Experimental Results:
304 stainless steel (UNS S30400) and an organic functionalized amorphous silicon surface were compared under static conditions to determine the impact of mercury and speciated mercury surface adsorption. Four, one gallon stainless steel sample cylinders (1800psi DOT rated, Swagelok Corp., Solon, OH) were used in the study. Two of the sample cylinders were treated with a functionalized amorphous silicon surface (SilcoTek Corporation, Bellefonte, PA). All sample cylinders were filled with a target concentration of 5 ug/m3 Hg standard. NIST traceable, internal mercury gas standards used in the study were supplied by Spectra Gases Inc. Alpha, NJ. Calibrated mercury standards were injected into the sample cylinders. The samples were stored at a nominal room temperature of 70°F. Samples were tested at day 0, day 7, day 19, and day 50. The samples were tested by direct interface gas sampling to an atomic absorption (AA) detector. Sample pathway, regulator, and tubing were treated with functionalized amorphous silicon to ensure a consistent sample pathway and experimental isolation of the sample cylinder test pieces. Figure 1 compares the average mercury response performance of functionalized silicon and untreated 304SS (UNS S30400) one gallon sample cylinder surfaces. The functionalized silicon cylinders show an initial mercury sample loss of 5% with sample stabilization within 7 days. Total 50 day sample loss was 10% (Table IV). The 304SS cylinders show an initial mercury sample loss of 42% at day 7 with no sample stabilization during the 50 day test period. Total sample loss after 50 days was 82%.
Figure 1: Average Mercury Response Comparison of Stainless Steel vs. Functionalized Silicon Surface
Table IV Tabulated response comparison: 304SS vs. Functionalized sample cylinders
| |
304SS cylinders |
|
Functionalized Si |
|
| |
ug/m3 |
|
ug/m3 |
|
| Test Day |
Avg response |
Loss vs. day 0 |
Avg response |
Loss vs. day 0 |
| 0 |
5.65 |
- |
6.45 |
- |
| 7 |
3.25 |
42% |
6.1 |
5% |
| 19 |
2.05 |
64% |
6 |
7% |
| 50 |
1 |
82% |
5.8 |
10% |
Experimental data show a 70 % greater loss of mercury when stored in 304SS sample cylinders over a period of 50 days. Significant and rapid mercury loss when exposed to a 304SS surface begins upon sample charging and continues throughout the test duration. Silicon coated surfaces show an initial mercury loss of 5% with sample loss stabilizing within 7 days. The test data demonstrate significant mercury adsorption due to active stainless steel surfaces. Amorphous silicon coated surfaces exhibit 70% less mercury loss compared to bare 304SS surfaces.
Chloride environments and chloride containing streams can greatly reduce the lifetime of process systems. Coatings and paints have been used to increase the lifetime of components in salt water and/or chloride containing environments. Table V are the results obtained from ASTM G48 Method B (1). This method uses immersion for 72 hours in a 6% Ferric Chloride solution to determine the amount of pitting and crevice corrosion. The amorphous silicon coated stainless steel shows greater than 10X the resistance than non-treated stainless steel in these environments. The use of PTFE, silicon and other coatings will greatly increase system life and reduce maintenance costs for systems in contact with chloride environments. PTFE systems however can allow Chloride streams to diffuse through the walls and have a damaging impact on surrounding materials. Use of silicon based materials will eliminate any diffusion in these applications.
Table V: Weight loss after 72 hour exposure to ferric chloride
| Sample |
Initial Weight (g) |
Final Weight (g) |
Weight Loss (g) |
Weight Loss (g/m2) |
| a-Si sample 1 |
10.4105 |
10.3710 |
0.0395 |
19 |
| a-Si sample 2 |
10.1256 |
10.0743 |
0.0513 |
25 |
| a-Si sample 3 |
10.1263 |
10.0742 |
00521 |
25 |
| Bare sample 1 |
10.0444 |
9.5655 |
0.4789 |
231 |
| Bare sample 2 |
10.1265 |
9.6923 |
0.4342 |
209 |
| Bare sample 3 |
10.1007 |
9.6276 |
0.4731 |
228 |
CONCLUSION:
To improve analytical system accuracy and durability in demanding applications, coatings are often used. Transfer of streams containing active and corrosive compounds will benefit from the use of properly chosen coatings. Physical system requirements such as pH, temperature and vibration can impact the performance and choice of coatings. By matching the physical properties of each coating with the application environment, system performance and durability improvements can lead to costs savings and reduce maintenance.
REFERENCES:
1.M. Zamanzadeh, M., Bayer Rhodes G., Smith D., Higgins M., "Laboratory Corrosion Testing of a Chemical Vapor Deposited Amorphous Silicon Coating", Matco Associates, Inc., SilcoTek Corporation, 2005.
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