Use of Nickel and 316L Stainless Steel Filter Assemblies By yaang.com

DataTime:[2015-10-12]

To ensure the highest quality gas at the point-of-use (POU), efforts have focused on the selection of the appropriate materials for construction of the distribution system. Traditionally, 316 stainless steel has been employed in the construction of subcomponents for use in reactive gas distribution systems. The proper selection of the stainless melt can improve thecorrosion resistance in UHP gas systems. In the case of high purity semiconductor gas filter assemblies, it is important to specify the proper chemistries, grain size and inclusions2. In addition, the suitability of the material to be formed, mechanically polished, and electropolished (EP) must also be evaluated. The careful selection of the 316L stainless steel utilized in the fabrication of filter assemblies is critical, as materials of similar composition can perform very differently.

Wang and coworkers noted variability in the corrosion behavior of steel alloys with bulk compositions that are virtually identical when exposed to moist HCl3. Smudde et al.4 observed that when moisture is below 1 ppmv, bromine from HBr is not incorporated beyond the native oxide of 316L stainless steel and no macroscopic degradation of the metal occurs.

Fine and coworkers confirmed the latter observations by investigating the effect of moisture content on the extent of HBr corrosion for 316L electropolished stainless steel. The scanning electron microscopy (SEM) and x-ray emmision spectroscopy (XES) analysis of the exposed sample coupons indicated no effect upon exposure to HBr containing less than 0.5 ppm of moisture. A moisture content of 10 ppm resulted in bromide incorporation and the onset of corrosion. The formation of corrosion pits was noted upon increasing the moisture level in the HBr to 100 ppm. A dense bromide scale was noted at a moisture concentration of 1,000 ppm.

Corrosion is typically quantified by techniques such as trace gas analysis, change of surface morphology, particle shedding and leakage. Wang and coworkers estimated the lifetime of EP 316L stainless steel tubing in HCl service, containing 1 ppm of moisture, to be of the order of 2-3 years based on particle shedding due to corrosion. The latter value is in agreement with field experience. The extrapolated lifetime of EP 316L stainless steel tubing at various moisture concentrations is shown in Table I. The extrapolated lifetime is based on the time required to produce 10 particles/scf at a flow rate of 3.531 scfm (100 slpm).

Table I
Extrapolated Lifetime of 0.25 - in. 316L stainless steel tubing [10 particles/scf at 3.531 scfm (100 slm)]

H2O (ppm)	Lifetime
5	170 Days
2	425 Days
1	2.3 Years
0.5	4.7 Years
0.2	11.6 Years
0.1	23.3 Years

A number of maintenance practices are recommended to eliminate the introduction of moisture into the corrosive gas distribution system7. It has been demonstrated that if adequate purge and evacuation procedures are followed to remove corrosive gases (such as HBr), EP 316L stainless steel can be exposed to moist air without diminishing the initial surface quality. However, if the purge and evacuation procedures are not followed, iron and bromine rich crystalline deposits form on the surface.
In order to maintain higher purity in corrosive gas service, new materials of construction have been investigated as a possible replacement for 316L stainless steel. One of the materials being investigated is nickel as it is corrosion resistant in aggressive environments. Nickel, however, is also a reactive material, commonly used as a hydrogenation catalyst.
The thermal decomposition characteristics of active specialty gases on various metal surfaces were investigated by Prof. Ohmi and his group at Tohuku University8. The metal surfaces investigated included nickel, oxygen passivated 316L stainless steel, chromium passivated 316L stainless steel, and 316L stainless steel with an electropolished (EP) surface. The thermal decomposition of the active specialty gases was monitored with the aid of a gas chromatograph (GC) and a Fourier Transform Infrared Spectrometer (FTIR). The FTIR was utilized to monitor the specialty gas concentration exiting the test sample (0.25" diameter, 1 m long 316L stainless steel tubing). In the case of phosphine, 100 ppm of phosphine in argon was passed through the 316L stainless steel tubing at a flow rate of 5 sccm. The nickel sample exhibited a strong catalytic effect on the phosphine decomposition (see Figure 1). The nickel surface reduced the phosphine concentration to undetectable levels at a temperature of 55°C. In contrast, the EP 316L stainless steel sample resulted in complete thermal decomposition of the phosphine gas at 260°C. The chromium passivated 316L stainless steel surface resulted in complete thermal decomposition at 370°C.

316L Stainless Steel Media and Nickel Media in Corrosive Gas Service
The purpose of the SEMI document “Test Method for Evaluation of Particle Contribution from Gas System Components Exposed to Corrosive Gas Service” is to provide a test method to compare gas handling components for potential particle generation in corrosive gas service. The document is intended as a practical means of generating performance data for a group of components to be compared in a selection process.
A flow chart of the sequence of exposure of gas handling components to corrosive service and the subsequent determination of the particle contribution is shown in Figure 3. The test sequence was used to compare the corrosion resistance of a gas filter assembly employing nickel media and a gas filter assembly employing 316L stainless steel media.

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