Corrosion Definition

Corrosion is one of the most common things we encounter daily. It's likely that you've noticed that some iron objects over time acquire an orange or reddish-brown coating on top of them. The chemical process that results in the formation of this layer is rusting, a type of corrosion.

Corrosion is a method that, in general, converts refined metals into more stable compounds such metal oxides, metal sulphides, or metal hydroxides. Similar to this, when iron rusts, air moisture and oxygen combine to generate iron oxides. If we examine the science of corrosion, we can conclude that it is an irreversible, spontaneous process in which the metals transform into considerably more stable chemical compounds, such as oxides, sulphides, hydroxides, etc. In this article, we'll go deeper into the idea of corrosion and examine its various aspects, including its definition, types, prevention, and more.

Corrosion definition

Corrosion can be summed up as a natural process that happens when pure metals react with elements like water or air to change into unwanted compounds. The metal is harmed and disintegrates as a result of this reaction, starting with the area of the metal that is exposed to the environment and progressing to the bulk of the metal as a whole.

Since it interferes with the beneficial qualities of the metal, corrosion is typically a phenomenon to be avoided. For instance, iron is recognised for its high stiffness and tensile strength (especially alloyed with a few other elements). However, iron objects that have been exposed to rusting become fragile, flaky, and structurally unsound. Corrosion, on the other hand, is a diffusion-controlled process that mostly affects exposed surfaces. So, in some instances, efforts are undertaken to decrease the activity of the exposed surface and raise the corrosion resistance of a material. It uses techniques like chromate conversion and passivation. But some rusting processes are both unanticipated and rarely visible.

Alternately, corrosion can be categorised as an electrochemical process because it often involves redox interactions between the metal and certain atmospheric agents such water, oxygen, sulphur dioxide, etc.

Do all metals corrode?

Higher reactive metals like iron, zinc, etc. corrode relatively quickly, whereas metals having lower reactivity like gold, platinum, and palladium do not corrode. The oxidation of metals during corrosion provides the explanation. There is virtually little tendency to become oxidised as the reactivity scale is descended (oxidation potentials is very low). Interestingly, while being reactive, aluminium doesn't corrode like other metals do. This is due to the fact that aluminium already has an oxide layer on it. It is shielded from additional corrosion by this coating of aluminium oxide.

Factors affecting corrosion

1. Metals that are exposed to gas-filled air, such as CO₂, SO₂, SO₃, etc.
2. Metals that have been exposed to water, especially saline water (which increases the rate of corrosion).
3. Salt and other contaminants are present (e.g. NaCl).
4. Temperature: Corrosion is accelerated by rising temperatures.
5. The type of initial oxide layer that is formed: Al₂O₃ forms an insoluble protective layer that can stop subsequent corrosion. Others, like rust, are simple to disintegrate and reveal the remaining metal.
6. The presence of acid in the atmosphere, which can quickly speed up corrosion.

Rate of corrosion

The creation of an oxide layer is frequently explained using the Deal-Grove model. In many different circumstances, this model aids in the prediction and management of oxide layer growth. The weight loss approach is an alternate way to quantify corrosion. In this method, a clean, weighed piece of metal or alloy is subjected to corrosive conditions for a set period of time. After that, a cleaning procedure removes the corrosion products. After then, the object is weighed once more to see how much weight has been shed.

The formula for calculating corrosion rate (R) is:

R= kW/ ρAt

Where,

k = constant,

W = weight loss of the metal in time t,

A = surface area of the metal exposed,

ρ = density of the metal (in g/cm³).

Types of Corrosion

1. Cervice Corrosion

Crevice corrosion, a localised type of corrosion that can happen whenever there is a difference in ionic concentration between any two local locations of a metal, can happen. For example, in restricted places, this type of corrosion predominates. Gaskets, washer undersides, and bolt heads are a few examples of places where crevice corrosion can happen. Crevice corrosion also affects stainless steels and aluminium alloys of all grades. This is mostly because a differential aeration cell has formed, which leads to corrosion developing within the fissures.

2. Stress Corrosion Cracking

It is the stress that causes deterioration. The term "cracking" (abbreviated as "SCC") refers to metal that has developed cracks as a result of being exposed to corrosive conditions and tensile strains. It frequently happens in hot environments. In chloride solution, for instance, austenitic stainless steel exhibits stress corrosion cracking.

3. Intergranular Corrosion

Intergranular corrosion is brought on by the presence of contaminants in the grain boundaries that separate the grains and are produced during the solidification of the metal alloy. It can also happen when the alloy is depleted or enriched at these grain boundaries. IGC has an impact on aluminium-base alloys, for instance.

4. Galvanic Corrosion

When two metals that have differing electrochemical properties and are in an electrolytic environment make electric contact with one another, galvanic corrosion may result. It explains the deterioration of one of these metals at a joint or junction. This type of corrosion is best demonstrated by the damage that occurs when steel and copper come into contact in a saltwater environment. Example: When carbon steel and aluminium are joined and submerged in seawater, steel is protected and aluminium corrodes more quickly.

5. Pitting Corrosion

Pitting corrosion is so unexpected, it is challenging to detect. It is thought to be one of the most dangerous types of corrosion. It begins locally and progresses to produce a corrosion cell that is encompassed by the normal metallic surface. Once this "Pit" has grown, it keeps growing and may take on several forms. The trench gradually penetrates metal from the surface in a vertical orientation, leading to structural failure in the end. Consider a water droplet on a steel surface as an example. At the centre of the water droplet, pitting will begin.

6. Uniform Corrosion

It is believed that this sort of corrosion, in which the environment assaults the metal's surface, is the most common. It is simple to determine the corrosion's extent. The performance of the material is only minimally impacted by this kind of corrosion. An object made of zinc and steel would typically disintegrate evenly and steadily when submerged in dilute sulfuric acid.

7. Hydrogen Grooving

This type of piping corrosion results in grooves that are created when a corrosive substance, corroded pipe components, and hydrogen gas bubbles interact. When the bubbles come into touch with the substance, they typically destroy the protective layer.

8. Metal Dusting

When sensitive materials are exposed to particular conditions with high carbon activity, such as synthesis gas, metal dusting, a destructive type of corrosion, develops. As a result of corrosion, bulk metal fragments become metal powder. Carbon monoxide (CO) in the vapour phase causes corrosion by depositing a graphite coating on the metals' surface. This graphite layer normally moves away from the surface of the metal after the production of meta-stable M₃C species (where M is the metal). No M₃C species may occasionally be seen. This indicates that the metal atoms have entered the graphite layer directly.

9. Microbial Corrosion

Microorganisms are the primary cause of microbial corrosion, sometimes referred to as microbiologically influenced corrosion (MIC). Most organisms are chemoautotrophs. In the presence or absence of oxygen, this corrosion can affect both metallic and non-metallic materials.

10. High-temperature Corrosion

As the name suggests, high-temperature corrosion is a type of heating-induced corrosion of materials, typically metals. Hot environments with gases like oxygen, sulphur, or other substances can cause metal to deteriorate chemically. These substances have the ability to quickly oxidise the materials (in this case, metals). For example, materials used in car engines must be able to endure prolonged exposure to high temperatures where they may be affected by an environment including corrosive combustion by-products.

Effects

There are many different things that corrosion can affect to differing degrees. As a result, it primarily results in resource waste. Additionally, it can lead to dangerous scenarios like weakening and unstable building structures, accidents from corroded parts, and other unintended failures like pipeline breaks, bridge collapses, transport vehicle disasters, or other calamities. Corrosion must thus be carefully monitored and avoided at all costs.