Method of forming a platinum layer on tantalum

A method of platinizing and heat treating tantalum metal is described. The platinizing of tantalum is accomplished by placing a piece of conductive corrosion resistant metal and a tantalum workpiece as electrodes in a platinizing solution. The electrodes are connected to a source of DC power supply and when the current is passed, platinum black is deposited on the tantalum workpiece. The platinized tantalum workpiece is then placed in a furnace maintained at a temperature of about 1050.degree. to 1150.degree. C. for a time sufficient to diffuse the coated platinum into the surface of the tantalum core. The method of using the electrode for the detection of faults in a glass lined apparatus is also disclosed.

BACKGROUND OF THE INVENTION 
This invention relates to a method of platinizing and heat treating 
tantalum and more particularly tantalum plugs to be used as electrodes in 
the detection of faults in reaction vessel protective linings. 
It is a common practice to use protective layers of glass enamel as an 
anti-corrosive lining on the inside of reaction vessels which contain 
corrosive liquids. In such vessels, a fault in the protective lining can 
result in serious corrosion of the vessel, and if such a fault remains 
undetected for some time, the vessel may become unusable and irreparable. 
In some cases, the contents of the vessel may even become contaminated. 
Thus, it can be seen that vessels of this type must be continuously 
monitored against damage to the protective lining. 
U.S. Pat. No. 3,858,114, which is incorporated herein by reference, 
discloses a method for detecting faults in reaction vessel linings. The 
method described in this patent involves disposing a platinum electrode in 
a conductive medium in the vessel and measuring any current flowing from 
the electrode to the iron or steel wall of the reaction vessel. At 
locations where damage has occurred to the lining, the current flow 
increases accordingly. The intensity of the current delivered by the so 
formed Pt-Fe cell indicates the degree of damage to the protective layer. 
The platinum metal used as an electrode in U.S. Pat. No. 3,858,114 is very 
expensive. It would therefore be worthwhile, if possible, to plate a 
tantalum core with platinum and use it as an electrode. Such an electrode 
would have a platinum surface and tantalum core and would be less 
expensive than conventionally platinum plated electrodes. Tantalum has 
been chosen as a core material because it is corrosion resistant and 
should the platinum layer of the electrode break down, the tantalum will 
not adversely affect the contents of the vessel. It has been found in 
practice, however, that conventionally plated platinum layers on tantalum 
wear out rather quickly, particularly when the contents of the vessel 
contain an abrasive material such as slurry. 
OBJECTS OF THE INVENTION 
Accordingly, it is an object of this invention to provide an abrasion 
resistant platinum layer on the outer surface of a tantalum workpiece. 
It is another object of this invention to provide a method of platinizing 
and heat treating a tantalum workpiece. 
It is yet another object of this invention to provide an improved electrode 
for a reaction vessel fault finder of high precision. 
It is a further object of this invention to provide a method of obtaining a 
thin stable layer of platinum on a tantalum workpiece. 
It is yet another object of the invention to provide an inexpensive 
platinum electrode and method for detecting faults in a glass lined 
apparatus. 
SUMMARY OF THE INVENTION 
These objects and others are accomplished in accordance with the present 
invention by providing a novel method of platinizing and heat treating a 
tantalum workpiece. The surface of the workpiece is first cleaned and then 
connected to the negative lead of a source of DC supply. The positive lead 
of the DC supply is then connected to a piece of corrosion resistant 
conductive metal, such as platinum, and both metals immersed in a 
platinizing solution. A current is then passed through the solution and 
platinum black is deposited on the tantalum workpiece. The tantalum 
workpiece is then removed from the platinizing solution and heated to a 
temperature of about 1050-1150.degree. C. in a vacuum furnace, or furnace 
containing an inert atmosphere, for a time sufficient to diffuse the 
platinum into the surface of the tantalum. This results in the formation 
of a tantalum workpiece which contains a stable outer layer of platinum 
diffused in the surface of the tantalum. This workpiece has utility as a 
preferred use as an electrode which is used to detect faults in a glass 
lined apparatus. This process may also be utilized to manufacture platinum 
coated parts such as electrodes, electrical probes, and repair plugs for 
glass lined vessels. 
It should be understood that tantalum and tantalum base alloys which may be 
platinized and heat treated are included within the scope of this 
invention. For example, tantalum alloyed with tungsten comprises a 
suitable alloy which may be used in place of tantalum in the process of 
the present invention.

DETAILED DESCRIPTION OF THE INVENTION 
One preferred embodiment of utilizing the platinum coated electrode of the 
present invention is described with reference to the following drawing. 
Referring to the sole FIGURE of the drawings, the apparatus 1 shown 
schematically in cross section has a steel wall 2, the inside surface of 
which is coated with an glass layer 3. A platinum coated electrode 5 which 
is made by the method of the present invention, and which is electrically 
insulated from the apparatus, is disposed inside the apparatus. The 
platinum electrode is connected to the wall 2 of the apparatus via a 100 
ohm measuring resistance 6 which lies outside the apparatus. A 
millivoltmeter and/or indicator device 7 is connected in parallel with the 
resistance. 
When a fault occurs in the glass layer and when there is an electrically 
conductive medium 8 in the apparatus, a Pt-Fe cell is formed between the 
platinum electrode 5 and the exposed steel surface 9. The current 
delivered by this cell then flows through the measuring resistance 6. This 
current, which is measured using the millivoltmeter 7 by measurement of 
the voltage drop across the measuring resistance 6, is an indication of 
the rate at which iron is going into solution and thus the extent of the 
damage to the glass layer. 
Within this disclosure, the terms platinum plated and platinized are 
specifically distinguished from each other. Platinum plating is carried 
out by conventional methods. A well cleaned tantalum workpiece is fastened 
in a chuck which is electrically charged. A sponge dipped in a platinum 
solution is held against the rotating tantalum core. The process is 
continued until the desired thickness of platinum is formed on the surface 
of the tantalum. 
In the platinizing and heat treatment process according to the present 
invention, a platinum black powder coating is deposited on the tantalum 
workpiece by electrolysis. The coated workpiece is then placed in a 
furnace which is heated to about 1050.degree. to 1150.degree. C. This heat 
treatment results in the diffusion of the platinum into the surface of the 
tantalum. This diffusion of the platinum into the surface of the tantalum 
results in a tantalum part containing a platinum surface which is 
electrically and mechanically superior to plated platinum parts. The 
process of the present invention results in a stable platinum layer on the 
tantalum surface and makes the electrode abrasion resistant. Further, the 
platinum layer made according to the present invention is thinner than 
conventionally plated platinum layers. 
In a preferred embodiment of the present invention a tantalum workpiece, 
suitable for ultimate use as an electrode in the detection of faults in a 
reaction vessel, is sandblasted to render the surface clean and free of 
dirt and grease. The tantalum core is in the form of a one piece 5/8 inch 
flat head screw and comprises 90% by weight tantalum and 10% by weight 
tungsten. The core is then brushed off and cleaned with a 20% HCl solution 
and washed. The core may be stored in distilled water if platinizing is to 
be done at a later stage. 
The following materials and apparatus were used in the platinizing process: 
1. Beaker (200 ml) 
2. Platinizing Solution (150 ml) 
3. Six volt DC power source 
4. Small strip of platinum metal (11/2.times.11/4.times.1/32 inches) 
The platinizing solution is prepared in the beaker. The solution contains 3 
gms of chloroplatinic acid, 0.02 gms of lead acetate per 100 ml of 
distilled water. The tantalum core described above is connected to the 
negative lead of the 6 V DC power supply and the platinum strip is 
connected to the positive lead of the 6 V DC power supply. Both the 
platinum strip and the tantalum core are suspended in the solution in the 
beaker. Only the area of the tantalum that is to be platinized is immersed 
in the solution. The voltage is then applied for six minutes during which 
time the platinum black is deposited on the tantalum core. The tantalum 
electrode is then carefully removed from the solution and rinsed with 
distilled water. The platinum black deposited is dull black and 
non-adherent at this stage. The electrode is then placed in a rack and 
placed in a furnace maintained at a vacuum of about 10.sup.-6 mm of 
mercury and a temperature of about 1150.degree. C. for 1/2 hour. This heat 
treatment results in the platinum being diffused into the surface of the 
tantalum which exhibits a dull gray appearance. Optionally, the tantalum 
core may be platinized and heat treated for a second time in order to form 
a more stable and adherent platinum layer. 
It has also been found that the quality of the platinzing and heat treated 
electrode is better when finer sand is used for blasting, rather than 
coarser. The surface roughness of the tantalum core after sandblasting was 
measured by ENGIS roughness meter type 6102E (Engis Equipment Co., Morton 
Grove, Illinois) and found to be 30-38 micro inches. The surface roughness 
of the completed platinized and heat treated electrode was measured as 
24-28 micro inches. 
The addition of lead acetate or other appropriate lead salt to the 
platinizing solution promotes the adherence of platinum to the tantalum 
electrode, and is essential for adherence of platinum black to the 
tantalum surface. Without the presence of a lead salt in the platinizing 
solution, the deposited platinum black flakes off the surface of the 
tantalum. Various voltages from about 3-24 volts and varied time periods 
from about 1/2 to 6 minutes were found to be satisfactory for the 
platinizing processes. Six volts for six minutes was found to be optimum 
from the viewpoint of cost and efficiency, but is not critical for the 
process. Higher voltages cause excessive platinum deposit on the electrode 
and lower voltages take longer times to form a platinum deposit on the 
electrode. 
For good quality electrodes it is essential during the heat treatment step 
to maintain the furnace temperature in the range of about 1050.degree. to 
1150.degree. C. for times ranging from about 30 minutes to 1 hour. It was 
found that below 1050.degree. C. the platinum black does not completely 
adhere to the tantalum electrode as characterized by a drop in their 
electrical sensitivity and at temperatures higher than about 1150.degree. 
C. the electrical sensitivity also drops off and the platinum black 
sometimes burns off. The furnace is maintained at a vacuum of 10.sup.-6 mm 
of mercury to prevent oxidation of platinum or tantalum, but vacuums 
outside this range are also satisfactory. If desired an inert atmosphere 
could also be used during the heat treating step. 
Superior quality of the platinized and heat treated electrode over platinum 
plated electrodes is further confirmed by the following test, based on the 
galvanic cell principle. An electrode formed by the above described 
process and constituting the positive lead is attached to one end of a 0-1 
mA ammeter and a piece of tantalum constituting the negative lead is 
attached to the other end of said ammeter. The electrode and tantalum are 
placed in a beaker containing 1% HCl water solution and the reading on the 
ammeter is noted. This reading is known as a no fault reading. The 
electrode and tantalum are allowed to remain in place for one week. After 
the one week period a piece of iron is connected to the tantalum and 
introduced into the HCl solution. This simulates the fault situation when 
the glass lining of the vessel is damaged and the contents are exposed to 
the metal. A reading is taken which indicates higher current flow. In the 
simulated fault situation, the tests have consistently shown higher 
current readings for platinized and heat treated tantalum electrodes than 
for platinum plated electrodes. The higher readings signify higher 
sensitivity of the platinized and heat treated electrodes to the plated 
electrodes. One such example showed the readings as follows: 
______________________________________ 
Fault Current-mA 
No Fault After one After one 
Electrode Current mA week initial 
week 10 min. 
______________________________________ 
Conventional 
Platinum Plated 
Ta 0.00 1.89 1.29 
Platinized and Heat 
Treated Ta 0.00 5.46 3.21 
______________________________________ 
Thus, it should be appreciated that the present invention of platinizing 
and heat treating tantalum produces electrodes which measure a fault in a 
glass lined vessel on a continued basis far more precisely than 
conventional platinum plated electrodes, even though the plated platinum 
coating is thicker than the platinized and heat treated platinum layer. 
Further, the superior performance of the platinized and heat treated 
platinum layer is achieved at a much lower cost than that of the platinum 
plating. 
Although particular embodiments of the present invention have been 
disclosed herein for purposes of explanation, further modifications or 
variations thereof will be apparent to those skilled in the art to which 
this invention pertains.