In this article, you will know the Does stainless steel rust? As well as when, how, and why rust appears on stainless steel and how to avoid rusting in stainless steel.
Stainless steel is a type of steel that is known for its corrosion resistance.
While making stainless steel, chromium, molybdenum, nickel, silicon, copper, etc are added to it but the amount of chromium in stainless steel is high.
Due to this a transparent layer gets deposited on the stainless steel and it becomes rust-proof.
But it can rust in under certain conditions which will be discussed further in detail.
So without wasting time let's get started.
Does Stainless Steel Rust?
Stainless steel can contain other elements such as nickel and manganese, but chromium is the key element that makes it rust-resistant.
As long as there is sufficient chromium present, the chromium oxide layer will continue to protect the stainless steel and prevent it from rusting.
Why Stainless Steel Doesn't Rust?
Chromium is the main element that protects steel from corrosion.
Will explain how chromium makes stainless steel, stainless.
In 1908, A German researcher, Philip Monnartz studied the effect of carbon content on the corrosion resistance of high-chromium steels.
His research showed that the passive layer of chromium oxide is responsible for this great increase in corrosion resistance.
He mentioned that at least 12% chromium by weight is required in a steel alloy to be stainless steel.
Till now you have understood that chromium forms a protective oxide layer that protects the steel from corrosion. But how this passive layer is formed.
This passive oxide layer forms naturally when stainless steel is exposed to air. However, this chromium oxide layer formed in this way may not be uniform. This is because of the presence of free ions and impurities on the free surface of stainless steel.
So a chemical treatment, known as passivation is carried out to make a uniform passive chromium oxide layer.
Now you may be thinking, what is passivation?
Passivation is a chemical process, which removes free iron from the surface of a stainless steel component, and at the same time promotes the formation of a thin, chromium oxide passive layer.
The chromium oxide passive layer acts as a protective barrier against a corrosive environment.
This passive film on stainless steels is composed of an outer iron-rich oxide, mainly Fe203, and iron hydroxide like Fe(OH)2 and Fe(OH)3, and an inner chromium-rich oxide, often stated as Cr203 and Cr(OH)3, with some molybdenum as well as silicon species.
The presence of chromium increases the stability of the oxide layer since it has a higher affinity for oxygen than does iron.
Chromium also forms a variety of chromium carbides in steels. These carbides are very hard and improve wear resistance to steel. Thus, in addition to corrosion resistance chromium also improves the wear resistance of stainless steel.
When, How, and Why Rust Appears on Stainless Steel?
When rust appears on the surface of stainless steel products, people are surprised; they think that stainless steel does not rust and there might be some problem with the quality of the product.
When the stainless steel is kept in contact with air and water for a maximum time, it gets rusted.
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Chemical Composition, impurities, internal stresses, manufacturing process, non-uniform environment are factors to increase the rate of corrosion.
The surface film of stainless steel products is damaged in many forms, with the common situation in production and usage in daily life being as follows:
- The processes of stamping, stretching, cutting, grinding, drawing, etc destroy the passivation film on the surface of stainless steel.
- Organic liquids such as vegetables, noodle soup, sputum, etc adhere to stainless steel surfaces and react in the presence of water and oxygen to produce organic acid corrosion on the metal surface persistently.
- Substances such as acids, alkalis, and salts cause local corrosion.
- Substances in the polluted air such as the atmosphere containing a lot of sulfides, oxidized carbon, and nitrogen oxide react in the presence of condensed water leading to the formation of sulfuric acid, nitric acid, and acetic acid, etc causing corrosion.
- If stainless steel is not cleaned properly, dirt and grime can accumulate on the surface, which can lead to rust.
- The specific type of alloy used in the stainless steel can greatly affect its rust resistance as 304 stainless steel, is highly resistant to rust, while others, 410 stainless steel, are more susceptible to corrosion.
Types of Stainless Steel Corrosion
- Uniform Corrosion
- Pitting Corrosion
- Crevice Corrosion
- Intergranular Corrosion
- Stress Corrosion
- Galvanic Corrosion
Uniform Corrosion
It is a very common and benign type of corrosion.
Their rate of corrosion is relatively judged and the impact of corrosion can be easily evaluated.
Stainless steel is generally resistant to uniform corrosion in a specific environment if the corrosion rate does not exceed 0.1 mm/year.
It occurs when the passive oxide layer is destroyed. Passive layer is destroyed due to acids and hot alkaline solutions.
Hydrochloric and sulfuric acids are more aggressive than stainless steel.
This corrosion is also called general corrosion.
Pitting Corrosion
Pitting corrosion is hard to detect, predict, and identify.
It is a very localized type of corrosion if the passive oxide layer is damaged locally. Pitting corrosion can initiate from that location.
This corrosion is difficult to identify easily and It generally occurs when stainless steel is exposed to environments rich in chlorides.
Crevice Corrosion
As the name suggests this type of corrosion occurs in confined spaces.
It occurs under the washer, nut bolt joints, gaskets, fastener thread, etc.
The oxygen (O2) supply in the confined space is limited and restricted and this prevents the oxygen (O2) from reacting with the chromium to form a chromium oxide layer on the surface of the stainless steel which facilitates corrosion attacks.
Intergranular Corrosion
It is caused by impurities at grain boundaries, formed during the solidification of alloy.
It is a very rare form of corrosion it occurs when the CO2 level is comparatively high, CO2 reacts with Cr and generates chromium carbide at temperatures around 450 to 850°C.
This process is also called sensitization and typically occurs during welding.
Stress Corrosion
It is generated when metal is exposed to a specific corrosive environment under tensile stress at elevated temperatures.
It occurs because of external stresses such as expansion and contraction due to high temperature.
It can also result due from residual stresses in steel generated during manufacturing such as cold forming, machining welding, etc.
It is very hard to detect.
Galvanic Corrosion
This corrosion is also called bimetallic corrosion.
It occurs when two different metals are in contact with each other in a moist environment.
In this corrosion, less corrosive resistant metal acts as an anode, and more corrosive resistant metal act as a cathode.
How to avoid Rusting in Stainless Steel?
The above condition as discussed can cause damage to the protective film on the stainless steel surface and lead to corrosion.
Therefore, to ensure that the metal surface is permanently bright and not corroded we suggest:
- After the production and processing of stainless steel products, the passivation repair protective layer should be applied.
- During use, the decorative stainless steel surface must be cleaned and scrubbed frequently to remove attachments, eliminating external factors.
- In coastal areas, 316 stainless steel should be used, which is resistant to seawater corrosion.
- Avoid exposing stainless steel to chemicals such as chlorine, sulfuric acid, and hydrochloric acid.
- Avoid exposing stainless steel to moisture by keeping it dry.
So Here you know the complete overview of Does stainless steel rust?
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