Cathodic Protection of Casing
Cathodic protection is used in many oilfields to protect the casing against external corrosion. Corrosion occurs at the anode, as electrons leave the anodic areas and move towards the cathodic areas. If electrons are forced into the anodic areas.
corrosion will not occur.
The first step in the control of external casing corrosion is to provide a complete cement sheath and bond between the pipe and the formation over all external areas of the casing strings as discussed previously.
Cathodic protection involves supplying electrons to the metal to make the potential more negative. Complete protection is achieved when all the surface area of the metal acts as a cathode in the particular environment.
The increase in electronegative potential can be achieved by use of sacrificial anodes (magnesium, aluminium and zinc) or by an impressed direct current. The potentials required for protection differ with the environment and the electrochemical reactions which are involved. For example. Blaunt (1970) noted that iron corroding in neutral aerated soil has a reduction potential of 0.579 V. The potential is limited by the activity and solubility of ferrous hydroxide. If iron is exposed to H2S in oxygen-free environment, the potential is increased to 0.712 V and is controlled by the solubility of ferrous sulfide.
Measurements of potential are made by use of reference half cells. The copper — copper sulfate half cell is widely used for potential measurements of pipe in soils. The criteria for protection of iron with this half cell is -0.85 V in aerated soil and — 0.98 V in an H2S system.
The theory of cathodic protection is illustrated in Fig. 6.3. As shown in Fig. 6.3, the polarization of cathodic areas of steel must be extended until the potential Ec of the cathodic surfaces reaches the potential Ea of the anodic surfaces. The current which is applied in cathodic protection (/") must exceed the equilibrium corrosion current (/’) of the metal in its corrosive environment without cathodic protection.
The two types of cathodic protection most commonly used are: galvanic and impressed-current. When anodes (e. g.. aluminium) are electrically coupled to steel (immersed in the same electrolyte), cathodic-protection current is generated. As a result of oxidation of aluminium, electrons are forced into the steel, because electrochemical potential of aluminium is higher than that of steel (see electromotive force series, Table 6.1). Inasmuch as aluminium is consumed in the process, it is called a “sacrificial anode”.
In the case of the impressed-current cathodic-protection. rectifiers are used to convert alternating current to direct current. The negative side of the direct current is connected to the casing, whereas the positive side is connected to the buried anodes. The anode material in this case is essentially inert (see Fig. 6.4).
Interference bond on an insulating flange at a cathodically protected casing is shown in Fig. 6.5. In the absence of bond, the interference current on the electrically isolated flowline would leave through the soil at point A, causing
|
Fig. 6.4: Deep well groundbed design using anode and carbonaceous backfill in open hole. (Courtesy of NACE, Houston. TX, Control of Pipeline Corrosion, fig. 8-12.) Can be used for either a pipeline or casing. |
|
variable RESISTANCE Fig. 6.5: Adjustable interference bond across an insulating (isolating), flange connecting a buried flowline to the cathodically-protected casing. (After Jones. 1988, p. 34, fig. 1.4-6; Courtesy of the OGCI Publications. Tulsa. OK.) |
As shown in Fig. 6.5. the insulating flange electrically isolates the casing from the surface equipment. This confines the cathodic protection current to the casing.