Corrosion and Electrochemical Corrosion Protection
2.2 Electrochemical Corrosion
2.2.4 Mixed Electrodes
In general, according to Eq. (2-10), two electrochemical reactions take place in electrolytic corrosion. In the experimental arrangement in Fig. 2-3, it is there- fore not the I(U) curve for one reaction that is being determined, but the total current-potential curve of the mixed electrode, E}. Thus, according to Eq. (2-10), the total potential curve involves the superposition of both partial current-potential curves:
2.2.4.1 Homogeneous Mixed Electrodes
To simplify matters, it is assumed that the current densities for the partial reac- tions are independent of position on the electrode surface. Equation (2-10') can then be used to designate the current densities:
Equation (2-38) is valid for every region of the surface. In this case only weight loss corrosion is possible and not localized corrosion. Figure 2-5 shows total and partial current densities of a mixed electrode. In free corrosion / = 0. The free corrosion potential UR lies between the equilibrium potentials of the partial reac- tions U*A and U*c, and corresponds in this case to the rest potential. Deviations from the rest potential are called polarization voltage or polarization. At the rest poten- tial JA = l/cl, which is the corrosion rate in free corrosion. With anodic polarization resulting from positive total current densities, the potential becomes more positive and the corrosion rate greater. This effect is known as anodic enhancement of cor- rosion. For a quantitative view, it is unfortunately often overlooked that neither the corrosion rate nor its increase corresponds to anodic total current density unless the cathodic partial current is negligibly small. Quantitative forecasts are possible only if the JC(U) curve is known.
Fig. 2-5 Partial and total current densities in electrolytic corrosion of a homogeneous mixed electrode.
When cathodic polarization is a result of negative total current densities Jc, the potential becomes more negative and the corrosion rate lower. Finally, at the equilib- rium potential U*A it becomes zero. In neutral water equilibrium potentials are undefined or not attainable. Instead, protective potentials Us are quoted at which the corrosion rate is negligibly low. This is the case when JA ~ 1 jJ,A cm"2 (w = 10 pm a"1) which is described by the following criteria for cathodic protection:
where 7S is the lowest protection current density and corresponds to the overall total density at the protection potential Us. In conjunction with Eq. (2-40), the fol- lowing concepts are identified.
The terms "protection current" and "protection current densities" refer to any values of total cathodic currents that meet the criterion in Eq. (2-40). However, in the field, and for designing cathodic protection stations, another term is of interest, the protection current requirement. This term is concerned with the lowest value of the protection current that fulfills the criteria in Eqs. (2-39) or (2-40). Since with an extended object having a surface S the polarization varies locally, only the current density for the region with the most positive potential Us has the value Js. In other regions \JC\ > Js. For this reason, the protection current requirement 7S is given by:
where J& is a constant for the system material/medium, whereas 7S is only defined for a given object.
Fig. 2-6 Current-potential relationships for a heterogeneous mixed electrode or for cell formation (explanation in the text).
2.2.4.2 Heterogeneous Mixed Electrodes and Cell Formation
This is the general case where the current densities of the partial reactions vary over the electrode surface. Equation (2-10') and not Eq. (2-38) applies. As a sim- plification, a heterogeneous mixed electrode consisting of two homogeneous re- gions is considered in what follows. Figure 2-6 shows the total current-potential curves Ia(U) and IC(U) (solid lines) and the relevant anodic partial current-potential curves IA a(U) and /A)C(t/) (dashed lines). The homogeneous regions have rest poten- tials UR a and UR c. The index c represents the homogeneous region with the more positive rest potential because it represents the local cathode. The index a repre- sents the corresponding quantities of the local anode. The metallic short circuit of the two homogeneous regions results in a heterogeneous mixed electrode and the cell current 7e flows as a result of the potential difference (UR c - UR a). In free corrosion, the potential of the local cathode is changed to £/c(-/e) and that of the local anode to t/a(/e) due to the internal polarization of the cell. The free corrosion potential of the heterogeneous mixed electrode is dependent on position. There is no rest potential because the local current densities are not zero. In the electrolyte the potential difference (Uc - £/a) exists as the ohmic voltage drop 7e x 7?e. If the conductivity is sufficiently high, this difference can be very small so that a hetero- geneous mixed electrode appears as a homogeneous electrode. The free corrosion potential that can be measured independent of position seems to be a rest potential of a homogeneous mixed electrode, but it is not.
There are no current-density-potential curves for mixed electrodes, only cur- rent-density-potential bands which can be represented in a three-dimensional J-U-x
diagram with x as the position coordinate [15]. Figure 2-7 shows as an example a U-x diagram for a cylindrical mixed electrode of copper with an iron ring in tap water. The parameter is the current related to the total area. In the case of free corrosion there exists a dip in the potential directly over the Fe anode. This dip becomes greater with increasing anodic polarization because the total current- potential curves of the homogeneous Cu and Fe regions become further apart at more positive currents. In contrast, the dip disappears with cathodic polarization.
At high cathodic currents, it can even be reversed so that the Cu is at this stage more negative than iron. The reason for this is that the total current-potential curves of the homogeneous Cu and Fe regions intersect at negative currents. This unusual effect is known as potential reversal. Potential reversal is also possible with anodic polarization if, for example, as a result of differential film formation the total cur- rent-potential curves intersect at positive currents. This is possible, for example, for Fe and Zn in warm tap water or in seawater [4]. Anodic potential reversal has to be taken into account in the cathodic protection of mixed installations (e.g., carbon
Fig. 2-7 Potential distribution curves for a cylindrical-shaped, mixed electrode of CuFeCu polarized in tapwater (K ~ 10~3 S cm"1).
Spacing between the electrode and probe 1 mm.
steel and stainless steel) and sacrificial anodes should be used in order not to dam- age the less noble component of the object being protected.
Local corrosion is generally the result of the formation of heterogeneous mixed electrodes where the change in the local partial current-density-potential curves can result from the material or the medium. Where this is caused by the contact of different metals, it is known as a galvanic cell (see Fig. 2-7) [16,17]. Local differ- ences in the composition of the medium result in concentration cells. These in- clude the differential aeration cell which is characterized by subsequent chemical reactions that stabilize differences in pH values; chloride and alkali ions are im- portant here [4]. Such corrosion cells can have very different sizes. In selective corrosion of multiphase alloys, anodes and cathodes can be separated by fractions of a millimeter. With objects having a large surface area (e.g., pipelines) the re- gions can cover several kilometers. It is immaterial whether the cathodic region is still part of the pipeline or an external component of the installation that is electri- cally connected to the pipeline. In the latter case this is referred to as a cathodic foreign structure. Examples of this are electrical grounds and reinforcing steel in concrete (see Section 4.3).
The ratio of the areas of cathodes to anodes is decisive for the potential dam- age resulting from cell formation [16,17]. Using the integral (mean) polarization resistances
The difference in rest potentials (see the practical potential series in Table 2-4) determines mostly the direction of the current and less of the level; for these the resistances are significant. In particular Re can be neglected in the external corro- sion of extended objects. In addition, the Ia(U) curve is usually steeper than the IC(U) curve (i.e., Ra < 7?c). By introducing the surface areas of anode and cathode 5a and 5C, it follows from Eq. (2-43) that:
where rc = Rcx Sc is the specific cathodic polarization resistance. At high cell cur- rents, the cathodic partial current at the anode can be neglected, so that 7A a ~ /a in
Table 2-4 Practical potential series
Metal (a) (b) Titanium (+181) (-111) Brass (SoMs) 70 +153 +28 Monel (+148) +12 Brass (Ms) 63 +145 +13 Copper +140 +10 Nickel 99.6 +118 +46 CrNi steel 1.4301 (-84) -45 AlMgSi (-124) -785 Aluminum 99.5 (-169) -667 Hard Cr plating (-249) -291 Tin 98 (-275) -809 Lead 99.9 (-283) -259 Steel -350 -335 Cadmium (anodes) -574 -519 Electrolytic zinc coating -794 -806 Zinc 98.5 -823 -284 Electron (AM) 503 -1460 -1355
Note: Rest potentials UH in mV for common metals in (a) phthalate buffer at pH 6 and (b) artificial seawater [18], 25°C, air saturated and stirred. The rest potentials of the values in parentheses tend to become more positive with time due to film formation. (The values are dependent on the medium and operating conditions.)
Fig. 2-5. Equation (2-44) indicates that the activity of the cathode as well as the surface ratio and the potential difference also have an influence.
Sometimes difficulties arise because the cathode of the cell has a more positive potential than the anode (see Fig. 2-6). This is because the definition of anode and cathode is based on processes in the electrolyte, whereas potential measurement is based on events on the metal. This fact is illustrated in Fig. 2-8. If electrodes Pt and Fe are both in the same electrolyte with potential 0^, then from the metal point of view, Pt is more positive than Fe. U is the electromagnetic force (emf) of the cell.
When the switch S is closed, electrons flow from Fe(-) to Pt(+). If both electrodes are initially connected and immersed in separate electrolytes, they both have the potentia!0Met. The electrolyte at Fe is now more positive than that at Pt. The volt- age U can be measured between two reference electrodes in the electrolytes. When the tap H is opened, a positive current flows in the electrolyte from Fe(+) to Pt(-).
This last process is the basis for the definition of anode and cathode. When current is flowing, the anode (Fe) is more negative on the metal side, but in the electrolyte it is more positive than the cathode (Pt).
2.2.5 Observations of Current Distribution
The distribution of current is of considerable interest in corrosion processes on heterogeneous mixed electrodes, particularly in the internal corrosion of tanks and complex shapes as well as generally in the application of electrochemical protec- tion. A primary current distribution can be obtained from the laws of electrostatics by integration of the Laplace equation (div grad 0 = 0) [10,19]. Polarization resis- tances at the electrodes are neglected in this treatment. Current distribution is exclu- sively related to the geometry. When polarization resistance is taken into account, secondary and tertiary current distributions can be distinguished in which activation polarization alone, or together with concentration polarization, has an effect [20].
This is relevant in, for example, electroplating where uniform metal deposition is required. Current distribution is more uniform as a result of polarization resistance than in primary current distribution [4,10,19,20]. A polarization parameter
is introduced for comparability in which the ratio of polarization resistance to elec- trolyte resistance is taken into account. It corresponds to the thickness of a layer of electrolyte, whose ohmic resistance is equal to the polarization resistance. If in the primary current distribution this thickness is accounted for in the geometry, a bet-
Fig. 2-8 Principle and potential diagram of a galvanic cell (explanation in the text).
ter current distribution is obtained. The polarization parameter is also a measure of the range (see Section 24.5).
In electrochemical protection the necessary range of protection current is achieved by an appropriate arrangement of the electrodes. It follows that measures which raise the polarization resistance are beneficial. Coated objects have a coat- ing resistance (see Section 5.2), which can be utilized in much the same way as the polarization resistance in Eq. (2-45). Therefore, the range in the medium can be extended almost at will by coatings for extended objects, even at low conductivity.
However, the range is then limited by current supply to the object to be protected (see Section 24.4).
In many practical cases the question arises whether geometric hindrances may prevent sufficient protection current from being successfully supplied to the metal surface. Current could be shielded by stones, crevices, and in particular by poorly adhering tapes or disbonded coatings (see Section 5.2.4). Geometrically controlled resistance for the protection current exists in equal measure also for current from corrosion cells, for stray current, and for access of oxidizing agents to the cathode reaction according to Eq. (2-9). The current densities for electric conduction and for diffusion are given by the similar Eqs. (2-11) and
A relation independent of geometry is given by the quotient:
For oxygen-controlled corrosion with z = 4,D = 1 cm2 d \ and Ac(O2) = -10 mg L \ it becomes:
Even for high-resistance media with K= 10~^ S cnr1, sufficient protection is ob- tained with only A0 = 0.1 V from the criterion of Eq. (2-40): Js « -Jc = 0.14 / (i.e., the current supplied, 7, is seven times greater than the current needed, /s).
Equation (2-47) applies exclusively to diffusion of the oxidizing component in stagnant medium and not for other possible types of transport, e.g., flow or aera- tion from the gas phase. Narrow crevices filled with stagnant water are less serious than stones that screen current (see Section 24.4.5).