Cathodic Protection

Cathodic protection:

Metals undergo corrosion due to the formation of galvanic cells. It is also anode which suffers corrosion. Therefore the principle involved in this method is to force the metal to be protected to behave as cathode, there by corrosion does not occur. This is done by converting anodic side into cathodic side. This phenomenon is known as cathodic protection. Cathodic protection can be achieved by following methods:

1. Sacrificial anode method.
2. Impressed current method

1. Sacrificial anode method:

In this method, the metal to be protected is converted into cathode by connecting it to a more active metal. The more reactive metals like Zn, Mg, Al and their alloys can be used as anodes, which when connected to the metal object undergoes preferential corrosion (sacrifices itself) protecting the metal structure. The sacrificial anode continuously corrodes and gives the electrons to the protected metal and forces it to act as cathode, which remains unaffected.  Exhausted sacrifial anodes are replaced by new ones as and when required.

Application:

a)    This method is used for protecting storage tanks with Mg blocks.

b)    Mg bars are fixed to the sides of ocean going ships to act as sacrificial anodes.

c)    This method is used for protecting buried iron pipe lines with Mg blocks.

(2) Impressed current method:

Another method providing cathodic protection is by applying a direct current larger than the corrosion current. The protected metal is made cathodic by connecting to the negative terminal of the d.c power supply. The positive terminal of the d.c power supply is connected to an inert electrode like graphite. The metal structure being cathodic does not undergo corrosion.  Anode being inert remains unaffected. 

Advantages:

(1)  One installation can protect large area of metal.

(2)  Low installation cost.

Applications:

Impressed current or impressed voltage method is used to protect

i) Buried oil pipe line.

ii) Marine pipes.

iii) Buried water storage tanks.

Corrosion inhibitors

Corrosion inhibitors:

Corrosion inhibitors are substances which when in small concentrations to a corrosive environment decreases the corrosion rate. The inhibitors provide protection against corrosion by retarding either anodic or cathodic reactions.

They are broadly classified as

          (a)  Anodic inhibitors  (b) Cathodic inhibitors

(a) Anodic inhibitors: During corrosion of metals, oxidation occurs at anodic side and if this oxidation is prevented, cathodic reactions also stop there by retarding corrosion reaction. This is achieved by addition of anions such as chromate, tungstate, molybdate, phosphate etc. These anions combine with metal ions formed at the anodes forming sparingly soluble respective salts. These salts get deposited on the anodic side and acts as a barrier between the metal surface and the corrosive environment. Thus further anodic reaction is prevented. Anodic inhibitors are found to be effective only when sufficient quantities of inhibitors are added.

(b) Cathodic inhibitors: The two important types of cathodic reactions are liberation of H₂ gas and absorption of O₂.Therefore there are two distinct methods in achieving inhibition by cathodic reactions i.e.1) Preventing liberation of H₂ gas 2) Preventing absorption of O₂.

Corrosion Control : Organic coatings

Organic coatings: 

 The most widely used corrosion control is to paint the articles to be protected with paints or lacquers. The function of the organic coatings is two-fold.

(i)   The organic coating acts as a barrier between the metal and the corrosion environment.

(ii)  The pigments (red lead, zinc chromate etc) or dry oils (linseed oil, wood oil etc) present in the paint often exert an inhibitive action on the corrosion process.