3 Factors of Stainless Steel Corrosion and Rust

Stainless steel, renowned for its corrosion resistance, is a material of choice for a wide range of applications. However, even the most rust-resistant metal can succumb to corrosion under certain conditions. In this article, we will explore 3 key factors that contribute to stainless steel corrosion and rust. Understanding these factors is crucial for effective material selection and longevity of stainless steel products.

3 Factors of Stainless Steel Corrosion and Rust:

There are several reasons for the rusting of stainless steel materials:

(1) The presence of chloride ions in the environment

Chloride ions are widely present, such as in table salt, sweat, seawater, sea breeze, soil, etc. In the presence of chloride ions, stainless steel corrodes quickly, even faster than ordinary low-carbon steel. Therefore, there are requirements for the use environment of stainless steel, and it needs to be cleaned and dried frequently to remove dust and keep it clean and dry. (This can be defined as “improper use”.)

There is an example in the United States: A wooden container was used to hold a certain chloride-containing solution. This container has been in use for nearly a hundred years. In the 1990s, it was planned to replace it because the oak material was not modern enough. After replacing it with stainless steel, the container leaked due to corrosion only 16 days later.

(2) No solid solution treatment

The alloying elements are not dissolved into the matrix, resulting in low alloy content in the matrix structure and poor corrosion resistance.

(3) Inherent intergranular corrosion

This material without titanium and niobium has a tendency for intergranular corrosion. By adding titanium and niobium and stabilizing treatment, intergranular corrosion can be reduced. Stainless steel is a type of high-alloy steel that is resistant to corrosion in the air or in chemical corrosion media. It has a beautiful surface and good corrosion resistance, and does not require surface treatment such as plating to achieve inherent surface properties of stainless steel.

It is widely used in various types of steel and is often referred to as stainless steel. Representative performance includes 13% chromium steel, 18-8 chromium nickel steel and other high-alloy steels. From the perspective of metallography, stainless steel forms a very thin chromium film on the surface due to the presence of chromium, which isolates the oxygen invading the steel and resists corrosion.

In order to maintain the inherent corrosion resistance of stainless steel, the steel must contain at least 12% chromium. It is used in welding applications. The lower carbon content minimizes the amount of carbides precipitated in the heat-affected zone near the weld, and the precipitation of carbides may lead to intergranular corrosion (welding erosion) in certain environments. This can damage the surface of stainless steel and attach iron powder, leading to rust formation.

Other Reasons of Stainless Steel Corrosion and Rust

In daily life, we sometimes find that the stainless steel on facilities such as street flags, bus shelters, light boxes, etc., show obvious rusting and pickling phenomenon. Since it is stainless steel passivation, why does it still rust? There are two reasons for these situations. First, the chrome content in the material is low, which belongs to inferior stainless steel. Second, it is not stainless steel at all, but it deceives users with electroplating. It is understood that many decorative materials are now processed with this electroplating process, and since the material is general steel, when the electroplating layer is peeled off, it will naturally rust.

Stainless steel has the ability to resist atmospheric oxidation – that is, it is not corroded, and it also has the ability to withstand corrosion in media containing acids, alkalis, salts, etc. – that is, corrosion resistance. However, the extent of its corrosion resistance varies with its own chemical composition, processing conditions, use conditions, and environmental medium type. For example, 304 steel pipes have excellent corrosion resistance in dry and clean atmospheres, but when moved to coastal areas with a lot of salt in the sea mist, they quickly rust; while 316 steel pipes perform well. Therefore, not any kind of stainless steel can resist corrosion and not rust in any environment.

Stainless steel relies on a very thin, strong, and dense stable chromium-rich oxide film (protective film) on its surface to prevent the further penetration of oxygen atoms and further oxidation, thereby acquiring anti-corrosion ability. Once for some reason, this film is continuously destroyed, oxygen atoms in the air or liquid will continuously penetrate or iron atoms in the metal will continuously precipitate out, forming loose iron oxide, and the metal surface will be continuously corroded. There are many ways to damage this surface film, and the following are the most common ones seen in daily life:

1. The stainless steel surface accumulates dust or adherents of other metal elements or foreign metal particles. In a humid atmosphere, the condensate water between the adherents and the stainless steel connects them into a micro-battery, which triggers an electrochemical reaction and destroys the protective film, which is called electrochemical corrosion.
2. The stainless steel surface adheres to organic juice (such as melon, tomato juice, phlegm, etc.), and in the presence of water and oxygen, it forms organic acids that corrode the metal surface over time.
3. The stainless steel surface adheres to substances containing acids, alkalis, salts (such as alkaline water and lime water splashed from wall decoration), which causes local corrosion.
4. In polluted air (such as air containing a large amount of sulfides, oxides, and nitrites), when cold condensate water forms sulfuric acid, nitric acid, and acetic acid spots, it causes chemical corrosion.

All the above situations can cause damage to the protective film of the stainless steel surface and initiate corrosion. Therefore, to ensure that the metal surface remains permanently bright and free from corrosion, the following suggestions are made:

1. It is necessary to clean and wipe the decorated stainless steel surface regularly to remove adherents and eliminate external factors that cause corrosion.
2. In coastal areas, it is recommended to use 316 material stainless steel, which can resist seawater corrosion.
3. Some stainless steel pipes on the market do not meet the corresponding national standards in terms of chemical composition, and cannot meet the requirements of 304 material, which may also cause corrosion. Therefore, users need to carefully choose products from reputable manufacturers.

Conclusion

Thank you for reading our article and we hope it can help you to have a better understanding of the factors of stainless steel corrosion and rust. If you want to find more information about stainless steel, we’d advise you to visit Sino Stainless Steel for more information.

As a leading supplier of stainless steel products across the world, Sino Stainless Steel provides customers with high-quality products such as stainless steel strips, stainless steel coils, stainless steel plates, stainless steel sheets, stainless steel bars, and stainless steel wires at a very competitive price.