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Corrosion Inhibitor

All know elements can be classified as metals, non-metals and metalloids. In the periodic table, the elements at the left side are metals whereas non-metals are placed at the right side of periodic table. Metalloids show intermediate properties and are placed between metal and non-metals as zig-zag line. 

Periodic Table

Metals are electropositive elements which have 1, 2, 3 valence electrons that can be lose to form metal ions (positive charge). Because of the presence of free mobile electrons and positively charged metal ions, they are good conductor of heat and electricity. The attraction force between free mobile electrons and metal ions is called as metallic bond. Due to metallic bonds, metals are hard, malleable and ductile. They usually exist in solid state whereas non-metals can exist in solid, liquid and gaseous state. Non-metals have tendency to accept electrons to form anions. They are electronegative elements with no conducting ability. Metalloids show intermediate properties of metal and non-metals. Overall we can conclude that metals are more reactive than other elements. We are using many object made from metals in our everyday life. 

Copper is a most common metal which is used in different fields of our life like in manufacturing of utensils, electric wires, water pipes etc.

Copper Wire

Other metal like aluminium is suitable for the bodies of planes, gold in electrical connections on circuit boards and in jewellery. Steel is an alloy of different metals in fixed proportion suitable for building material and for utensil manufacturing. Have you seen the rusted screws in pipes, rusted ship or iron nails? Why don’t they have metallic shine? What is the red coating on them? 

It is called as corrosion. It can be defined as the deterioration of a metal due to chemical reactions between metal and environment. It is mainly the reaction between metal ions and oxygen gas in the presence of atmosphere moisture. Type and reactivity of metal determine the form and rate of deterioration. Almost all metals corrode less or more. Metals like pure iron corrode quickly whereas stainless steel which is an alloy of iron with other metals corrodes very slowly. Noble Metals such as gold, palladium, platinum are much less reactive than others.

 

Corrosion Inhibitor Definition

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Different metals corrode at different rate and for different types of compounds which coated the metal surface. The rate of corrosion also depends on environmental conditions such as moisture, aerobic and non-aerobic conditions, presence of other metal and chemical compounds etc. On this basis, corrosion can be classified in different types. 

Uniform attack corrosion:
It is the most common type of corrosion which is caused by chemicals. It is an electrochemical reaction that results the deterioration of the metal surface which is exposed and deteriorates to the point of failure. It is predictable, manageable and preventable form of corrosion. 
Localized corrosion:
 It is a localized corrosion which targets at one specific area of the metal structure. It can be in the form of pitting, crevices or filiform corrosion. Pitting forms a small hole in the metal. This small hole acts as anode and remaining metal surface acts cathode. Overall it produces a localized galvanic reaction and penetrates the metal.  Crevice corrosion occurs at a specific location. Filiform corrosion occurs under painted surfaces or can also begin at small defects in the coating.
Galvanic corrosion:
When two different metals are located together in a corrosive electrolyte, galvanic corrosion occurs. Here one metal acts as the anode and the other as cathode. Anode is a sacrificial metal here and corrodes faster than cathode.
Environmental cracking:
Different environmental conditions like stress corrosion cracking, corrosion fatigue, hydrogen-induced cracking and liquid metal embrittlement are some examples of environmental cracking which occur due to environmental conditions of change of temperature, presence of chemicals etc.
Flow-accelerated corrosion:
When a protective layer of oxide on a metal surface is removed by some external effects such as water, the exposed metal surface corrodes further and deteriorates.
Inter-granular corrosion: 
Inter-granular corrosion is an electrochemical attack on the grain boundaries of metal surface which occurs due to presence of impurities. Impurities tend to be present in higher contents near grain boundaries therefore they can be more corrosion than the bulk.
Selective leaching corrosion:
It is a selective corrosion of a specific element in an alloy such as de-zincification of un-stabilized brass. It forms a deteriorated and porous copper.
Fretting corrosion:
It occurs due to repeated wearing or vibration on a rough surface. It forms pits and grooves on the surface. It is one of the common corrosion in rotation machinery, bolted assemblies and on the surfaces exposed to vibration.
High-temperature corrosion:
Gas turbines contain vanadium or sulfates which form corrosive compounds have low melting point during combustion. It can cause by high-temperature oxidization and carbonization. Overall corrosion deteriorates the metal surface and causes the damage of metal surface. The chemicals which react with a metallic surface and provide a level of protection are called as corrosion inhibitors. They adsorb on metallic surface and form a protective film which prevents the interaction between atmospheric oxygen and metal surface. They distribute on metal surface in the form of solution or dispersion and slow down the corrosion processes by either increasing the anodic or cathodic polarization or by reducing the movement of ions to the metallic surface. Some of the corrosions inhibitors can also increase the electrical resistance of the metallic surface.

Corrosion Inhibitors Examples

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Today metals are used in different forms in various field so metal corrosion is definitely now accepted specially at industrial level. This was mother of invention of different techniques which can manage, slowed or stopped the deterioration of metal surface.

Some common corrosion prevention techniques are listed below.
  • Environmental Modifications
  • Metal Selection and Surface Conditions
  • Cathodic Protection
  • Corrosion Inhibitors
  • Coating and plating
Environmental modification can inhibit the interaction between metal and atmospheric gases. It can immediately reduce the rate of corrosion. It includes the limiting contact with rain or direct manipulation of the environmental and also reduces the S, Cl and O content in the environment. Metal selection and surface conditions are another method of corrosion inhibition which involves monitoring the environmental conditions. Production of new alloys can also prevent the corrosion. Cracks and crevices are more susceptible to corrosion so monitoring of surface conditions also protects the deterioration of metal surface. Cathodic protection is one of the best ways of metal protection.  

Galvanic corrosion occurs when two different metals are situated together in an electrolyte.  When two different or same metals in close proximity submerged in seawater or in moist soils, it forms a galvanic cell. It converts the unwanted anodic sites on the surface of metal to cathodic sites that protects the metal deterioration.

Corrosion inhibitors chemical compounds react with the metal surface that prevents corrosion by interrupting the reaction of corrosion. Some examples of corrosion inhibitors are amines, urea, heterocyclic nitrogen compounds, Mercaptobenzothiazole (MBT), ascorbic acid, succinic acid, tryptamine, caffeine, benzotriazole, toliotriazol, aldehydes, sulfur-containing compounds, acetylenic and caffeine.

Volatile Corrosion Inhibitors

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The chemical compounds which are transported in a closed environment to the site of corrosion by volatilization from given source are called as Volatile Corrosion Inhibitors (VCI) or Vapor Phase Inhibitors (VPI). Morpholine and hydrazine are examples of Volatile Corrosion Inhibitors which are volatile basic compounds and are transferred in boilers with steam to prevent corrosion in the condenser tubes.

These basic compounds neutralize acidic compounds like carbon dioxide that shifts the surface pH and reduces the corrosive values. Other examples of Volatile corrosion inhibitors are salts of dicyclo-hexylamine, cyclohexylamine and hexamethyleneamine which are used in shipping containers. These volatile inhibitors forms vapor when come in contact with metal surface. The reaction of vapor with metal surface results hydrolysis by any moisture and liberates protective ions. 

Corrosion Inhibitor Mechanism

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The degradation of a metal surface due to a reaction with its environment is called as corrosion. Overall it is a great lose of metal surface. Since metal is one of the most important materials used in our everyday life therefore protection of metallic substances is essential. Today we know several methods to protect the corrosion.  The inhibition of environmental conditions of corrosion can prevent the corrosion. 

Corrosion Inhibitor Mechanism

Usually corrosion inhibitors act in aqueous or partly aqueous systems.  The corrosion inhibitors adsorb on surface of metal and act as barrier for the reaction of oxygen with metal surface. It inhibits the interaction between atmospheric oxygen and metal surface that reduces the rate of corrosion.

Corrosion Inhibitor Oil and Gas

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In gas and oil production, corrosion control is a complex issue as injection and multiphase solutions are jeopardizing the stability of surface pipes and equipment. To control such corrosion, Different corrosion inhibitors are designed which can retard the effects of corrosive acid gases such as $H_{2}S$ and $CO_{2}$.  These corrosion inhibitors reduce the corrosion in flow lines, pipelines and down-hole equipment. They also minimize the unexpected failures of lost production that definitely increased costs. The water extracted from oil wells contains different minerals and can directly react with metal surface. 

The presence of corrosion inhibitors can prevent the interaction between water and metal surface of pipelines from which the water is extracted from oil field. Usually water soluble corrosion inhibitors are used for this purpose.
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