silicon for the chemical and solar industry XIII

Corrosive Agent

The process consists of hydrogen chloride and some chloride salts formed from the

impurities present in metallurgical silicon. Due to the nature of the reactor, it is not

possible to eliminate the chlorination sources.

Tensile Stress

The cracks in the reactor were only present in isolated areas, where the lifting lug was

welded and some areas of the knuckle radius, where the steel plate was formed to

make a dished shape. However, the knuckle was not uniformly covered in cracks, they

were only located in isolated areas, indicating that the required stress was only located

in specific areas. This indicates that the post-weld heat treatment was inadequate in

these specific locations

Solution

Although there were several areas with stress corrosion cracking, it was only found in

areas where high stress was present during construction (bending or welding) and it

was not uniformly distributed around the knuckle. It is difficult to determine the

residual stress in the reactor without destructive testing, but the distribution of SCC

indicates that it is possible to use carbon steel in the environment if the stress is

correctly relieved through heat treatment.

Records indicate that the conditions specified for post-weld heat treatment exceeded

pressure vessel design code minimum requirements, but were not ideal. Due to

placement of thermocouples, it is highly probable that the area where the SCC

occurred was not heated to a sufficient temperature. The implemented solution is to

install several thermocouples, controlling the heating rate and temperature gradient

across the entire section to ensure that all parts, especially the thickest sections and

the most highly stressed areas, have uniformly attained sufficient temperature.

Conclusion

It was found that carbon steel is susceptible to stress corrosion cracking in a

trichlorosilane reactor. The combination of carbon steel in a chlorinating environment

with residual tensile stress allowed cracks to slowly propagate up to 11 mm into

reactor wall. The solution is to place more focus on a stringent heat treatment regime,

effectively relieving the residual tensile stress and eliminating a pre-requisite for

stress corrosion cracking.

References

1. Cottis RA. Guides to Good Practice in Corrosion Control - Stress Corrosion

Cracking

2. Treseder RS, Wachter A. Corrosion. 1949;5: 383-91.

3. Strauss MB, Bloom MC. Corrosion. 1960;16: 109-12.

Acknowledgements

We would like to thank Dow Corning Corporation for permission to publish this

work.

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