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SEMI F77-0703 © SEMI 2003 2 4.2 ASTM 1 Standar ds ASTM G3 — Standard Practice for Conventions Applicable to Electroch em ical Measurements in Corrosion Test ing ASTM G15 — Standa rd Terminol ogy Relati ng to Corrosion an…

SEMI F77-0703 © SEMI 2003 1

SEMI F77-0703

TEST METHOD FOR ELECTROCHEMICAL CRITICAL PITTING

TEMPERATURE TESTING OF ALLOY SURFACES USED IN

CORROSIVE GAS SYSTEMS

This test method was technically approved by the Global Gases Committee and is the direct responsibility of

the North American Gases Committee. Current edition approved by the North American Regional Standards

Committee on April 11, 2003. Initially available at www.semi.org June 2003; to be published July 2003.

1 Purpose

1.1 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 intended to

differentiate between alloy compositions and processes

intended to enhance the corrosion performance of the

wetted surfaces.

2 Scope

2.1 This test method describes a procedure, based on

the electrochemical critical pitting temperature (CPT),

which is used to rank the pitting corrosion resistance of

wetted surfaces of tubing or test coupons of

representative 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.

2.2 This test method is an adaptation of ASTM G150.

The adaptation describes a method for performing the

test method on coupons or wetted-surface sections cut

from gas supply system components such as tubing. It

is an aqueous immersion method.

2.3 The test method is reproducible and provides a

metric (critical pitting temperature) in addition to a

qualitative (visual) evaluation of corrosion resistance.

2.4 This test method applies to materials as specified in

SEMI Standards referenced in Section 4.1, and to welds

of these materials.

2.5 This test method may also be used for other

corrosion resistant alloys and their welds not referenced

in Section 4.1.

NOTICE: This standard does not purport to address

safety issues, if any, associated with its use. It is the

responsibility of the users of this standard to establish

appropriate safety health practices and determine the

applicability of regulatory or other limitations prior to

use.

3 Limitations

3.1 Only analysts familiar with the instrumentation and

technique should use this test method.

3.2 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.

3.3 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.

3.4 Since alloy composition and surface parameters

can affect the results of the test (e.g., surface finish), all

variables other than the one being tested must be fixed

during the test in order to obtain reproducible and

comparable results.

3.5 The bias for this test has not yet been determined.

4 Referenced Standards

4.1 SEMI Standards

SEMI F2 — Specification for 316L Stainless Steel

Tubing for General Purpose Semiconductor

Manufacturing Applications

SEMI F17 — Specification for High Purity Quality

Electropolished 316L Stainless Steel Tubing,

Component Tube Stubs, and Fittings made from Tubing

SEMI F19 — Specification for the Finish of the Wetted

Surfaces of Electropolished 316L Stainless Steel

Components

SEMI F20 — Specification for 316L Stainless Steel

Bar, Extruded Shapes, Plate, and Investment Castings

for Components Used in High Purity Semiconductor

Manufacturing Applications

SEMI F77-0703 © SEMI 2003 2

4.2 ASTM

Standards

ASTM G3 — Standard Practice for Conventions

Applicable to Electrochemical Measurements in

Corrosion Testing

ASTM G15 — Standard Terminology Relating to

Corrosion and Corrosion Testing

ASTM G150 — Test Method for Electrochemical

Critical Pitting Temperature Testing of Stainless Steels

NOTICE: Unless otherwise indicated, all documents

cited shall be the latest published versions.

5 Terminology

NOTE 1: Unless otherwise stated, the sign conventions used

in this test method are in agreement with ASTM G3, and the

terminology relating to corrosion and corrosion testing is as

defined in G15.

5.1 Definitions

5.1.1 CPT-Critical Pitting Temperature — the lowest

temperature at which stable propagating pitting occurs

on the test surface under the specified test conditions, as

indicated by a rapid increase beyond a specified limit of

the measured anodic current density of the test surface.

5.1.2 passive potential range — the potential range

over which the current density is independent of

potential. The current is a very low value due to

formation of an oxide layer.

5.1.3 pitting — corrosion of a metal surface, confined

to a point or small area, that takes the form of cavities.

5.1.4 pitting potential range — the range of measured

potentials where pitting is initiated. This potential

range only exists above the minimum CPT.

5.1.5 potential dependent CPT — the CPT determined

at a potential within the pitting potential range of the

tested surface.

5.1.6 potential independent CPT — the CPT

determined at a potential above the pitting potential

range, but below the transpassive potential range.

5.1.7 temperature ramp — the rate (° C/min) at which

the test temperature is increased during the test.

5.1.8 transpassive potential — the potential above the

passive potential range, in which the current density

increases rapidly as the potential increases.

5.1.9 wetted surface — surfaces of a component in

contact with the contained fluids.

1 Available from American Society for Testing and Materials, 100

Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-

2959. Fax: 1-610-832-9555. Website: http://www.astm.org.

6 Summary of Method

6.1 The test method measures the critical pitting

temperature (CPT) by using a potentiostatic technique

and a temperature scan. Specific types of specimen

holders are used to minimize crevice corrosion, which

is another type of corrosion mechanism that can

complicate the CPT result. The test is performed in an

aqueous electrolyte solution, such as 1 M NaCl. After

polarizing the specimen to a potential above the pitting

potential, the temperature scan begins at 0°C. The

solution is heated at a constant rate of 1°C/minute while

the current is measured by means of a

potentiostat/galvanostat instrument. The CPT, as

defined in Section 5.1.1, is the temperature at which the

current density increases above 100 µ A/cm

for 60

seconds. Pitting is normally confirmed by visual or

microscopic observation of the specimen after testing as

shown in Figure 1. Refer to Appendix X1 in ASTM

G150 for a description of the relationship between the

pitting potential and the critical pitting temperature.

Figure 1

Examples of Pitting on Type 316 Stainless Steel

After a Pitting Potential Test Performed at 25°C in 1

M NaCl (11 × Magnification)

(Ref 17.1)

7 Possible Interferences

7.1 Specimens must be examined after testing to

confirm pitting. A possible interference is failure by

crevice corrosion in which case the test is invalid.

Crevice corrosion can occur at the seal of the specimen

holder. Figure 2 shows the mechanism by which

crevice corrosion occurs, and Figure 3 gives an

example of the typical morphology of crevice corrosion

from a pitting potential test performed on type 316

stainless steel using masked edges. It should also be

noted that the critical crevice temperature (CCT) for

AISI 316L is less than 10°C.

Figure 2

Mechanism for Crevice Corrosion

(See Section 17.2.)

Figure 3

Examples of Crevice Corrosion (Dashed Circles) on

Type 316 Stainless Steel After a Pitting Potential

Test in 1 M NaCl at 35°C. 11 × Magnification (See

Section 17.1.)

8 Apparatus

8.1 In general, refer to ASTM G150 for a detailed

description of the apparatus and accessories necessary

to carry out the CPT evaluation. Specifically, Sections

6.1 to 6.9 of ASTM G150 describe the potentiostat, the

potential measuring instrument, the current measuring

instrument, the temperature controller, and the

temperature measurement instrumentation.

8.2 Flat Specimens — The test cell for flat specimens

(consisting of two options, G5-type cell and a flushed-

port cell), are given in Appendices X2 and X3 of

ASTM G150 with a detailed description of each.

8.3 Tubular Specimens — The test cell can be modified

for testing tubular specimens. The modified test cell

consists of a 2-L glass beaker cut to 4.5 in. high and

capped with a polyethylene cover. A three-electrode

set up is used consisting of the test specimen, an SCE

reference electrode inserted in a Luggin probe, and two

graphite rods as counter electrodes. See Figure 4 for the

modified test cell set-up.

Figure 4

Modified Test Cell for CPT Testing of Tubular

Specimens; Tubing Specimen is the Working

Electrode

9 Test Specimens

9.1 Flat Specimens — In general, refer to ASTM G150

for a detailed description of the flat specimen.

9.2 Tubing Specimens — Tubing specimens consist of

2.0-in. lengths of tubing. The tubing should be faced,

and the cut ends should be masked with a non-reactive

lacquer.

9.3 All specimens should be masked to isolate the test

to the surface of interest only. This can be

accomplished by masking with a non-reactive lacquer,

which can be baked to ensure optimum adhesion.