Information Notice No. 94-59: Accelerated Dealloying of Cast Aluminum-Bronze Valves Caused by Microbiologically Induced Corrosion

UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C.  20555

August 17, 1994


NRC INFORMATION NOTICE 94-59:  ACCELERATED DEALLOYING OF CAST ALUMINUM-BRONZE
 VALVES CAUSED BY MICROBIOLOGICALLY INDUCED
 CORROSION


Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the potential consequences of increased
corrosion rates in aluminum-bronze valves in service water systems when
microbiologically induced corrosion (MIC) is present.  It is expected that
recipients will review the information for applicability to their facilities
and consider actions, as appropriate, to avoid similar problems.  However,
suggestions contained in this information notice are not NRC requirements;
therefore, no specific action or written response is required.

Description of Circumstances

On October 12, 1993, operators making their rounds at Surry Power Station
Units 1 and 2 noted varying degrees of corrosion on a total of 22 Jamesbury
cast aluminum-bronze ball valves in the 1- and 2-inch nominal diameter
service water lines to the charging/safety injection pumps.  The corrosion
conditions ranged from a slight seepage of water to a buildup of corrosion
products on the external valve parts.  There are approximately 25 such valves
for each unit.  The maximum valve leakage noted was a few drops of water
through one valve body in a 1-hour period.

Five valves were removed and submitted to the site metallurgical laboratory
for evaluation of the failure mechanism.  Sectioned valves exhibited
dealloying to varying degrees.  Dealloying of aluminum-bronze is the selective
removal of aluminum from the structure.  Cross-sections through the leaking
areas examined under the light microscope revealed that the aluminum-rich
phases had been leached out, leaving a porous structure through which the
weeping occurred.  An analysis of the corrosion product on the outside of the
valves revealed a large percentage of aluminum, thus confirming the dealloying
process.  The dealloying was most severe under corrosion nodules.  The
location of the leak appeared to coincide with a corrosion nodule.  Bacterial
analysis of the corrosion nodules revealed several bacteria types, e.g.,




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sulfate-reducing and acid-producing.  The licensee concluded that once a
nodule was formed by the bacteria, an acidic condition was created under the
nodule and this resulted in an accelerated rate of corrosion/dealloying of the
alloy.

Discussion

The service water valves at Surry Power Station were installed in 1986.  They
were made of cast aluminum-bronze, alloy C 95400.  Chemical analysis of the
cast valves showed that they met the chemical composition requirements of
American Society for Testing Materials, Specification B 148, "Standard
Specification for Aluminum-Bronze Sand Castings," (ASTM B 148).  This material
contains a nominal 11-percent aluminum.  Aluminum-bronzes with aluminum
contents above approximately 9 percent require heat treatment for optimum
corrosion resistance.  ASTM B 148 specifications require no heat treatment.
Thus, the as-cast material is not optimally conditioned to resist corrosion.
Corrosion of this material often occurs by dealloying.

At Surry Power Station, the affected portion of the service water system
provided brackish cooling water to the charging/safety injection pump
lubricating oil and seal cooler heater exchangers.  The system flow is
normally approximately 189 liters [50 gallons] per minute, but varies on a
seasonal basis.  The flow velocity can be as low as 0.6 to 0.9 meter [2 to 3
feet] per second.  Flow below approximately 1.5 meters [5 feet] per second
lends to the system the potential for fouling which can promote the formation
of microbiologically induced corrosion nodules as found in the subject valves.

Cast aluminum-bronze valves continue to be used successfully in fluids from
fresh water to brackish water when fouling is inhibited.  However, any valve
or component manufactured from C 95400 alloy may experience dealloying under
certain corrosive conditions.  Visual leakage and corrosion deposits are the
first indications of dealloying.

The dealloying of the aluminum-bronze valve body reduces the tensile strength
and the toughness of the material and increases the susceptibility to
transient and impact loading.  A failure of the valve body could reduce the
flow of cooling water to the charging/safety injection pump lubricating oil
and seal cooler heat exchangers.  After analyzing the five removed valves, the
licensee for Surry Power Station determined that sufficient safety margin
remained to warrant continued operation with the installed valves until the
next scheduled refueling outage.  At that time, the licensee intends to
replace the affected valves with valves more resistant to the conditions in
the service water system.

Related Generic Communications

NRC Information Notice 85-30, "Microbiologically Induced Corrosion
of Containment Service Water Systems," dated April 19, 1985.

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August 17, 1994
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This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.

/s/'d by BKGrimes


Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation

Technical contacts:  John W. York, RII
   (404) 331-5536

   Morris W. Branch, RII
   (804) 357-2101

   Geoffrey P. Hornseth, NRR
   (301) 504-2756

Attachment:
List of Recently Issued NRC Information Notices