Product characteristics of zinc alloy sacrificial anodes

Pei Yingying   1862587  9268

When cathodic protection is applied to a system, meaning a cathodic current is applied to polarize the cathode, the overall potential of the system shifts in the negative direction, such as to point 1. At this point, the total current on the cathode is I1, corresponding to the segment E1Q. Part of this current is externally applied, equivalent to segment PQ, while another portion still arises from the high corrosion of the anode, equivalent to segment E1P. It can be observed that the corrosion current I at the anode is smaller than the corrosion current Ic, indicating a reduction in the anode's corrosion rate and thus achieving a certain degree of protection.

As the externally applied current continues to increase, the system's potential further shifts in the negative direction. When the potential reaches the equilibrium potential of the anode, the anode's corrosion current becomes zero, achieving complete protection. At this point, the cathodic current Ip (equivalent to the segment) is entirely externally applied. This externally applied current is referred to as the minimum protective current, and the corresponding potential is called the minimum protective potential (generally, polarizing the metal from its stable potential by 200–300 mV in the negative direction in seawater can achieve complete protection).

(1) Main Parameters of Cathodic Protection: Protective Potential

Protective potential refers to the potential value required to halt metal corrosion during cathodic protection. To completely stop corrosion, the protected metal must be polarized to the active anode's open-circuit potential. For steel structures, this potential is the equilibrium potential of iron in the given electrolyte solution.

The protective potential has a specific range. For example, the protective potential for iron in seawater ranges from -0.80 to -1.0 V (relative to a silver/silver chloride electrode). When the potential is more positive than -0.80 V, iron cannot achieve complete protection, so this value is also referred to as the minimum protective potential.

The protective potential value is often used as a criterion to determine whether cathodic protection is complete. By measuring the potential values at various parts of the protected structure, the protection status can be assessed. Therefore, the protective potential value is an important indicator for designing and monitoring cathodic protection.

2) Protective Current Density

The current density required to reduce the metal's corrosion rate to an acceptable level during cathodic protection is called the minimum protective current density. The minimum protective current density corresponds to the minimum protective current. To achieve the minimum protective potential for the metal, the current density must not be lower than this value; otherwise, satisfactory protection cannot be attained. If the applied current density significantly exceeds this value, overprotection may occur, leading to excessive energy consumption and reduced protective effectiveness.

As one of the main parameters of cathodic protection, the minimum protective current density depends on factors such as the type of metal being protected, the nature of the corrosive medium, the total circuit resistance in the protection system, the presence and type of coatings on the metal surface, and external environmental conditions. It must be determined based on experience and practical circumstances. Table 3-8 lists the required current densities for steel in different environments.

(2) Factors Influencing Cathodic Protection

Since cathodic protection is based on cathodic polarization of the protected metal's surface, the polarization characteristics of the original corrosion cell significantly impact cathodic protection.

1) When the cathodic polarization is high and the anodic polarization is low, the corrosion of the protected metal is primarily cathodically controlled. In such cases, complete protection is easier to achieve. If the cathodic polarization curve falls within the diffusion-controlled region and the protective potential lies within this range, Ip is approximately equal to Icorr.

2) When the cathodic and anodic polarization rates are equal, or when the anodic polarization is high, as shown in the polarization curve in Figure 3-7, achieving complete protection by bringing the system to the protective potential EA requires Ip >> Icorr, meaning the protective current must be much larger than the corrosion current.

3) An increase in the corrosiveness of the surrounding medium will raise the required protective current. Increases in chloride ion concentration, oxygen content, and medium agitation speed (e.g., increased flow velocity in seawater) all reduce cathodic polarization, thereby increasing the current required for polarization and raising the protective current.

Variations in the chemical composition of the medium, oxygen content, types of ions, and the amount of suspended matter all affect cathodic protection. Differences in oxygen content or changes in medium conductivity can alter the anodic polarization rate, causing the required cathodic protection current to change. An increase in suspended matter can cause abrasion on the cathode surface, reduce cathodic polarization, increase the corrosion current, and correspondingly raise the protective current.

Temperature also affects polarization rates. An increase in temperature reduces oxygen solubility but accelerates diffusion and electrode reaction rates, ultimately increasing the protective current density.

(3) Anti-Corrosion Coatings

Anti-corrosion coatings on metal surfaces can isolate the metal from the surrounding medium. However, since coatings are not entirely impermeable, they contain tiny pores and defects where localized corrosion can occur. The combined use of anodic protection and coatings protects these localized areas, resulting in a much lower required protective current density compared to bare metals during cathodic protection.

During cathodic protection, a calcareous deposit layer forms on the anode surface due to an increase in pH, known as an electrolytic coating. This coverage increases surface resistance, reduces the corrosion current density, and enhances anti-corrosion effectiveness.

Since the formation of a calcareous deposit layer significantly lowers the protective current density and makes potential distribution more uniform, the cathodic protection process often initially involves applying a higher current density to quickly form a dense calcareous layer on the surface, followed by a lower current density for maintenance.

Pei Yingying   1862587   9268