GB2218520A

GB2218520A - Multichannel corrosion rate measurement apparatus

Info

Publication number: GB2218520A
Application number: GB8811320A
Authority: GB
Inventors: Raymond Paul Gill; R John Stuart Gill
Original assignee: APPLIED CORROSION MONITORING L
Current assignee: APPLIED CORROSION MONITORING L
Priority date: 1988-05-13
Filing date: 1988-05-13
Publication date: 1989-11-15
Also published as: GB2218521A; GB8811320D0; GB8812999D0

Abstract

Corrosion rates of a number of metals 4 in contact with a common electrolyte 3 are measured in an apparatus which comprises a potentiostat 1 connected to a reference and an auxiliary electrode located in the electrolyte and a zero resistance ammeter 2 for each metal 4. The zero resistance ammeters and potentiostat have a common earth. Basic corrosion potentials can be measured with the potentiostat switched off. <IMAGE>

Description

XULTICHANEL DYEAXIC ZERO RESISTANCE AMMETER.

This invention relates to a multichannel dynamic zero resistance ammeter for electrochemical and associated measurements.

Zero resistance ammeters (ZRA's) are commonly used in the measurement, by scientists and engineers, of electrochenical phenomena. They allow the current flowing between two metal electrOdes immersed in an electrically conducting solution (called an electrolyte in this application) to be measured whilst coupled by the ZRA without introducing an electrical resistance.

In conjunction with a potentiostat a single ZRA is sometimes used to record the current flowing between a counter electrode and the electrode under investigation (ie. the working electrode (W3))when the WE is polarised to a set potential. In this way electrochemical parameters of the WE are determined.

While fine for studies of single metals the single channel Z9A-potentiostat combination doe not allow real multi-metal systems to be studied. As an example consider a domestic central beating system with copper pipes, mild steel radiators and an aluminium boiler. ]t is required to find the rate of corrosion (and hence expected life) of each of the metals whilst in a common electrolyte and electrically coupled to each other. The single ZRApotentiostat can only measure the overall corrosion rate of all these metals - not the individual response of each.

According to the present invention there is provided a means of obtaining individual electrochemical responses of any number of metals coupled together in a common electrolyte. Comprising a single reference electrode input, a single counter electrode output and a number of ZRA's (one per metal under test). The whole is incorporated into one box. Automatic operation is used via an IEEE-488 interface from a computer. Common grounds are provided for each ZRA ensuring all 'VE's are electrically connected via the virtual earth of each ZRA.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which Fig 1 shows a block arrangement for performing potentiodynamic/potentiostatic experiments on 4 working electrodes.

Fig 2 shows block details for providing sweep voltages to the system.

Fig 3 shows block arrangements for sensing current measuring resistor setting and mi-asuris.g FA current.

Fig 4 shows details of potentiostat circuit.

Fig 5 shows details of one of the ZRA circuits.

Fig shows front panel arrangement.

Referring to fig 1 the 4 channel Dynamic ZRA comprises a single potentiostat (1) and 4 ZRA's (2). Both potentiostat reference voltage and each ZRA are referenced to the same ground, it is this common ground that forms tbe key to the system. Any metal (4) immersed in a common electrolyte (3) and connected to one of the ZRA's is held at the same potential as any other metal also in the common electrolyte and connected to one of the ZRA's. Thus they behave as if physically in direct electrical contact. If they are then polarised by the potentiostat they all will move to the same potential (of the metal with respect to ground).

However the current required to hold each metal at the set potential will not be the same for each. The individual ZRA associated with each metal will allow the recording of the individual currents. This may be done with any number of working electrodes.

Sweep voltages are set by two 16 bit Digital to Analog convertors, fig 2, one covering a range of +5.5 Volts(l) and the other a range of i150 mVolts(2). This allows the setting of the couple potential on the 5.5 V range and the generation of wide sweeps (again using the i5.5 V DA convertor) or fine sweeps using the +150 mV DA convertor. Sweeps are generated by the host computer and transmitted to the invention over a standard IEEE-488 interface. To isolate the counter electrode from the cell until a dynamic sweep is required wn isolate-run ch5ns2.e cver relay is ro d t'iz Fig 3 refers to the measurement arrangement of the system.Three parameters need to be measured: (a) Couple rest potential (5), (b) Switch setting (6), and (c) ZRA Current (7), all referenced to ground. Three 16 bit Analog to digital convertors are used, one for each function above.

They cover a range of +10 V each and are provided with scaling multipliers, for measuring small voltages, of *10 and 100. Each ZRA has 5 ranges set manually by a front panel switch (1). The ranges cover: 1. (300 mA, 2. < 30 mA, 3. < 3 mA, 4. < 300 A, 5. < 30 A.

Switch position sensing is performed using a double pole switch, the first pole of which sets the range, the second acts as wiper on on a i8 V ladder of resistors. Thus a known voltage is measured for each switch settling.

The r-est potential is measured from a very high impedance reference electrode buffer ( > 1000 G# input impedance) (4).

The current readings (voltage across the range resistor) for each ZRA is taken through a 5th order low.pass filter (2) to reduce noise ( < 25 Hz, -3 dB pass).

Both switch setting and current reading are fed into an analog multiplexer (3) (in this case 4 channels each) and the resulting pair of outputs digitally controlled.

Fig 4 shows the details of the potentiostat circuit. A high impedance reference electrode buffer (1) feeds a summing amplifier (@) which 5 also fed with a swee@ volta@e. The summing amplifier drives a @4 Volt @ Amp unity gain buffer (3) which feeds into the iso-run relay (4) which leads to the counter electrode output on the front panel.

A single ZRA circuit is shown in fig 5. A chopper stabilised op-amp (1) is used to balance the current flowing in the WE at it's virtual earth (2).

This is achieved by driving a 325 mA unity gain buffer (3) which feeds into the count resistor (4) and back to (2). A sense line (5) is included for each ZRA to compensate for any resistive voltage drops in the connecting leads. The voltage at the output of the buffer is fed into the 5th order filter (6) and then to the multiplexer (fig 3,3).

Essential connections are (fig 6); 1. Counter electrode, 2. Reference electrode, and 3. (for each ZRA) sense lead, driver lead.

The whole instrument is controlled via suitable computer enabling automatic determination of corrosion rate for individual elements in a mixed system.

Claims

CLAIM

1 An electronic Dynamic Zero Resistance Ammeter instrument comprising a device capable of polarising any number of metals simultaneously in-a common electrolyte for electrochemical and corrosion studies by utilising one potentiostat with common reference electrode and common counter electrode in conjunction with an individual ZRA for each metal under test.

2 A Dynamic Zero Resistance Ammeter as claimed in Claim 1, wherein any number of metals may be coupled electrically together by connecting each metal to a ZRA built into the instrument.

3 A Dynamic Zero Resistance Ammeter as claimed in Claim 1 or Claim 2, wherein metals coupled together via ZRA's within the instrument can be swept simultaneously over a range of potentials to monitor each metals current response at that potential.

4 A Dynamic Zero Resistance Ammeter as claimed in Claim 2, wherein a perturbing voltage may be applied to all Working electrodes by means of a single potentiostat circuit.

5 A Dynamic Zero Resistance Ammeter as claimed in claim 4 where a potentiostat circuit is included with a high impedance ref@@@@@@e electrode input and a high current counter electrode output.

6 A Dynamic Zero Resistance Ammeter as claimed in Claim 5, wherein a means of isolating the counter electrode via a relay is provided.

7 A Dynamic Zero Resistance Ammeter as claimed in Claim 5 or Claim 6, wherein an external input is provided to the potentiostat stage for use with impedance analysers.

8 A Dynamic Zero Resistance Ammeter as claimed in Claim 1 or Claim 2, wherein each ZRA built into the instrument has a range of count resistors.

9 A Dynamic Zero Resistance Ammeter as claimed in Claim 8, wherein each ZRA range setting may be sensed electronically via analog to digital convertors.

10 A Dynamic Zero Resistance Ammeter as claimed in Claim 1 or Claim 2, wherein the voltage across each ZRA count resistor may be measured electronically via analog to digital convertors.

11 A Dynamic Zero Resistance Ammeter as claimed in Claim 10 wherein voltage across the count resistor of each ZRA is provided as an external output for use with an impedance analyser.

12 A Dynamic Zero Resistance Ammeter as claimed in Claim 1 or Claim 2, wherein each ZRA and the potentiostat are all referenced to the same ground.

13 A Dynamic Zero Resistance Ammeter as claimed in any preceding claim, wherein sockets are provided for each ZRA for drive and sense leads.

14 A Dynamic Zero Resistance Ammeter as claimed in any preceding claim, wherein sockets are provided for reference electrode input and counter electrode output.

15 A Dynamic Zero Resistance Ammeter as claimed in any preceding claim, wherein potential sweeps are generated by way of internal digital to analog convertors.

16 A Dynamic Zero Resistance Ammeter as claimed in sny preceding claim, wherein the whole instrument may be used from com=nds sent over a standard computer interface.

17 A Dynamic Zero Resistance Ammeter as claimed in any preceding claim, wherein the instrument may be powered via msins electricity or from batteries or both.

18 A Dynamic Zero Resistance Ammeter as claimed in any preceding claim, wherein the potentiostat stage may be automatically or manually disabled and the instrument used in a Static multichannel ZRA mode where the galvanic current between each of the metals under test may be monitored at their free couple potential.

19 A Dynamic Zero Resistance Ammeter as claimed in any preceding claim, wherein one ZRA only may be selected to allow the use of the instrument in a conventional potentiostat mode.

20 A Dynamic Zero Resistance Ammeter as claimed in any preceding claim, wherein more than one potentiostat stage may be added to the instrument for use in more than one common electrolyte.

21 A Dynamic Zero Resistance Ammeter as claimed in any preceding claim, wherein the system ground may be switched from mains ground to floating ground via a switching mechanism.

22 A Dynamic Zero Resistance Ammeter substantially as described herein with reference to figures 1-6 of the accompanying drawing.