What is corrosion and how to prevent it?

The process of corrosion is among the most ubiquitous things we see on a regular basis. Eventually, a coating of orange or reddish-brown colour develops on the surface of iron objects. This coating develops due to corrosion in the form of a chemical reaction known as rusting. In this article, we will go over the different types of corrosion as well as the techniques that may be used to prevent it.

What is corrosion?

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Corrosion is the process by which a refined metal undergoes a natural transformation into a more stable form, such as the oxide, hydroxide, or sulphide state of the metal. This process results in the material’s degradation. Corrosion is a major concern that may be quite expensive to fix. Structures like bridges and buildings may give way, oil pipelines can burst, chemical facilities can spring leaks, and even the smallest of bathrooms can get inundated.

 

Corrosion: Causes

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Electrochemical reactions are the most prevalent cause of corrosion. The process of general corrosion takes place when the majority of atoms on a single metal surface get oxidised, which damages the whole surface. The bulk of metals is readily oxidised, which means that they have the propensity to give up electrons to oxygen (as well as to other compounds) when exposed to oxygen in the air or water. When oxygen is reduced, which results in it gaining electrons, it develops an oxide with the metallic surface.

Corrosion: Types

There are many different types of corrosion. These include:

Stress corrosion cracking

After inspecting your part with an eyelet, you detect several small, hardly visible cracks in the corners. That’s a classic symptom of stress corrosion, one of the worst kinds of corrosion. When a metal is subjected to tensile loads in addition to a corrosive medium, stress corrosion occurs. For each given metal, there exists a certain tensile range or restriction beyond which stress corrosion becomes more prone to occur.

Pitting corrosion

Whenever there is even the tiniest break in the metal, corrosion of this kind might begin to set in. After this process, the metal around the hollow becomes cathodic, while the hole or cavity becomes anodic. As a result, the metal suffers from a concentrated galvanic reaction and eventually breaks. Because of the presence of additional components formed during corrosion’s creation, pitting may be difficult to locate and manage.

Erosion corrosion

Erosion corrosion is the outcome of mechanical abrasion brought about by the mobility of metal surfaces coming into contact with corrosive fluids. The metal’s surface wears out, and cavities form as a result of the abrasion caused by the fast-moving fluids. This kind of corrosion is common in metal tubes used to transport fluids. The metal substrate has a distinctive directional pattern of swirls, troughs, gullies, and dips that sets it apart from other materials.

Crevice corrosion

This type of corrosion forms in fissures and other cracks on the metal’s surface. Anything that the metallic surface is “attached” to, whether it be soldered metal on metal or metal bound to non-metal items, is vulnerable to crevice corrosion. These crevices are often filled with impurities that expedite the corrosion rate. The presence of liquid water is the primary concern and serves as a catalyst for the chemical reaction that will eventually lead to the material’s breakdown and failure.

Selective leaching

When an alloy is exposed to a corrosive environment, selective corrosion occurs when one of the metals is de-alloyed. Zinc de-alloying causes this kind of corrosion in brass alloy pipes, which is common since zinc is often present as an extra component. In a similar fashion, nickel may be selectively eroded out of tubes made of the copper-nickel alloy under certain conditions.

Uniform corrosion

The metal will deteriorate and disintegrate over time due to corrosion, reducing its “thickness.” It will lead the metal to become brittle and prone to cracking. Carbon steel degrades uniformly throughout its whole surface due to this corrosion when it reacts chemically or electrochemically with its surroundings.

Intergranular corrosion

As the name implies, intergranular corrosion occurs between grains rather than inside them. It arises when the reactivity to pollutants varies noticeably across grain boundaries and grains. Poor welding and heat treatments of steel, copper, etc., may produce a noticeable variation in their reactivity.

Galvanic corrosion

When two dissimilar metals come in contact for an extended period of time, bimetallic corrosion develops. As a consequence of the disparity in electrode potency, bimetallic pair cells are generated. One metal acts as the anode, and the other serves as the cathode in a bimetallic pair cell. Increasing the separation between two metals in the galvanic series increases the potential influence on bimetallic corrosion.

 

What affects corrosion?

There are many factors that affect the rate of corrosion. Some of the major ones include:

 

Diffusion

In most cases, the rate of corrosion is influenced by the diffusion of reactants from and to the surface of the metal. For instance, freshly exposed steel surfaces corrode faster than surfaces covered by a thin layer of rust. The rate of corrosion is affected by the diffusion of oxygen to the metal surface through water.

Temperature

The rates of corrosion are also influenced by the temperature. This is because temperature heavily affects the rates of diffusion, which in turn, has an effect on corrosion rates. Metals corrode much faster at higher temperatures as compared to lower temperatures. 

Conductivity

Corrosion only occurs when there is a conductive medium between the corrosion reaction’s two parts. For instance, corrosion does not happen in distilled water but its speed increases as the presence of ions in the solution increases. This is because the presence of ions boosts the solution’s conductivity. 

Metal reactivity

The more reactive a metal is, the higher the rate and risk of corrosion. Pure metals are usually more reactive and thus more susceptible to corrosion as compared to metal alloys. 

 

Effect of temperature on corrosion: What is high-temperature corrosion?

As mentioned above, corrosion occurs at a much faster rate at higher temperatures than it does at lower temperatures. Also referred to as dry corrosion, high-temperature corrosion is a kind of corrosion that doesn’t require the presence of a liquid electrolyte. In this form of corrosion, a metal directly reacts with the atmosphere’s gaseous atoms instead of the ions in a solution. In high-temperature corrosion, the temperatures range over 650°C. 

 

Corrosion: Prevention techniques

Even while certain metals will be more prone to corrosion than others, there are still things you can do to slow down the rate at which corrosion occurs, including the following:

• Use sacrificial metals 

This is often placed as a protective layer that will wear away over time but will stop the corrosive substance, in this instance, water, from damaging the primary structure while it is still there.

• Zinc or Magnesium blocks 

The addition of such softer metal blocks has the effect of making your equipment’s alloy mostly irrelevant while the blocks are uniformly etched and attacked.

• Employ primer

The majority of newly manufactured apparatus will arrive with either a Zinc protective coating or a primer that is resistant to corrosion applied to all surfaces that are intended to be submerged while the apparatus is in operation.

• Impressed current

Equipment that uses impressed current is intended to generate a reverse current, which acts as a defence against the corroding current.

 

FAQs

Which kind of corrosion is the most severe?

Pitting corrosion is particularly dangerous because it is often difficult to foresee, detect, and describe. In pitting, a local anodic point or, more typically, a cathodic point forms a tiny depletion layer with the surrounding surface, resulting in localised corrosion.

What is the leading cause of corrosion?

The exposure of most metals to the elements is the primary factor that results in the corrosion of these materials. Therefore, if you store your metal goods outside where the air may get to them, they will rust. They will oxidise at a rapid rate due to the fact that they are subjected to the elements of the environment, such as moisture, wind, and precipitation.

How can we prevent corrosion?

Utilise metals that won't rust or corrode, such as aluminium or stainless steel. It is important to keep the surface of the metal clean and dry. Make use of drying agents. Make use of a covering or barrier substance like grease, oil, paint, or a carbon fibre coating, for example.

Does temperature have any effect on corrosion?

Due to the correlation between temperature and the progression of corrosion processes, it is important to safeguard metal commodities, equipment, and structures that operate at high temperatures.

What are the consequences of corrosion in daily life?

Corrosion is being caused all across the globe by pollution in the air, and the problem is becoming worse. One of the repercussions of corrosion is a rise in the danger of catastrophic equipment breakdown, as well as a major decline in the condition of heritage and natural monuments.

What characteristics make a substance resistant to corrosion?

The core metal of corrosion-resistant metals, notably stainless steel, is protected from oxygen by a very thin coating of chromium oxide, which is formed by these metals. This is significant because rust, also known as iron oxide, can only be created when iron is combined with oxygen.