Zirconium and zirconium alloy passivation process

A zirconium or zirconium alloy passivation process comprises providing an electrolyte which is capable of removing nickel, nickel alloys and alloys containing nickel from the surface of a zirconium or zirconium alloy article, keeping the dissolved metal in solution while simultaneously anodizing the article surfaces. Such nickel, if not removed provides a window for hydride accumulation to occur, detrimentally affecting the alloy properties when subject to a nuclear reactor environment. An article placed in the electrolyte in proximity to a cathode and connected to a power source has the trace nickel, nickel alloys and alloys containing nickel removed to background levels and reduces the potential for hydride accumulation within the article in a nuclear reactor environment, and provides for increased article life.

TECHNICAL FIELD 
This invention relates to zirconium and zirconium alloys for use in nuclear 
reactor assemblies and more particularly to methods for increasing the 
hydride resistance of the zirconium and zirconium alloys. 
BACKGROUND 
Zirconium and zirconium alloys have structural and other characteristics 
which make them desirable for use in nuclear reactor assemblies. For 
example, such materials have a low neutron cross section, good mechanical 
properties at elevated temperatures and relatively low co-efficients of 
thermal expansion. However, such alloys do have several drawbacks, such as 
the susceptibility to hydriding in an aqueous environment at elevated 
temperatures. For example, amounts of hydrogen as low as about 70 ppm in 
zirconium can produce an embrittlement effect, which with time, reduces 
the strength and integrity of the zirconium component. 
In U.S. Pat. No. 3,864,220, zirconium alloy objects are anodized in an 
aqueous solution containing preferably 1% phosphoric acid. After 
anodizing, the object is heat treated in an oxygen containing atmosphere 
to produce an oxidized film, for example, by heat treating for 16 hours at 
370.degree. C. in air. 
In U.S. Pat. No. 3,909,370, a process for surface treatment of zirconium 
alloys is disclosed which includes pickling in a fluoride bath and then 
adding a protective coating by oxidation. An anodizing step eliminates any 
fluoride contaminants from the surface before autoclaving in water to add 
the oxide film. 
When a zirconium alloy is subjected to high temperature water or steam, the 
zirconium reacts with the water to form zirconium oxide and liberated 
hydrogen. Some of this hydrogen enters the zirconium alloy slowly, but 
diffuses rapidly through the alloy. Small quantities of hydrogen can 
dissolve in the alloy without reacting with it and some reacts to form 
zirconium hydride. This hydride is typically uniformly distributed through 
the zirconium alloy and, within limits, has no detrimental effect. 
Nickel, nickel alloys, and/or other metal alloys containing nickel may be 
deposited on zirconium and zirconium alloy surfaces during part 
fabrication and finishing processes, or during nuclear fuel manufacturing 
operations. Most fuel component manufacturing operations utilize stainless 
steel, an iron alloy containing nickel, whenever possible because of 
stainless steel's corrosion resistance. Fuel components made of zirconium 
and zirconium alloys may be pushed or pulled across stainless steel 
equipment and have small quantities of stainless steel deposited or 
imbedded in their surfaces. Nickel alloy transfer by this mechanism 
results in small non-uniform deposits on the zirconium and zirconium alloy 
surfaces. 
Nickel, nickel alloys, and alloys containing nickel can also become 
imbedded on a zirconium or zirconium alloy surface indirectly through grit 
blasting operations which are common finishing steps in the nuclear 
component fabrication industry. For example, by using alloys containing 
nickel pipe or tubing to conduct the blasting grit and fluid to the work 
piece, abrasive grit traveling through the alloy tube can impact the alloy 
tube and pick up small quantities of the alloy on the grit surface. When 
the grit exits the tube and contacts a zirconium and zirconium alloy part, 
the alloy on the grit surface can be imbedded into the part surface. Very 
small quantities of alloy can be uniformly deposited on the part surface 
by this material transfer mechanism. 
While uniform hydriding is acceptable, it has been found that the presence 
of nickel, nickel alloys, or alloys containing nickel on or near the 
surface of a zirconium article can provide a window through which hydrogen 
easily enters the zirconium alloy at a much higher rate than it enters the 
zirconium surfaces without such a window. When the absorption of hydrogen 
into the zirconium alloy is rapid, zirconium hydride may form and collect 
near the surface where the hydrogen enters and form a thick zirconium 
hydride deposit or "rim" inside the metal near the surface. The zirconium 
hydride is brittle and occupies a higher volume than a zirconium oxide and 
can change the physical properties of the part as a whole, making it more 
prone to failure. A flexible and ductile piece of zirconium alloy tubing 
thus becomes a brittle and inflexible piece of zirconium hydride at lower 
temperatures than for the alloy. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method for treating 
zirconium and zirconium alloys to reduce hydride rim formation. 
It is a further object to provide a process which eliminates nickel and 
nickel alloys from the surface of a zirconium or zirconium alloy article 
to minimize detrimental hydriding in an aqueous environment. 
It is a further object to increase the hydride resistance of a zirconium or 
zirconium alloy article in one step. 
These and other objects of the present invention are achieved by providing 
a zirconium or zirconium alloy article, placing the article in an 
electrolyte bath, the bath comprising an electrolyte which is capable of 
dissolving nickel and nickel alloys and maintaining them in solution and 
simultaneously anodizing the zirconium surfaces to increase hydride 
resistance of the article, and, anodizing the zirconium or zirconium alloy 
article.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, a tube 1 is composed of a zirconium or a zirconium 
alloy such as Zircaloy-2 or -4. For purposes of this application, the 
terms "zirconium" and "zirconium alloy" are used interchangeably and 
without limitation to refer to articles having zirconium as a major 
constituent. For exemplary purposes, the article is a tube, which may be 
used as a control rod guide tube or an instrument tube in a nuclear 
reactor assembly. Of course, any zirconium article could also be treated 
according to the invention. 
The tube 1 is connected to a power supply 2 by a wire 3 and acts as an 
anode. The tube is located in an electrolyte 4. A cathode 5 is located 
within the tube and is separated from the tube by an insulator 6. The 
cathode 5 is connected by a wire 7 to the power supply 2 to complete the 
circuit for forming an electrolytic cell. 
By applying potential across the cell, anodizing of the exposed article 
surfaces takes place. However, by the proper choice of electrolyte, 
simultaneously, excess nickel, nickel alloys, and alloys containing nickel 
are removed from the article surfaces to allow the underlying zirconium 
and zirconium alloys to be completely anodized to limit rapid hydriding 
during use. The term "nickel", "nickel alloy", and "alloys containing 
nickel" are used interchangeably throughout the specification, and 
encompass pure nickel, nickel compounds such as nickel oxides and alloys 
having nickel as a constituent, particularly stainless steel. 
The combined anodizing and nickel removal steps are hereafter termed 
"passivation" which produces a zirconium oxide film, of about 500 
Angstroms in thickness, on the article surface while electrochemically 
etching the nickel from the surface. 
The electrolyte preferably comprises an oxalic acid solution, more 
preferably a mixture of oxalic acid and nitric acid, though other 
materials may be used. For example, other organic acids such as citric 
acid or acetic acid can be substituted for the oxalic acid. Oxalic acid is 
preferred as the main electrolyte component as, not only does it assist in 
removing nickel while anodizing zirconium, but it also is a good chelating 
agent that will hold the dissolved metal ions in solution until removed 
during rinsing. Being an organic acid, any residual acid left after 
rinsing would be destroyed quickly by radiolysis. Similarly other 
inorganic acids such as phosphoric acid or sulfuric acid can be 
substituted for the nitric acid. While oxalic acid can be used alone, a 
blend is preferred to assure consistent oxidizing and etching at shorter 
exposure times. 
Typically, a direct current potential greater than approximately 1.6 volts 
applied across a Zircaloy anode in a electrochemical cell using an 
appropriate electrolyte causes zirconium alloy oxidation with the oxide 
film thickness produced being a function of the applied voltage and 
typically equals about 20 Angstroms per volt. A direct current potential 
greater than approximately 0.75 volts applied across a nickel alloy 
(stainless steel) anode in an electrochemical cell using an appropriate 
electrolyte will cause the stainless steel to dissolve. Zircaloy located 
near a nickel or nickel alloy rich area will not anodize until the nickel 
is dissolved at which point the zirconium alloy will then anodize. 
The anodizing process itself provides a means for process monitoring, as 
the current in the electrochemical cell is very high at the start of the 
reaction. As the nickel is removed and the zirconium alloy begins to 
anodize, the cell current decreases as the resistance across the 
developing oxide film increases. Consequently, the anodizing reaction is 
self limiting and when the current decreases to a predetermined level, the 
process is complete and the part is removed from the bath. 
Typically, the process is run until the cell current decreases to a 
residual current level established at about 1-5 amps at 24 volts. However, 
to assure completion of the reaction, it is recommended that the process 
be continued for several minutes after the current decreases to the 
residual value. 
COMATIVE EXAMPLE 1 
A small electrochemical cell was constructed using a Zircaloy -4 tube known 
to contain trace quantities of stainless steel (iron alloy containing 
nickel) as the anode. A copper tube was inserted into the Zircaloy tube to 
act as the cathode. The cathode was covered with small pieces of rubber 
tubing to act as insulators. A 600 milliliter beaker was the cell vessel 
and the electrolyte was 0.10% by weight sodium hydroxide. 
A 24 volt DC potential was applied across the electrochemical cell. A small 
initial current was observed. After a few seconds, bubbling inside the 
Zircaloy stopped and the Zircaloy began turning blue, indicating that 
anodizing was taking place. The anodized tube was rinsed, dried, and had a 
piece cut for analysis. An analysis for iron was performed, as the 
presence of iron indicates the presence of stainless steel on the tube, 
with iron analyzed using a scanning electron microscope (SEM). Iron 
content on the tube inner surface was reduced from 0.5% to 0.19%, which is 
the background value for iron in the Zircaloy (0.18% to 0.24%). 
As shown in Table I, additional testing showed that stainless steel was not 
reproducibly removed in sodium hydroxide. Test samples were placed in an 
autoclave at autoclave conditions chosen to simulate a reactor environment 
i.e. exposure at 270.degree. C., for 16 hours, at 500 psi hydrogen over 
pressure in an aqueous solution chemical content equaling 2.2 ppm lithium. 
Hydride rims formed in the test pieces during the autoclave test. 
TABLE I 
______________________________________ 
HYDRIDING 
SAMPLE IRON LEVEL % UNIFORM RIMS 
______________________________________ 
180 Minute 
Processing Time 
Top 0.39% Yes Yes 85.mu. 
Bottom 1.01% Yes Yes 50.mu. 
______________________________________ 
COMATIVE EXAMPLE II 
Conditions similar to those used in Comparative Example 1 were followed 
except a stainless steel cathode was used, 0.1% nitric acid was used as 
the electrolyte, and the tube was a full size guide tube known to contain 
trace amounts of stainless steel on the tube inner surface. The initial 
current was approximately 70 amps, at 24 volts, and decreased rapidly to 
about 48 amps then increased again to 58 amps, then decreased slowly to 17 
amps. The current did not decrease further. The test was stopped after 30 
minutes and samples taken for evaluation. Iron was not analyzed as hydride 
rim formation appeared to be the harsher test. The results, shown in Table 
II, show hydride rims did form. 
TABLE II 
______________________________________ 
HYDRIDING 
SAMPLE IRON LEVEL % UNIFORM RIMS 
______________________________________ 
Thirty minute 
Processing Time 
Top Not determined 
Yes No 
Mid-1 " Yes Yes 50.mu. 
Mid-2 " Yes Yes 55.mu. 
Bottom " Yes Yes 45.mu. 
______________________________________ 
COMATIVE EXAMPLE III 
The same procedure as Comparative Example II was followed except 0.5% 
oxalic acid alone was used as the electrolyte. Initial current was 
approximately 35 amps at 6 volts which decreased quickly. As the voltage 
was increased, in 6 volt steps up to 24 volts, the current increased then 
decreased rapidly. The test was stopped after 5 minutes at a current of 
about 5 amps and samples taken for evaluation. As shown in Table III, Iron 
was removed to background levels which indicate that most of the nickel 
alloy was removed, yet hydride rims did form. 
TABLE III 
______________________________________ 
Hydriding 
IRON 
SAMPLE LEVEL % UNIFORM RIMS 
______________________________________ 
Zero minute 
Processing 
Time 
1 Top 0.78 Yes Yes 30.mu. 
2 Bottom 0.85 Yes Yes 40.mu. 
Five minute 
Processing 
Time 
1 Top 0.23 Yes No 
2 Bottom 0.26 Yes No 
3 Mid Point 
0.23 Yes Hint 
4 Mid Point 
0.23 Yes Yes 0-23.mu. 
______________________________________ 
EXAMPLE IV 
The same procedure as used in comparative Example III was followed except 
for a longer processing time, 10 minutes. As shown in Table IV, the longer 
processing time in oxalic acid produces acceptable autoclave test 
hydriding results. 
TABLE IV 
______________________________________ 
HYDRIDING 
SAMPLE IRON LEVEL % UNIFORM RIMS 
______________________________________ 
Ten Minute 
Processing Time 
1 Not Determined 
Yes No 
2 " Yes No 
3 " Yes No 
4 Yes No 
______________________________________ 
EXAMPLE V 
The same procedure as comparative Example III was followed except an 
electrolyte of 0.5% oxalic acid and 0.1% nitric acid was used. The 
potential was initially at 6 volts and increased in one-minute intervals 
to 12 and then 24 volts. Samples were taken after 10 minutes and then 
after 20 minutes. As shown in Table V, the mixed electrolyte removed iron 
to indicate that the nickel alloy was removed and prevented hydride rim 
formation during the autoclave test. 
TABLE V 
______________________________________ 
HYDRIDING 
IRON 
SAMPLE LEVEL % UNIFORM RIMS 
______________________________________ 
Zero Minute 
Processing Time 
Top 0.79 Yes Yes 
Bottom 0.65 Yes Yes 
Ten Minute 
Processing Time 
Top 0.22 Yes No 
Bottom 0.16 Yes No 
Twenty Minute 
Processing Time 
Top 0.16 Yes No 
Bottom 0.16 Yes No 
______________________________________ 
EXAMPLE VI 
In view of the success with the 0.5% oxalic acid electrolyte and mixed 0.5% 
oxalic acid/0.1% nitric acid, additional testing was done to establish the 
parameters of the process. 
In separate tests, five samples were randomly taken through a tube length 
before testing to determine the base iron, and therefore the base 
stainless steel level. The test was started at 6 volts, increased to 12 
volts after one minute and to 24 volts, 1 minute later. After 5 minutes, 
two tube samples were taken; the test then resumed for three more minutes, 
stopped and two additional samples taken. The test then resumed and 
stopped again at ten minutes total processing time and 8 samples taken. 
The test resumed and stopped at fifteen minutes with 4 samples taken. 
The samples were cut in half and half the samples were tested for iron, the 
other half autoclaved to determine hydride formation. The results are 
shown in Table VI for 0.5% oxalic acid and in Table VII for 0.5% oxalic 
acid/0.1% nitric acid. 
TABLE VI 
______________________________________ 
HYDRIDING 
IRON 
SAMPLE LEVEL % UNIFORM RIMS 
______________________________________ 
Zero Minute 
Processing Time 
1 0.70 -- Yes 
2 0.80 -- Yes 
3 0.67 -- Yes 
4 0.57 -- Yes 
5 0.45 -- Yes 
Five Minute 
Processing Time 
1 0.19 -- No 
2 0.10 -- No 
Eight Minute 
Processing Time 
1 0.17 -- No 
2 0.22 -- No 
Ten Minute 
Processing Time 
1 0.19 -- No 
2 -- -- No 
3 -- -- No 
4 -- -- No 
5 -- -- No 
6 -- -- No 
7 -- -- No 
8 0.15 -- No 
Fifteen Minute 
Processing Time 
1 0.17 -- No 
2 -- -- No 
3 -- -- No 
4 0.17 -- No 
______________________________________ 
TABLE VII 
______________________________________ 
HYDRIDING 
IRON 
SAMPLE LEVEL % UNIFORM RIMS 
______________________________________ 
Zero Minute 
Processing Time 
1 0.63 -- Yes 
2 0.59 -- Yes 
3 0.74 -- Yes 
4 0.70 -- Yes 
5 0.63 -- Yes 
Five Minute 
Processing Time 
1 0.18 -- No 
2 0.21 -- No 
Eight Minute 
Processing Time 
1 0.18 -- No 
2 0.17 -- No 
Ten Minute 
Processing Time 
1 0.18 -- No 
2 -- -- No 
3 -- -- No 
4 -- -- No 
5 -- -- No 
6 -- -- No 
7 -- -- No 
8 0.15 -- No 
Fifteen Minute 
Processing Time 
1 0.18 -- No 
2 -- -- No 
3 -- -- No 
4 0.22 -- No 
______________________________________ 
The amount of oxalic acid, whether alone or in the mixture may be in the 
range of about 0.25-0.75%. The amount of nitric acid in the mixture may 
vary from about 0.05%-0.15%, though the 0.5% oxalic/0.1% nitric mixture is 
preferred. 
Optionally, a wetting agent is added to the electrolyte to increase 
effectiveness. Various wetting agents are known in the art for use in 
metal surface treatment operations such as etching or pickling. For 
example, 0.75% PLURONIC L-43 wetting agent, made by BASF Corporation, 
which is a polyoxypropylene-polyoxyethylene condensate, was added to a 
mixed acid electrolyte and it was found that current decreased quicker and 
to a lower level evidencing enhanced reactivity through improved surface 
contact. Of course, other wetting agents could also be used. 
With time and use, the electrolyte may lose its effectiveness. Testing 
confirmed that even at 50% of initial strength, the electrolyte was still 
effective at removing nickel and nickel alloys. However, it is recommended 
that a more conservative approach be used and the electrolyte be changed 
when the concentration reaches 75% of the initial concentration. 
Utilizing the inventive method, a zirconium alloy article is produced which 
is resistive to hydride accumulation, yet allows normal uniform hydriding 
to occur. Such an article thus maintains its mechanical properties for 
extended periods in a nuclear reactor environment. 
While preferred embodiments of the present invention are shown and 
described, it will be understood by those skilled in the art that various 
changes and modifications could be made without varying from the scope of 
the invention.