Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to the use of treated
niobium or tantalum as a connector and such a connector
and has particular reference to the treatment of
niobium or tantalum to improve its resistance to a
previously unknown crevice corrosion attack.
It has already been proposed to use niobium
as a connector, particularly for use in a saline
solution such as seawater. Such a connector is
described in detail in British Patent No. 2 001 807,
published on February 7, 1979 in the name of
Marston Excelsior Limited. Niobium is chosen as a
particularly suitable material for such a connector
for a number of reasons. The purpose of the connector
is to permit a dismantelable connection to be made in
an electrical line feediny positive current to an
underseas anode in an impressed current cathodic
protection system.
The electrical connector proposed in Patent
Specification No. 2 001 807 has a number of distinct
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advantages over previous eLectrical connectors in that
it is easily dismantlable and assemblable and by
comparison to previous connectors is comparatively
inexpensive. Niobium is chosen as a particularly
suitable material for such an electrical connector as
it is normally recognised that niobium can be
connected and operated as an anode in a saline
solution, such as seawater, without corroding at
anodic voltages less than approximately lnO volts.
The anodic properties of niobium are well-known and it
is well-known that niobium forms a resistant and
insulating niobium oxide film at applLed voltages of
up to 100 volts. Above that volta~e, known as the
breakdown voltage, ~he film fails and the material
corrodes rapidly. Before the present invention,
however, it was considered that bulk niobium was
self-passivating at voltages below the breakdown
voltage. Unexpectedly, however, it was found that
when connectors formed of niobium were tested they
could, in certain conditions, corrode rapidly. Why
this should happen was not to be found in any
literature known to the applicants. An answer to the
problem has now been found and basically the solution,
which forms one feature of the present invention, is
to pre-treat the niobium by pickling to remove
contamination and surface oxlde films and subsequently
to form an oxide film on the surface of the niobium.
As a result of investigations carried out by
the applicants it has been found that such a
pre-treatment is known per se but has only been
proposed as a method of forming a smooth surface,
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for example in the treatment of superconductor cavity
resonators.
Thus, in British Paten-t Specification No
1 335 165 there is described the treatmen~ of the
internal niobium surface of a superconducting cavity
resonator by pickling in a mixture of nitric acid and
hydrofluoric acid and subsequently anodising the
resonator surface in an aqueous ammonia solution.
There is no indication, however, from this prior
patent specification that the material so treated is
particularly suitable for use as a connector in a
saline solution or that such a material would be
resistant to corrosion. It is apparent, therefore,
that the inventors have discovered an unexpected
property of niobium when treated in a manner known
per se. The fact that the niobium is, when in use,
connected as an anode and yet can suffer from
corrosion, increases the novelty of the invention
insofar as a pre-treatment comprising pickling and
anodisation substantially prevents corrosion at a
later date.
It has also been found, as 21 result of work
carried out by the inventors, that pickliing alone is
not sufficient, nor is anodisation alone sufficient.
Thus, it is necessary to have the combination of steps
before the beneficial effects of the invention are to
be found.
It is, of course, well-known to anodise
niobium, as is described, for example, in British
Patent Specification No 1 228 939 or US ]?atent
Specification No 3 496 076, and it iis also known to
pickle and subsequently anodise niobium or use as a
superconducting cavity resonator as is described in
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British Patent Specification No 1 335 165 referred to
above.
It appears that the particular problem
associated with the use of niobium as a connector
occurs when the niobium is located in a saline
solution and is connected anodically. Thus, when
niobium is inserted in a cold saline solution but is
not connected as an anode it does not corrode.
However, when connected as an anode, in certain
circumstances it has been found that corrosion can
occur. To the best of the inventors' ~nowledge this
fact was never known prior to the making of the
present invention.
In U5 Patent Specification No 3 730 856 there
is described a method of anodising titanium or niobium
to remove surface ion contamination so as to improve
! the corrosion resistance of chemical plant where the
metal surfaces are in contact with hydrogen. However,
there is no reference to the particular advantages to
be obtained from using pickled and anodised niobium as
a connector in a saline environment.
It has also been proposed see, for example,
British Patent Specification No 1 430 185, to reduce
the susceptibility of titanium to crevice corrosion
by abrading and pickling in hydrofluoric and nitric
acid mixtures to remove ion surface contamination.
However, this specification does not point out the
advantages of the present treatment of niobium.
In US Patent Specification No 3 876 136
niobium (ie columbium) is used in place of titanium
where titanium is said to be susceptible to crevice
- attack. The specification does not draw the
conclusion that pickling and anodisation of the
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niobium is necessary~ In other words, from this
specification it will be concluded that niobium is
itself resistant to crevice corrosion without the need
for any treatment of it.
In US Paten~ Specification No 3 A69 975 it is
stated that the problem of containing halide solutions
is complicated by the complete unpredictability of
susceptibility ~of materials) to crevice corrosion.
This reference is to be found in column 2, lines 1 to
3, of the specification. In lines 27 to 32 there is
further reference to the complete unpredictability of
materials to crevice corrosion.
Although crevice corrosion of materials is
well-known for materials such as steel - see, for
example, a paper published by W D France Jr in a
symposium presented at the Seventy-Fourth Annual
Meeting of the American Society for Testing and
Materials, 27 3une to 2 July 1971, published as ASTM
Special Technical Publication 516, pages 164 to 196 -
there was no indication that niobium was a materialwhich could suffer such a corrosion. Furthermore, in
a publica~ion by the Centre Belge d'Etude de la
Corrosion published in Brussels, June 1957, reference
NZ.55 JVM.144, entitled "Electrochemical Resistance of
Niobium" by J Van Muylder, N de Zoubor and M Pourbaix
it is stated that the resistance of niobium to
corrosion is that of a refractory metal, unattacked in
air, oxygen and water, that it is not attacked by
normal acids, such as hydrochloric, sulphuric and
nitric acids or their mixtures, that aqua regia has no
action and that the same is true of caustic alkali
solutions. Because it is so inert to reactive
chemicals it is necessary to use fused caustic alkalis
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or alkaline carbides or their complex solutions to attack it.
The paper goes on to say that hydrofluoric acid attacks
niobium only slowly but the rate of attack can be increased if
the niobium is contacted with platinum. Alternatively, the rate
of attack can be increased by adding nitric acid to the hydro-
fluoric acid. It is believed that the niobium forms complex
fluorides or oxy-fluorides.
The paper concludes by saying that the known resistance
of niobium to the action of chemical agents gives rise to the
conclusion that the niobium oxide which forms on the metal
constitutes an effective protective oxide. The paper ends by
stating that in the absence of complexing substances the metal
niobium is virtually non-corrodihle.
Thus, contrary to the prior art discussed above, it has
been found that the treatment of niobium known per se can give
unexpected increases in the resistance of niobium to crevice
corrosion, particularly when anodically connected as an electrical
connector in ~ saline solution.
According to the present invention, there is provided a
method of preparing an electrical connector for use in an electrical
line of a cathodic protection system said electrical line feeding
positive current to an anode, the connector being formed of a metal
selected from the group consisting of niobium and tantalum, the
method comprising treating the connector before use by a two stage
process:
(a) pickling to remove contamination and surface oxide
films and
(b) forming an oxide film on the surface of the connector
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by oxidising the surface of the connector.
In another aspect, the invention provides an elongate
electrical connector incorporating at least a surface portion of
niobium or tantalum having been pre-treated by pickling to remove
contaminated surface oxide film and subsequently having had its
surface oxidised to form an oxide film.
The invention also provides an electrical connector adapted
to be immersed in sea water and anodically polarised, the connector
being constructed of a metal selected from the group consisting of
niobium and tantalum, the surface of the metal having been pickled
to remove contamination and surface oxide films and having a fresh
oxide surface film formed thereon by oxidation of the surface of
the connector.
Thus, use is made of niobium or tantalum, which has been
treated in a manner known per se by pickling to remove contamination
and surface oxide films, and subsequently forming an oxide film on
its surface, as a connector in a saline solution. The present
invention also provides for the use of niobium in which the oxide
is formed by anodisation in a manner known per se. The connector
may be used when anodically polarised in the saline solution. The
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saline solution may be an aqueous saline solution and
the niobium may be polarised at a voltage in the range
30-100 volts. The present invention t--urther provides
a niobium or tantaium member exposed, in use, to a
saline solution and having been treated by pickling to
remove contaminated surface oxide films and
subsequently having formed on its suriace an oxide
film.
The present invention further provides a
cathodic protection system incorporating such a
connector.
By way of example the treatmen~ of niobium and
the advantages to be obtained thereby ~11 be
described below.
A piece of extruded niobium was partially
covered with a piece of plastics tape and was inserted
in a hot saline solution and connected as an anode at
50 volts. It was found that after a short period of
time white niobium oxide corrosion products were to be
found adjacent the piece of plastics tape. Two
similar pieces of niobium were then treated in the
following manner. A first piece was treated by a
method not in accordance with the present invention.
It was pickled in a solution of hydrofluoric acid and
nitric acid and then washed in water. The material
was then again wrapped in tape and connected as an
anode at 44 volts in a saline solution containing
250g/1 Na~l at a pH of 2. Again, corrosion products
were found on the surface.
The second piece of niobium was then pickled
in a hydrofluoric acid and nitric acid mixture to
remove 40 microns of niobium, washed in water and
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su~seqently anodised in an ammonium sulpha-te solution
at 40 volts to form an anodic film on the surface of
the niobiu~.
This sample was then partially wrapped with a
plastics tape and treated in the manner of the first
sample mentioned above. This piece of niobium, which
had been treated in accordance with the present
inventionr was found to suffer no attack whats~ever.
By way of comparison third and fourth samples
o extruded niobium were merely anodised in aqueous
NaCl containing 20g/1 and 220g/1 NaCl at 40 volts and
covered with plastics tape. These samples were then
tested in accordance with the first-mentioned sample.
This sample, again not in accordance wiih the present
invention, was also ound to corrode.
It can thus be seen that the combination of
pickling and forming the oxide layer - which could be
formed by air oxidation of the,niobium - leads to
unexpected improvements in the corrosion resistance of
niobium when used as a connec-tor, particularly as an
electrical connector in a saline solution, such as
seawater. It will be realised that hot water was used
to accelerate the effect of corrosion. Tantalum
behaves in a similar manner to niobium, havin~ a
similar normal breakdown potential.
The connectors were particularly useful for
cathodic protection installations.
