Reviewed by Anurag Mishra (Sr. Technical Consultant)
Corrosion of metals refers to the degradation of metal caused by chemical changes or electrical reactions with its surroundings. It results in material loss or loss of built-in durability due to less stable compounds like oxides or hydroxides. It occurs naturally and can be accelerated by humidity, oxygen, and corrosive substances. Corrosion is a natural process that turns metal into a more stable form, like rust.
Corrosion slowly damages metals over time due to chemical or electrical reactions to their surroundings. It is not new; it is something we often see in everyday life. You might have noticed that some iron objects develop an orange or reddish-brown layer over time. This happens because of rusting, a chemical process called rusting, which is a type of corrosion.
Types of metal corrosion include uniform corrosion, galvanic corrosion, pitting corrosion, crevice corrosion, and stress corrosion cracking. Understanding these types helps in preventing, controlling, and mitigating material degradation. There are several types of corrosion, which will be discussed one by one in a detailed and descriptive manner:
Uniform corrosion is the most basic type of corrosion and is known as damage across the exterior of the material. It is also the mildest, as the severity of impact is commonly judged very easily, the resulting consequences are assessed easily and fairly due to the ability to continuously replicate and test the process.
Pitting is one of the most harmful types of corrosion, as it can be difficult to analyze, identify, and characterize. It is a restricted type of corrosion in which a small corrosion cell forms when either a local anodic spot or, more commonly, a cathodic spot interacts with the surrounding surface.
Crevice corrosion is a type of localized corrosion that happens in small, stagnant spaces. It occurs when there is a difference in ion concentration between two areas of a metal. It occurs in narrow spaces where access to the large environment is prohibited, frequently resulting in a rapid metal breakdown.
Stress corrosion cracking occurs when metal is exposed to both stress and a corrosive environment, especially at high temperatures. This stress can come from external forces, temperature fluctuations, or the manufacturing process.
Unlike regular corrosion, SCC does not damage the whole surface but rather creates small cracks that are hard to see. These cracks weaken the metal and can lead to sudden failure.
Galvanic corrosion happens when two different metals touch in a wet or salty environment, causing one metal to corrode faster. Factors like temperature and surface finish also affect corrosion. It is a large structure with different metals, like buildings or machines, and can suffer from this if not developed carefully.
Metals corrode when they react with substances such as oxygen, water, acids, and other chemicals. Corrosion can also occur when metals such as steel are kept under too much stress, causing the materials to crack.
One of the main causes of corrosion can be when metals begin to corrode when they react with substances such as oxygen, hydrogen, Moisture, or chemical agents. Corrosion can also be accelerated by exposure to electric currents, bacteria, or contaminants like dirt.
Additionally, certain metals like steel can undergo stress corrosion cracking when subjected to excess mechanical stress, leading to material failure. There are primarily three main causes of metal corrosion:
Environmental Factors
Environmental factors leading to metal corrosion include moisture, oxygen, temperature, pollutants, and soil components. Excess moisture speeds up oxidation, while acidic or saline environments improve electrochemical reactions.
Water Exposure
Water exposure is considered to be one of the biggest risks for metals. This is the reason the metals that are used in underwater applications like plumbing pipes and submarine hulls must exhibit exceptional resistance to corrosion due to their continuous exposure to corrosion-inducing factors.
Air Exposure
Air-based corrosion generally takes longer to occur compared to other types, but it remains a significant cause of metal deterioration. A metal surface corrodes more quickly in the presence of dirt and contaminants. While it is impossible to eliminate exposure to air, there are preventive measures that can be taken to mitigate this issue.
The rate of corrosion is the speed at which any specified metal degrades in a particular environment. The rate of corrosion depends on environmental factors and the metal’s type and condition. Corrosive rates are based on the number of microns it progresses each year. To determine the corrosion rate, the following data must be gathered:
The formula for calculating corrosion rate is CR=(K*Weight loss) /( Density*Surface Area*Time)
Here, K is the conversion factor, and CR is the corrosion rate and is expressed in units such as mpy (mils per year) or mmpy (millimeters per year).
Examples include Rusting of Iron, Tarnishing of silver, Green patina on copper, Corrosion of metal structures and food, and corrosion in marine environments. These examples are necessary to understand corrosion, prevent failures, ensure safety, and reduce maintenance costs. Here are some real-world examples of Corrosion in Metals:
Rusting of Iron is a common real-life example of metal corrosion, occurring when iron reacts with moisture and oxygen in the environment. This electrochemical process forms hydrated iron oxide, weakening the metal’s structure over time. It results in material degradation, reduced strength, and potential failure in buildings, vehicles, and infrastructure.
The tarnishing of silver is a common example of metal corrosion, occurring when silver reacts with sulfur compounds in the air, forming a blackish layer of silver sulfide. The chemical reaction reduces the metal’s luster and appeal. Tarnishing is assessed by humidity, pollution, and exposure to substances like rubber.
The green patina on copper is a real-life example of corrosion, occurring due to prolonged exposure to oxygen, moisture, and pollutants like sulfur dioxide. This reaction forms copper carbonate or copper sulfate, giving a characteristic greenish layer. While it protects the underlying metal, it also indicates surface oxidation over time.
The corrosion of metal structures, such as bridges, pipelines, and buildings, occurs due to prolonged exposure to moisture, oxygen, and pollutants. This weakens the structural integrity, leading to safety hazards, costly repairs, and potential failures. Protective coatings, cathodic protection, and material selection help mitigate corrosion and extend structural lifespan.
Corrosion in marine environments occurs due to constant exposure to saltwater, oxygen, and humidity, accelerating metal degradation. Ships, offshore platforms, and pipelines suffer from electrochemical reactions, leading to rust, pitting, and structural weakening. Protective coatings, cathodic protection, and corrosion-resistant alloys help mitigate damage and extend metal lifespan in harsh conditions.
Corrosion testing and monitoring are important for identifying and minimizing material degradation, guaranteeing asset integrity, and avoiding costly failures by assessing corrosion rates and the performance of control measures.
Corrosion Testing: Corrosion testing means assessing a material’s resistance to corrosion under a replicated or controlled environment, such as in a laboratory setting.
Corrosion Monitoring: Corrosion monitoring mainly concentrates on observing and evaluating the ideal corrosion behavior of materials in their working conditions, utilizing various techniques and tools.
Corrosion testing and corrosion monitoring are important because they prevent material degradation, ensuring safety, durability, and cost-effectiveness in various industries and environments.
Corrosion can cause structural deterioration, decreased performance, and safety hazards, leading to costly repairs, equipment failure, and environmental risks. Monitoring and testing are essential for early detection, ensuring durability, reliability, and operational efficiency.
Understanding corrosion rates and applying effective control strategies can significantly extend the lifespan of assets. By reducing maintenance costs, preventing unexpected failures, enhancing safety, and ensuring optimal performance in industries like construction, transportation, and manufacturing.
Proactive corrosion monitoring and testing aid in optimizing maintenance schedules, minimizing downtimes, and reducing repair costs. It also improves asset reliability, enhancing safety, preventing environmental hazards, and ensuring compliance with industry standards and regulations.
Corrosion can weaken structures and equipment, creating safety risks that can be reduced through effective monitoring and testing. It helps prevent failures, extend asset lifespan, reduce maintenance costs, ensure regulatory compliance, and enhance overall operational efficiency.
Preventing metal corrosion involves several methods, such as using coatings, inhibitors, cathodic protection, selecting appropriate materials, controlling environmental conditions, and performing regular maintenance. Here are some effective methods to prevent corrosion of metal:
By applying these methods, we can enhance the durability and performance of metal structures.
Corrosion is a natural electrochemical process that causes metal deterioration through reactions with moisture, air, or environmental substances. Some of the factors that affect corrosion are:
The reactivity of a metal plays a crucial role in its susceptibility to corrosion. More reactive metals tend to corrode faster. For example, alkali metals and alkaline earth metals are highly reactive and will corrode more readily than noble metals like gold and platinum.
The presence of electrolytes, such as salts in water, can significantly increase the rate of corrosion. Electrolytes facilitate the movement of ions, which enhances the electrochemical reactions that lead to corrosion. For instance, saltwater is known to accelerate the corrosion of metals compared to pure water.
Temperature is another critical factor affecting corrosion. As the temperature increases, the rate of corrosion typically increases as well. Higher temperatures can enhance the kinetic energy of the molecules involved in the corrosion process, leading to faster reactions.
The exposure of metals to corrosive gases, such as carbon dioxide, sulfur dioxide, and sulfur trioxide, can also encourage corrosion. These gases can react with moisture in the air to form acids, which can further corrode the metal surfaces.
Corrosion is the gradual deterioration of metals due to chemical reactions in the environment. It is important as it affects material durability, safety, economy, and industrial processes, necessitating prevention and management strategies.
Corrosion in everyday life includes various examples to maintain the durability and reliability of metals. Some examples are rusting of iron gates, tarnishing of silverware, corrosion of pipelines, green patina on copper, and battery leakage, all impacting durability, safety, and aesthetics.
The types of corrosion are included to classify degradation mechanisms systematically. The types involve uniform, galvanic, pitting, crevice, intergranular, and stress corrosion. They differ by appearance, causes, affected areas, and mechanisms, impacting material degradation in various environments and conditions.
Corrosion can be prevented using various methods to prevent hazardous risks.
Some of them are protective coatings, galvanization, cathodic protection, corrosion inhibitors, material selection, and environmental control, ensuring enhanced durability, safety, and cost-effectiveness in industrial and everyday applications.
In general, corrosion is detrimental and harmful but it also has advantages such as natural surface passivation and biodegradability of metals. It is also beneficial in controlling material removal in machining and aiding geological mineralization processes, contributing to environmental and industrial applications.
+91 9313 140 140 
info@testronixinstruments.com