A NOVEL PROCESS FOR THE FABRICATION OF SILVER / SILVER CHLORIDE ELECTRODE
FIELD OF INVENTION: This invention relates to the process of fabricating a silver / silver chloride electrode of suitable composition and strength for potential measurement of the immersed structures in seawater environments.
PRIOR ART
Underwater portion of ships and offshore installations are protected against the corrosive attack from the aggressive marine environments by cathodic protection. There are two methods of cathodic protection available namely sacrificial anode and impressed current cathodic protection (ICCP) systems. For effective functioning of the ICCP systems, it is mandatory that the potential of the structure is to be accurately determined continuously and the corresponding current is fed to the structure so that it is cathodically protected and does not suffer corrosive damage. Thus the integrity of the hull structure of the ship and that of offshore installations depends on the reliability of the measured underwater structure potential and hence in this context, a rugged and reliable reference electrode for use in such structures attains importance.
Zinc reference electrode has been in use for the measurement of potential of the immersed offshore structures since long time. Even though the zinc reference electrodes is rugged, they are highly sensitive to iron impurities. In case the iron impurities is more than 14 ppm, it leads to the formation of Zn(OH)2, shifting the reference potential to the noble side thereby leading to erroneous values. Here the equilibrium reproducible reference potential is the potential of the zinc electrode in equilibrium with its ions and the value is around - 1050 mV vs SCE in natural seawter .
In the case of silver-silver chloride reference electrode, the standard equilibrium potential is due to the reversible reaction occurring at the electrode surface with respect to chloride ions and the corresponding potential in seawater ( or 3.5 wt% NaCl ) is around 5 mV vs SCE.
For applications in electrochemical signal detections namely potential of underwater immersed structures such as ships hull and offshore structures, electrodes with high conductivity and low electrode polarization are important. Because of the ability of Ag/AgCl electrodes to maintain a constant and standard electrode potential, Ag/AgCl is widely used as a reference electrode. Also the silver - silver chloride reference electrode is reversible with chloride ions and the potential thus depends on the activity of chloride ions in the medium. Thus the silver-silver chloride is a preferred choice for use a reference electrode in marine environments. This is also due to the fact that in seawater media, there is abundance of chloride ions and the equilibrium potential of these electrodes are attained very quickly in such environments. Also any damage to the surface gets healed fast by the conversion of the underneath silver layer to silver chloride by chemical reaction with seawater.
United States Patent 5,234,570 ( August 10,1993 )Taylor, et al. describes a process of making silver /silver chloride reference electrode with ceramic housing for use in high temperature and high pressure environments. Also improved stability in the reference electrode potential was obtained by provision of an annular liquid junction between the electrolyte within the electrode, and the liquid in which the electrode is immersed in. The drawback of the process is the use of ceramic housing which cannot be used for open seawater environments and is not cost effective.
United States Patent 5,238 ,53 (August 24, 1993) Hettiarachchi, et al. describes the fabrication of Ag/AgCl reference electrode for use in measuring corrosion potential and pH of water in a drum boiler at supercritical temperatures. The electrode consists of an alumina or zirconia tube packed with a bulk solid electrolyte. The electrolyte consists of a mixture of silver chloride and glass. A solidified silver chloride interfaces with the bulk electrolyte and a conductive silver electrode is embedded which completes the circuit. The drawback of the process is the fragile nature of the tube with solid electrolyte.
United States Patent5,217,596 (June 8,1993) Indig, et al details an electrode probe suited for employment as an electrical potential reference electrode in an aqueous, high pressure, high temperature, and high radiation field environment such as the core of a nuclear reactor. The electrode with made a silver/silver chloride is a brazed and welded assembly consisting of only ceramic and metal parts including a sapphire base which is brazed to a kovar/stainless steel housing, welded in turn, to a coaxial cable assembly for signal transfer. The base incorporates an integrally formed pedestal through which a conductor wire extends and over which is positioned a selectively coated cylindrically shaped sealing retainer. The drawback of the process is the cumbersome fabrication technique.
United States Patent 5,334,305 (August 2, 1994) Okada, et al. describes an internal liquid for a reference electrode containing lithium chloride (LiCl) and ammonium nitrate (NH.4 NO3) in a dissolved state at a ratio relates to the invention of a reference electrode which will give a reference electrical potential for a system for potentiometrically measuring an ion concentration in a solution. More particularly, it relates to an electrode which may be used as a reference (control) electrode or as an internal reference electrode of an ion electrode (ion-selective electrode). The drawback of the process is the use of costly chemical and selective utility of the electrode.
United States Patent5,565,143 October 15, 1996 Chan details an invention that relates to polymeric compositions containing a water based polymeric binder, silver particles and silver chloride particles for use in making electrochemical and biomedical electrodes. These conductive compositions were made for use in printing silver/silver chloride coatings on plastic dielectric film substrates to make disposable electrodes, which finds use in electrochemical and biomedical applications, such as electrocardiograph and blood sensors. The drawback of the process is the high cost of processing and limited use.
United States Patent 5,964,992 October 12, 1999 Swette, et al. describes a graphite reference electrode for use in the cathodic protection of steel embedded in concrete. This electrode has been produced with a stable catalyzed structure that when equilibrated with air or oxygen and an electrolyte reaches a reproducible and reversible redox potential. The embedded catalyzed graphite reference electrodes can be used in impressed cathodic protection systems or monitoring the corrosion condition of embedded steel to provide an early warning of impending damage. The drawback of the process is that this electrode is not suitable for use in marine environments.
United States Patent 6,495,012 (December 17, 2002) Fletcher , et at details an invention of a sensor with a reference electrode based on Ag/AgCl electrode and a flowing electrolyte. This is particularly useful for measuring the ion concentration of a process solution. The invention includes a sensor having a pressurized reservoir, which provides flow of an electrolyte, a non-metallic solution ground and a resistance temperature device bonded to a non-metallic solution ground. The invention provides sensors with greater accuracy and stability by minimizing or eliminating ingress of contaminants from a process solution through the external junction of the sensor. The limitation of this technique is that it is ideal for laboratory use and not for field measurements.
United States Patent 6,554,982 (April 29, 2003) Shin, et al. invention relates to a solid-state reference electrode for a potentiometric electrode system and, more particularly, to a miniaturized solid-state reference electrode characterized by employing an ion-sensitive membrane containing an ion-selective material as protective membrane for its inner reference solution and having an enlarged area of junction contacting with the sample solution, whereby operational abnormality may be checked by itself and fast activation may be achieved. The drawback here is that the electrode is not suitable for use in contaminated seawater environments for the measurement of underwater potential.
Further all the above referred studies by various authors are for reference potential measurements in various environments such as boiler, high pressure and high temperature environments in nuclear applications etc., using silver -silver chloride reference electrode with suitable housing based on the environments. No reported patents relates to the development of silver/silver chloride reference electrode with desired electrochemical properties and suitable housing for use in ships and offshore structures in the cathodic protection systems for measurement of hull potential.
NEED FOR THE PRSENT INVENTION:
There exists a need for the development of highly reliable reference electrode based on Silver /Silver Chloride with desirable mechanical properties and electrochemical reversible potential for use in Ships and offshore structures.
OBJECTIVE OF THE PRESENT INVENTION
Silver -Silver chloride reference electrodes are used in electrochemical and biomedical applications due to the electrode's high conductivity and low electrode polarization which helps one to achieve low noise and high signal sensitivity.
The objective of the present invention is to fabricate a suitable Silver/Silver reference electrode pf desired strength and reversible electrochemical potential tor use in ships and offshore structures. The electrode so fabricated forms part of the impressed cathodic piotection systems (ICCP) and /or sacrificial anode (SA) systems. In ICCP systems, the sensed potential of the structure by the Ag/AgCl reference electrode is used by the auto control unit to impress the desired amount of current to the cathodic structure in order to bring it to the protected potential and in SA systems, the sensed potential indicate the level of protection offered by the sacrificial anodes to the structure.
STATEMENT OF INVENTION
The present invention relates to a process of preparing silver-silver chloride electrode comprising:
- mixing 60-90% powdered silver with 40-10% powdered silver chloride,
- feeding the mixture into alloy steel die,
- a connecting silver rod is embedded in the mixture and is compacted,
- the compacted electrode is sintered,
- sintered electrode is rinsed and cleaned in ultrasonic bath, and the electrode is chloridised.
DESCRIPTION OF THE INVENTION
In this invention, powders of silver and silver chloride were compacted and sintered to make a pellet of suitable dimension and strength for use as reference electrode in chloride bearing marine environments. A silver rod of appropriate dimension is embedded in the pellet which establishes the desired electrical connection.
High purity silver (min 99.94 % pure) and silver chloride (99.94% pure) of particle size . in the range of 5 to 75 microns, preferably around 35 microns, were used for making the electrode The impurities in the fine powders does not exceed the limits as shown below.
Impurities Percentage
(Elements) (max)
Copper 0.050
Lead 0.004
Iron 0.002
Nickel 0.001
These powders were taken in different weight ratios of Ag/AgCl viz., 60% to 90 % Ag and 40 % to 10 % AgCl and mixed thoroughly in a mortar. The powders are then compacted in a special die (made of alloy steel and hest treated to obtain 60 HRc hardness) for making pellets of dimension shown in Fig. 1.
A connecting rod made of high purity silver (min.99.94 %) with diameter in the range of 1.7-1.9 mm is used for establishing the electrical connection to the pellet (electrode) The dk. used for making the Ag/AgCl electrode has provision for embedding the connecting rod in the mixed powder bed of Ag and AgCl before compacting the same. Using a hydraulic press, the powder mixture of Ag and AgCl is then compacted at a pressure of 150 to 250 MPa, preferably at 200 MPa for a period of 45 - 120 seconds, preferably for a period of 60 seconds.
The compacted pellet with the embedded connecting silver rod is then sintered in the temperature range of 400 to 450 C for a period of 1 - 2 hrs and then furnace cooled to room temperature. The sintered pellet was found to have compressive strength in the range of 1400 to 1600 MPa.
The sintered electrode is washed with acetone initially and then cleaned in an ultrasonic bath with distilled water for a period of 30 to 60 minutes. The electrode is then placed in a solution of 0.03 - 0.06 N KC1 (potassium chloride) , preferably 0.05 N KC1. in 1 N HC1 (hydrochloric acid) ,
The fabricated reference electrode is subjected to a anodic current density of 0.1 to 2 mA/cm2 for a period of 60-90 minutes. After this chloridisation process, the electrode is washed with distilled water and placed in saturated KC1 for 48 hours
Chloridised electrode is then fixed in a specially fabricated Nylon housing , for holding the fabricated electrode, which can be directly used as reference electrode for ships and offshore structures.(Fig 2), using epoxy resin LY 556 and hardener HY 951 (100:12 by weight) and cured for 24 hours. A lug connected to the silver rod embedded in the electrode is used for further connections of the electrode with other cables of the ICCP system in the ship completes the circuit.
The reproducible potential of electrode is tested against Saturated Calomel Electrode (SCE) in saturated potassium chloride solution. The potential difference shall be in the range of 43 ±3 mV vs. SCE.
Example 1. (70Ag/30AgCl)
Silver powder (avg.particle size 30 microns) of weight 7.7 gms and 3.3 gms of silver chloride powder is mixed thoroughly in mortar. The mixture is fed into a suitable alloy steel die, made with a provision to insert the connecting silver rod. The silver rod is pushed into the mixture of Ag and AgCl powder. The powder along with the inserted rod is then compacted at a pressure of 175 Mpa. The compacted pellet (electrode) is then removed-and sintered at 410 C for a period of 60 minutes. The sintered pellet is rinsed with acetone and cleaned in an ultrasonic bath with distilled water for a period of 45 minutes.
The electrode is then placed in a solution of 0.05 N KC1 in IN HC1 solution and
2
chloridised at an anodic current density of 1 mA/cm for a period of 75 minutes.
The potential of the chloridised Ag/AgCl reference electrode is 42 mV vs. SCE in saturated KC1 solution..
Example 2. (80Ag /20AgCl)
Silver powder (avg.particle size 30 microns ) of weight 8.8 gms and 2.2 gms of silver chloride powder is mixed thoroughly in mortar. The mixture is fed into the die specially made for making pellets with connecting silver rod. The silver rod is pushed into the mixture of Ag and AgCl powder. The powder along with the silver rod is then compacted at a pressure of 200 MPa. The compacted pellet (electrode) is then removed and sintered at 420 C for a period of 75 minutes. The sintered pellet is rinsed with acetone and cleaned in an ultrasonic bath with distilled water for a period of 50 minutes.
The electrode is then placed in a solution of 0.05 N KC1 in IN HC1 solution and chloridised at an anodic current density of 1 mA/cm for a period of 60 minutes.
The chloridised reference electrode reversible electrode potential is 43 mV vs. SCE in saturated .KG solution.

Claim:
1. A process of preparing silver-silver chloride electrode comprising:
mixing 60-90% powdered silver with 40-10% powdered silver
chloride,
feeding the mixture into alloy steel die,
a connecting silver rod is embedded in the mixture and is
compacted,
the compacted electrode is sintered,
sintered electrode is rinsed and cleaned in ultrasonic bath, and
then electrode is chloridised.
2. A process as claimed in claim 1, wherein powdered mixture is compacted at a pressure of 150-250 Mpa with hydraulic press.
3. A process as claimed in claims 1 and 2, wherein compaction is carried for 45-120 seconds.
4. A process as claimed in claim 1, wherein compacted electrode is sintered at 400-450°C.
5. A process as claimed in claims 1 and 4, wherein compacted electrode is sintered for a period of 60-120 minutes.
6. A process as claimed in claim 1, wherein sintered electrode is rinsed with acetone.
7. A process as claimed in claim 6, wherein rinsed electrode is cleaned in an ultrasonic bath with distilled water.
8. A process as claimed in claim 7, wherein rinsed electrode is cleaned for 30-60 minutes with distilled water in ultrasonic bath.
9. A process as claimed in Claims 1 and 8, wherein cleaned electrode
is chloridised in 0.03-0.06 N KC1 in IN HC1 with an anodic
current.
10. A process as claimed in claim 9, wherein anodic current's density is 0.1-2mA/Cm2
11. A process as claimed in claims 1 and 10, wherein chloridisation with anodic current is carried out for 60-90 minutes.
12. A process for preparing silver-silver chloride electrode substantially as herein described and illustrated with reference to foregoing examples and accompanying drawings.