Cathodic protection by sacrificial anodes

A more detailed treatment of cathodic protection by sacrificial anodes is given in Section 10.2. 

Environment Reduce kinetics of cathodic reaction Lower potential of metal Cathodic inhibition Reduce a , reduce O2 concentration or concentration of oxidising species lower temperature, velocity agitation Cathodically protect by sacrificial anodes or impressed current sacrificially protect by coatings, e.g. Zn, Al or Cd on steel Formation of calcareous scales in waters due to increase in pH additions of poisons (As, Bi, Sb) and organic inhibitors to acids... 

General performance of apphed cathodic protection by sacrificial anodes can be evaluated as follows ... 

It is interesting that the first large-scale application of cathodic protection by Davy was directed at protecting copper rather than steel. It is also a measure of Davy s grasp of the topic that he was able to consider the use of two techniques of cathodic protection, viz. sacrificial anodes and impressed current, and two types of sacrificial anode, viz. zinc and cast iron. 

The forms of corrosion which can be controlled by cathodic protection include all forms of general corrosion, pitting corrosion, graphitic corrosion, crevice corrosion, stress-corrosion cracking, corrosion fatigue, cavitation corrosion, bacterial corrosion, etc. This section deals exclusively with the practical application of cathodic protection principally using the impressed-current method. The application of cathodic protection using sacrificial anodes is dealt with in Section 10.2. 

Steel tanks are often protected by sacrificial anodes. These anodes need to be monitored and replaced regularly. Tanks made from reinforced isopolyesters are inherently corrosion resistant and non-conductive. They do not require cathodic protection. 

Magnesium, zinc, or aluminum blocks are anached to ships hulls, oil and gas pipelines, underground iron pipes, and gasoline storage tanks. These reactive metals provide cathodic protection by acting as a sacrificial anode. 

Impingement attack at the inlet ends can be minimized by providing a suitable amount of cathodic current. While this current does not enter the tube ends to any great depth, it protects the first few inches. The current may be provided by sacrificial anodes made from iron, zinc, aluminum, or magnesium. 

FIGURE 26.35 Current-voltage Tafel plot showing cathodic protection by nse of a sacrificial anode. = equilibrium potential of the redaction reaction = equilibrium potential of the primary metal dissolution reaction E a = equilibrium potential of the sacrificial anode oxidation reaction. 

The ohmic drop exerts a sensible influence on the evaluation of the electrochemical parameters as well as on the definition of the reaction scheme that is most suitable for describing the behaviour of a metal in a given environment. It also determines the success of many operations, such as cathodic protection by means of sacrificial anodes or impressed current and corrosion rate monitoring. 

When more and less noble materials are placed in contact, the more noble material offers an extra area for the cathodic reaction. Therefore flie total rate of the cathodie reaction is increased, and this is balanced with an increased anodic reaction, i.e. increased dissolution of the less noble material (galvanic corrosion. Section 7.3). If the more noble material (the cathodic material) has a large surface area and the less noble metal (the anodic metal) has a relatively small area, a large cathodic reaction must be balanced by a correspondingly large anodic reaction concentrated in a small area. The intensity of the anodic reaction, i.e. the corrosion rate (material loss per area unit and time unit) becomes high. Thus, the area ratio between the cathodic and the anodic materials is very important and should be kept as low as possible. It should be mentioned that in a galvanic corrosion process, the more noble material is more or less protected. This is an example of cathodic protection, by which the less noble material acts as a sacrificial anode (see next section). 

The most common example of beneficial application of galvanic elements in corrosion engineering is cathodic protection by use of sacrificial anodes (Section 10.4). A coating of a material less noble than the substrate gives the same effect. 

Figure 10.13 Cathodic protection by a) sacrificial anodes and b) impressed current.

C. Rousseau, F. Baraud, L. Leleyter, O. Gil. Cathodic protection by zinc sacrificial anodes Impact on marine sediment metallic contamination. Journal of Hazardous Materials, Vol. 167, Nos. 1-3, pp. 953-958, 2009. 

Cathodic protection by impressed current involves the use of a rectifier connected to a power line. Contrary to sacrificial anodes, which operate at a fixed potential, the use of a rectifier permits to adjust the voltage (or the current) to the particular requirements of a protection scheme. This not only allows one to optimize the electrochemical conditions for protection, but the method is also well suited to protect large surfaces. On the other hand, protection by impressed current needs more maintenance than the use of sacrificial anodes. In order to protect buried structures by impressed currents one uses consumable anodes such as scrap iron or, more often, non-consumable anodes made of iron-silicon alloy, graphite or of titanium coated with noble-metal oxides. 

The most frequently used materials for buried metal structures are the carbon steels. For prevention of their corrosion the most recommended, economical, and effective method is cathodic protection (CP). The use of CP is now standard procedm-e for long-term corrosion protection of imderground pipelines, oil and gasoline tanks, and other structures. With a shift of the metal potential to more of a negative value of -0.85 V versus a C11/CUSO4 reference electrode, it is possible to make the metal surface a cathode, which ensures an immune (no corrosion) state of the carbon steel. Cathodic polarization is achieved by direct current, which can be supplied either by sacrificial anodes in galvanic contact with the steel structure, or by impressed current from a rectifier. 

Coatings such as a zinc silicate primer covered with a layer of an epoxy-based polymer are routinely applied to steel structures to protect them against corrosion. However, cracks or flaws in the coating expose Fe which then undergoes oxidation in an anodic process. To prevent this, a second protection system is put in place cathodic protection. By placing a block of a more electropositive metal on the surface, this second metal is preferentially oxidized. This is the same principle as the use of zinc in galvanized steel (see Section 6.7). From Table 8.1, you can see why Zn, A1 and Mg (or alloys of these metals) are typically chosen as sacrificial anodes. The most electropositive metals (Li, Na, K and Ca) are unsuitable because they react with cold water. The relevant half-equations (at pH 7) are now ... 

Sacrificial Anode The Alaskan pipeline is cathodically protected by a parallel zinc cable. 

In addition, cathodic protection based on sacrificial anodes requires a cell potential as the driving force for a self-inposed spontaneous protective current imparted by sacrificial anodes liberating electrons at a specific rate [42]. The driving force for this current is the potential difference (overpotential) between the sacrificial anode and cathode that is, E = Ec + o.In fact, the resultant... 

Submerged marine structures. Cathodic protection of submerged marine structures such as steel jackets of offshore oil and gas platforms and pipelines is widely provided by sacrificial anode systems. A... 

On the other hand, water composition is a factor of paramount importance blackening occurs if the water contains bicarbonates HCO and if its pH is between 8 and 9. Blackening is said to be caused by the adsorption of bicarbonate ions at the porous natural oxide film. Blackening may also appear when aluminium is coupled with a less electronegative metal in water silver, stainless steel, or copper. On the other hand, aluminium that is cathodically protected by a sacrificial magnesium anode does not blacken in water [10]. 

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