Corrosion and Rancidity

Chemical reactions may be found everywhere around us, from our bodies’ food metabolism to how the light we receive from the sun is produced through chemical reactions. It’s crucial to understand physical and chemical changes before starting with chemical reactions. The best example of physical and chemical change is a burning candle. 

Take a candle and put it on the table. We can see how the candle turns to wax as time goes on. The candle will go out if you cover it with a jar. The burning of the candle is a chemical change, whereas the conversion of the candle to wax is a physical change in the demonstration. A physical change primarily results in a change in the state of the substance, whereas a chemical change primarily results in the formation of a new substance in which energy is either released or absorbed. As a result, we can deduce that chemical changes are followed by physical modifications.

Corrosion

One of the most typical phenomena we see in our daily lives is corrosion. You’ve probably seen that over time, some iron objects become covered in an orange or reddish-brown-coloured layer. This layer is formed as a result of a chemical reaction known as rusting, which is a type of corrosion.

Corrosion is the process by which refined metals are transformed into more stable compounds such as metal oxides, metal sulphides, and metal hydroxides. The development of iron oxides occurs as a result of the action of air moisture and oxygen on iron. Corrosion is commonly regarded as a bad phenomenon since it compromises the metal’s good characteristics. 

Iron, for example, is recognised for its tensile strength and stiffness (especially alloyed with a few other elements). Rusting, on the other hand, causes iron items to become brittle, flaky, and structurally unsound. Corrosion is an electrochemical process because it usually involves redox interactions between the metal and certain atmospheric agents including water, oxygen, and Sulphur dioxide, among others.

Do All Metals Corrode?

Metals with a greater reactivity series, such as iron and zinc, corrode quickly, whereas metals with a lower reactivity series, such as gold, platinum, and palladium, do not corrode. The reason for this is because corrosion requires the oxidation of metals. The tendency to oxidise decreases as we progress down the reactivity series (oxidation potentials is very low). Interestingly, although being reactive, aluminium does not corrode like other metals. This is due to the fact that aluminium is already covered with an oxide layer. It is protected from further corrosion by this layer of aluminium oxide.

Factors Affecting Corrosion

• Metals are exposed to gases such as CO₂, SO₂, and SO₃ in the air.
• Metals exposed to moisture, particularly saltwater (which increases the rate of corrosion).
• Impurities such as salt are present (e.g. NaCl).
• Temperature: As the temperature rises, so does the rate of corrosion.
• The nature of the first oxide layer that forms: some oxides, such as Al₂O₃, generate an insoluble protective coating that can prevent further corrosion. Rust, for example, crumbles readily and exposes the rest of the metal.
• Presence of acid in the atmosphere: acids have the ability to speed up the corrosion process.

Types of Corrosion

The following are the types of corrosion types:

1. Crevice Corrosion: A limited kind of corrosion known as crevice corrosion can occur whenever there is a difference in ionic concentration between any two local locations of a metal. Gaskets, the underside of washers, and bolt heads are all places where crevice corrosion can occur. Crevice corrosion occurs in all grades of aluminium alloys and stainless steels, for example.
2. Stress Corrosion Cracking: Corrosion Due to Stress SCC refers to the breaking of metal as a result of the corrosive environment and the tensile stress exerted on it. It happens a lot when the weather is hot. In a chloride solution, stress corrosion cracking of austenitic stainless steel is an example.
3. Intergranular Corrosion: The presence of contaminants in the grain boundaries that separate the grain generated during the solidification of the metal alloy causes intergranular corrosion. Depletion or enrichment of the alloy at these grain boundaries can also cause it. IGC, for example, has an impact on aluminum-base alloys.
4. Galvanic Corrosion: Galvanic corrosion can occur when an electric contact develops between two metals that are electrochemically different and are in an electrolytic environment. It describes the breakdown of one of these metals at a joint or junction. The degradation that occurs when copper comes into contact with steel in a saltwater environment is a good illustration of this form of corrosion. When aluminium and carbon steel are linked and submerged in seawater, the aluminium corrodes faster while the steel is protected.
5. Pitting Corrosion: Pitting Corrosion is unpredictably unpredictable, making it difficult to detect. It is regarded as one of the most hazardous forms of corrosion. It starts at a single location and progresses to the production of a corrosion cell encircled by the regular metallic surface. Once established, the ‘Pit’ continues to develop and can take on a variety of shapes. The pit progressively eats away at metal from the surface in a vertical direction, eventually leading to structural failure if not addressed. Consider a droplet of water on a steel surface; pitting will begin near the water droplet’s centre (anodic site).
6. Uniform Corrosion: This is the most prevalent type of corrosion, in which the environment attacks the metal’s surface. The degree of rusting can be seen clearly. This sort of corrosion has a minimal impact on the material’s performance. A piece of zinc or steel immersed in diluted sulphuric acid would normally dissolve at a constant rate throughout its whole surface.

Corrosion Examples and Reactions

Here are some common examples of corrosion, which are typically encountered in metals.

•  Copper Corrosion: When copper metal is exposed to the environment, it combines with oxygen in the air to produce copper (I) oxide, which is a reddish-brown substance.

2Cu + 1/2 O₂ → Cu2O

Cu₂O is oxidised further to generate CuO, which is black in colour.

Cu₂O+ 1/2O₂ → 2CuO

CuO interacts with CO₂, SO₃, and H₂O in the environment to produce Cu₂(OH)₂ (Malachite), a blue mineral, and Cu₄SO₄(OH)₆ (Brochantite), a green mineral. The colour of the copper plating on the Statue of Liberty, which has turned blue-green, is a good example of this.

• Silver Tarnishing: Silver combines with Sulphur in the air to form silver sulphide (Ag₂S), which is a dark substance. Exposed silver reacts with H₂S in the environment, which is present due to some industrial processes, to generate Ag₂S.

2Ag + H₂S → Ag₂S+ H⁺₂

• Corrosion of Iron (Rusting): When iron comes into touch with air or water, rusting occurs, which is the most typical occurrence. The reaction resembles that of a normal electrochemical cell.

Metal iron loses electrons and is converted to Fe²⁺ in this process (this could be considered as the anode position). The electrons that are lost will travel to the opposite side and interact with H+ ions. H+ ions are emitted in the atmosphere by either H₂O or H₂CO₃ (this could be considered as the cathode position).

H₂O ⇌ H⁺ + OH^–

H₂CO₃ ⇌ 2H⁺ + CO₃²

Prevention from Corrosion: Corrosion prevention is critical in order to avoid significant losses. Metals make up the majority of the structures we employ. Bridges, autos, machines, and home items such as window grills, doors, and railway lines are all examples. Electroplating, galvanization, painting and lubrication, and the use of corrosion inhibitors are just a few of the popular methods for preventing corrosion.

Rancidity

The term “rancidity” refers to the process of food containing fat and oil coming into contact with ambient oxygen and undergoing auto-oxidation, which results in a foul odour and a change in taste. 

Almost any meal has the potential to go rotten. The word is especially applicable to oils. Oils are especially vulnerable to rancidity due to their chemistry, which makes them vulnerable to oxygen attacks. 

• Metabolic interaction between fats and oxygen causes the oxidation of fats. Long-chain fatty acids are destroyed and short-chain molecules are generated during this process. 
• Butyric acid is one of the reaction products, and it is this acid that gives a rotten taste. 
• The degradation of fats, oils, and other lipids by hydrolysis, oxidation, or both is known as rancidification. 
• In glycerides, hydrolysis separates fatty acid chains from the glycerol backbone. These free fatty acids can subsequently be auto-oxidized further. 
• Unsaturated fats are oxidised largely through a free radical-mediated mechanism. In rancid foods and oils, these chemical processes can produce highly reactive molecules, which are responsible for unpleasant and toxic aromas and flavours. Food nutrients may be destroyed as a result of these chemical reactions. Vitamins in food can be destroyed by rancidity in some circumstances.

Types of Rancidity

There are two sorts of rancidity:

• Oxidative Rancidity: “Oxidative rancidity” refers to rancidity that occurs as a result of oxygen damage to foods. During the process, oxygen molecules interact with the oil’s natural structure, altering its odour, taste, and safety for consumption, i.e. fat is oxidised and decomposes into compounds with shorter carbon chains, such as fatty acids, aldehydes, and ketones, all of which are volatile and contribute to the rancid fat’s unpleasant odour. The development of both unpleasant and dangerous chemicals is caused by oxidative rancidity. There are three types of substances found in oxidised fat that have been proven to be toxic:
  • Fatty acids that have been peroxidized (peroxidized fatty acids destroy both vitamin A and E in foods).
  • Material made of polymers (under normal food processing conditions these appear in small enough quantities to be insignificant).
  • Sterols that have been oxidised (thought to be involved in the causation of atherosclerotic disease).
• Hydrolytic Rancidity: Enzymatic hydrolysis of fats results in the release of free fatty acids from glycerides, resulting in a rotten odour. This is known as hydrolytic rancidity. In glycerides, hydrolysis separates fatty acid chains from the glycerol backbone. Further auto-oxidation of these free fatty acids results in oxidative rancidity.

Factors Affecting Rancidity

• Oxidation: Because lipids are eight times more soluble in oxygen than water, the oxidation that results from this exposure is the primary source of rancidity. Unsaturated fats are oxidised largely through a free radical-mediated mechanism. In rancid foods and oils, these chemical processes can produce highly reactive molecules, which are responsible for unpleasant and toxic aromas and flavours. Auto-oxidation, often known as oxidative rancidity, is the name given to this process.
• Hydrolysis: Under the right conditions, triglycerides react with water to generate diglycerides and free fatty acid residues. Monoglycerides and fatty acids are formed when diglycerides mix with water. Finally, the monoglycerides hydrolyzed entirely, yielding glycerol and fatty acids. This is known as hydrolytic rancidity.
• Presence of Microorganisms – Microbial Lipase: Lipase is a hydrolytic enzyme produced by certain bacteria that directly interferes with the breakdown of triglycerides to create glycerols and fatty acids. These fatty acids become rancid due to auto-oxidation. For its activity on fats and oils, microbial lipase requires the right pH and other conditions.
• Presence of Unsaturation in Fatty Acid Chain: When unsaturated components of a fatty material are exposed to air, they are transformed into hydroperoxides, which then break down into volatile aldehydes, esters, alcohols, ketones, and hydrocarbons, some of which have unpleasant odours. The aforementioned process, as well as hydrolysis, which releases volatile and malodorous acids, mainly butyric acid, causes butter to get rancid. Saturated fats, such as beef tallow, resist oxidation and turn rancid at room temperature.
• Polyunsaturation: The higher a fat’s Polyunsaturation, the faster it will go rancid. Animal fats must become several times more rancid than vegetable oils. Oils and fats with polyunsaturated fatty acids are more prone to rancidity than monounsaturated and other forms of saturated fatty acids.
• Chemical Structure of Oils and Fats: Oils and fats that are chemically more complex and include a greater number of double bonds, carboxyl or hydroxyl groups have a greater likelihood of becoming rancid. Auto-oxidation is aided by the double bonds found in fats and oils. Auto-oxidation is particularly common in oils with a high degree of unsaturation. Determining the peroxide value is the best test for auto-oxidation (oxidative rancidity). In the auto-oxidation reaction, peroxides are intermediates. The peroxide value of an oil or fat is used to determine how far rancidity reactions have progressed during storage.
• Temperature and pH: These are the major factors that cause fat- and oil-rich foods to go rancid. The hydrolytic action of microbial lipase requires a specific temperature and alkaline pH. The auto-oxidation and hydrolysis are influenced by temperature and pH in an indirect manner.
• Heat and Light: Heat and light speed up the rate at which lipids react with oxygen, i.e. heat speeds up auto-oxidation. The formation of free radicals in the rancidity and reversion of oils and fats is fueled by heat and light.

Prevention from Rancidity

Rancidity can be avoided in a number of methods:

• Addition of Antioxidants: Antioxidants are the most effective way to keep food from becoming rancid. Antioxidants are added to fat-containing foods to prevent rancidity from forming due to oxidation. There are five types of antioxidants:
  • Natural antioxidants- Flavonoids, polyphenols, ascorbic acid (vitamin C), and tocopherols are all-natural antioxidants (vitamin E). 
  • Synthetic antioxidants- Butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl-3, 4, 5-trihydroxybenzoate (also known as propyl gallate), and ethoxyquin are examples of synthetic antioxidants. 
  • Semi-synthetic antioxidants – gallic acid, propyl gallate.
  • Metal chelators – citric acid, phos­phoric acid.
  • Oxygen scavengers – ascorbic acid.

Natural antioxidants have a short shelf life, whereas synthetic antioxidants have a longer shelf life and perform better. Water-soluble antioxidants are ineffective in stopping direct oxidation within fats, but they are useful in intercepting free radicals that pass via food’s watery portions. A mix of water-soluble and fat-soluble antioxidants, usually in a fat-to-water ratio.

• Addition of Sequestering Agents: Metals are bound by sequestering agents, which prevent them from initiating auto-oxidation. EDTA (ethylene diamine tetraacetic acid) and citric acid are examples of sequestering agents.
• Proper Storage of Fats and Oil Food: Another strategy to prevent food from becoming rancid is to store it properly, away from the effects of oxygen. Because heat and light accelerate the rate of reactivity of lipids with oxygen, rancidification can be reduced by storing fats and oils in a cold, dark environment with little exposure to oxygen or free radicals. Do not add new oil to vessels that already have old oil in them. The old oil will cause a reaction, causing the new oil to get rancid faster than if it were stored in a clean, empty vessel. Allow tanks to drain and dry thoroughly before use, as this will speed up the problems related to oxidation.

Sample Questions

Question 1: What is rancidity?

Answer:

The term “rancidity” refers to the process of food containing fat and oil coming into contact with ambient oxygen and undergoing auto-oxidation, which results in a foul odour and a change in taste. Almost any meal has the potential to go rotten. The word is especially applicable to oils. Oils are especially vulnerable to rancidity due to their chemistry, which makes them vulnerable to oxygen attack. A metabolic interaction between fats and oxygen causes oxidation of fats. 

Long-chain fatty acids are destroyed and short-chain molecules are generated during this process. Butyric acid is one of the reaction products, and it is this acid that gives the rotten taste. The degradation of fats, oils, and other lipids by hydrolysis, oxidation, or both is known as rancidification. 

In glycerides, hydrolysis separates fatty acid chains from the glycerol backbone. These free fatty acids can subsequently be auto-oxidized further. Unsaturated fats are oxidised largely through a free radical-mediated mechanism. In rancid foods and oils, these chemical processes can produce highly reactive molecules, which are responsible for the unpleasant and toxic aromas and flavours. Food nutrients may be destroyed as a result of these chemical reactions. Vitamins in food can be destroyed by rancidity in some circumstances.

Question 2: What do you understand by Corrosion?

Answer:

Corrosion is the process by which refined metals are transformed into more stable compounds such metal oxides, metal sulphides, and metal hydroxides. 

The development of iron oxides occurs as a result of the action of air moisture and oxygen on iron. Corrosion is commonly regarded as a bad phenomenon since it compromises the metal’s good characteristics. Iron, for example, is recognised for its tensile strength and stiffness (especially alloyed with a few other elements). 

Rusting, on the other hand, causes iron items to become brittle, flaky, and structurally unsound. Corrosion is an electrochemical process because it usually involves redox interactions between the metal and certain atmospheric agents including water, oxygen, and Sulphur dioxide, among others.

Question 3: Do all metals corrode?

Answer:

Metals with a greater reactivity series, such as iron and zinc, corrode quickly, whereas metals with a lower reactivity series, such as gold, platinum, and palladium, do not corrode. The reason for this is because corrosion requires the oxidation of metals. The tendency to oxidise decreases as we progress down the reactivity series (oxidation potentials is very low). Interestingly, although being reactive, aluminium does not corrode like other metals. This is due to the fact that aluminium is already covered with an oxide layer. It is protected from further corrosion by this layer of aluminium oxide.

Question 4: What are the factors that affect corrosion?

Answer

The factors that affect corrosion are as follow:

• Metals are exposed to gases such as CO₂, SO₂, and SO₃ in the air.
• Metals exposed to moisture, particularly salt water (which increases the rate of corrosion).
• Impurities such as salt are present (eg. NaCl).
• Temperature: As the temperature rises, so does the rate of corrosion.
• The nature of the first oxide layer that forms: some oxides, such as Al2O3, generate an insoluble protective coating that can prevent further corrosion. Rust, for example, crumbles readily and exposes the rest of the metal.
• Presence of acid in the atmosphere: acids have the ability to speed up the corrosion process.

Question 5: How do you prevent metals from getting corrode?

Answer:

Corrosion prevention is critical in order to avoid significant losses. Metals make up the majority of the structures we employ. Bridges, autos, machines, and home items such as window grills, doors, and railway lines are all examples. Electroplating, galvanization, painting and lubrication, and the use of corrosion inhibitors are just a few of the popular methods for preventing corrosion.

Question 6: What will happen when copper metal is exposed to the environment?

Answer:

When copper metal is exposed to the environment, it combines with oxygen in the air to produce copper (I) oxide, which is a reddish-brown substance.

2Cu + 1/2 O₂ → Cu2O

Cu₂O is oxidised further to generate CuO, which is black in colour.

Cu₂O+ 1/2O₂ → 2CuO