Information Notice No. 96-54: Vulnerability of Stainless Steel to Corrosion when Sensitized

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                             WASHINGTON, DC  20555

                               October 17, 1996

                               CORROSION WHEN SENSITIZED


All materials licensees.


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to make licensees aware of the vulnerability of stainless steel to
corrosion when sensitized.  This issue is of particular concern to the NRC
when it involves sources or devices constructed of stainless steels.  Although
the corrosion in the incident described below involved only 316L stainless
steel, the inherent concerns extend to other types of stainless steel that may
be subjected to temperatures that sensitize the material.  It is expected that
recipients will review the information for applicability to their programs. 
However, suggestions contained in this information notice are not NRC
requirements; therefore, no specific action nor written response is required.

Description of Circumstances

Recently, the NRC was informed of the discovery of corrosion found on a
particular manufacturer�s 316L stainless steel irradiator source used in
Category IV irradiator facilities.  These sources were similar in design and
construction to other manufacturers� sources; however, their sources showed
signs of intergranular corrosion, whereas other manufacturer�s sources located
in the same irradiator pool did not.

Analysis of test data and reports provided by the manufacturer revealed that
the 316L stainless steel sources were heat sensitized as a result of improper
loading of the sources in the shipping container.  The heat generated by these
high activity sources resulted in higher than normal temperatures inside the
container.  The sensitized stainless steel was then vulnerable to
intergranular corrosion.  The intergranular corrosion was initiated due to the
sensitized material being in an environment favorable for corrosion (i.e.,
poor pool water quality).


Although stainless steels are considered as having excellent corrosion
resistance, they can be susceptible to corrosion when exposed to certain
conditions.  Stainless steels are protected against corrosion by alloying them
with protective elements such as chromium.  For example, 316L stainless steel,

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which is considered to have high corrosion resistance, contains 17 percent
chromium.  However, if the level of these protective elements is reduced below
the minimum percentage necessary, the protective properties are reduced and
the material becomes vulnerable to corrosion.

One of the ways that these protective properties can be compromised occurs
when the stainless steel is subjected to temperatures within a certain
critical temperature range.  The critical temperature range for this to occur
varies for different stainless steels, and is typically higher than the normal
operating temperatures that the material would be subjected to.  Stainless
steels subjected to temperatures within this critical range for a sufficient
length of time will become heat sensitized.  Heat sensitization refers to a
condition where the protective elements in the material structure of the
stainless steel are reduced to levels below the minimum necessary for
corrosion protection.  The amount of time before heat sensitization occurs is
temperature and time dependent, and is typically longer for temperatures at
the lower end of the temperature range.  For example, a material may need to
be exposed to temperatures at the low end of the range for a year or longer
before heat sensitization would occur, while at the upper end of the scale,
heat sensitization may occur within seconds.  These higher than normal
temperatures may occur in a number of ways, including, but not limited to,
loading too many sources in a shipping or storage container, loading the
sources in an improper configuration within the shipping or storage container,
and use or accident conditions which result in the material experiencing high
temperatures.  If these sensitized materials are placed in an environment
favorable to corrosion, such as an electrolyte or conducting liquid medium,
then intergranular corrosion, which is a type of corrosion that occurs along
the boundaries between the grains of the stainless steel, can occur.  The rate
of intergranular corrosion is hard to predict and is based on a complex
relationship involving time, temperature, and environment.

All licensees should be aware of the potential for sensitization and resulting
corrosion in stainless steel, and should consider their own circumstances and
take action appropriately.  Licensees should be particularly sensitive to past
and potential situations involving transportation, and to situations involving
accident conditions and incident response.  Licensees should avoid subjecting
their sources and devices to conditions that could cause sensitization. 
Licensees who suspect that a source or device may have been subjected to
conditions that could cause sensitization, should monitor the source or device
for signs of corrosion.  Sources or devices showing signs of corrosion should
be isolated and placed in an environment not favorable to corrosion.  They
should be evaluated before continued use.  Licensees may contact the
manufacturer or a knowledgeable consultant to obtain assistance in evaluating
whether sensitization and corrosion is a concern for their particular

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This information notice requires no specific action nor written response.  If
you have any questions about the information in this notice, please contact
the technical contacts listed below or the appropriate regional office. 

                                          Donald A. Cool, Director 
                                          Division of Industrial and 
                                            Medical Nuclear Safety
                                          Office of Nuclear Material Safety
                                            and Safeguards

Technical contacts:  Michele Burgess, IMAB
                     (301) 415-5868

                     Thomas Rich, IMAB
                     (301) 415-7893

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