Steam, steam condensate and boiler feedwater are present in almost all CPI
plant utilities. The technology of handling them is mostly well-established.
Stress-corrosion cracking failures have, over the years, usually been attributed
to impurities in the steam or in the water. However, water with very little else
in it is still an environment which must be considered with stress-corrosion
cracking in mind. There has been widespread cracking of 304 stainless steel
nuclear power plant piping and recent laboratory findings show that even carbon
steel is not completely immune to SCC in high-purity water.
- The world's fossil-fuelled steam plants use
thousands of miles of steel piping to carry steam, condensate and boiler
feedwater. Stress-corrosion cracking is not a problem in well-maintained
systems. In poorly maintained and monitored systems, impurities occasionally
concentrate and cause cracking.
In steam turbines, stress-corrosion cracking is a serious source of forced
outages in power plants. The cracking is related to contaminants in the steam or
in the water. The most common contaminants that cause SCC in turbines are
caustic, chlorides and sulfite. Sodium sulfite is used as an oxygen scavenger in
boiler feedwater treatment up to 600 psig. Thermal decomposition of the sulfite
to H2S has caused sulfide stress cracking of the low-alloy steel used in turbine
rotors. Use of hydrazine instead of sodium sulfite has largely eliminated this
source of SCC.
High-strength alloy steels such as AISI 4140 and 4340 can crack even in
distilled water at room temperature if their hardness exceeds about Rc40. As
noted in the discussion on chlorides versus steels, this cracking is a form of
hydrogen- assisted cracking. Unlike most other forms of environmental cracking,
HAC is most severe around room temperature and tends to be less severe as
Boiling Water Reactor
- BWR Nuclear power plants pose different problems.
Oxygen levels in fossil-fired steam generators are usually controlled at less
than 5 ppb. This is not possible in BWR nuclear power plants, because the
radiolytic decomposition of water inevitably frees some oxygen; boiling water
reactors operate from 50 to 288°C (122 to 550°F) and typical oxygen
levels range from 0.02 to 8 ppm.
At these oxygen levels, general corrosion rates on carbon steel generate
considerable amounts of undesirable iron oxide under boiling water reactor
conditions. Consequently, carbon steel is not widely used.
However, transgranular SCC has been observed in the laboratory under
simulated BWR conditions when oxygen contents exceed 1 ppm and the temperature
Low alloy steels are more susceptible than carbon steels to this sort of
- In the fossil-fired steam generators common to CPI plants, austenitic
stainless steels have been known to suffer SCC in ostensibly deaerated boiler
feedwater containing chlorides. SCC does not occur in clean steam nor in
deaerated, demineralized boiler feedwater, but may be encountered in steam
condensate, if contaminated with chlorides and oxygen.
- In BWR nuclear power plants, the consistent presence of from 0.02 to 8 ppm
oxygen in the water (due to radiolytic decomposition) introduces another
problem; intergranular SCC of sensitized material.
Most of the second generation of commercial BWR nuclear power plants were
built with regular carbon 304 stainless steel piping and vessels. Over the
years, up to 5% of the welds in these plants have cracked due to intergranular
- Necessary conditions for cracking are:
1. High residual stresses
(unlike transgranular chloride SCC, IGSCC will not propagate at typical
2. A sensitized microstructure
3. A critical
combination of oxygen and temperature (see Diagram below).
- SCC of sensitized 304 stainless steel in pure water as a
function of oxygen and temperature
- New plants, built after these problems surfaced, use L-grade or N-grade
(nuclear grade) stainless steels. Older plants use hydrogen additions to
eliminate the oxygen, corrosion-resistant cladding to protect the pipe, or
induction-heating stress-improvement (IHSI) or last-pass heat-sink welding
(LPHSW) to eliminate the tensile residual stresses from welding.
- SCC of 12Cr stainless steel buckets, bucket covers
and tie wires is one of the most frequent sources of outages in steam turbines.
Parts per billion of contaminants in the steam become concentrated to injurious
levels due to evaporation and drying, and deposition from the superheated steam.
The most common injurious species are caustic and sulfates.
- Silicon bronze and aluminum bronze D are quite
susceptible to SCC in live steam. Whether it is the steam itself or contaminants
therein that causes the SCC has never been established. One major petro-chemical
company found that over the years fully one-third of its CA 655 silicon bronze
pressure vessels had stress-cracked, either due to mercury (liquid metal
embrittlement) or SCC in steam. Aluminum bronze D also stress-cracks readily in
steam at temperatures over 120° C (248° F).
- High-purity water and steam do not stress-crack the
low-strength aluminum alloys most common in CPI plants. However, there have been
reports of stress-cracking of the Al-Cu (2xxx series) alloys in distilled water.
- Titanium and its alloys do not stress-crack in
steam or high-purity water.
- Zirconium alloys are the standard materials for
fuel cell elements in boiling water nuclear reactors. No problems have been
encountered on the water side in these applications.