When brown spots appear on the surface of stainless steel, people are often surprised and assume that stainless steel is rust-free. They believe that if it rusts, it must be due to poor quality. However, this is a misconception due to a lack of understanding of 304 stainless steel. In fact, 304 stainless steel can rust under certain conditions.

Stainless steel is resistant to rust, but it can still rust under certain conditions. The ability of stainless steel to resist rust is closely related to its composition. In addition to iron, stainless steel contains elements such as chromium, nickel, aluminum, and silicon. The chromium content in most stainless steels is usually at least 12%, and can go up to 18% in some grades. Adding chromium and other elements improves the steel's properties, making its molecular structure more uniform and allowing a dense oxide protective film to form on the surface, which significantly enhances its corrosion resistance.

It is resistant to atmospheric oxidation, i.e. "stainless", and also has corrosion resistance when exposed to acids, bases, or salts. The corrosion resistance of steel varies based on its chemical composition, the state in which it is treated, the usage conditions, and the kind of environment medium. 304 stainless pipes, for example, have excellent corrosion resistance when in clean, dry air, but rust rapidly in areas with salty mist. 316 stainless performs much better in these environments. Not all stainless steel will be corrosion-resistant or rust-free in every environment.

A protective oxide layer that forms over the stainless steel surface is essential in stopping rust. The chromium-oxide film is a thin, stable, dense layer theat prevents oxygen from penetrating and causing rust. If this protective film is damaged oxygen atoms can penetrate the surface and dissolve iron atoms, leading to rusting. Here are a few examples of how this protective film may be damaged:

1.Electrochemical corrosion Dust or metal particles from other elements can collect on stainless steel surfaces. When the air is moist, condensation occurs between these particles and stainless steel, creating a small battery. The electrochemical reaction that occurs is what breaks down the protective layer.

2.Acids is a term used to describe organic substances, such as soups, vegetable juices or phlegm, that can stick on the stainless steel surface. These substances, when combined with oxygen and water, can produce organic acids that corrode metal surfaces over time.

3.Alkaline or Acidic Substances Substances such as alkaline or salty water used for wall decorations can cause corrosion.

4.Corrosion In environments with high levels of sulfur dioxide, nitrogen oxides or carbon oxides (such as those in polluted cities), condensation can lead to acidic drops such as sulfuric, nitric, or acetic acids. These droplets cause chemical corrosion.

We recommend the following to maintain stainless steel's permanent shine and avoid rust:

1.Cleaning: Wipe the surface frequently to eliminate contaminants.

2.Use of 316 Stainless steel Near the Seaside 316 stainless is resistant to corrosion by seawater and suitable for coastal areas.

Is Stainless Steel Magnetic?

Often, people believe that stainless steel is not authentic or of low quality if the magnet adheres to it. If it does not attract a magnet, then it's assumed that it is high quality. It is a simplistic method that does not accurately identify stainless steel.

There are many types of stainless steel with different properties and different use environments.

For example: 304 steel pipe has absolutely excellent corrosion resistance in a dry and clean atmosphere, but if it is moved to the coastal area, it will quickly rust in the sea fog containing a lot of salt; while 316 steel pipe performs well.

Stainless steel can be classified into different series based on its composition: Cr-series (SUS400), Cr-Ni-series (SUS300), Cr-Mn-Ni-series (SUS200), and precipitation-hardening-series (SUS600). Below is a breakdown of the main types:

200 Series - Chromium-Nickel-Manganese Austenitic Stainless Steel

SUS200: Chromium, nickel, and manganese-based stainless steel. This series is more cost-effective than the 300 series while maintaining good corrosion resistance.

300 Series -Chromium-Nickel Austenitic Stainless Steel

SUS301: Known for good ductility and used in forming products. It can be hardened quickly through mechanical processing and has good weldability. Its wear resistance and fatigue strength are superior to 304 stainless steel.

SUS302: Has the same corrosion resistance as 304 but with higher carbon content, which increases its strength.

SUS303: Contains small amounts of sulfur and phosphorus, making it easier to machine compared to 304.

SUS304: Also known as 18/8 stainless steel, with the GB grade being 0Cr18Ni9. It is the most commonly used grade for general applications.

SUS309: Has better temperature resistance than 304.

SUS316: The second most widely used steel grade after 304. It is mainly used in the food industry and for surgical instruments. The addition of molybdenum gives it a unique corrosion-resistant structure. It is particularly resistant to chloride corrosion, which is why it is also used as "marine steel." SS316 is commonly used in nuclear fuel reprocessing equipment. The 18/10 grade stainless steel typically meets this application level.

SUS321: Similar to 304 but with the addition of titanium, which reduces the risk of corrosion at welded seams.

400 Series -Ferritic and Martensitic Stainless Steel

SUS408: Known for good heat resistance but weak corrosion resistance. Contains 11% Cr and 8% Ni.

SUS409: The most affordable model (common in the US and UK), primarily used in automobile exhaust systems. It is a ferritic stainless steel (chromium steel).

SUS410: Martensitic (high-strength chromium steel) with good wear resistance but poor corrosion resistance.

SUS416: Contains sulfur, improving machinability.

SUS420: "Tool-grade" martensitic steel, similar to high-chromium Brinell steel. It is one of the earliest types of stainless steel and is used for surgical instruments. It can be polished to a high gloss.

SUS430: Ferritic stainless steel, mainly used for decoration, such as automotive trim. It has good formability but relatively poor heat resistance and corrosion resistance.

SUS440: High-strength tool steel with slightly higher carbon content. After appropriate heat treatment, it achieves higher yield strength and can reach a hardness of up to 58 HRC, making it one of the hardest stainless steels. Common applications include razor blades. Common grades include 440A, 440B, 440C, with 440F being the machinable variant.

500 Series -Heat-Resistant Chromium Alloy Steel

This series is designed for high-temperature applications, offering excellent heat resistance.

600 Series- Martensitic Precipitation-Hardening Stainless Steel

SUS630: The most commonly used precipitation-hardening stainless steel, also known as 17-4. It contains 17% Cr and 4% Ni and is widely used for applications requiring high strength and corrosion resistance.

Martensitic and ferritic steels, however, are magnets. Austenitic stainless steels, such as 304, used for decorative plates and pipes are non-magnetic. Due to differences in the chemical composition or processing during manufacture, these steels may have magnetic properties. It does not necessarily mean that the steel is substandard or counterfeit.

What causes this? Austenitic stainless is normally non-magnetic. However, if components are segregated during the melting process or heat treatments, then small amounts of ferritic and martensitic phases may form. The result is a weak magnetic property. Cold working of stainless steel 304 can also lead to a transformation in its structure into martensite and increase the magnetic response. More cold-working will increase the magnetic response of the steel. When manufacturing different diameter pipes from the same batch, greater deformations (or shapes like square sections or square tubes) can lead to stronger magnetic properties.