High-Purity Water and Steam

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.


High-purity Water & Steam versus Carbon- & Low-alloy Steels

Description
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 temperatures increase.

Boiling Water Reactor (BWR)
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 exceeds 175°C.
Low alloy steels are more susceptible than carbon steels to this sort of attack.

Up


High-purity Water & Steam versus Austenitic Stainless Steels

Up


Introduction
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.
back

BWR
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 SCC (IGSCC).
back

Necessary conditions
Necessary conditions for cracking are:
1. High residual stresses (unlike transgranular chloride SCC, IGSCC will not propagate at typical design stresses)
2. A sensitized microstructure
3. A critical combination of oxygen and temperature (see Diagram below).

Diagram:
SCC of sensitized 304 stainless steel in pure water as a function of oxygen and temperature

back

Prevention
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.
back

Up


High-purity Water and Steam vs. Ferritic and Martensitic Stainless Steels

Description
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.

Up


High-purity Water & Steam versus Copper and its Alloys

Description
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).

Up


High-purity Water & Steam versus Aluminum Alloys

Description
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.

Up


High-purity Water & Steam versus Titanium and its Alloys

Description
Titanium and its alloys do not stress-crack in steam or high-purity water.

Up


High-purity Water & Steam versus Zirconium and its Alloys

Description
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.

Up