Corrosion deteriorates materials due to chemical interactions as surface atoms readily oxidize in certain conditions. Sometimes, it protects the materials but results in discoloration, reduced structural integrity, and decreased life span. Therefore, this post will help you understand different corrosion types and their effect on various materials faster.
Why do materials corrode?
Corrosion is a natural process that converts pure metal into undesirable substances when it reacts with water or air. Metals corrode naturally, but conditions they are subjected to, such as environmental or chemical ones, can speed up the process. Some stable metals, like platinum and gold, are less likely to rust easily because of their unique chemical nature.
Can most victims of corrosion be metalheads?
Generally, metals corrode quickly due to their unstable nature and oxide under different environmental conditions. However, metals placed in lower reactivity areas, like gold and platinum, do not corrode, and those placed in higher reactivity areas, such as zinc and iron, corrode easily. Interestingly, aluminum prevents corrosion by forming a layer of aluminum oxide, which protects it from oxidation.
Non-metals corrode, but corrosion is not termed because they have strong ionic or covalent bonds and few free atoms. However, Non-metal materials such as plastics materials, ceramic materials, and polymers can rust when exposed to sunlight.
Factors Affecting the Corrosion
Temperature: An increase in temperature significantly increases the corrosion rate in industries like automotive, aerospace, refining, airport and petrochemical.
Moisture and Humidity: When metals are exposed to areas that contain high amounts of moisture, they develop different types of rust. It happens because the moisture content increases the rate of oxidation and corrosion. Therefore, dry and non-humid conditions are ideal for metals to prevent corrosion and rust.
Reactive Metals: Highly reactive metals have a higher rate of corrosion. Therefore, materials with higher electrode potential have the lowest corrosion rate than materials with lower electrode potential.
Presence of Impurities: The rate of corrosion increases with the presence of impurities such as salt.
Nature of the First Layer: Some materials, like aluminum oxide, form a protective layer that prevents them from corrosion. Other metals easily rust and expose the rest of the material.
Pollutants in the atmosphere: Metals near the sea and industries corrode faster because of atmospheric pollutants like carbon dioxide, HCl, and sulfur dioxide. Moreover, the presence of sea salt in the coastal areas increases the liquid presence on the surface of metals.
9 Different Types of Corrosion Affecting Materials
Galvanic Corrosion
Bimetallic or galvanic corrosion occurs when two different types of materials with dissimilar electrical potential contact in an electrolytic environment. A galvanic couple forms between these metals, where one becomes the cathode and the other anode. The active anode undergoes corrosion faster than the more stable cathode.
Galvanic corrosion is more common in the sea, where saltwater is an electrolyte between aluminum and steel sheets. When the difference between the two metals increases in the presence of electrolytes, the passive layer begins to break down.
In addition, Galvanic corrosion canoften be seen in the battery terminals, the battery terminals arecoroded will affect the service lifeof the battery. lt should be cleanedand prevented in time.
A sacrificial or galvanic anode that is more active than the metals is used to prevent or lessen corrosion. It protects the metals from oxidation by providing their free electrons for corrosion. Additionally, choose materials with a voltage difference of 0.2V to prevent galvanic corrosion.
Pitting Corrosion
Pitting corrosion is an unpredictable localized form of the most destructive corrosion that creates holes in the metal. These holes are small or large in diameter and sometimes isolated or so close together that they look like a rough surface. It results from de-passivation or removal of the surface from a small area. This area becomes anodic, while the other part remains cathodic.
Structural defects, moisture, non-uniformities, and damage to the protective layer of metal cause this type of corrosion. It’s considered more dangerous because only a tiny amount of material is lost while the whole metal structure is damaged. Pitting corrosion is observed in steel, aluminum, and nickel alloys. So, it’s necessary to use polished metal because uniform metal surfaces resist pitting corrosion.
High-Temperature Corrosion
The corrosion causes metal deterioration under extreme temperatures (above 400⁰C /750⁰F) that contain gases like oxygen, sulfur, or other compounds. High-temperature corrosion is common in industries such as gas turbines and furnaces. The compounds are very corrosive toward metal alloys resistant to high temperatures.
Atmospheric contaminants may leave molten salts and ash deposits, enhancing corrosion. Therefore, heat-resistant alloys and the cooling mechanism are essential to prevent high-temperature corrosion.
Intergranular Corrosion
To understand intergranular corrosion, consider all metals consisting of grains, and atoms are systematically arranged within each grain. The corrosion affects the grain boundaries because this area is more reactive than the matrix. It’s caused by impurities at the grain boundaries and enrichment or depletion of one of the alloying elements in the boundary area.
Environmental Cracking
Environmental cracking or corrosion results from diverse environmental conditions like chemical, stress-related, and temperature. The following types of corrosion deteriorate materials:
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Stress Corrosion Cracking
Stress corrosion is hazardous among different types of corrosion because it causes the material to fail without warning. It occurs when tensile strength is applied to the material at extreme temperatures in a corrosive environment. Metal expansion and contraction from these temperatures weaken the structural integrity of the metal.
First, fine cracks appear at the surface, which develop over time, causing structural failure. This type of corrosion can be seen in stainless steel when stressed in chloride environments.
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Fatigue Corrosion
Like stress corrosion, fatigue corrosion prematurely fractures different materials under a corrosive environment. It ruptures the passive protective layer and accelerates the corrosion process. Fatigue stress can be controlled by lowering or eliminating cyclic stresses and avoiding vibration transmission design.
Uniform Corrosion
General or uniform corrosion occurs across the metal surface, and the absence of a protective layer significantly causes it. The chemical or electrochemical reaction happens uniformly over the entire surface, making the metal thin and weak. Moreover, the extent of corrosion is easily visible and has a low impact on the metal’s performance.
For instance, metals such as aluminum, zinc, iron, steel, and lead corrode uniformly with continuous exposure to corroding agents.
Microbial Corrosion
Often referred to as microbiologically influenced corrosion (MIC), microbial corrosion is caused by microorganisms such as chemoautotrophs. These microorganisms, including bacteria, algae, and fungi, can cause corrosion to metals and non-metals in various environments.
Because of its rapid destruction, MIC is a rising concern for marines, oil and gas industries, and wastewater management. Some organisms can consume oil and release toxic acids that cause vessel corrosion. Therefore, oil must be purified enough to remove water content to control microbial growth.
Erosion Corrosion
Erosion corrosion results from mechanical abrasion caused by the relative movement between the corrosive fluid and metal surfaces. In this case, the metal surface gradually deteriorates due to the abrasion of fast-moving fluids.
Corrosion usually exhibits a directional pattern and occurs inside the metal tubes that transfer liquids. Once the pit is formed, it can take several shapes and slowly penetrate the metal from inside in a vertical direction.
Fretting Corrosion
Fretting corrosion occurs when two joined metals undergo small movements caused by vibrations and slips. The protective surface is removed during the oscillations, and freshly exposed metals stick firmly. This corrosion can be seen in clamped surfaces, bolted and riveted joints, etc.
Coating, lubrication, and another protective layer are necessary to protect metals from fretting corrosion. Moreover, to avoid fretting issues, using metals with different softness, such as a pair of hard and soft metals, is necessary.
Corrosion resistant materials
Corrosion changes materials’ natural properties and structural integrity and reduces their average life. Corrosion-resistant materials are designed to prevent or minimize this process through chemical composition or a protective outer layer. These materials are used when longevity, durability, and safety are crucial, especially in infrastructure and construction.
Choosing the best corrosion-resistant material depends on the specific application and type of corrosion to which the material will be exposed. However, some of the commonly used corrosion-resistant materials are:
Stainless Steel: Known for flexibility and high strength, stainless steel is a common choice for its high resistance to oxidation and corrosion. The different grades of stainless steel have various compositions, each providing a unique resistance level.
Aluminum is a non-toxic, lightweight, and strong metal that, due to its oxide layer, provides natural resistance to corrosion.
Copper, Brass, and Bronze: These metals ensure corrosion resistance throughout a component’s life cycle. Copper is more resistant to corrosion in environments with low acidity.
Galvanized Steel: This type of steel has been coated with a thin layer of zinc to protect against corrosion. The zinc is a barrier against water and oxygen from reaching the steel. The zinc protects the underlying areas through cathodic conservation even if the protective layer is scratched.
Ceramics Coatings: The coatings are thin layers of ceramic materials that can be applied to various metal surfaces to protect them from corrosion.
Titanium: Titanium is more robust and lightweight than steel and has a higher tensile strength. When titanium interacts with corrosive compounds and hot temperatures, it forms a protective titanium oxide layer. This layer protects from rusting and is extensively used in aerospace, chemical, and marine industries.
Advanced Ceramics
These metals and alloys often succumb to corrosion in unstable conditions and aggressive environments. Advanced ceramics like alumina and silicon carbide stand out from these materials’ unique resistance properties. The ceramics are immune to chemical reactions and adverse environmental conditions, ensuring durability and longevity.
Advanced ceramics are crucial in chemical processing, aerospace, and semiconductor manufacturing. These ceramics are tailored to meet specific design requirements and show superior resistance to temperature and corrosion.
Learn more about advanced ceramics
Conclusion
Different materials corrode differently, but corrosion occurs when placed in unfavorable chemical or environmental conditions. Metals are more prone to corrosion when exposed to corrosive compounds or an oxygenated environment. Therefore, corrosion-resistant materials or a protective layer is essential to minimize the risks of corrosion or rusting.