Regulatory Guide 3.37

Guidance for Avoiding Intergranular Corrosion and Stress Corrosion in Austenitic Stainless Steel Components of Fuel Reprocessing Plants

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Issue date:	09/30/1975
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U.S. NUCLEAR REGULATORY COMMISSION September 1975 REGU LATORY GUIDE

OFFICE OF STANDARDS DEVELOPMENT

REGULATORY GUIDE 3.37 GUIDANCE FOR AVOIDING INTERGRANULAR CORROSION AND STRESS CORROSION

IN AUSTENITIC STAINLESS STEEL COMPONENTS OF FUEL REPROCESSING PLANTS

A. INTRODUCTION

are designed to allow one to detect and control (a)

improperly processed or fabricated materials and (b)

Section 50.34, "Contents of applications: technical conditions that could lead to intergranular corrosion and information," of 10 CFR Part 50, "Ucensing of Produc- stress corrosion cracking during fabric'n or service of tion and Utilization Facilities," requires, among other austenitic stainless steels. The recc crendations of this things, that each application for a construction permit guide apply to austenitic stainlesROLused in safetyt for a fuel reprocessing plant include sufficient informa- related process equipment wit assodW vessels and tion relative to construction materials tqyrovide reason- piping, radioactive waste d storage systems, able asrance that the final design will conform to the metal liners of proc te storage tank design 'bases with adequate margin for safety. Section vaults, and other sa -rela tructures, systems, and

50.34 also requires a discussion of how the applicable components of fue requirements of Appendix B, "Quality Assurance Cri teria for Nuclear Power Plants and Fuel Reprocessing USSiON

Plants," to 10 CFR Part 50 will be satisfied. As used in Appendix B, "quality assurance" comprises D11 those e e and decontamination of radiochem planned and systematic actions necessary to provide i ce u ment, t structures, and components can adequate confidence that safety-related structures, sys- be t and hazardous. Cracks, crevices, and tems, and components will perform satisfactorily inr ies on the surfaces of radiochemical equip service. Appendix B requires, in part, that measures be y retain radioactive solutions or particulates.

established to ensure materials control and con s complicates decontamination both for decommis special processes, including welding and heat ating si g and for direct repair, maintenance, or inspection.

and to ensure the performance of re ng Cracks and defects can also cause equipment leakage by programs. These special processes must b MP initiating accelerated crevice corrosion from either the by qualified personnel using qualified p ures. operation or decontamination environment. To ensure public health and safety; the defense-in-depth concept Austenitic stainless steels are widely use mpon- dictates that process equipment, structures, and corn eats of fuel reprocessing plants because of their corro- ponents important to safety be designed, constructed, sion resistance to the sol encountered. However, operated, inspected, and maintained to keep their improper use, fabricati ning could result in integrity under all normal and accident conditions.

corrosion failure of e a nii tainless steel during fabrication or in acceptable practices for Welding is used extensively in the fabrication of controlling t e of ai eni 'c stainless steels can be radiochemical equipment because of the requirement for followed W vo te anular and stress corrosion. high quality construction and to minimize discontin These p t , iffer significantly because of uities, crevices, and leakage. Because the austenitic insufficien idance toward standardization. In the stainless steels, American Iron and Steel Institute (AISI)

interest of siardization, this guide specifies proce- 3XX series, generally have good resistance to corrosion dures acceptable to the NRC staff for controlling the use by nitric acid solutions and other chemically aggressive and testing of austenitic stainless steels to avoid inter- solutions used in the reprocessing of spent nuclear fuels, granular corrosion and stress corrosion. The procedures these steels are widely used in process equipment.

USNRC REGULATORY GUIDES Comments should be sent to the Secretary of the Commission. U.S. Nuclear Regulatory Guides are issued to describe and make available to the public Regulatory Commission. Washington. D.C. 2MM. Attention: Docketing and Service Section.

methods acceptable to the NRC staff of implementing specific parts of th Commission's regulations. to delineate techniques used by the staff In evalu- The guides are issued in the following ten broad divisions:

ating specific problems or postulated accidents. or to provide guidance to appli cants. Regulatory Guides are not substitutes for regulations, and compliance 1. Power Reactors S. Products with them Is not required. Methods and solutions different from those 5et out in j. Research end Test Reactors 7. Transportation the guides will be acceptable if they provide a basis tor the findings requisite to I Fuels and Materials Facilities "11 Occupational Health the issuance or continuance of a permit or license by the Commission. 4. Environmental and Siting S. Antitrust Review Comments and suggestions for improvements in these guides are encouraged S Materials and Plant Protection 10. General at all times. and guides will be revised, as appropriate, to accommodate com ments and to reflect new information or experence. However, comments on Copies of published guides may be obtained by written request indicating the this guide, if received within about two months after its Issuance, will be par. divisions desired to the US. Nuclear Regulatory Commisslo-s. Washington. D.C.

ticularly useful in evaluating the need for an early revision. 2M. Attention: Director. Office of Standards Development.

structures, and components of fuel reprocessing plants. ambient temperatures the alloy will remain in metastable However, unexpected, unpredictable, and unnecessary equilibrium, i.e., the structure will remain austenitic corrosion failure of austenitic stainless steels can result with the carbon in solid solution because the diffusion from their improper application, fabrication, cleaning, of carbon is virtually nonexistent at these temperatures.

and welding. For example, the improper selection and However, when the austenitic stainless steels are sub welding of an austenitic stainless steel can leave the jected to elevated temperatures, e.g., in the range of 800

mnaterial in a sensitized condition susceptible to inter to 1600*F, carbon diffusion rates are high and the alloy granular corrosion in its service environment. This will tend to reach stable equilibrium. The carbon corrosion can lead to roughening of surface and to grain diffuses to the grain boundaries where surface is avail boundary separation which allows radiochemicals to able for the nucleation of a new phase. The carbon then concentrate, making decontamination difficult and precipitates in the form of very small and thin chromium eventually leading to accelerated and unexpected inter carbides which grow at grain boundaries and into granular corrosion failure and leakage. Similarly, adjacent grains.

improper control of environment can lead to failure by stress corrosion cracking. The following discussions The chromium atoms that combine to form carbides present the metallurgical conditions and causes leading are directly adjacent or close to the carbon atoms near to sensitization of austenitic stainless steel and steps that the grain boundaries. With increased time at temperature can be taken to avoid sensitization. Controls to avoid and carbon levels above 0.02 wt% (the solubility limit of stress corrosion cracking will also be discussed briefly. carbon at these higher temperatures), more carbides form at the grain boundaries and thus more chromium is Through these discussions, the engineer who must depleted from these areas. This results in a band of steel design fuel reprocessing plants, but who is not neces surrounding the grain boundaries whose chromium level sarily an expert in the metallurgy of stainless steels, can is substantially lower than that required to form a gain a better 'understanding and appreciation of inter passive film. The corrosion properties of the band granular corrosion and stress corrosion. These discus therefore approach those of plain carbon steel. The sions should also aid the engineer in controlling the diffusion of chromium at these temperatures is slow, un application and use of austenitic stainless steels to avoid like that of carbon, and chromium atoms from the bulk or minimize unexpected and unnecessary corrosion of the grain do not reach the chromium-depleted zone in failures of fuel reprocessing plant process equipment, appreciable numbers. The metallurgical microstructure structures, and components. of austenitic stainless steel thus exposed to elevated temperatures consists of austenitic grains whose bulk L Sensitization and Intergranular Corrosion of Austen chromium composition is nearly equal to the average itic Stainless Steel chromium composition of the alloy. This structure is surrounded by an envelope (sometimes discontinuous)

Steels become "stainless," i.e., resistant to corrosion, of chromium-depleted steel in contact with chromium by. virtue of their chromium content. A minimum carbides at and near the grain boundaries.

amount in the vicinity of 12' percent chromium by weight is required for a steel to exhibit stainless The chromium-depleted zone and the bulk of the properties. Above this chromium level, a protective grain form a dissimilar metal couple. The chromium oxide film forms on the exposed stainless steel surface. depleted zone is the anode of this couple and has a much smaller area than the remainder of the cathodic grain.

At ambient and mildly elevated temperatures, prop When stainless steel of this structure is exposed to erly processed commercial austenitic stainless steels exist chemically aggressive solutions such as oxidizing acids.

in metastable equilibrium. The equilibrium structures of the chromium-depleted layer is actively corroded while plain iron-carbon-chromium-nickel stainless steels con the major portion of the grain remains unattacked. The tain two phases (1) austenite, the face-centered cubic attack completely removes the depleted layer surround form of iron with chromium, nickel, and less than 0.01 ing the grains until the grains fall out of the structure.

weight percent (wt%) carbon in solid solution and (2) This gives rise to a high rate of corrosion penetration chromium carbide, (Cr, Fe) 2 3 C6 . Properly processed and to eventual failure.

commercial austenitic stainless steels, however, consist only of the austenite phase. They are metastable because Thermal treatments or exposures that cause chrom they contain carbon in solid solution in excess of their ium carbide precipitation at the grain boundaries with solubility limits for these temperatures, i.e., they are the accompanying chromium-depleted zones are called supersaturated with carbon. These stainless steels are sensitization treatments or exposures. They leave the single phase because they have been rapidly cooled from stainless steel in a sensitized condition or with a elevated temperatures where the solubility of carbon is sensitized structure. This means that the stainless steel is high and therefore is in solid solution. The rapid cooling sensitive to selective corrosion at the grain boundaries.

prevents precipitation of excess carbon as the solubility The corrosion of the chromium-depleted layer surround of carbon decreases with decreasing temperature. At ing the grain boundaries is called intergranular corrosion.

K

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Temperature exposures for the commercial 3XX series ture-metallurgical reaction relationships for the carbides austenitic stainless steels leading to sensitized structures in 3XX stainless steels:

susceptible to intergranular corrosion in most cases are in the range of 800 to 1600WF. In practical applications, a. Between ambient temperature and the lower sensitization of austenitic stainless steels may result from bound temperature of the sensitization range, no carbide hot forming operations, stress relieving heat treatments, precipitation takes place.

inservice exposure to elevated temperatures, and welding. b. In the sensitization temperature range, only the chromium carbide precipitates.

Several metallurgical schemes are used commercially to control or counteract sensitization and the resulting c. Between the upper temperature of the sensitiza.

susceptibility to intergranular corrosion cracking of tion range and approximately 2250°F, chromium austenitic stainless steels. Many of the normal carbon carbide dissolves and the stabilizing element carbide (of grades 3XX series stainless steels contain a maximum of titanium, niobium, or tantalum) forms.

0.08 wt% carbon, and a few types (e.g., 310) contain as much as 0.25 wt% carbon. One method of controlling d. Above about 2250°F both the stabilizing element sensitization is to lower the carbon content of the carbide and chromium carbide dissolve.

stainless steel to a level near its solubility limit at the sensitizing temperature range. Commercial grades of the If the stainless steel with a stabilizingelement is held at a

3XX series stainless steels are produced called the "L" temperature in the range between the upper sensitization grades, meaning low carbon content, (e.g., 304L) which temperature and approximately 22500 F (e.g., 19500 F),

contain less than 0.03 wt% carbon. At these carbon only the stabilizing element carbide forms and the excess levels, the stainless steel will not sensitize in many carbon is tied up by its formation. The rate of cooling practical and properly executed forming and welding following this exposure does not significantly affect the processes when it is subjected to sensitizing temperatures precipitation of other carbides. Since the stabilizing for a reasonably short time. However, even the "L" (low carbide is also stable at lower temperatures, no carbon is carbon) grades of austenitic stainless steels will precipi available for formation of. chromium carbide in the tate chromium carbides and become sensitized when sensitization temperature range. Therefore, if a stabilized subjected to sensitizing temperatures for longer times. stainless steel is reheated in the sensitization temperature range where chromium carbides would normally form, Another method for controlling sensitization in there is no carbon available for their formation, and austenitic stainless steel is the addition to its composi sensitization due to chromium depletion does not occur.

tion of elements that, under certain conditions, are The commercial stabilized stainless steels (e.g., 321 and stronger carbide formers than chromium. Elements used 347) are given a stabilization heat treatment at the steel for this purpose are titanium, niobium (columbium), and mill and their as-received structure consists of a stabil tantalum. Stainless steels containing these elements that izing carbide in an austenitic matrix.

have been properly processed to precipitate their carbides are called stabilized grades. Type 321 is an A third method of controlling intergranular corrosion austenitic stainless steel that has been stabilized with the susceptibility in sensitized austenitic stainless steel is addition of titanium. Type 347 is a grade stabilized with through a solution anneal and quench heat treatment.a niobium or niobium plus tantalum. These stabilizing This consists of heatini the stainless steel to a tempera.

elements are also strong oxide, sulfide, and nitride ture of 1950 to 2050 F, maintaining that temperature formers. Because the steel is normally deoxidized (by long enough for all the carbides to go into solid solution, the addition of aluminum, for example) prior to the and then quenching or otherwise cooling the steel addition of the stabilizing element, very little of the quickly enough through the sensitization temperature stabilizing element is used up by the formation of its range to avoid reprecipitation of carbides. This treat oxide. However, some of the stabilizing element added ment will provide a stainless steel with a single solid to the stainless steel combines with the sulfur and solution phase not susceptible to intergranular corrosion nitrogen present In the melt to form sulfides and attack (unless it is again heated In the sensitization nitrides. Therefore, to tie up all of the excess carbon temperature range).

present in the stainless steel with the stabilizing element, more than the stolchiometric amount necessary to form its carbide is required. In the commercial grades of Two important phenomena associated with intergran stabilized stainless steels (e.g., 321 and 347), a minimum ular corrosion of fabricated stainless steel components level of stabilizing element based on the carbon content is specified.

aA solution anneal and quench heat treafment means heating to An understanding of how stainless steel is stabilized a suitable temperature, holding at the temperature long enough to cause all carbides to enter into solution, and then cooling can be gained by consideration of the following tempera. rapidly enough to keep the carbon in solution.

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are (1) weld decay and (2) knife-line attack. Weld decay steels should be solution heat treated and quenched is the intergranular corrosion in the heat-affected zone following welding for service in corrosive media. If a of the base metal at some distance from and parallel to solution annealing and quenching treatnent of the K

the weld bead. Weld decay usually occurs in unstabilized component is not feasible, low carbon grades or stabil austenitic stainless steel of normal carbon composition ized stainless steels should be used. However, as discus as. a result of temperature exposure from the heat of sed, improper welding of these steels can also cause welding and subsequent service in a corrosive environ sensitization. Significant sensitization does not normally ment. When a weld is made, some of the heat is result when typical welding procedures and material dissipated through the base metal. The temperature is chemistry are used and when no further heating of highest in the molten weld pool, and a complex thermal material occurs. The welding procedures and material gradient exists through the weld and adjoining material. chemistry should be controlled to prevent undue sensiti The temperature distribution in the metal surrounding zation during welding. Control should be exercised to the weld is a function of the base metal thickness and include the following: (1) maintaining low heat input to conductivity, preheat and interpass temperature, welding weld joint by controlling current, voltage, and arc travel heat input and speed, and other factors. However, at speed, (2) limiting interpass temperature, (3) using some distance from the weld bead in the heat-affected stringer bead technique and the limiting of weaving, and zone, the base metal is exposed to temperatures in the (4) limiting the carbon content of the material where sensitizing range. In the normal carbon grades and section thickness results in increased time at unstabilized stainless steels, a band of chrondum sensitization temperatures.

depleted steel parallel to the weld bead develops due to the precipitation of chromium carbides; the band is thus sensitized. If the weldment is subsequently exposed to In nitric acid solutions and possibly in other highly aggressive solutions, intergranular corrosion occurs in oxidizing solutions, severe intergranular corrosion has this band of material parallel to the weld. This been associated with the precipitation of sigma phase in phenomenon is called weld decay. molybdenum-containing types 316 and 317 and in the carbide stabilized types 321 and 347 stainless steels. The Knife-line attack is another phenomenon that may sigma phase is a metallurgical reaction product of ferrite occur in the stabilized grades of austenitic stainless steels and forms in the temperature range 1150 to 1550WF.

as a result of welding. It is due to the sensitization of a Often the sigma phase is submicroscopic. Also, in nitric very thin layer, a few grains wide, directly adjacent to acid the austenitic stainless steels can undergo severe and on either side of the weld bead. During welding this layer of stainless steel is exposed to temperatures just end-grain corrosion. This is accelerated corrosion of the stainless steel when the grains are exposed to the K

below its melting point. At these temperatures the solution in transverse cross section, for example, the solubility of carbon is very high and the stabilizing through thickness transverse cross section of rolled plate.

carbides go into solid solution. Subsequently this zone cools quickly enough through the temgerature range It should also be recognized that intergranular corro (approximately 2250eF down to 1600 F) where the sion of austenitic stainless steels can take place in the stabilizing carbides form and the carbon and stabilizing nonsensitized condition when these steels are exposed to element remain in solution. This layer is then susceptible strongly oxidizing solutions. This corrosion occurs in to sensitization. The layer will sensitize if the stainless exposures of these steels to strong boiling nitric acid steel is exposed to the sensitization temperature range solutions containing the Cr6 + ion. This ion can be where chromium carbide forms but the stabilizing present and can accumulate in the solution hs a result of carbide does not form. This exposure can result (1) from the corrosion process of the stainless steel itself The deposition of multiple weld beads, (2) when the thick presence of any of the ions, C6+,Fe3+ ,Ce4+Xn1+ or ness of the welded section is great enough to slow the V5+, in solutions of boiling nitric acid can cause cooling rate through the sensitization temperature range, intergranular corrosion of nonsensitized austenitic stain or (3) if welded parts are given a postweld heat less steels. The phenomenon can be attributed to the treatment, such as a stress relief, in the sensitization segregation of elements to the grain boundaries, thereby temperature range. If stabilized stainless steels that have altering the electrochemical properties of the boundary been thus fabricated are exposed to corrosive media, with respect to the grain and causing accelerated intergranular corrosion in a very thin band on either side corrosion of grain boundary areas in the solutions of the weld, called knife-line attack, may result. mentioned above. Silicon and phosphorus can have a significant effect on the resistance of nonsensitized As indicated by the previous discussion, intergranular austenitic stainless steels to intergranular corrosion.

corrosion may occur for several diverse reasons and many opportunities exist for stainless steels to be Proper choice of austenitic stainless steels is impor sensitized by the fabrication process or the service tant; both the fabrication procedures and inservice environment. Stainless steels of normal carbon content environmental exposure conditions should be considered will usually be sensitized by welding. Therefore, these in the selection. It should be shown by corrosion testing

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of as-received and fabricated materials and by qualifica.

tion of procedures that stainless steels are not placed in- Control can be accomplished by testing as-received I material to ensure that it has been properly processed service in fuel reprocessing plants in a sensitized t and heat treated and that it is of the proper composi condition. tion. For example, results of the corrosion screening test can indicate whether low carbon grades are low enough

2. Intergranular Corrosion Testing in carbon content and whether stabilized grades are properly processed and contain enough stabilizing Quality control testing for intergranular corrosion of elements. Also, these tests should be used to qualify austenitic stainless steels is needed to ensure that steels the welding process and procedure including, when that have been sensitized and are susceptible to inter required, any postweld solution or stabilization heat treatment.

granular corrosion are not used for fuel reprocessing This is to ensure that stainless steel structures, systems, plant components exposed to aggressive solutions. These and components are not placed in service in a sensitized quality control corrosion tests are not designed to reproduce condition. Test weldments from which corrosion inservice conditions, but rather to detect testing metallurgical microstructural conditions known to cause coupons are obtained should be made from each heat of inservice intergranular corrosion in aggressive solutions. stainless steel and for each welding and heat treatment The value of these tests, therefore, is to enable control process or procedure. Base metal of the same thickness of the use of sensitized stainless steel for service and the same filler metal as the production weld should applications, not to predict the service life of stainless be used in making the test weldment. The same welding process and variables, such as heat input, travel steel components. speed, number of weld passes, and preheat and interpass Several corrosion tests are available for detecting temperature, should be used for both the test weldment susceptibility of stainless steel to intergranular corrosion and the production weld. The test weldment (prior to due to sensitization by chromium carbide and/or sigma obtaining corrosion coupons) should be heat treated or precipitation. Some of the more widely accepted tests otherwise exposed to time-temperature cycles to simu have been standardized and are described in American late any thermal exposure or treatment imposed on the Society for Testing and Materials (ASTM) Standard A production weldment and component after fabrication.

262-70, "Recommended Practiees for Detecting Suscep The test weldment must be large enough for the tibility to Intergranular Attack in Stainless Steels."k corrosion testing coupons taken from it to simulate the These tests have been proven useful by comparisons of microstructure and condition of(1) the bulk production laboratory and inservice test results. ASTM Standard weld and its associated heat affected zone and (2) the A finished base material.

262-70 describes recommended practices for five tests:

The boiling 65 wt% nitric acid test (Practice C, ASTM

a. PracticeA - Oxalic acid etch test for classification A 262-70) is sometimes considered too severe of etch structures of stainless steels, for stainless steels that are used in less corrosive media.

The test detects intergranular corrosion due to chromium b. Practice B - Ferric sulfate-sulfuric acid test for depletion and sigma phase. It can also show increased detecting susceptibility to intergranular attack in corrosion rates due to end-grain corrosion and attack stainless steels, of chromium carbides and of titanium carbides in stabilized stainless steels. Also a product of stainless steel corrosion c. PracticeC - Nitric acid test for detecting suscepti itself, the highly oxidizing Cr6 + ion increases the bility to intergranular attack In stainless steels, corrosiveness of the nitric acid solution. However, all of the above considerations are Important for stainless steel d. Practice D - Nitric-hydrofluoric acid test for service in nitric acid. Therefore, the boiling nitrid acid detecting susceptibility to intergranular attack in test (Practice C, ASTM A 262-70) should be used for the molybdenum-bearing austenitic stainless steels, intergranular corrosion testing of austenitic stainless steel exposed to nitric acid solutions in fuel reprocessing e. Practice E - Copper-copper sulfate-sulfuric acid plants.

test for detecting susceptibility to intergranular attack in stainless steels. For details on how to perform the boiling nitric acid test, refer to the ASTM Standard A 262-70 practice. A

If these tests are used as recommended in, ASTM brief description of the test is given here to facilitate A

262-70 and the limitations of the various practices for following discussions. A cleaned and weighed stainless specific alloys are observed, the results of these tests can steel specimen is exposed to the boiling 65 wt% nitric allow control of the use of austenitic stainless steels acid solution for five consecutive 48-hour testing in fuel reprocessing plants. periods. Between each 48-hour test period the specimen is cleaned, reweighed, and then tested in fresh solution aCopies of this Standard may be obtained from ASTM, 1916 for each period. The test results consist of (1) weight Race .Street, Philadelphia, PA 19103. loss data for each 48-hour period and (2) the average weight loss for the full 240-hour test.

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which it has no effect. If either control of the stress or The ASTM standard practice does not give criteria to control of the chemical, or both, can be accomplished, enable determination of whether the test results indicate .then stress corrosion cracking can be avoided. In those intergranular corrosion susceptibility of the stainless cases where neither control of the stress nor control of steel tested. When the data show increasing weight losses the chemical Can be accomplished, a different material for successive 48-hour test periods, this should indicate must be sought that is not susceptible to stress corrosion susceptibility of the stainless steel tested to intergranular (or other types of corrosion) in the particular corrosion. Corrosion rates calculated from the weight environment.

loss data, exceeding 0.018 inches per year, should also indicate intergranular corrosion susceptibility. Metallo If the stainless steel is sensitized, intergranularstress graphic examination of the corrosion tested specimen corrosion cracking sometimes occurs under the condi may also be performed to determine if intergranular tions that normally lead to transgranular stress corrosion corrosion took place during the test. cracking in the annealed stainless steel.

The other practices'of ASTM A 262-70 may also be used, as applicable, for intergranular corrosion testing of Intergranular stress corrosion cracking has occurred both in solution-annealed and in sensitized stainless stainless steels for service in solutions less corrosive than steels and in environments that may not necessarily nitric acid. Of these other practices, Practices B and E

induce transgranular stress corrosion cracking. In these are preferred, in that order. Practice A, the oxalic acid instances the role of the stress may be only to open the screening test can be used, as applicable, except for crack and allow solution to reach fresh metal while the stainless steel service in nitric acid solutions when the failure is induced by intergranular corrosion. Examples boiling 65 wt% nitric acid test, Practice C of ASTM A of interest are intergranular stress corrosion cracking of

262-70, is the recommended test. austenitic stainless steel in high temperature-high purity

3. Stress Corrosion water, cracking caused by fluoride-containing weld fumes, and cracking from fluoride residual contamina The austenitic stainless steels in the annealed tion remaining on surfaces that have been pickled with condition are susceptible to transgranularastress corro nitric acid-hydrofluoric acid solutions. In situations similar to these, where stress-assisted intergranular corro sion cracking. The environments producing stress corro sion occurs, those treatments that improve intergranular sion cracking normally havd no detrimental effect on the general corrosionb resistance of the stainless steel. These corrosion resistance can also improve the intergranular stress corrosion resistance. However those treatments do environments include caustic solutions and chloride solutions, even at chloride concentrations of a few parts not necessarily eliminate intergranular stress corrosion crackin&

per million in the presence of an oxidizer or oxygen of a few parts per million concentration. The physical state of the exposure is important. For example, under Process controls should be exercised during all stages alternate wetting and drying exposure when traces of of component manufacturing and construction to chlorides have a chance to concentrate on the stainless minimize exposure of stainless steel to contaminants steel surface, accelerated stress corrosion occurs. Stress that could lead to stress corrosion cracking. Since some corrosion cracking requires simultaneous action of both degree of material contamination is inevitable during a tensile stress and some particular corrosive agent these operations, halogens and halogen-bearing (although this agent would not normally result in general compounds (e.g., die lubricants, marking compounds, corrosion of the stainless steel). The stress may be and masking tape) should be avoided to the degree applied or it may be a residual stress left In the structure practical.

from a fabrication operation.

All cleaning solutions, processing compounds, To avoid stress corrosion cracking in those systems degreasing agents, liquid penetrant examination mater where it occurs, the stress can be eliminated or reduced ials, and other foreign materials should be completely to a level below a certain critical stress that is dependent removed at any stage of processing prior to any elevated upon the exposure temperature, the solution concentra temperature treatment and prior to any pressure testing.

tion, and the composition and microstructure of the Reasonable care should be taken to keep (1) fabrication stainless steel. Alternatively, the responsible chemical and construction areas clean, (2) components protected can be inhibited, eliminated, or reduced to a level below and dry during storage and shipment, and (3) all crevices and small openings protected against contamination.

Pickling of stainless steel should be avoided. Special aTransgranular means through the grains; Intergranua*r means precautions should be taken to avoid surface contamina along or at the grain boundaries. " tion with fluorides from welding rod coatings and fluxes.

bAs used In this guide, general corrosion Is the deterioration of The quality of water used for final cleaning or flushing metal by chemical or electrochemical reaction with its environ of finished surfaces should meet, as a minimum, the ment which proceeds uniformly over the exposed surface (e.g.,

the rusting of steel In the atmosphere).

requirements of ANSI Standard N45.2.1-1973, K.

337-6

"Cleaning of Fluid Systems and Associated Components number of weld passes and postweld heat treatment During Construction Phase .of Nuclear Power Plants."a practices; and (d) for any other important change in the Liquid penetrant examination materials should meet the fabrication of components. Test sections from which requirement of T-630 of Article 6 of Section V of the corrosion coupons are tested should be welded and American Society of Mechanical Engineers Boiler and treated to represent the production weldments and Pressure Vessel Code.b finished products. All important variables should be reproduced for fabrication and treatment of qualifica tion test sections as discussed in Section B of this guide.

C. REGULATORY POSITION

Austenitic stainless steels of the AISI Type 3XX 5. Stainless steel base materials used for component series used for safety-related process equipment with its and system manufacture should be in a solution heat associated vessels and piping, radioactive waste handling trnated condition or in a stabilized condition for the and storage systems, metal liners of process cells and stabilized grades. Nonsusceptibility to intergranular cor waste storage tank vaults, and other safety-related rosion should be shown by corrosion testing of starting structures, systems, and components of fuel reprocessing material, unless the entire component or system is to be plants should meet the following conditions: solution heat treated or given a stabilization heat treatment after fabrication. In this case, both the weld

1. Stainless steels should not be placed in service in a with Its heat affected zone and the base material should sensitized condition that is susceptible to intergranular be qualification corrosion tested in their final fabricated corrosion. and heat treated condition.

2. Nonsusceptibility to intergranular corrosion should be verified using ASTM A 262-70, "Recom 6. If fabricated components cannot be solution heat mended Practices for Detecting Susceptibility to Inter treated, the normal carbon grades stainless steels should granular Attack in Stainless Steel," Practice C, the not be used. Low carbon or stabilized stainless steels boiling 65 wt% nitric acid test, for stainless steel service should be used if the fabrication process can be qualified in solutions of nitric acid. For other service, Practice C by corrosion testing to show nonsusceptibility to inter or the other practices of ASTM A 262-70 may be used as granular corrosion.

applicable and as discussed in Section B of this guide.

3. Nonsusceptibility to intergranular corrosion 7. Heat treatments in the sensitization temperature should be shown for stainless steel process equipment, range (800 to 1600*F) after component fabrication and structures, and components in their final fabricated prior to corrosive environment exposure or inservice condition prior to service in corrosive environments. exposures to these temperatures should not be allowed This should be shown by corrosion testing of base unless it can be shown by appropriate tests or extensive material and by qualification corrosion testing of weld inservice experience that the stainless steel will not be ments and their associated heat-affected zones. made susceptible to intergranular corrosion by these actions. Components on which inservice experience is

4. Quality control testing for intergranular corrosion based should be destructively tested (e.g., metallography should be performed' on the base materials of the and mechanical testing) to show positively that inter austenitic stainless steel product forms produced from granular corrosion did not result in the component base each different heat and final heat treatment practice. materials, weldments, and heat-affected zones during Qualification intergranular corrosion testing for fabri service.

cated components should be performed (a) for each change in stainless steel type as represented by a different AISI 3XX designation, using either base mater 8. Stainless steels should be suitably cleaned and ial having the maximum carbon content anticipated. or suitably protected against contaminants capable of base material from each heat; (b) for each change in causing stress corrosion cracking during fabrication, nominal thickness of the sections welded; (c) for each shipment, storage, construction, testing, and operation change in welding method or procedure including of process equipment, structures, and components. The changes in joint design, filler metal type and size, quality of water used for final cleaning or flushing of finished surfaces should meet, as a minimum, the requirements of ANSI Standard N45.2.1-1973, "Clean aCopies may be obtained from the American National Standards ing of Fluid Systems and Associated Components During Institute, Inc., 1430 Broadway, New York, N.Y. 10018. Construction Phase of Nuclear Power Plants."a Liquid penetrant examination materials should meet the re bCopies of the 1974 Edition may be obtained from American quirements of T-630 of Article 6 of Section V of the Society of Mechanical Engineer:, United Engineering Center, American Society of Mechanical Engineers Boiler and j 345 East 47th Street, New York, N.Y. 10017.

Pressure Vessel Code.b

3.37-7

D. IMPLEMENTATION

submittals for operating license or construction permit applications docketed after May 31, 1976. 11-.

The purpose of this section is to provide information to applicants and licensees regarding the staff's plans for using this regulatory guide. If an applicant wishes to use this regulatory guide in Except in those cases in which the applicant proposes developing submittals for applications docketed on or an alternative method for complying with specified. before May 31, 1976, the pertinent portions of the portions of the Commission's regulations, the method application will be evaluated on the basis of this described herein will be used in the evaluation of guide.

UNITED STATES

NUCLEAR REGULATORY COMMISSION

WASHINGTON. 0. C. 20555 POSTAGE AND FEES PAID

UNITEO STATES NUCLIEAR

OFFICIAL BUSINESS NEGULA'AON V "

COMMISSI 4$tON

PENALTY FOR PRIVATE USE. $300

K.

337-8