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What is corrosion or deterioration of metal?

In this article discuss about the corrosion or deterioration or degradation of metal, forms of corrosion, protection against corrosion and how corrosion affect nation’s economy.

Introduction: Degradation of metal by chemical or electrochemical reaction with the environment is called corrosion. In other words, corrosion is defined as the destructive and unintentional attack of a metal; it is electrochemical and ordinary begins at the surface. The consequences of corrosion are all too common as example rusting of automotive body panels and radiator and exhaust components.

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Role of standard emf series:

The electromotive force(emf) series is generated by coupling to the standard hydrogen electrode, standard half cell for various metals and ranking them according to their voltage.

Bellow the table represents the corrosion tendencies for the several metal; those at the top such as gold and platinum are noble or chemically inert. All the substances appearing toward the bottom are good oxidizing agent or more reactive in nature. Sodium and potassium have highest re-activities.

The standard emf series

Forms of corrosion:

Corrosion of metal is classified according to the manner in which it is manifest, generally metallic corrosion classified in to eight forms.

  1. Uniform corrosion: It is a most common form of corrosion. As per name it is occur uniformly on the metallic surface. Or in the microscopic sense, the oxidation and reduction reaction occur randomly on the surface. It can be prevent by Use paints or metallic coatings such as plating, Corrosion inhibitors or modifying the environment.
  2. Galvanic corrosion: Occurs when two metals with different EMF are electrically connected, of which one gets corroded. Counter measures include: insulation; pairing metals with less difference between their EMF; altering the corrosive environment; tailoring the design of components with respect to their EMF.

For example, steel screw corrode when in contact with brass in a marine environment or if steel and copper tube are joined in a domestic water then steel will be corrode in vicinity of the junction.

For the prevention of galvanic corrosion; both metals should be similar corrosion potentials. Apply coating on both the metals. The coating on the cathode is the most important and must be in good condition, otherwise there will be galvanic corrosion.

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3. Crevice corrosion: This corrosion occurs as the result of concentration differences of ions or dissolved gases in electrolyte solution and between two regions of the same metals. 

Crevice corrosion may be prevented by using welded instead of riveted or bolted joints, using non absorbing gaskets when possible, removing accumulated deposits frequently, and designing containment vessels to avoid stagnant areas and ensure complete drainage.

(1). uniform corrosion, (2). Galvanic corrosion, (3). Crevice Corrosion, (4). Pitting

4. Pitting corrosion: Pitting is another type of localized corrosion attack in which small pits or holes form. These are formed from top of a horizontal surface downward in nearly vertical direction. Stainless steel are somewhat susceptible to this form of corrosion; however, alloying with about 2% molybdenum enhances their resistance significantly.

5. Intergranular corrosion: It occurs along the grain boundaries for some alloys and in specific environment. The net result about is that a macroscopic specimen disintegrates along its grain boundaries. Such type corrosion found in stainless steel. Stainless steel heated between 500 to 800˚C for sufficiently long time periods, it becomes sensitized to intergranular attack.

Stainless steel may be protected by this corrosion by some parameters; subjecting the sensitized material to a high temperature heat treatment in which all the chromium carbide particles are redissolved.

Lowering the carbon content below the 0.3% C so that carbide formation is minimal, alloying the stainless steel with another metal such as niobium or titanium which has greater tendency to form carbides than that chromium and because of this Cr remains in solid solution.

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6. Selective leaching: It is found in solid solution alloy and it happen when one element or constituent is preferentially removed as a consequence of corrosion. In other words, it occurs in selective metals. It is selective removal of a particular metal from the component. The most common example is dezincification of brass. Hence zinc is selectively leached from copper zinc brass alloy.

7. Erosion corrosion: This is the acceleration of corrosion due to mechanical actions. All metals and alloys are prone in this form of corrosion. More damaging in case of metals with passive scale. It can be reduce by change the components design, clearing the fluid for particulate.  

8. Stress corrosion: It is also known as stress corrosion cracking, form the combine action of an applied tensile stress and a corrosive environment; both influences are necessary. Counter measures: environments of change, cathodic protection, addition of inhibitors.

(5). Intergranular (6). Selective Leaching (7). Erosion corrosion (8). Stress Corrosion

Economical losses due to corrosion:

The effects of corrosion in our daily lives are both direct, in that corrosion affects the useful service life of our assets, and indirectly, the costs of corrosion to producers and suppliers of goods and services that they pass on to consumers. At home, rust is easily recognized on automobile body panels, charcoal grills, outdoor furniture, and metal tools. Preventive maintenance such as painting protects such items from corrosion.

The corrosion of steel reinforcing bar in concrete can proceed out of sight and suddenly result in failure of a section of highway, the collapse of electrical towers, and damage to buildings, parking structures, and bridges, etc.

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According to International Zinc Association (IZA) report (2021); India losses around 5-7% of its Gross Domestic Product (GDP) every year due to corrosion. The association also stressed upon the need for immediate and appropriate measures by authorities to control further damage.

The global cost of corrosion is then determined for each economic region by sector by summing the countries in the region. The global cost of corrosion is estimated to be US$2,505 billion, which is equivalent to 3.4% of the global GDP (2013). In addition, these costs typically do not include individual safety or environmental consequences.

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There are many direct and indirect economic consequences of corrosion, some of which are as follows:

  • Over design to allow for corrosion
  • Shutdown of equipment due to corrosion failure
  • Replacement of corroded equipment
  • Preventive maintenance, for example, painting

Methods to control corrosion:

These are some primary methods to use control corrosion;

  • Material selection
  • Coatings
  • Inhibitors
  • Cathodic protection
  • Design

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K Yashdeep

Hye there..! I am Yashdeep Kamal. I completed my Bachelor in Technology(Materials Science and Metallurgical Engineering). Engineer by passion, writer by choice. I have been writing about Composites, Ceramics, polymers, nanotechnology, advance materials and metallurgy etc. You can read about these topics here. Hope it may help you.

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