can be used to indi- cate whether the application of a corrosion inhib- itor to a process stream has adequately controlled corro
43、sion at a specific location. Section 3: Guidelines for Conducting a Field Corrosion Test Program General 3.1.1 The type of corrosion test program to be con- ducted depends on the nature of the process environ- ments involved and the metals being considered. Potential corrosion problems for each envi
44、ronment should be assessed, and metals with varying degrees of corrosion resistance should be selected for evalu- ation. Other factors, such as cost, mechanical proper- ties, and weldability, should also be considered and weighed against corrosion resistance. Plant exposure location and exposure per
45、iod are also important fac- tors. Corrosion specimen testing can be used to moni- tor various operations of a unit, e.g., diverse batch processes, shutdown, and chemical cleaning. Selecting Test Specimens and Exposure Locations 3.2.1 The type of test specimen and metal being evaluated depends on the
46、 form of corrosion expected. For example, if low pH, high-chloride solutions are pre- sent, then pitting and crevice corrosion are potential problems. Thus, test specimens and test racks should be designed with these forms of corrosion in mind. Significant concentrations of chlorides or caustic at e
47、le- vated temperatures also may cause concern for SCC and require exposure of stressed test specimens. 3.2.2.2 The metals that would be expected to develop significant corrosion within the planned expos u re period ; 3.2.2.3 The metals that are likely to be resistant to the process; 3.2.2.4 The equi
48、valent cast form of the metal if testing pertains to cast components. 3.2.3 Duplicate test specimens of each metal should be included. Duplicate test specimens should not be exposed next to one another on the test rack but should be separated widely. Test results for duplicate specimens should be re
49、ported separately and not averaged. 3.2.4 The selection of the test specimen exposure location in the process is an extremely important aspect of the test program. The four basic exposure locations are (see Figure 1 and Paragraph 5.4.1): 3.2.4.1 Liquid phase. 3.2.4.2 Vapor phase. 3.2.4.3 Splash, spray, or tidal zone. 2 3.2.2 The metals to be evaluated should consist of a 3.2.4.4 Waterline. selection of the following: 3.2.5 As another aspect of the test program, special 3.2.2.1 The metals currently used in the process consideration should be given to velocity, impingement, equipment where
