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Background Statement for SEMI Draft Document 5876 New Standard: Test Method for Determining the Critical Pitting Temperature of Stainless Steel Surfaces Used in Corrosive Gas Systems by Use of a Ferric Chloride SolutionNotice: This background statement is not part of the balloted item. It is provided solely to assist the recipient in reaching an informed decision based on the rationale of the activity that preceded the creation of this Document.

Notice: Recipients of this Document are invited to submit, with their comments, notification of any relevant patented technology or copyrighted items of which they are aware and to provide supporting documentation. In this context, “patented technology” is defined as technology for which a patent has issued or has been applied for. In the latter case, only publicly available information on the contents of the patent application is to be provided.

BackgroundA simple Test Method is needed for comparing corrosion resistance of stainless steels intended for fabrication of semiconductor gas supply components. The Test Method is a chemical procedure that does not require expensive analytical equipment.

This Test Method describes a procedure that may be used to rank the pitting corrosion resistance of wetted surfaces of tubing or components of finished surfaces intended for use in corrosive gas systems. Pitting corrosion is believed to be a major corrosion failure mode in semiconductor gas delivery systems, particularly in components and tubing welded and exposed to corrosive gases and moisture.

This Test Method is an adaptation of ASTM G48 Method E, Critical Pitting Temperature Test for Stainless Steel. The adaptation describes a method for performing the Test Method on coupons or wetted surface sections cut from gas supply system components or tubing. It is an aqueous immersion method in an acidified ferric chloride solution.

This Test Method is reproducible and provides a metric (i.e., critical pitting temperature) in addition to a qualitative (i.e., visual) evaluation of corrosion resistance.

The ballot results will be reviewed and adjudicated at the meetings indicated in the table below. Check http://www.semi.org/standards under Calendar of Events for the latest update.

Review and Adjudication Information

Task Force Review TC Chapter AdjudicationGroup: Materials of Construction of Gas Delivery

Systems Task ForceNA Gases TC Chapter

Date: Monday, November 2, 2015 Tuesday, November 3, 2015Time & Time Zone: 08:00 – 09:30 Pacific Time (Tentative) 9:00 – 12:00 Pacific TimeLocation: SEMI Headquarters in conjunction with the

Fall NA Standards Meetings SEMI Headquarters in conjunction with the Fall NA Standards Meetings

City, State/Country: San Jose, California/USA San Jose, California/USALeader(s): Tim Volin (Parker Hannifin) Tim Volin (Parker Hannifin)

Mohamed Saleem (Fujikin)Standards Staff: David Bouldin

972.965.0340/dbouldin@semi.org

David Bouldin 972.965.0340dbouldin@semi.org

This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an official or adopted Standard or Safety Guideline. Permission is granted to reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.

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*This meeting’s details are subject to change, and additional review sessions may be scheduled if necessary. Contact the task force leaders or Standards staff for confirmation.

Telephone and web information will be distributed to interested parties as the meeting date approaches. If you will not be able to attend these meetings in person but would like to participate by telephone/web, please contact Standards staff.

This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an official or adopted Standard or Safety Guideline. Permission is granted to reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.

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SEMI Draft Document 5876 New Standard: Test Method for Determining the Critical Pitting Temperature of Stainless Steel Surfaces Used in Corrosive Gas Systems by Use of a Ferric Chloride Solution1 Purpose1.1 The purpose of this Test Method is to determine the relative resistance to pitting corrosion of the wetted surfaces of components intended for use in corrosive gas distribution systems for semiconductor manufacturing. This Test Method is an adaptation of American Society for Testing and Materials (ASTM) G48 and is intended to differentiate between alloy compositions or processing that may affect the corrosion performance of the wetted surfaces. Detailed information on the method is found in ASTM G48.

2 Scope2.1 This Test Method describes a procedure which may be used to rank the pitting corrosion resistance of wetted surfaces of tubing or components of finished surfaces intended for use in corrosive gas systems. Pitting corrosion is believed to be a potential corrosion failure mechanism in semiconductor gas delivery systems, particularly in components and tubing welded and exposed to corrosive gases with moisture present.

2.2 This Test Method is an adaptation of ASTM G48 Method E, Critical Pitting Temperature Test for Stainless Steel. The adaptation describes a method for performing the Test Method on coupons or wetted surface sections cut from gas supply system components or tubing. It is an aqueous immersion method in an acidified ferric chloride solution.

2.3 This Test Method is reproducible and provides a metric (i.e., critical pitting temperature) in addition to a qualitative (i.e., visual) evaluation of corrosion resistance.

NOTICE: SEMI Standards and Safety Guidelines do not purport to address all safety issues associated with their use. It is the responsibility of the users of the Documents to establish appropriate safety and health practices, and determine the applicability of regulatory or other limitations prior to use.

3 Limitations3.1 The test conditions of this Test Method do not simulate actual service in a corrosive gas supply system. Thus the test results may not correlate to relative corrosion resistance in a specific corrosive gas or corrosive gas supply system.

3.2 The Critical Pitting Temperature (CPT) is valid only in a range of 10°C to 95°C. A CPT result greater than 95°C is not considered to be valid since it is approaching the boiling point of the solution. A critical pitting temperature (CPT) below 10°C shall only be reported as <10°C and may be an indication of crevice corrosion, which renders the test invalid.

3.3 Since alloy composition and surface parameters can affect the results of the test, all variables other than the one being tested must be fixed during the test in order to obtain reproducible and comparable results.

3.4 The test is destructive of the component tested. The component should not be placed in service following test.

This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an official or adopted Standard or Safety Guideline. Permission is granted to reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.

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4 Referenced Standards and Documents4.1 SEMI Standards

SEMI F19 — Specification for the Surface Condition of the Wetted Surfaces of Stainless Steel Components

4.2 ASTM Standards1

ASTM G15 — Standard Terminology Relating to Corrosion and Corrosion Testing

ASTM G48 — Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution

NOTICE: Unless otherwise indicated, all documents cited shall be the latest published versions.

5 Terminology1: Unless otherwise stated, the terminology relating to corrosion and corrosion testing is as defined in ASTM G15.

5.1 Abbreviations and Acronyms

5.1.1 AISI — American Iron and Steel Institute

5.1.2 ASTM — American Society for Testing and Materials

5.1.3 CPT — critical pitting temperature

5.1.4 GP — general purity [GP]

5.1.5 HP — high purity

5.1.6 UHP — ultra-high purity

5.2 Definitions

5.2.1 crevice corrosion — corrosion at a cavity between adjoining pieces or at the edge of a seal or masking paint. The chemical mechanism of crevice corrosion is similar to that of pitting, the cavity being a ‘preformed’ pit. As corrosion is initiated at the cavity, the critical temperature for crevice corrosion is always lower than the critical pitting temperature (CPT).

5.2.2 critical pitting temperature (CPT) — the lowest temperature at which pitting occurs on the test surface under the specified test conditions.

5.2.3 pitting — corrosion of a metal surface, confined to a point or small area, that takes the form of cavities.

5.2.4 wetted surface — surfaces of a component in contact with the contained fluids.

6 Summary of Test Method6.1 The test specimen is immersed in a solution of 6 % FeCl3 by mass and 1 % HCl at 10°C for 24 hours followed by visual examination for pitting. If pitting is not present the test is repeated at 5°C higher temperature increments until pitting is observed. The temperature at which pitting is observed is reported as the critical pitting temperature (CPT).

7 Possible Interferences7.1 A possible interference is failure by crevice corrosion in which case the test is considered invalid.

7.2 Crevice corrosion can occur at any point of contact of the area under test with another object, such as a specimen holder or container, or at the edge of the masking media. The crevice corrosion sites exist at the contact sites or the masking boundaries. Figure 1 gives an example of the typical appearance of crevice corrosion sites from a critical pitting test performed on type 316 stainless steel using masking.

1 American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, Pennsylvania 19428-2959, USA. Telephone: 610.832.9585; Fax: 610.832.9555; http://www.astm.org

This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an official or adopted Standard or Safety Guideline. Permission is granted to reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.

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7.3 Crevice corrosion also has a critical temperature that is always less than the CPT. The critical crevice corrosion temperature for American Iron and Steel Institute (AISI) 316L is less than 10C. Test results for AISI 316L less than 10C achieved by the use of this Test Method are likely due to crevice corrosion and are considered invalid.

7.4 Sulfur is added to stainless steel to improve machinability and welding penetration and exists as manganese sulfide precipitate inclusions. These precipitates are preferred sites for the initiation of pitting when revealed on the surface under test. Surfaces that are not parallel to the mechanical working direction of the stainless steel may intersect a large number of the inclusions elongated by the mechanical working and therefore exhibit high pitting densities and deep pitting.

8 Apparatus8.1 The apparatus necessary for this Test Method consists of a test container, a constant temperature bath, and a specimen holder.

8.1.1 Test container – glass flask or test tube

8.1.1.1 Glass flask requirements – 1,000 mL wide-mouth laboratory flask

8.1.1.2 Test tube requirements – If the test specimens are small, a test tube may be used of sufficient size to completely immerse the test surface of the test specimen and have a solution volume of at least 150 mL and 5 mL/cm2 (30 mL/in.2) of the test surface area.

8.1.2 Constant temperature bath – A water or oil bath having the capability of maintaining a constant temperature between 5°C and 85°C ± 1°C.

8.1.3 Specimen holder – If an unmasked test specimen is used, it can be supported on cradles or hooks as described in ASTM G48. A test specimen that is masked per ¶ 9.6 can be placed on the bottom of the test container as long as the unmasked area of test interest is not touching the container, such that crevice corrosion may be induced.

This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an official or adopted Standard or Safety Guideline. Permission is granted to reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.

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Figure 1Examples of Pitting and Crevice Corrosion (Dashed Circles) on Type 316

Stainless Steel After a Pitting Test

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9 Test Specimens9.1 Test specimens are required to be representative of a product or process lot.

9.2 Specimen preparation should preferably be done by the component manufacturer. The specimen(s) are to be cleaned and packaged per the manufacturer’s standard final cleaning and packaging procedures.

9.3 If specimen preparation is done by a party other than the manufacturer, the specimen(s) may be cleaned by degreasing with a suitable detergent, rinsing in deionized water, followed by ethanol or similar solvent, and air drying. After degreasing, the specimen must be handled with clean gloves, soft clean tongs, or equivalent preventive measures, to avoid surface contamination. If the specimen(s) are not to be analyzed immediately, they should be packaged by wrapping in clean metal foil or sealing in cleanroom quality nylon bags. If specimen preparation is done by a party other than the manufacturer, this shall be stated in the report narrative and the analytical results are not to be interpreted as indicative of the manufacturer’s quality of cleaning and packaging procedures. A note to this effect shall be included in the reported results.

9.4 After preparation, specimens should be analyzed promptly, with allowance for shipping times and queuing time at the analyst.

9.5 Tubing specimens shall consist of 5.0 cm (2.0 in.) lengths of tubing sectioned axially. Other types of components shall be sectioned to expose the surface of interest to the test solution. The cut surfaces shall be smoothed sufficiently to be effectively masked per ¶ 9.6.

9.6 All specimens should be masked to isolate the test to the surface of interest only. This can be accomplished by masking with a nonreactive lacquer, which can be baked to ensure optimum adhesion. Alternately, plater’s tape can be used.

9.7 The finish of the surface to be tested shall be consistent with the requirements of the surface roughness and surface defects of the intended specification grade (i.e., general purity [GP], high purity [HP], or ultrahigh purity [UHP]) of SEMI F19. Since surface finish can affect the CPT results, surface finishing should be done as consistently as possible within a specimen set. Details of the surface finishing process, parameters, and characterization should be provided in the test report as outlined in ¶ 13.

9.8 A minimum of three specimens shall be tested, and preferably five or more to obtain statistical significance in the testing results.

9.9 The minimum test area is 1 cm2. This minimum test area is required for both flat specimens and tubing specimens. Certain types of components may not have sufficient test area to meet this requirement. This should be noted in the test report and the calculated test area reported.

10 Reagents And Materials10.1 Reagent grade chemicals and deionized water shall be used in the preparation of test solutions.

10.2 The ferric chloride test solution is prepared by dissolving 68.72 g of reagent grade ferric chloride (FeCl3·6H2O) in 600 mL of deionized water and adding 16 mL of reagent grade concentrated (36.5–38.0%) hydrochloric acid (HCl). The solution can be made up in bulk and stored in a sealed container for up to five days at room temperature.

11 Procedure11.1 Transfer 600 mL of the test solution to a flask or the amount required per ¶ 8.1.1.2 to a test tube. Cover with a watch glass and place in the constant temperature bath set at 10°C. Allow the test solution to equilibrate to the set temperature.

11.2 Immerse one or more test specimens in the test solution and cover the container with a watch glass for the duration of the test.

11.3 After 24 hours, remove the specimen(s) from the test solution, rinse thoroughly in deionized water followed by methanol, and dry.

11.4 Examine the specimen(s) for pitting. If pitting is observed, the test temperature shall be reported as the CPT.

11.5 If pitting has not occurred, repeat ¶¶ 11.1 through 11.4 at 5°C higher temperature increments until pitting is observed.

This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an official or adopted Standard or Safety Guideline. Permission is granted to reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.

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11.6 When test is completed, remove the specimen(s) from the test solution and rinse with deionized water.

11.7 Examine the test area of the specimen(s) under an optical microscope at 20X magnification.

11.8 Confirm the existence of pits and the absence of crevice corrosion, as indicated by the absence of pits touching the masked-off or lacquered area (see ¶ 7.1 and Figure 1). Any crevice corrosion observed on the specimen after testing means that the test results are invalid and must be discarded.

12 Data Analysis12.1 The CPT is the lowest temperature at which pitting without crevice corrosion is observed by visual examination.

13 Reporting Results13.1 Information Required in the Test Report

13.1.1 Test specimens(s) identification

13.1.1.1 Manufacturer

13.1.1.2 Part number (if applicable).

13.1.1.3 Lot number

13.1.1.4 Specification Grade per F19 or surface processing history (e.g., mechanical polish, electropolish, passivation).

13.1.1.5 Welded specimens must be identified and the welding process specified.

13.1.1.6 Tubing diameter for tubing specimens.

13.1.2 Test identification number and date of test.

13.1.3 Critical pitting temperature (CPT). CPTs below 10°C shall only be reported as below 10°C.

13.1.4 Number of specimens in which pitting is observed, if more than one specimen is tested.

13.1.5 The scatter or deviation in the CPT values based on multiple runs, if performed14 Related Documents14.1 SEMI Standards

SEMI F20 — Specification for 316L Stainless Steel Bar, Forgings, Extruded Shapes, Plate, and Tubing for Components Used in General Purpose, High Purity and Ultra-High Purity Semiconductor Manufacturing Applications

NOTICE: SEMI makes no warranties or representations as to the suitability of the Standards and Safety Guidelines set forth herein for any particular application. The determination of the suitability of the Standard or Safety Guideline is solely the responsibility of the user. Users are cautioned to refer to manufacturer’s instructions, product labels, product data sheets, and other relevant literature, respecting any materials or equipment mentioned herein. Standards and Safety Guidelines are subject to change without notice.

By publication of this Standard or Safety Guideline, SEMI takes no position respecting the validity of any patent rights or copyrights asserted in connection with any items mentioned in this Standard or Safety Guideline. Users of this Standard or Safety Guideline are expressly advised that determination of any such patent rights or copyrights and the risk of infringement of such rights are entirely their own responsibility.

This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an official or adopted Standard or Safety Guideline. Permission is granted to reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.

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