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Theories of Corrosion, Types of Corrosion and Their Prevention

When a single piece of metal (for example, Fe) comes into contact with an acid (for example, HCl), little galvanic cells form on the surface.

Theories of Corrosion

Corrosion Reaction on Single Metal

When a single piece of metal (for example, Fe) comes into contact with an acid (for example, HCl), little galvanic cells form on the surface. Each galvanic cell is made up of two parts: anode and cathode.



Reaction at anode - A metal formed by the transfer of two electrons from iron will form the ion Fe++.

Because the Fe++ ion is soluble in water, it is released into the medium. As a result, the iron surface corrodes.

Reaction at cathode - The emitted electron is conducted into the cathode area by the metal component. A pair of electrons and two protons (H+) are combined to create two H atoms. Because hydrogen atoms are unstable, two of them will join to form a stable H2 molecule. Water dissociates to produce H+ ions in the absence of acid.

2H+ + 2e– ----> H2

Hydrogen (H2) bubbles develop on the metal surface. Hydrogen generation at slow rates can result in a coating of H2 bubbles, slowing the cathode reaction and thereby corrosion rates. As hydrogen generation proceeds slowly, a layer of hydrogen bubbles will form on the cathode surface, thereby slowing corrosion. As a result, corrosion occurs quickly.

Corrosion Involving Oxygen

Water is generated when oxygen in the electrolyte is combined with hydrogen stored in the electrolyte. As hydrogen is depleted (reduced), it allows corrosion to occur.

At cathode - O2 + 2H2 ----> 2H2O

A reaction such as the one above occurs in acidic medium. Oxygen is absorbed by alkaline and neutral media. Moisture makes corrosion more likely.

Types of Corrosion

Fluid Corrosion – General

The general corrosion of a metal surface is when it is generally present as a whole. During general corrosion, corrosion occurs uniformly throughout the whole surface area. For example, plastic materials may swell, crack, soften, etc.

Fluid Corrosion – Biological

A material can be corroded by metabolic action of micro-organisms in either a direct or indirect manner, for example:
  • On the metal surface, create electrolyte concentration cells.
  • Effecting the rate of anodized/cathodic reactions.
  • The reducing bacteria on sulfates convert them into hydrogen peroxide (H2S). Ferrous sulphide (FeS) is formed when this reacts with iron. This causes the iron to corrode.

Prevention

Following methods can be used to prevent or reduce corrosion:

Proper design of equipment

The following conditions can minimize corrosion:
  • Drainage of liquids is designed to be complete.
  • Easy to clean design.
  • It is designed for easy maintenance and inspection.
  • A metal shouldn't come into direct contact with another metal. Metal can be isolated.

A corrosion inhibitor's use

Corrosion-by-corrosion inhibitors prevent metals from corroding. It is critical that the inhibitors be applied with less than 0.1%. Chromate, phosphate, and silicate compound solutions protect iron and steel from corrosion in aqueous solutions. The organic sulphides and amines in steel and iron protect them when they are exposed to acidic environments. In sulphuric acid diluted in hot water, copper sulphate protects stainless steel from corrosion.

Cathodic and anodic protection

Cathodic protection can be achieved in two ways.

Sacrificial anode methods
The cathode (copper) is in contact with the anode (metal), causing the anode to scarify. Zinc, aluminum, magnesium and their alloys act as sacrificial anodes to prevent corrosion of iron and steel tanks.

Impressed emf methods
The electrodes and tanks are connected by an external voltage. The anode remains positive at all times. The effect of natural galvanic current is prevented. There is no loss of anode. All metals or noncorrodible alloys can be used. Anodes are buried in the ground and compressed with sulfuric acid and deionized water. These methods are advantageous because they are simple, efficient, low cost, and versatile for storing mildly corrosive liquids. Anodic corrosion control, on the other hand, is a more recent development compared to cathodic protection. An electric potential is applied to metals. At first, metal dissolves or corrodes as current increases. Passivation takes place at a critical point. At a critical point, the potential known as passivating potential develops. Current flow drops to a minimum area above passivating potential. Passivating current is the result. Small currents are part of the advantage of anodic protection. It is used to transport sulphuric acid in a concentrated form.

By changing the environment

To prevent corrosion caused by water, air is removed from boiler feed water. In nickel-based alloys, oxygen or air content is reduced when inert gas is injected. Acidic media lessen the corrosion of stainless steel alloys when they are aerated. Reduced exposure time can also reduce corrosion in addition to reducing the temperature and moisture.

Design of equipment

Corrosion maintenance and repair are less costly and time consuming when the design is appropriate. Dead spaces or crevices frequently lead to corrosion, which is why it is highly recommended to eliminate or reduce them when designing. In order to prevent corrosion, there should be no crevices in the equipment, that equipment should be properly cleaned, that liquids should be drained completely after use, and that proper inspections of equipment should be done periodically.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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