Rate of Corrosion

Corrosion is an irreversible process and naturally occurring process in which pure and unstable metals (except the noble metals)  are converted into chemically stable compounds like oxides, sulphide, hydroxides, etc. The most common example of corrosion is rusting of Iron (Fe2O3.xH2O).

Factors affecting the rate of Corrosion

1 – Metal reactivity: If metal is more reactive, it is more prone to corrosion.

2 – Metal striation: Corrosion is a common occurrence in metal cuts and bends.

3 – If  Impurities are present: If there are impurities in the pure metal, it will corrode more quickly.

4 – Electrolyte presence: Metal quickly corrodes in saline water (electrolyte).

5 – Moisture and air: The best illustration is of rusting of iron.

6 – Acidity: The acidic pH of 7 is more corrosive than the basic or neutral pH.

7 – Temperature: The rate of corrosion increases as the temperature rises.

Electrochemical theory of corrosion

When metals are exposed to air, moisture, soil, and other environmental factors, they get corroded by an electrochemical process. On the metal surface, discrete anode and cathode zones are formed during this process. Corrosion is always caused by an oxidation reaction and the release of an electron at the anodic regions of metal. The freed electrons from the anode travel to the cathode, where they are reduced. The overarching theory of corrosion is known as the electrochemical theory. Corrosion happens when moisture is present, that is why it is known as the “Wet Theory of Corrosion”. 

The following processes control the flow of free electrons in this corrosion:

a) Hydrogen Liberation

b) Absorption of Oxygen

Corrosion Prevention 

1 – Metal Coating

A metal coating procedure was used to preserve the basic metal against corrosion.

The following procedures can be used to coat metals:

A. Galvanization:

Galvanization is also called Anodic metal coating. It is a metal coating that is anodic to the “base metal”. Any metal which is anodic with higher in the galvanic series, like Zn, is coated on the “surface of a metal” It protects from corrosion. A good example of “anodic metal coating” is Galvanization. It is a process of “covering a base metal” with molten “Zn” by dipping the “base metal” which is cathodic. 

B. Cathodic metal coating (Tinning):

Cathodic metal coating is cathodic to the underlying metal. Any metal which lies at a lower position in a galvanic series, including copper, Nickel, etc, is used to coat the metal surface to protect it from corroding.

2. Scarification of anodic protection: 

The metallic structure(cathodic) is protected by “scarification of anodic protection”, mostly in areas of high corrosion intensity to protect it from corrosion. Sacrificial anode refers to high reactive metal that is used. On being completely corroded, the sacrificial anode is replaced by a new one. Mg, Zn, Al, and their alloys are often used as sacrificial anodes because they have a low reduction potential (which makes them more reactive) and are at the top of the electrochemical series. Water pipelines and tanks are buried underground.

The sacrificial anode strategy protects them as well.

3. Impressed cathodic protection:

In the “impressed cathodic protection” the corroding anodic metal becomes cathodic and corrosion-resistant. The external DC source supplies the impressed current. The positive terminal of the DC source is linked to the anode, while the negative terminal is attached to metal to protect it. Graphite, stainless steel scrap iron, and other materials can be used as anodes in this application. Water tanks, water and oil pipelines, transmission line towers, and other structures are protected by the impressed current.

4. Electroplating

Electroplating is the process of depositing metal (anode) on the surface of metals and alloys by using electrolysis (cathode or base metal). The two electrodes are submerged in an electrolyte solution containing the metal ions that will be deposited on the base metal. Metals such as Au, Zn, Sn, Ag, Cr, Ni, and others can be electroplated onto a base metal’s surface (Cathode). 

Conclusion 

When metal comes into contact with oxygen, it creates a chemically unstable condition. The metal oxide is the stable state that corresponds. For obvious geometrical considerations, metal can only react with oxygen where it comes into touch with it (i.e. on its surface). That is why all non-noble metals have a layer of metal oxide on them (although this layer often is so thin that it is invisible). Metals covered in a protective oxide have qualities that allow them to form a barrier against oxygen in the environment, making them potentially helpful for manufacturing metal objects. Hardness, ductility, and other qualities are, of course, determinants of their utility. The metal’s nobility (i.e., its p