Sacrificial protection coatings

The modern procedure to minimise corrosion losses on underground structures is to use protective coatings between the metal and soil and to apply cathodic protection to the metal structure (see Chapter 11). In this situation, soils influence the operation in a somewhat different manner than is the case with unprotected bare metal. A soil with moderately high salts content (low resistivity) is desirable for the location of the anodes. If the impressed potential is from a sacrificial metal, the effective potential and current available will depend upon soil properties such as pH, soluble salts and moisture present. When rectifiers are used as the source of the cathodic potential, soils of low electrical resistance are desirable for the location of the anode beds. A protective coating free from holidays and of uniformly high insulation value causes the electrical conducting properties of the soil to become of less significance in relation to corrosion rates (Section 15.8). 

As the general standard of protective coatings has improved (materials, application and construction standards) the use of sacrificial anodes has extended to soil of ca. 10 000 ohm cm or more for buried pipelines. By using anodes in extruded ribbon form and placing them as close as possible to the structure to be protected they can be used in soils of high resistivity. 

There are two main reasons why zinc is chosen as a protective coating for iron and steel. The first is the natural resistance of zinc itself against corrosion in most atmospheric conditions, and the second is the fact that zinc is electronegative to iron and can protect it sacrificially. ... 

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... 

Various strategies are employed to prevent corrosion. The use of paint as a protective coating is described in our chapter introduction. A metal surface can also be protected by coating it with a thin film of a second metal. When the second metal is easier to oxidize than the first, the process is galvanization. Objects made of iron, including automobile bodies and steel girders, are dipped in molten zinc to provide sacrificial coatings. If a scratch penetrates the zinc film, the iron is still protected because zinc oxidizes preferentially ... 

Protective coating with ablative or sacrificial finishes... 

Because corrosion can cause considerable damage, it is important to investigate ways to prevent rust and deterioration. Paint or another protective coating is one way to protect steel structures from corrosion. Q Sacrificial anodes of magnesium or other active metals are also used to prevent corrosion. 

It is essential to make an important decision on the selection of the CP system to be instaUed. The choice of the CP system is based on the environment in which the pipeline is operated. The merits of each system are iUustrated in Section 15.2.3. Sacrificial protection is generaUy employed in environments with low soil resistance (10,000 Q cm) with a relatively low protective current requirement because of the low driving voltage. WeU-coated and isolated pipeUnes favor the instaUation of a sacrificial system in high resistive soils. On the other hand, ICS are usuaUy instaUed in conditions where the current requirements are higher and for pipelines that are bare or poorly coated. In aU cases, the final decision is based on the total instaUation and annual maintenance cost of the systems. 

Sacrificial coatings are based on the principle of galvanic corrosion for the protection of metals against corrosion. The substrate is protected by a metal or alloy that is electrochemically more active than the material to be protected. Coatings formulated with metallic zinc powder have been... 

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