Corrosion reference electrodes

This handbook deals only with systems involving metallic materials and electrolytes. Both partners to the reaction are conductors. In corrosion reactions a partial electrochemical step occurs that is influenced by electrical variables. These include the electric current I flowing through the metal/electrolyte phase boundary, and the potential difference A( = 0, - arising at the interface. and represent the electric potentials of the partners to the reaction immediately at the interface. The potential difference A0 is not directly measurable. Therefore, instead the voltage U of the cell Me /metal/electrolyte/reference electrode/Me is measured as the conventional electrode potential of the metal. The connection to the voltmeter is made of the same conductor metal Me. The potential difference - 0 is negligibly small then since A0g = 0b - 0ei ... 

Practical measurements providing data on corrosion risk or cathodic protection are predominantly electrical in nature. In principle they concern the determination of the three principal parameters of electrical technology voltage, current, and resistance. Also the measurement of the potential of metals in soil or in electrolytes is a high-resistance measurement of the voltage between the object and reference electrode and thus does not draw any current (see Table 3-1). 

Even with the superposition of the ac with a cathodic protection current, a large part of the anodic half wave persists for anodic corrosion. This process cannot be detected by the normal method (Section 3.3.2.1) of measuring the pipe/soil potential. The IR-free measurable voltage between an external probe and the reference electrode can be used as evidence of more positive potentials than the protection potential during the anodic phase. Investigations have shown, however, that the corrosion danger is considerably reduced, since only about 0.1 to 0.2% contributes to corrosion. 

Rectifiers working according to the control diagram in Fig. 8-6 are used for anodic corrosion protection in passivatable systems that go spontaneously from the passive to the active state when the protection current is switched off [12]. The predetermined nominal voltage between reference electrode and protected object is compared with the actual voltage f/j in a differential display unit D. The difference AU = is amplified in a voltage amplifier SV to VqAU. This... 

Measurement of the cable sheathing/soil potential can be used to assess the corrosion danger from stray current interference (see Section 15.5.1). Since the measured values vary widely and the stray currents cannot be switched off, IR-free potential measurements are only possible with great effort. In order to keep the IR term of the potential measurement low, the reference electrode must be placed as close as possible to the measured object. With measurements in cable ducts (e.g., underneath tramway tracks), the reference electrodes can be introduced in an open duct. 

Potential control with zinc reference electrodes presented a problem because deposits of corrosion products are formed on zinc in hot water. This caused changes in the potential of the electrode which could not be tolerated. Other reference electrodes (e.g., calomel and Ag-AgCl reference electrodes) were not yet available for this application. Since then, Ag-AgCl electrodes have been developed which successfully operate at temperatures up to 100°C. The solution in the previous case was the imposition of a fixed current level after reaching stationary operating conditions [27]. 

Six iron anodes are required for corrosion protection of each condenser, each weighing 13 kg. Every outflow chamber contains 14 titanium rod anodes, with a platinum coating 5 /tm thick and weighing 0.73 g. The mass loss rate for the anodes is 10 kg A a for Fe (see Table 7-1) and 10 mg A a for Pt (see Table 7-3). A protection current density of 0.1 A m is assumed for the coated condenser surfaces and 1 A m for the copper alloy tubes. This corresponds to a protection current of 27 A. An automatic potential-control transformer-rectifier with a capacity of 125 A/10 V is installed for each main condenser. Potential control and monitoring are provided by fixed zinc reference electrodes. Figure 21-2 shows the anode arrangement in the inlet chamber [9]. 

Corrosion Potential—the potential of a corroding surface in an electrolyte relative to some reference electrode. 

The thermodynamic driving force behind the corrosion process can be related to the corrosion potential adopted by the metal while it is corroding. The corrosion potential is measured against a standard reference electrode. For seawater, the corrosion potentials of a number of constructional materials are shown in Table 53.1. The listing ranks metals in their thermodynamic ability to corrode. Corrosion rates are governed by additional factors as described above. 

These values are roughly constant across a range of electrolyte environments except where noted but the variations between alloys, heat treatment conditions, etc. creates a range for each metal. For some metals such as iron and steel the range is low ( 100 mV), but for lead, nickel, stainless steels a range is given. The corrosion potential is reported with respect to the saturated calomel reference electrode. 

Fig. 1.22 Spontaneous corrosion of zinc in acid illustrated by the reversible cell ZnlZn IH30", H2 Pt. The individual potentials of the electrodes are determined by a reference electrode (ReQ and a Luggin capillary to minimise the IR drop in the solution...

By means of a resistance in the circuit the spontaneous corrosion reaction can be made to proceed at a predetermined rate, and the rate can be measured by means of an ammeter A. At the same time the potentials of the individual electrodes can be measured by means of a suitable reference electrode, a Luggin capillary and high-impedance voltmeters and Kj. At equilibrium there is no net transfer of charge (/ = A = 0). the e.m.f. of the cell is a maximum and equals the difference between the reversible potentials of the two electrodes... 

A typical Evans diagrams for the corrosion of a single metal is illustrated in Fig. 1.26a (compare with Fig. 1.23 for two separable electrodes), and it can be seen that the E -I and E -I curves are drawn as straight lines that intersect at a point that defines and (it is assumed that the resistance for the solution is negligible). E can of course be determined by means of a reference electrode, but since the anodic and cathodic sites are inseparable direct determination of /co by means of an ammeter is not... 

However, in the case of stress-corrosion cracking of mild steel in some solutions, the potential band within which cracking occurs can be very narrow and an accurately known reference potential is required. A reference half cell of the calomel or mercury/mercurous sulphate type is therefore used with a liquid/liquid junction to separate the half-cell support electrolyte from the process fluid. The connections from the plant equipment and reference electrode are made to an impedance converter which ensures that only tiny currents flow in the circuit, thus causing the minimum polarisation of the reference electrode. The signal is then amplifled and displayed on a digital voltmeter or recorder. 

Voltmeters and potentiometers The instruments described here are generally referred to as corrosion voltmeters. As mentioned previously, the current flowing through any potential-measurement circuit must be small to avoid errors due to polarisation. Moreover, if the current flow is too large, errors will be introduced owing to the voltage drop caused by the contact resistance between the reference electrode and the electrolyte. It is thus clear that the prime requirement of a potential measurement circuit is high resistance. 

The potential difference across the electric double layer A. This cannot be determined in absolute terms but must be defined with reference to another charged interface, i.e. a reference electrode. In the case of a corroding metal the potential is the corrosion potential which arises from the mutual polarisation of the anodic and cathodic reactions constituting the overall corrosion reaction see Section 1.4). 

Various types of reference electrodes have been considered in Section 20.3, and the potentials of these electrodes and their variation with the activity of the electrolyte are listed in Table 21.7, Chapter 21. It is appropriate, however to point out here that the saturated calomel electrode (S.C.E.), the silver-silver chloride electrode and the copper-copper sulphate electrode are the most widely used in corrosion testing and monitoring. 

The simplest procedure in studying galvanic corrosion is a measurement of the open-circuit potential difference between the metals in a couple in the environment under consideration. This will at least indicate the probable direction of any galvanic effect although no information is provided on the rate. A better procedure is to make similar open-circuit potential measurements between the individual metals and some appropriate reference electrode, which will yield the same information and will also permit obser-... 

Measurements of open-circuit potentials relative to some reference electrode have been assumed on occasion to provide a means of rating metals as to their relative resistance to corrosion on the basis that the more negative the measured potential, the higher will be the rate of corrosion, but this assumption is obviously invalid, since it disregards polarisation of the anodic and cathodic areas. 

Choice of an appropriate reference electrode remains an enigma for non-corrosion-aware personnel-although commercially available polarisation-resistance probes can be adapted. An interesting aspect concerns localised corrosion in that, for some materials, localised corrosion only occurs within characteristic potential ranges. 

Reference Electrode an equilibrium (reversible) electrochemical half-cell of reproducible potential against which an unknown electrode potential can be measured. Examples of those commonly used in corrosion are the Pt, H /H (the hydrogen electrode), Hg/Hg Clj/Cl" (the calomel electrode), Cu/CuS04/Cu, Ag/AgCl/Cl", all with fixed activities of the dissolved ions. 

Equilibrium electrode potentials are readily established when metal electrodes are in contact with melts. However, two difficnlties arise in attempts to measnre them suitable, sufficiently corrosion-resistant reference electrodes must be selected, and marked diffusion potentials develop at interfaces between different melts. 

Figure 1. Schematic representation of potential profile and charge distribution across an anodic oxide film of thickness S on aluminum (a) hypothetical situation in the absence of any current (b) in the presence of an anodic current caused by corrosion or by an external source. RE, reference electrode to which the potential of aluminum is referred.

In many STM studies little effort has been made to control the atmosphere within the electrochemical cell. Yet oxygen is known to exert a major role in the chemistry and corrosion of many transition metals. For example, several STM studies have used the copper/copper ion reference electrode, yet the electrode is known to be polarized from its reversible condition by oxygen, leading to significant dissolution [154]. These effects become particularly significant in the smdy of metal deposition and dissolu-... 

Figure 17.11 Transmission spectroelectrochemistry cell designed for use with room-temperature haloaluminate melts and other moisture-reactive, corrosive liquids, (a) Auxiliary electrode and reference electrode compartments, (b) quartz cuvette containing the RVC-OTE, (c) brass clamping screw, (d) passageway between the separator and OTE compartment, (e) fritted glass separator, (f) A1 plate, (g) lower cell body (Teflon), (h) upper cell body (Teflon). This cell is normally used inside a glove box and is optically accessed with fiber optic waveguides. [From E. H. Ward and C. L. Hussey, Anal. Chem. 59 213 (1987), with permission.]...

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