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Corrosion and Its Prevention

Metal corrosion occurs when a metallic material reacts with its environment, causing measurable changes to the material.

Corrosion

Metal corrosion occurs when a metallic material reacts with its environment, causing measurable changes to the material that may lead to loss of function or failure of a system. Surfaces become corrosion-prone by being exposed to water, air, and caustic chemicals. As the surfaces of equipment corrode, they interfere with the performance and fabrication. Corrosion can be classified as wet or dry, depending on the environmental conditions.

Prevention of Corrosion

Corrosion can be prevented or reduced by using the following methods:

Material Selection

  • The downside to pure materials is that they are expensive and soft, and they tend to pit less readily. The only pure material is aluminium.
  • The addition of corrosion-resistant elements can improve corrosion resistance. Intergranular corrosion, for instance, occurs in stainless steel. Addition of titanium to steel can reduce the corrosion tendency.
  • The industries that use nickel, copper, and their alloys are typically those that do not have oxidizing environments, whereas those that have oxidizing environments use alloys containing chromium.
  • For fabrication, electrochemically similar materials should be used.
Corrosive materials are constructed with the following materials:

Materials that are corrosive and those that are used in construction

Corrosive material 

Suitable material

Nitric acid

Stainless steel

Hydrofluoric acid

Monel metal

Distilled water

Tin 

Dilute sulphuric acid

Lead 

Caustic

Nickel 


Proper design of equipment

Corrosion can be reduced if the following conditions are met:
  • The liquids should be completely drained.
  • Designed to be easily cleaned.
  • Designed to be easy to inspect and maintain.
  • Direct contact between metals should be avoided. Insulate if necessary.

Coating and Lining

Corrosion is more likely to occur in metals. Coatings or linings that are non-metallics should be used to combat metal corrosion. Electrical cladding, organic coatings, and electroplating can also be employed. By coating the anodes and cathodes with barrier coatings or by isolating them both, galvanic corrosion can be controlled. Organic coatings can also be applied to tanks, pipes, and shipping containers. Those metals are bonded together through coating. Usually, two metal sheets are rolled together. As another method to combat corrosion, steel is clad with an alloy.

By changing the environment

Steel gets protection from corrosion caused by water by removing air from boiler feed water. Nickel-based alloys are reduced in oxygen or air by pumping inert gas. Stainless steel alloys are less likely to be corroded by acidic media when aeration is applied. Besides reducing the temperature and reducing the moisture, reducing the exposure time can also reduce corrosion.

Use of corrosion inhibitors

Metals are protected from corrosion-by-corrosion inhibitors. A critical number of inhibitors is used (less than 0.1% by weight). Iron and steel are protected in aqueous solutions with compounds such as chromates, phosphates, and silicates. Iron and steel are protected in acidic environments with organic sulphides and amines. Stainless steel is protected from corrosion by copper sulphate in a hot diluted solution of sulphuric acid.

Cathodic and anodic protection

There are two ways to achieve cathodic protection.

Sacrificial anode method
A cathode (anode) is in contact with a protected metal (anode), leading to anode scarification. The corrosion protection of iron and steel tanks is provided by sacrificial anodes such as zinc, aluminium, magnesium and their alloys.

Impressed emf method
Electrodes and tanks are connected with external voltage using this method. Positive potential is always maintained at the anode. It is a way to prevent natural galvanic action. This prevents anode consumption. Any metal or noncorrodible alloy can be used. When sulphuric acid and deionized water are used, anodes are buried in the ground, while graphite and high-silicon steel are compressed when sulphuric acid is used. In addition to being simple, effective, and economical, this method is also suitable for storage of mildly corrosive liquids.

Anodic corrosion protection is a more recent corrosion control method compared to cathodic protection. The metal is subjected to a predetermined potential during anodic protection. In the beginning, metal corrosion or dissolution occurs as current increases. Passivation occurs at a critical point. Passivating potential is what develops at this point. Current flow is reduced to minimum value above the passivating potential. We call this passivating current. Anodic protection is most efficient when it requires a small current. Concentrated sulfuric acid is transported using this method
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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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