IVC-17/ICSS-13 and ICN+T2007
Journal of Physics: Conference Series 100 (2008) 092023

IOP Publishingdoi:10.1088/1742-6596/100/9/092023

Effect of Chemical Polishing in Titanium Materials for LowOutgassing
K Ishizawa1,2, H Kurisu1, S Yamamoto1, T Nomura2, N Murashige2
1

Graduate School of Science and Engineering, Yamaguchi University,Ube, Yamaguchi 755-8611, Japan2
CT division San-ai Plant Industries, Ltd, Kawasaki, Kanagawa 210-0821, Japan
E-mail: kishizawa@san-ai-oil.co.jp
Abstract. A chemical polishing using a nitric acid solution was found to be the most suitable
for the titanium materials. 1.8 nm of small surface roughness was observed in a microscopic
range in 1 µm square, and 7 nm of a thin oxide layer was shown to exist for the chemicallypolished titanium. The surface processing for the titanium was developed combining thechemical polishing and the precision cleaning. The chemically polished pure titanium of JISgrade 2 showed extremely low outgassing rate below 10-12 Pams-1 after baking process, whichis two orders of magnitude smaller than that for standard vacuum materials under the samebaking condition. Outgassing rates of the titanium is about 1/5 of that for a stainless steel
without baking process.

1. Introduction
Recent years, titanium has been paid attention as a high vacuum material because of lightweight, nonmagnetism, a low Yong’s modulus, and a small thermal expansion coefficient. The vacuum
characteristics for some kinds of titanium materials, pure titanium of JIS grade 2, Ti-3Al-2.5V and Ti0.35Fe-0.35O, have been investigated and their outgassing rates are clarified to be lower than that forthe standard materials.[1-3]The surface polishing process, which improves smoothness, cleanness and passivation layer at thematerial surface, is necessary to obtain the low outgassing property. For the stainless steels andaluminum alloys, various surface polishing processes have been developed, and the electrolytic
polishing [4-8], the chemical polishing [9,10] and the electro-chemical buffing [5,7,11] are clarified tobe suitable for reduction in the outgassing rate. However the surface polishing for titanium materials
were hardly discussed, although the mechano-chemical polishing was found to be effective to improve
the outgassing rate.[2]In this paper a newly developed chemical polishing method for titanium materials is described. The
outgassing properties and surface conditions of chemically polished titanium materials were examined,
and the excellent outgassing properties have been obtained.

c 2008 IOP Publishing Ltd1

IVC-17/ICSS-13 and ICN+T2007
Journal of Physics: Conference Series 100 (2008) 092023

IOP Publishingdoi:10.1088/1742-6596/100/9/092023

2. Chemical Polishing
In generally, mechanical polishing, wet polishing and the wet polishing combined with mechanicalpolishing are applied for the surface treatment of the vacuum materials. The chemical polishing andthe electrolytic polishing give a smooth surface, thin and uniform surface layer and non-residual
impurities, and these polishing methods have advantages for a complex shaped samples and a cost.
Therefore, these polishing processes were applied to the pure titanium of JIS grade 2 and the effects
were discussed in this study. The electrolytic polishing was difficult, since some electrolytic solutions
were insufficient to improve surface conditions, and other solutions are chemically unstable.
Figure 1 shows the mechanism of a chemical polishing. A surface of the metal immersed inchemical polishing solution dissolves into metal ions and the ions diffuse in the polishing solution. In
the case that the metal dissolves faster than the ion diffusion, the metal ions concentrate at the concave,
and a density of cations, mainly hydrogen ions (H+), in polishing solution decreases at the concave asshown in Fig. 1 (a). This gives smooth surface, because an etching rate at the convex is larger than thatat the concave as shown Fig. 1 (b) and (c). In addition, substances un-removed by a precision cleaning
can be removable by the chemical polishing as shown in Fig. 1 (a) and (b). Where un-removable
substances are sticking impurities such as powders produced in cutting or polishing process, the stain
due to osmosis and diffusion of atoms and ions into vacuum materials, and chemically modified
surface layer such as an oxidation scale formed in welding process as shown in Fig. 1 (a) – α, β and γ.
The effects of chemical polishing for the three kinds of the solutions, a sulfuric acid bath, ahydrochloric acid bath and a nitric acid bath, were evaluated. The process conditions, such as chemicalcomposition of the polishing solutions, temperature and process time, were optimized. The chemical
polishing with a nitric acid bath, which is stable and workable, was found to be the most suitable forthe titanium materials.
Figure 2 shows the flow chart of the developed chemical polishing process using the nitric acid bathand precision cleaning, named “precision chemical polishing”. Here, the process 2 is the chemical
polishing and the other processes are precision cleaning. In the precision cleaning, the process 1
removes organic materials such as oil. Process 3 removes metallic salts and heavy metals after thepolishing process. The process 4 is the wet wiping. The removal effects of impurities can be further
improved by adding the physical cleaning process between the chemical cleaning processes. Processes
5 and 6 remove particles and the inorganic salts. Moreover, in order to protect the contamination, thematerials are treated in a clean room for the process 5. The materials are dried out by nitrogen gas,
inspected and packed in clean atmosphere in processes 7 and 8.
(a)Receiving inspection

High densityγ1Polishing solution

Water rinse

Low density

αβDegreasing

2Metal ion

3(c)Fig. 1 Mechanism of a chemical polishing.

6Hot
Pure water:
pure watersoaking 18MΩ-cm

7

N2 gasblowing

Nitric acid soaking
Water rinse

45 Pure water Clean roomsoaking (Class-10000)

Chemical polishing
Water rinse(b)Water rinse

WipingClean room(Class-1000)

Final inspection
8Packing

Clean bench(Class- 10)

Shipping Inspection

Fig. 2 Flow chart of developed chemical
polishing and precision cleaning process.

2IVC-17/ICSS-13 and ICN+T2007
Journal of Physics: Conference Series 100 (2008) 092023

IOP Publishingdoi:10.1088/1742-6596/100/9/092023

3. Experimental Procedure
1-mm-thick plates of pure titanium, JIS grade 2, were employed as sample materials. Unpolished basis
metal Ti (Ti(BM)), buffed Ti (Ti(BP)), mechano-chemically polished Ti (Ti(MCP)) and chemically polished
Ti (Ti(CP)) were prepared. Here alumina abrasive grains (#400) were used for the buffing, and themechano-chemically polishing consists of the wet chemical polishing combined with the mechanicalpolishing. The values of surface roughness (Ra) of the samples were estimated by an atomic force
microscopy (AFM) in 10 µm2 and 1 µm2 area. In order to measure the surface oxide layer thickness,
depth profiles were analyzed by an auger electron spectrometer (AES) with Ar etching gas, andtransmission electron microscope (TEM) images were observed. The distribution of hydrogen atoms
around the surface layer was analyzed by a time of flight - secondary ion mass spectrometry (TOFSIMS) with Ar etching gas.
The outgassing rates were estimated by a modified orifice method, switching between two pumpingpaths[12]. The detection limit of the system was 7×10-13 Pams-1 since the upstream and downstreampressures attained to 10-9 Pa and below, the orifice conductance C0 was set to 6.1×10-3 m3･s-1 and thesample area was 8.9×10-1 m2 [3]. The outgassing rate was measured at room temperature under nonbaking and after baking process. Here the baking condition was 393K for 20 hours and 48 hours
natural cooling process.
4. Results and Discussions
Table 1 shows the Ra and thickness of surface oxide layer for polished pure titanium samples. In themacroscopic area of 10 µm2, Ra for Ti(CP) is smaller than those for Ti(BM) and Ti(BP), and 10 times larger
than that of Ti(MCP). However, the surface of Ti(CP) in microscopic area of 1 µm2 is as smooth as that ofTi(MCP), and the difference in Ra is considered due to the macroscopic undulation. Ra for Ti(CP) issmaller than twice the Ra for Ti(MCP) in microscopic area of 1 µm2. Since the amount of the adsorption
gas is influenced by the surface roughness in a microscopic area, the surface of Ti(CP) is considered tobe smooth enough.
The thickness of the surface oxide layer in Ti(CP) is smaller than those of the other polished samples
as seen in Table 1, where the thickness was estimated by the half maximum value of the AES depth
profile of oxygen (O). The surface oxide layer is confirmed to be an amorphous by the cross sectional
TEM observation. The thin oxide surface layer is considered to act as barrier for bulk gas, mainlyhydrogen, diffusion, resulting in low outgassing.
Table 2 shows the outgassing rates of the samples measured at 5 and 50 hours after the pumping
without the baking process, and those rates with the baking process. Outgassing rates for Ti(CP) andTi(MCP) without baking are 1/5 of that for SUS(CP), and the outgassing rate of Ti(BP) is comparable to that
Table 1 Surface roughness and thickness ofsurface oxide layer of various polished puretitanium samples
Surface Roughness(Ra)
210 µm
1.0 µm2range (nm) range (nm)Table 2 Outgassing rates for various
polished titanium samples and chemical
polished stainless steel
Non-Baking(Pams-1)
5h50 hThicknessof Surface
OxideLayer (nm)Ti(BP)

3.3×10-8

2.8×10-9WithBaking(Pams-1)−Ti(BM)108−

13Ti(MCP)

9.3×10-9 4.6×10-10

7×10-13

Ti(BP)

44−

10Ti(CP)

7.0×10-9 5.1×10-10

7×10-13Ti(MCP)

2.5

1.03

11SUS(CP)

4.7×10-8

1.0×10-10

Ti(CP)

25

1.80

83

3.1×10-9

IVC-17/ICSS-13 and ICN+T2007
Journal of Physics: Conference Series 100 (2008) 092023

IOP Publishingdoi:10.1088/1742-6596/100/9/092023

for SUS(CP). The outgassing rates for Ti(CP) and Ti(MCP) withSmooth surface
baking process reach to 7×10-13 Pams-1, which is two orders(Ti(CP), Ti(MCP))
magnitude smaller than that of SUS(CP) under the same preSurface oxide layer
baking process. This means that chemical polishing is also(〜10 nm, amorphous )suitable as the surface treatment for titanium materials.
In order to discuss the mechanism for the low outgassing
Distribution ofproperty in the titanium material, the TOF-SIMS depthhydrogen atoms
profiles were measured for Ti(CP) and Ti(MCP). The depth(Ti(CP), Ti(MCP))
profile of hydrogen ions showed the peak at the boundarybetween the surface oxide layer and the bulk titanium. ThisBulk Titanium
result suggests that the hydrogen atoms localize at theboundary region as schematically shown in Fig. 3. ThisFig. 3 Schematic diagram for theboundary region is considered to prevent the bulk hydrogen
low outgassing titanium material.
diffusion. And also the smooth surface and the thin oxidesurface layer are considered to be important for the lower
outgassing property because the rough surface has many adsorbed sites for gases and the thick oxide
layer has many defects and dislocations which adsorb and dissolve gasses. Consequently smoothsurface, thin oxide surface layer and barrier region preventing the bulk hydrogen diffusion give theextremely low outgassing rate for the titanium materials.
5. Summary
The chemical polishing of the pure titanium of JIS grade 2 for UHV systems were discussed in thisstudy. The chemical polishing using a nitric acid is clarified to be stable, and the chemically polishedtitanium showed smooth surface in a microscopic area, thin oxide surface layer and formation ofbarrier region preventing the bulk hydrogen diffusion, resulting in low outgassing property (see Fig. 3).
The surface processing composed of the chemical polishing and the precision cleaning, named“precision chemical polishing”, was developed for the titanium material.
The outgassing property of the chemically polished titanium is excellent, and the outgassing rate
without or with the baking process is smaller than that of standard vacuum materials, and comparable
to that of mechano-chemically polished titanium with a mirror surface.
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