Measurement of Outgassing Rates of Steels

Summary

A protocol for the measurement of outgassing rates of hydrogen from ordinary steel vacuum chambers using the rate-of-pressure rise method is presented.

Abstract

Steels are commonly used materials in the fabrication of vacuum systems because of their good mechanical, corrosion, and vacuum properties. A variety of steels meet the criterion of low outgassing required for high or ultrahigh vacuum applications. However, a given material can present different outgassing rates depending on its manufacturing process or the various pretreatment processes involved during the fabrication. Thus, the measurement of outgassing rates is highly desirable for a specific vacuum application. For this reason, the rate-of-pressure rise (RoR) method is often used to measure the outgassing of hydrogen after bakeout. In this article, a detailed description of the design and execution of the experimental protocol involved in the RoR method is provided. The RoR method uses a spinning rotor gauge to minimize errors that stem from outgassing or the pumping action of a vacuum gauge. The outgassing rates of two ordinary steels (stainless steel and mild steel) were measured. The measurements were made before and after the heat pretreatment of the steels. The heat pretreatment of steels was performed to reduce the outgassing. Extremely low rates of outgassing (on the order of 10⁻¹¹ Pa m³ sec⁻¹ m⁻²) can be routinely measured using relatively small samples.

Introduction

Steels are routinely used in construction because of their good mechanical properties. Certain steels (ferrous steels, in particular) are preferred materials for applications involving vacuum. Depending on the type and grade, these steels have sufficiently low outgassing rates essential for high vacuum (HV, 10⁻⁷ < p < 10⁻⁵ Pa) or ultrahigh vacuum (UHV, 10⁻¹⁰ < p < 10⁻⁷ Pa) systems. Further, extensive research has been conducted toward the development of special pretreatment procedures that reduce outgassing^1-3. The pretreatment measures are designed to minimize the pumping investment or to improve the vacuum from HV to UHV or from UHV to extreme-high vacuum (p < 10⁻¹⁰ Pa).

Although many practical methods have been proposed to reduce the outgassing rate of ferrous steels, recent methods are focused on reducing the time and temperature required to obtain a lower outgassing rate. Heat treatment at 350 °C-450 °C rather than vacuum firing at 800 °C-950 °C, is a good example of this approach.^1,4,5 Furthermore, choosing the ideal material for a specific vacuum application is critical; for example, selecting a ferritic material with a very low outgassing rate for use in magnetic field shielding.^6,7

During such investigations, precise measurement of the outgassing rate is a prerequisite for the screening of candidate materials or verifying the effectiveness of various pretreatment procedures.^8,9 The most common experimental techniques used for the measurement of outgassing are the throughput and rate-of-pressure rise methods.¹⁰ Recently, various experiments have been conducted to measure the hydrogen outgassing rate based on the RoR method using spinning rotor gauge (SRG).^{1, 11-13} The RoR method using SRG is highly suitable for measuring very low hydrogen outgassing rates that often limit the lowest pressure achievable in a vacuum system made of steel. This is because the SRG has negligible pumping or outgassing action. Further, the SRG also has excellent accuracy and good linearity in high vacuum and ultra-high vacuum range.¹⁴

Given that the published literature on RoR experiments is limited, it is worthwhile to describe the experimental details to develop a deeper understanding of the method. In this video article, we describe in detail the process of setting up the experiment and provide detailed instructions to perform outgassing measurements using the RoR method. To demonstrate the efficacy of the method, the outgassing rates of two commonly used steels (stainless steel 304 and mild steel S20C) were measured before and after a preheat treatment to reduce the hydrogen outgassing rate. The pre- and post-treatment values were compared. Typical experimental results using a rather simple setup are presented to demonstrate the efficacy of the method optimized for evaluating low hydrogen degassing rates.

Protocol

Caution: Please follow all appropriate safety practices while assembling the equipment and sample chambers. Please wear personal protective equipment (safety glasses, gloves, safety shoes, etc.).

1. Fabrication of a Sample Vacuum Chamber

1. Design and fabrication of the vacuum chamber
   1. Prepare and submit design drawings to a commercial vendor or an in-house machine shop for manufacturing the sample vacuum chamber. A representative example of the design drawing for a vacuum chamber made of S20C steel is shown in Figure 1. The chamber designed in this experiment is very basic and is commonly employed by vacuum companies.
      NOTE: The manufacturer should have a basic knowledge of UHV systems.
   2. Inspect all dimensions to ensure compliance with the drawing.
   3. After shaping (machining) the vacuum chamber, cover the end CF flanges with plastic to prevent damage during transportation.
2. Cleaning
   NOTE: Follow local environment, health, and safety regulations during chemical cleaning. Wear personal protective equipment. Handle the parts using vinyl gloves. Do not touch the parts with bare hands.
   1. Clean the steel parts following the UHV cleaning procedure for steels. A typical cleaning procedure is described below.
   2. Degrease the parts using a solvent, such as acetone, at room temperature for 5 min.
   3. Clean the parts in an ultrasonic bath for 20 min using a BN cleaner (pH 13).
   4. Rinse the parts with tap water for 10 min, followed by a thorough rinse with deionized water for 20 min.
   5. Rinse thoroughly with alcohol and blow dry using dry nitrogen gas.
   6. Wrap the parts in clean, lint-free paper and allow the parts to air-dry for one day.
3. Welding
   NOTE: Do not touch the parts with bare hands. The welder should be trained in UHV welding.
   1. Place the parts on a welding bench.
   2. Preassemble the parts and align the parts as per the design drawing.
   3. Back-purge with argon gas (5 L/min) to prevent corrosion during welding.
   4. Tack weld the end flanges using the Tungsten inert gas (TIG) weld technique (argon flow rate: 8-9 L/min).¹⁵
   5. Weld the end flanges completely using the TIG technique and a turning jig. Allow the heat-affected zone to cool down to room temperature. Stop the argon gas flow.
4. Leak test
   1. Seal one end of the chamber with a CF blank flange.
   2. Connect the other end to a helium leak detector (HLD).
   3. Pump down the sample vacuum chamber using the HLD.
   4. Place the heat-welded seam in a vinyl bag and fill the bag with helium gas.
   5. Measure any change in the helium level. Ensure that the chamber is leak-proof. The helium leak rate should be <1 × 10⁻¹¹ Pa m³ sec⁻¹ (1 × 10⁻¹⁰ mbar L sec⁻¹).
   6. If no leak is observed, vent the chamber. Otherwise, reweld the vacuum chamber after venting (repeat steps 1.3.3 through 1.4.5).

2. Fabrication of the Oven

1. Prepare and submit the design drawings to a vendor or an in-house machine shop for manufacturing the oven. Refer to the picture shown in Figure 2.
2. Procure the required parts and equipment described in Temperature Control of List of Specific Materials/Equipment.
3. Connect the cooling line.
4. Supply cooling water to a chiller. Run the chiller and check for water leaks. Stop the chiller.

3. Experimental Setup for RoR Measurements

1. Collect the required vacuum equipment/hardware specified in the List of Specific Materials/Equipment. The test setup primarily consists of a SRG, a residual gas analyzer (RGA), a turbomolecular pump (TMP) equipped with a roughing pump (RP), an all-metal angle valve (AV_Ch), a tee (CF35), and a reducer (CF35 to CF63). Additional items may include a helium leak detector and an UHV gauge, as listed in the List of Specific Materials/Equipment. The AV_Ch should have a rotatable flange on the seat (sealing) side to level off the SRG.
2. Collect the wrenches (M6 and M8), copper gaskets (CF35 and CF63), and bolts/nuts (M6 and M8) required for the assembly.
3. Use an industrial level meter to assemble the SRG.
4. Handle the parts using vinyl gloves. Do not touch the surface that is exposed to vacuum with bare hands. Wear safety shoes.
5. Assembly of the experimental apparatus
   1. Assemble the vacuum components sequentially using copper gaskets from the pump side to the sample side, as shown in Figure 3. Always place the valve seat side facing the sample chamber. Tighten all flange joints face-to-face with a copper gasket (CF35) using a wrench (M6), except the joint between the sample chamber and the chamber isolation valve (AV_Ch).
   2. Adjust the SRG flange assembly and the sample chamber so that the axis of the SRG head is vertical using the level meter; within ±2° (max) (Figure 4). Tighten the flange joint between the sample chamber and the AV_Ch, face-to-face, while maintaining the SRG flange's level. Refer to the SRG user's manual for detailed instructions.
   3. Connect the RP and the HLD with isolating valves (AV_Ro, AV_HLD) to the clamp flange (KF) port of the exhaust end of the TMP.
      Caution: Ensure that there is no mechanical shock to the SRG flange assembly or rotor.
6. Leak test
   1. Turn on the HLD and wait until the detector is ready. Open the AV_HLD and close the AV_Ro.
   2. Pump down the setup using the HLD. Refer to the HLD manual for the correct operating procedure. Wait for ~30 min to pump out the residual helium gas from the setup. Ensure that the helium level is within the minimum detectable limit of the HLD.
   3. Spray helium gas through the leak-test groove on the flanges.
   4. Measure any change in the helium level. Ensure that the chamber is leak tight. The helium leak rate should be <1 × 10⁻¹¹ Pa m³ sec⁻¹ (1 × 10⁻¹⁰ mbar L sec⁻¹).
   5. In case of leakage from the flanges, retorque the flanges.
   6. If no leaks are found, stop the leak testing and vent the vacuum system. Open the AV_Ro and close the AV_HLD.

4. Measurement of Outgassing Rates

1. Pump down procedure
   1. Pump down the vacuum system by switching on the TMP and the RP at the same time.
   2. While the pumping process is on, collect the necessary items for bakeout; electrical heater tapes, heater controllers, handheld multimeter, Al foil, and temperature sensors/cables.
2. Bakeout procedure
   1. Remove the SRG head from the flange assembly. Wrap vacuum components (between the SRG flange assembly and the inlet flange of the TMP) in band heaters.
   2. Check and ensure that there is no electrical short circuit between the heaters and the vacuum parts using handheld multimeter.
   3. Connect the heaters to the respective controllers and wrap the chamber in Al foil.
   4. Raise the temperature to 150 °C at a ramp rate of 1 °C-2 °C/min.
   5. Hold the chamber at 150 °C for 24-48 hr using bakeout program controller. Maintain the temperature of the RGA electronics under 50 °C using cooling fan.
   6. Degas each of the RGA filaments by electron bombardment for at least 5 min.
   7. Measure the RGA spectrum from 1 to 50 m/e to ensure that the H₂O peak (m/e = 18) is less than one-half of the H₂ peak (m/e = 2). If not, continue the bakeout.
   8. Allow the system to cool down to room temperature at a ramp rate of 1 °C-2 °C/min. Check for leaks referring to the RGA spectrum measured during the cool down.
   9. Analyze the residual gas in the sample chamber. Measure the RGA spectrum. Close the AV_Ch and measure the RGA spectrum again. The RGA spectrum of the sample chamber corresponds to the difference between the spectra acquired before and after closing the AV_Ch.
   10. Verify that the sum of all impurity gases, such as H₂O, CO, and CO₂, is below 5%; otherwise, repeat the bakeout again.
3. Operating the SRG
   NOTE: Proper operation of the SRG is very important. Refer to the SRG operating manual for instructions.
   1. Assemble the SRG head on the SRG flange assembly.
   2. Ensure that the axis of the SRG head is within ±2° (max) (Figure 4). Use a level meter for reference.
   3. Start the SRG and wait for stabilization of the residual drag, a pressure-independent signal of the SRG, which usually takes a few hours.
   4. Enter the proper input parameters, such as gas (H₂), temperature (24 °C), and measurement interval (10 sec).
4. Procedure for initiating temperature control
   1. While waiting for the signal to stabilize, stabilize the temperature of the sample. Switch on the chiller to run cooling water through the system. Set the fluid temperature to 15 °C.
   2. Start the heater controller for the sample. Set the target temperature to 24 °C. Wait for the temperature to stabilize within ±0.1 °C after closing the door of the oven.
5. Procedure for acquiring the SRG signal
   1. Verify that the variation of the offset value of the signal is within ±1 × 10⁻⁹ Pa/sec; otherwise, disassemble the SRG from the system and change the rotor or flange assembly, then repeat 3.5-4.4. (If this is not possible, acquire the slope of the offset for 8-24 hr. This will be subtracted from the measured outgassing rate data.)
   2. Check the signal level provided by SRG controller; it should be at least −10 dB. Ideally, it should be between 0 and 6 dB. However, values of up to 12 dB are acceptable. If the signal is >14 dB, stop the SRG operation. Detach the head and heat the thimble to 200 °C. Repeat the entire operation starting from step 4.5.1.
   3. Check the damping level provided by SRG controller; the optimum value is between −35 and −60 dB, which is normally satisfied in the system using TMP and scroll pump which is laid on a rubber pad. Otherwise, stop all running equipment and remove sources of any mechanical vibration.
6. Acquiring the RoR data
   1. Gently close the AV to start the pressure buildup. Be careful not to subject the SRG to a mechanical shock.
   2. Close the door of the oven and acquire pressure data for 8-24 hr using a computer.
   3. Precheck the measured data to verify that the variation in temperature is within ±0.1 °C after stabilization and that the pressure rise is linear within 10% error. If these criteria are met, stop the measurement. Otherwise, continue to measure until the pressure rise becomes linear within 10% error for at least 16 hr.
   4. Turn off all equipment.
7. Calculation of the outgassing rate
   1. Select the pressure set after the temperature stabilization.
   2. Fit the pressure rise data using linear least squares fitting and calculate the slope. The slope dP/dt is the measured rate of pressure rise after the valve is closed.
   3. Calculate the outgassing rate, q (H₂ equivalent), using the equation
      q = (V/A) (dP/dt) [Pa m³ sec⁻¹ m⁻²],
      where V is the volume of the sample chamber (m³) and A is the geometrical surface area of the chamber (m²).

Results

As expected, the residual gas after the bakeout was mostly hydrogen.⁷ The pressure rise measured using the SRG was linear over a long period of time (Figure 5). Thus, the readsorption effect might be negligible and the intrinsic outgassing rate (q) for the steels tested in this study can be evaluated using the RoR method.¹⁰ The measured pressure rise data was analyzed using the linear least squares fitting method. The outgassing rates of the...

Discussion

Numerous methods for the measurement of outgassing rates have been reported in the literature. Experimental methods include the throughput, conductance modulation, two-path, RoR, and variations of these methods. However, no one method is ideal for obtaining the necessary outgassing data.¹⁰ The RoR method using SRG, however, became the method of choice for measurement of low outgassing materials.^11-13 SRG¹⁷ is often used as a secondary standard in high vacuum systems without erroneous pump...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Sample chamber
Stainless steel, 304	POSCO (www.posco.co.kr)
Mild steel, D3752	Xiangtan Iron&Steel co.,LTD (http://www.hnxg.com)
Mild steel, D3752	SeAh Besteel (www.seahbesteel.co.kr)
Name	Company	Catalog Number	Comments
Cleaning
Cleaning bath	Samill IDS		Ultrasonic cleaning, heating, timer, concentration control
Acetone	Samchun Chemical (www.samchun.com)	A1759	HPLC grade (99.7%)
Tekusolv	NCH Co.        (www.nch.com)	0368-0058J	Solvents
BN cleaner	Henkel surface technologies (na.henkel-adhesives.com)	6610263775	Alkaline, pH 13
Ethanol	Fisher Scientific (www.fishersci.com)	A995-4	HPLC Reagent (99.9%)
Deionized water (Electro deionizer SYSTEM)	A.T.A        (www.atagroup.co)	EDI SYSTEM
Liquid N2 gas	Hanyoung (www.gasmaster.co.kr)	B/T 176 L	LN2 dewar, purity 99.999%
Name	Company	Catalog Number	Comments
Welding
Tungsten Inert Gas wedling machine	Thermal Arc (www.victortechnologies.com/thermalarc)	400GTSW	Ar gas preflow and postflow 8 L/min, backflow 5 L/min
turning jig	Vactron (www.vactron.co.kr)	Made to order	Made to order
Ar gas	Lindekorea (www.lindekorea.com)		Purity 99.999%
Name	Company	Catalog Number	Comments
Leak test
Leak detector	Adixen (www.adixen.fr/en/)	ASM380	Pumping Speed (air): 9.7 L/sec
He gas	Lindekorea (www.lindekorea.com)		Purity 99.999%
Name	Company	Catalog Number	Comments
Vacuum equipment
Spinning rotor gauge	MKS Instruments (www.mks.com)	SRG-3	Controller, head, and thimble set
Oscilloscope	Tektronix (www.tek.com)	TDS2012B
Residulal gas analyser	Balzers	QMA200	m/e 0-100
TMP (HiPace 80)	Pfeiffer Vacuum (www.pfeiffer-vacuum.com)	PMP03941	Pumping Speed (N2): 67 L/sec
Scroll pump	Anest Iwata (www.anest-iwata.co.jp)	ISP 90	Pumping Speed (Air): 1.8 L/sec
All-metall easy close angle valve (CF35)	VAT Inc. (www.vatvalve.com)	54032-GE02-0002	Rotatable flange
Angle valve (KF25)	MDC Vacuum Inc. (www.mdcvacuum.com)	KAV-100
Name	Company	Catalog Number	Comments
Temperature control
Chiller	JEIO Tech (www.jeiotech.com)	RW-2025G
Cooling line	LS Metal (www.lsmetal.biz)	C1100	Level Wound Coil, Diameter 10 mm
Heater controllers	HMT	Made to order	Bakeout program controller
Electrical heater tapes	Brisk heat (www.briskheat.com)	BIH101080L
Thermocouple (K type)	miraesensor (www.miraesensor.com)	MR-2290
Handheld multimeter	Saehan (www.saehan.co.kr)	3234
Data recorder (Temp.)	Yokogawa (www.yokogawa.com)	GP10-1E1F-UC10