Information Notice No. 84-71: Graphitic Corrosion of Cast Iron in Salt Water

                                                           SSINS No.:  6835 
                                                           IN 84-71        

                               UNITED STATES 
                       NUCLEAR REGULATORY COMMISSION 
                    OFFICE OF INSPECTION AND ENFORCEMENT 
                            WASHINGTON, DC 20555 

                             September 06, 1984 


Information Notice No. 84-71:   GRAPHITIC CORROSION OF CAST IRON IN SALT 
                                   WATER 

Addressees: 

All holders of a nuclear power reactor operating license (OL) or 
construction permit (CP). 

Purpose: 

This notice is provided to inform licensees and applicants of a potentially 
significant problem pertaining to graphitic corrosion of cast iron in salt 
or brackish water. It is expected that recipients will review the 
information for applicability to their facilities and consider actions, if 
appropriate, to preclude or ameliorate a similar problem from occurring at 
their facilities. However, suggestions contained in this information notice 
do not constitute NRC requirements and, therefore, no specific action or 
response is required. 

Description of Circumstances: 

On May 3, 1984, Baltimore Gas and Electric Co. reported that a through-wall 
corrosion attack had been identified in the salt water side of a component 
cooling system heat exchanger at Calvert Cliffs Unit 2. Visual examination 
of corresponding components for Unit 1 (then operating at 100% power) 
disclosed evidence of leakage. Unit 1 was shut down on May 6 because of 
concerns about the integrity of the component cooling system. On May 14, 
1984, the licensee determined that corrosion damage to the salt water heat 
exchangers was so extensive in both units that these systems could not 
survive a design-basis seismic event. This required that Unit 2 suspend 
refueling and restore containment integrity and that Unit 1 remain in cold 
shutdown with full containment established. These requirements remained in 
effect until repairs (structural, as well as leak tightness) could be 
completed. 

At Calvert Cliffs, each unit has two auxiliary cooling water systems that 
contain condensate grade water with corrosion inhibiting additives. The 
"component" cooling system serves most of the nuclear plant cooling needs 
and the "service water" cooling system principally serves balance-of-plant 
cooling needs, but also serves the emergency diesel generators, the spent 
fuel pool, and containment air coolers. Thus, both systems are 
safety-related. 


8409040187  
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                                                        IN 84-71           
                                                        Page 2 of 3        
                                                        September 06, 1984 

The plant cooling systems are, in turn, cooled by a salt water cooling 
system. This system consists of redundant and separable supply headers and 
redundant discharge headers, which ultimately combine in a single overboard 
discharge pipe. Each supply header is served by a dedicated pump and a spare
"swing" pump can be connected to either supply header. Each supply header, 
in turn, supplies a full capacity heat exchanger for each of the plant 
cooling systems. 

Materials of the salt water system are: cast iron for pump casings, valve 
bodies and water boxes for the heat exchangers; pipe is flanged cement lined
cast iron; and the once-through tube bundles are 90-10 copper-nickel with 
aluminum bronze tube sheets. 

Discussion: 

Graphitic corrosion is a long-known phenomenon where an electrolytic cell is
established between the graphite and the iron within the cast iron itself 
when in contact with water containing enough dissolved salts to act as an 
electrolyte. This may be sea water or brackish water. Generally the 
phenomenon occurs on ocean-going ships and in coastal or tidal estuary 
industrial plants. It can, however, occur at inland plants if the raw 
cooling water is sufficiently contaminated. The corrosion can be accelerated 
by the presence of copper-based alloys (the tube bundles). On the other 
hand, the attack on the cast iron can be minimized by the installation of 
"sacrificial" zinc plates to take the corrosion attack. 

Other ways of reducing the corrosion attack consist of using alternate (but 
more expensive) materials to replace the cast iron, or applying a corrosion 
resistant coating to the cast iron. This latter approach must be very 
carefully implemented, particularly in the presence of copper-based parts, 
because a small break in the coating concentrates the attack and accelerates
the local rate of corrosion. Where corrosion resistant coatings are used, 
good practice indicates use of sacrificial zinc plates also. Direct current 
electric potential (cathodic protection) may also be used, but must be 
carefully monitored or it may do more harm than good. 

Graphitic corrosion is often not readily apparent from visual examination. 
As the iron is dissolved away, the graphite structure remains, and, 
particularly where covered by a film of silt or marine growth, the surface 
may appear relatively sound, even though it is highly permeable to water and
has no strength. 

Examination for graphitic corrosion historically has been performed by use 
of a test hammer on the wetted surfaces to determine the depth and extent of
attack. At Calvert Cliffs, the licensee reports that use of an ultrasonic 
thickness gage from the outside could locate the interface between sound 
metal and corroded material. At 2.25 megahertz discrimination was not 
adequate, but at 1/2 to 1 megahertz the L-wave technique produced acceptable
accuracy. 
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                                                     IN 84-71              
                                                     Page 3 of 3           
                                                     September 06,  1984   

Replacement water boxes were available and installed on three component 
cooling water heat exchangers. Temporary repairs were made to others. All 
water boxes, after cleaning, were coated with a coaltar-epoxy anticorrosion 
compound. A full complement of replacement water boxes is now on site, and 
further replacements will be made when appropriate outages occur. A long-
term program for monitoring the integrity of cast iron components in salt 
water service is being developed. 

NRC requirements do not include specific provisions for checking the degree 
of corrosion in safety-related components and in some cases extensive 
corrosion may not be visually apparent. In this case, graphitic corrosion 
progressed to the point of seismic and structural inadequacy and caused an 
extended forced outage of both units. Accordingly, licensees are advised to 
review their maintenance programs in light of the need to preclude 
substantial degradation of safety related components due to corrosion. 

We are advised that the Institute of Nuclear Power Operations plans a 
similar publication on this event, providing additional detail. No written 
response or specific action is required by this notice. If you have any 
questions about this matter, please contact the Regional Administrator of 
the appropriate NRC regional office or this office. 


                                   Edward L. Jordan, Director 
                                   Division of Emergency Preparedness 
                                     and Engineering Response 
                                   Office of Inspection and Enforcement 

Technical Contact:  J. B. Henderson, IE 
                    (301) 492-9654 

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