What is corrosion and how can it be prevented?

Corrosion is the deterioration and loss of metal, or material by chemical, or electrochemical reactions with its environment.

Example: rusting of iron and steel.

Corrosion occurs when metals come in contact with corrosive atmosphere. Thus, corrosion can be prevented by:

1. Changing the metal, or/and design of parts

2. Modification of corrosive environment.

3. Separating the metal from the environment

4. Changing the electrode potential of the metal.

The most effective and economical method of preventing corrosion is to choose the most suitable geometrical design, most suitable material and then chose the most suitable protective method.

*Geometrical design*

Design should consider mechanical and strength requirements alongwith an allowance for corrosion. Slight change in design to prevent the corrosion mah prove highly economical in many cases.

Some simple rules should be kept in mind which designing a part:

1. Welded joints be preferred over riveted joints.

2. Avoid sharp bends in pipes when high velocities if fluids are involved.

3. Avoid contact with absorbent materials.

4. Avoid protruind parts.

5. Provide thicker section to take care of impingement attack.

*Proper selection of material*

Though mechanical strength, cost, fabricability, etc are the main factors in choosing a material, slight variations in compos or structure can help preventing corrosion. Use of purer metals (devoid of S, P in steel, Fe, Cu, Si in aluminium) improves the resistance but pure metals are expensive, soft and weak. Alloying elements are preferred as a choice to improve corrosion-resistance of metals.

Some factors to be considered before choosing a material for a particular service condition:

1. Metals on anodic end of galvanic series have less corrosion resistance than the metals on cathodic end ( more noble). Pure metals have more corrosion resistance than impure metals.

2. Coarse grained materials have higher corrosion resistance than fine grained materials.

3. Cold worked metal is anodic to annealed metal.

*Modification of environment*

1. Dehumidification of air: It is a process of reducing the moisture from air so that corrosion is almost prevented. Freeze drying

2. Removal of acid from water by neutralization such as by adding lime.

3. De-aeration and de-activation: Elimination of O2 is essential from the electrolyte to prevent the corrosion.

Inhibitors:

Inhibitor is a substance which when added in a small quantity to the corrosive environment decreases the rate of corrosion. Inhibitor either acts as a barrier by forming an adsorbed layer or retard the cathodic and/or anodic reactions.

Cathodic protection:

Sacrificial Anode method: In this method, another metal, which has more negative electrode potential than the structure to be protected is connected electrically to the structure, which now acts like a cathode.

Anodic protection:

If active -passive metals, particularly nickel, iron, chromium, titanium and their alloys (even alloys having these metals) are supplied a carefully controlled anodic current, they get passivated as protective film forms on anode material, the rate of corrosion decreases, or the corrosion is prevented.

Coatings:

Metallic coatings: Protecting the metals by metal coating is done as these are more durable, more decorative and provide better protection against corrosion.

Cathodic coating: The coating metal with respect to the bas metal. These coatings provide a physical barrier between the metal and the environment.

Anodic coatings: The coating metal is anodic to the underlying metal. The anodic coatings provide galvanic protection to the base metal, in addition to acting as a physical barrier between the base metal and the environment.