METHODS OF CORROSION PREVENTION .1 Modifying the environmental conditions

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1.3.8 Stress corrosion

Stress corrosion cracking (SCC) refers to the cracking caused by the simultaneous presence of tensile stress and a corrosive environment. The stressed region acts as the anode and develops fine cracks which migrate in the metal in a direction perpendicular to the direction of the stress applied.

1.4 METHODS OF CORROSION PREVENTION

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1.4.3 Surface coating

Corrosion of metal surfaces is a common phenomenon. To protect a metal surface from corrosion, the contact between the metal and the corrosive environment is to be cut off. This is done by coating the surface of the metal with a continuous, non-porous material, inert to the corrosive atmosphere. Such a coating is referred to as surface coating or protective coating. In addition to protective action, such coatings also give a decorative effect and reduce wear and tear.

Objectives of coating surfaces

 Corrosion prevention.

 Enhancement of wear and scratch resistance

 Increasing the hardness

 Insulating electrically/thermally

 Imparting decorative colour

1.4.3.1 Metallic coatings

Surfacing coatings made up of metals are known as metallic coatings. These coatings separate the base metal from the corrosive environment and also function as an effective barrier for the protection of base metals. The metal which is coated upon is known as the base metal. The metal applied as a coating is referred to as coat metal.

The different methods used for metal coating are:

i) Hot dipping

 Galvanization

 Tinning ii) Metal spraying iii) Cladding.

iv) Cementation

 Sherardizing – Cementation with zinc powder is called sherardizing.

 Chromizing - Cementation with 55% chromium powder & 45%

alumina is called chromizing

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 Calorizing – Cementation with Aluminium and alumina powder is called calorizing

v) Electroplating or electrodeposition.

1.4.3.2 Anodizing

Anodizing is an electrochemical oxidation process for producing a stable Aluminium oxide layer on the surface of Aluminium and its alloys. Anodizing process is carried out in a wide verity of the electrolytes by applying AC, DC or a combination of both. In order to produce a stable Aluminium oxide layer, the Aluminium or its alloys must be used as anode and another appropriate metal or alloy as a cathode.

Advantages of the anodizing process

 Increases the corrosion resistance

 Increases in surface hardness and abrasion resistance

 Provides better adhesion for paint primers

 Provides electrical and thermal insulation

 Porous layer allows for colouring and sealing of the coating

There are three principal types of aluminium anodizing processes. Type I is chromic acid anodizing, Type II is sulphuric acid anodizing and Type III is sulphuric acid hard anodizing (sulphuric acid alone or with additives). The other less frequently used anodizing processes, use sulphuric acid with phosphoric acid, tartaric acid, oxalic acid, sulfophthalic acid sulfosalicyclic acid. The acidic anodizing process produces the porous oxide film on the surface. These pores in the oxide film permit the corrosive ions and water to reach the substrate surface and initiate the corrosion. The porous oxide layer is sealed by prolonged immersion in hot water. In this process oxide is converted into its hydrate form, the resulted swelling reduces the porosity of the layer.

The change in properties produced by sealing are basically

 Reduction of primer coating adhesion

 Reduction in hardness and abrasion resistance

 Reduction of thermal resistance

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 Reduction of the retention of dyes

 Increases in the corrosion resistance

 Increases in impedance of the oxide film 1.4.4 Cathodic protection

The principle involved in cathodic protection is to force the metal to behave like a cathode. Since there will not be any anodic area on the metal, corrosion does not occur.

There are two methods of cathodic protection.

(a) Sacrificial anodic method.

(b) Impressed voltage method (a) Sacrificial anodic method

In this technique, a more active metal is connected to the metal structure to be protected so that all the corrosion is concentrated at the more active metal and thus saving the metal structure from corrosion. This method is used for the protection of sea- going vessels such as ships and boats. Sheets of zinc or magnesium are hung around the hull of the ship. Zinc and magnesium being anodic to iron get corroded. Since they are sacrificed in the process of saving iron (anode), they are called sacrificial anodes.

The corroded sacrificial anode is replaced by a fresh one when consumed completely.

Important applications of sacrificial anodic protection are as follows:

 Protection from soil corrosion of underground cables and pipelines.

 Magnesium sheets are inserted into domestic water boilers to prevent the formation of rust.

(b) Impressed voltage method

In this method, an impressed current is applied in the opposite direction to nullify the corrosion current and converting the corroding metal from anode to cathode. This can be accomplished by applying a sufficient amount of direct current from a battery to an anode buried in the soil and connected to the corroding metal structure which is to be protected. The anode is in a backfill (composed of gypsum) so as to increase the

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electrical contact with the soil. Since in this method, current from an external source is impressed on the system, this is called the impressed current method.