Protection by Sacrificial Anodes

Representation of variables involved in galvanic interaction between iron and zinc 

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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... 

Floating platforms are generally protected by CP (sacrificial anodes or impressed current) combined with a high-quality paint system. Pipelines with an organic coating and an external concrete coating are also protected by sacrificial anodes. 

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. 

The hot brine found in crystallization plants can be quite corrosive. Inconel, Monel, and stainless steel are used in most of the major equipment. Nonmetallic piping, including FRP, is common, within its serviceable temperature range, and Schedule 80 carbon steel pipe is often used in pump lines. The soil in a salt-processing plant can itself become corrosive as a result of spillage. Underground carbon steel lines therefore are protected by sacrificial anodes of magnesium or zinc or by application of direct electrical current. 

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. 

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 19.6 (See color insert.) Potential distributions along a pipeline caused by sacrificial anodes protecting areas with coating breakdown in the pipeline [5]. 

Sacrificial anodes Metal protected by sacrificial wastage of more electronegative metal Magnesium, aluminium or zinc (iron for copper and copper alloys) Faradaic equivalent of sacrificial metal — in practice the efficiency is seldom 100% Extremely simple Very improbable providing anodes properly located with respect to surface being protected... 

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. 

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... 

This is the area to be protected by the anode for 10 years. Thus, the average self-imposed spontaneous protective current (Table 8.4) and current density imparted by the sacrificial Mg-alloy anode are, respectively... 

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... 

Buried aluminum npelines are usually protected by saoificial anodes—zinc for coated lines and magnesium for uncoated lines. It is generally accepted that such coatings as exbuded polyethylene or a tape wrap should be applied to aluminum pipes for underground service. Because of the effectiveness and longevity of sacrificial anode systems and the need to avoid overprotection, impressed current (rectifier) systems generally are not used to protect aluminum pipelines. 

Galvanic corrosion can be controlled by the use of sacrificial anodes. This is a common method of controlling corrosion in heat exchangers with Admiralty tube bundles and carbon steel tube sheets and channel heads. The anodes are bolted direcdy to the steel and protect a limited area around the anode. Proper placement of sacrificial anodes is a precise science. 

The most significant chemical property of zinc is its high reduction potential. Zinc, which is above iron in the electromotive series, displaces iron ions from solution and prevents dissolution of the iron. For this reason, zinc is used extensively in coating steel, eg, by galvanizing and in zinc dust paints, and as a sacrificial anode in protecting pipelines, ship hulls, etc. 

Examples of the sacrificial-anode method include the use of zinc, magnesium, or aluminum as anodes in electrical contact with the metal to be protected. These may be anodes buried in the ground for protection of underground pipe lines or attachments to the surfaces of equipment such as condenser water boxes or on ship hulls. The current required is generated in this method by corrosion of the sacrificial-anode material. In the case of the impressed emf, the direct current is provided by external sources and is passed through the system by use of essentially nonsacrificial anodes such as carbon, noncor-rodible alloys, or platinum buried in the ground or suspended in the electrolyte in the case of aqueous systems. 

CoiTosion prevention is achieved by correct choice of material of construction, by physical means (e.g. paints or metallic, porcelain, plastic or enamel linings or coatings) or by chemical means (e.g. alloying or coating). Some metals, e.g. aluminium, are rendered passive by the formation of an inert protective film. Alternatively a metal to be protected may be linked electrically to a more easily corroded metal, e.g. magnesium, to serve as a sacrificial anode. 

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