1.4 Effects of Stray Current on Metallic Structures
1.4.1 At the Current Discharge Location
Identification of the current discharge site receives considerable attention in stray current investigations because it is the location where corrosion damage is most likely to occur on all metallic structures. When a current transfers from a metallic structure to earth (Figure 1-8), it must do so via an oxidation reaction that converts electronic current to ionic current.
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Figure 1-8: Current Discharge from a Metal Structure to Earth via an Oxidation Reaction
The generic oxidation reaction is the corrosion of the metal as in Equation 1-2.
Mo Æ Mn+ + ne– [1-2]
For steel, the oxidation reaction is:
Feo Æ Fe++ + 2e– [1-3]
A stray current discharge from a metallic structure may not cause corrosion attack if the structure is receiving CP (Figure 1-9). Whether the superposition of a stray current discharge and a CP current pick-up at a metal/electrolyte interface causes corrosion will depend on time and the relative magnitudes of these two currents.
O
Figure 1-9: Superposition of a Stray Current and a Cathodic Protection Current at a Metal/Electrolyte Interface
CP current transfers across the metal/earth interface via a reduction reaction, which produces hydroxyl ions in either of the three following reactions:
H3O+ + e– Æ HO + H2O [1-4]
O2 + 2H2O + 4e– Æ 4OH– [1-5]
2H2O + 2e– Æ H2Ç + 2OH– [1-6]
In the presence of a high concentration of hydroxyl ions, a possible oxidation reaction is given in Equation 1-7. The reaction involves the oxidation of hydroxyl ions to oxygen and water.
4OH– Æ O2 + 2H2O + 4e– [1-7]
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This latter reaction does not consume metal atoms; therefore, there is no corrosion damage. Hence, as long as the polarized potential at the structure electrolyte interface is not driven more electropositive than the CP criterion (e.g., –850mVcse for iron or steel), significant corrosion would not be expected.
If the metal has a surface passive film or is a relatively inert material (such as some of the materials used for impressed current anodes), then not all of the stray current need transfer through a corrosion reaction. If the stray current polarizes the metal surface electropositively to the oxygen line on the Pourbaix diagram, then the hydrolysis[13] of water molecules by the following reaction 1-8 is likely.
2H2O Æ 4H+ + O2Ç + 4e– [1-8]
This oxidation reaction does not result in the consumption of the metal surface, but it does produce an acidic pH from the generation of hydrogen ions.
On an iron or steel structure without CP, the oxidation reaction is usually the dissolution of the metal according to Equation 1-9
Feo Æ Fe++ + 2e– [1-9]
The severity of corrosion depends on the magnitude of the stray current and time as related by Faraday’s Law:
corr
t I
F M
W t
= n [1-10]
where:
Wt = total weight loss at anode or weight of material produced at the cathode (g)
n = number of charges transferred in the oxidation or reduction reaction
Icorr = the corrosion current (A)
F = Faraday’s constant of approximately 96,500 coulombs per equivalent weight of material (where equivalent weight =
n M)
M = the atomic weight of the metal that is corroding or the substance being produced at the cathode (g)
t = the total time in which the corrosion cell has operated (s)
13 Hydrolysis is defined as a double decomposition reaction involving the splitting of water into its ions and the formation of a weak acid or base or both. CRC Handbook of Chemistry and Physics, CRC Press, 53rd Edition, 1972-1973, PF-83.
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Given the atomic weight of pure iron as 55.85 g and assuming 100% efficiency and pure DC, the consumption rate of iron as illustrated in Table 1-1 is 9.13 kg/A-y.
Table 1-1: Theoretical Consumption Rates of Various Metals and Substances
Reduced
On pipelines, the total weight loss is usually less important than the penetration rate. By re-arranging Faraday’s Law, the weight loss per unit time per unit area is shown to be directly proportional to current density (i = I/A) as in Equation 1-11.
n i
Dividing this equation by the density (d) of the metal or alloy produces the corrosion rate (rcorr), which can be expressed in mm/y (Equation 1-12).
d
n = number of charges transferred in corrosion reaction i = current density (μA/cm2)
k = unit correction term ≈ 3.156 x 108 mm s/cm yr d = density (g/cm3)
rcorr = penetration rate in (mm/yr)
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Example: Using Equation 1-12 to calculate the penetration rate based on a current density of 1 A/m2 (10-4 A/cm2):
Table 1-2 gives the penetration rate, in mpy and 10-3 mm/y, equivalent to a current density of 1μA/cm2 for a number of common pure metals.
Table 1-2: Electrochemical and Current Density Equivalence with Corrosion Rate for Some Common Pure Metals
Penetration Rate Equivalent to 1 μA/cm2[1]
Metal/Alloy
The foregoing corrosion rates apply to stray current situations involving a continuous DC discharge. Corrosion rates decrease for periodic reversals of DC and are substantially less for 60Hz AC (Figure 1-3).
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The low corrosion rate for a 60Hz current is attributed to the relatively low impedance of the interfacial capacitance. The structure/electrolyte interface can be modeled electrically by a Randle’s Circuit shown in Figure 1-10.
Cdl
Re = resistance of steel surface to remote earth
Rp = polarization resistance (1-104Ω-cm )2
Eoc = potential difference (volts) Iac = total AC crossing the interface Ia,rp =
Ia,dl =
total AC through polarization resistance
total AC through double-layer capacitance
Iac Iac,rp
Iac,dl
Figure 1-10: Randle’s Electrical Circuit Model of a Metal/Electrolyte Interface
This circuit model illustrates that the interface is not simply a resistance but a parallel combination of the polarization resistance (Rp) and a capacitor (Cdl) called the layer capacitance. Unlike DC, AC can pass through the double-layer capacitance. There is no mass transfer in this current path and hence no corrosion polarization results from current transfer in this path. The proportion of AC (Iac,dl) through the double-layer capacitor is a function of the relative impedance of this path compared to the polarization resistance.
The reactance (Xcdl) of the double-layer path is given by the following equation:
2
Assuming a 1cm2 surface area and mid-range values of both the polarization resistance and the double-layer capacitance as follows, then the proportion of AC through the capacitor can be calculated.
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The total impedance Zt to 60Hz AC of the parallel combination of the polarization resistance (Rp) and the double-layer capacitance is therefore:
54
Then the proportion of AC current through the double-layer capacitance is:
Xc
Accordingly, only approximately 2.6% of the AC would pass through the polarization resistance and only the positive half-cycle of the current would be involved in the corrosion reaction.