Does stainless steel really not rust

What is stainless steel?

As aluminum alloy is an alloy based on aluminum, stainless steel is also an alloy, but it is based on iron.
The alloy elements of stainless steel include chromium, nickel, carbon, titanium, etc.
Among them, chromium addition is the key to stainless steel rust free.
This is determined by the characteristics of chromium.

In stainless steel, the chromium content is at least 10.5%. It can form a protective self repairing redox film on the surface of stainless steel, which is why stainless steel is corrosion resistant.

Self repair means that if the protective film is damaged, it will form oxide film again in aerobic environment and continue to provide corrosion protection.
So many times, when we choose surface treatment, we usually use chrome plating or galvanizing, especially for martensitic stainless steel.
Of course, when it is used in practice, more zinc is chosen, because it is cheaper. The world produces 12 million tons of zinc a year, half of which is used for galvanizing.
This is not the same as that of carbon steel or low alloy steel, because after the scratch of the surface coating of such steel, the corrosion resistance is greatly weakened or even absent.

So, is stainless steel really rust free?

Of course not, stainless steel is not rusty is conditional.
In most conventional conditions, stainless steel has corrosion resistance, but it is not rust free in all environments.
For example, in high temperature, in the environment of sulfuric acid and hydrochloric acid containing 50%, serious corrosion will occur, which will cause the rupture of protective film on the metal surface.
Of course, we can also increase corrosion resistance by adding other alloy elements, such as molybdenum and chromium, which increase the resistance to chloride penetration, and nickel can improve the corrosion resistance in some acidic environments.

Several types of stainless steel corrosion

Stainless steel has many types of corrosion, such as point corrosion, crevice corrosion, stress corrosion, electrical corrosion, contact corrosion, etc.
Here I just talk about a few corrosion that is easy to appear in general environment.
(1) Ordinary corrosion
Common corrosion is usually a uniform solution to materials, such as when stainless steel is used in chemical plants to contain strong acids.
(2) Spot corrosion
Point corrosion is the corrosion that starts from a little to spread out slowly.
In the environment of strong sulfuric acid, in the environment of high concentration chloride, such as sodium chloride in seawater or in medium and high temperature acidic environment, it is easy to cause point corrosion, resulting in air leakage and leakage of pipelines, joints, etc.
Once the point corrosion begins, it is easy to continue to develop, because there is potential difference between the point corrosion part and the non corrosive part.
In many cases, 316 and 317 stainless steel can be used to enhance resistance to point corrosion. In addition, the surface with coarseness less than 0.5um has better resistance to spot corrosion.
(3) Crack corrosion
The crevice corrosion is caused by the local difference of oxygen concentration in the gap.
The corrosion of cracks is related to the deposits in the cracks such as metal surface, gasket, joint, etc. here, a small amount of liquid will gather and stagnate, which will induce crack corrosion.
It is reported that wood, plastic, rubber, glass and active organisms all cause crevice corrosion. Once erosion begins in the crevice, it develops rapidly, especially in chloride environment.
Therefore, molybdenum containing stainless steel, such as 316317, is usually used to minimize crack corrosion. Of course, the best solution is to eliminate the gap design.
(4) Stress corrosion
Stress corrosion is caused by the joint action of tensile stress and corrosion.
As we all know, many alloys will undergo stress corrosion cracking, such as brass in ammonia, carbon steel in nitrate solution, aluminum alloy in seawater, etc.
Stainless steel is easy to crack due to stress corrosion in chloride environment.
At present, the mechanism of stress corrosion is not completely clear, but the method of reducing stress corrosion has been found through experiments and experience.
For example, the use of duplex stainless steel 2205 can improve the resistance to stress corrosion, because it has excellent resistance to chloride stress corrosion cracking.

Classification of stainless steel

According to different microstructure, stainless steel can be divided into martensite, austenite, ferrite and duplex stainless steel (austenite and ferrite duplex).

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Families of stainless steels

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Chromium nickel content of different kinds of stainless steel

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Stainless steel family

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Austenitic stainless steel and duplex stainless steel

Austenitic stainless steel

Austenitic stainless steels contain at least 16% chromium and 6% nickel.
For example, the famous 304 stainless steel is austenitic stainless steel, which contains 18% chromium and 8% nickel, also known as 18 / 8 stainless steel. With the addition of nickel, austenitic stainless steel can also be used at low temperature without brittle fracture (hard and brittle at low temperature).
Austenitic stainless steels are divided into 3 series and 2 series stainless steels. 300 series stainless steels contain chromium and nickel, while 200 series stainless steels contain chromium, nickel and manganese.
This kind of stainless steel can be hardened by cold work, but not by heat treatment.
In the annealed state, austenitic stainless steel is basically non-magnetic, but some may become slightly magnetic due to cold working.
Austenitic stainless steel has excellent corrosion resistance and excellent formability.
The commonly used austenitic stainless steels are 303304316317, etc.
304 stainless steel: it can withstand common corrosion in buildings, resist food processing environment (except high acid and chloride high temperature conditions), and also resist organic chemicals, dyes and a variety of inorganic chemicals.
304 L stainless steel (L = low, low carbon): good resistance to nitric acid and sulfuric acid at medium temperature and concentration, widely used for storage of liquefied gas, household appliances and other consumer goods, such as kitchen equipment, hospital equipment.
316 stainless steel: this kind of stainless steel contains more nickel and 2-3% molybdenum than 304 stainless steel, so it has better corrosion resistance than 304 stainless steel, especially in the chloride environment which is easy to cause pitting corrosion.
317 stainless steel: it contains 3-4% molybdenum and more chromium than 316, which can better resist pitting and crevice corrosion.

Ferritic stainless steel

Ferritic stainless steel is straight chromium 400 series stainless steel, which contains 10.5% – 25% chromium and has medium strength, corrosion resistance and poor processability. For example, 430,409 is ferritic stainless steel.
These stainless steels cannot be hardened by heat treatment, but can only be moderately hardened by cold working. They are magnetic conductive, have good ductility, corrosion resistance and oxidation resistance.
430 is the most basic ferritic stainless steel.

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Ferritic and martensitic stainless steels

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Ferritic stainless steel (Image from nickelinstitute.org)

Martensitic stainless steel

Martensitic stainless steel is a 400 series stainless steel. Its main alloy element is also chromium. Compared with ferritic stainless steel, martensitic stainless steel has higher carbon content and lower chromium content. For example, 410 and 416 only contain 12% chromium.
These alloys can be hardened by heat treatment, have magnetism and have medium corrosion resistance. And they have good ductility, the tensile strength can reach 1379 MPa after heat treatment.

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Martensitic stainless steel (Image from nickelinstitute.org)

410 stainless steel is the basic alloy of martensitic stainless steel series. It has the lowest alloy content among the three kinds of general stainless steel. It is generally used for high stress parts that need the combination of strength and corrosion resistance, such as fasteners.
410 stainless steel has corrosion resistance in mild atmosphere, steam and many mild chemical environments.

Duplex stainless steel (austenite + ferrite)

The microstructure of duplex stainless steel is the mixture of austenite and ferrite, so it has the characteristics of both austenitic stainless steel and ferritic stainless steel.
For example, although it is not as resistant to stress corrosion as ferritic stainless steel, it also has high resistance to chloride stress corrosion cracking, excellent resistance to pitting corrosion and crevice corrosion, and its yield strength is about twice that of traditional stainless steel.
Type 329 and 2205 are typical dual phase alloys.
In addition, the corrosion resistance of general duplex stainless steel is equal to or better than 304 and 316, and in general, its pitting corrosion resistance is better than 316, because these alloys have very high chromium content, with about 22% to 25% CR.

Precipitation hardening stainless steel

These are also chromium nickel steels. The biggest advantage is that they can provide high tensile strength under “solution treatment”. The most common in this group is “17-4 PH”, also known as 630 stainless steel, which consists of 17% chromium, 4% nickel, 4% copper and 0.3% niobium.
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Comparison of properties of stainless steel

Characteristics of common stainless steel 303, 304, 316, 420, 440

303, 304, 316 these three kinds of stainless steel are austenitic stainless steel, the main alloy elements are chromium and nickel, most of the time there is no magnetic, slightly magnetic after cold processing, mechanical and physical properties are almost the same, the main difference lies in the corrosion resistance and price.
410, 420, 440, 440c belongs to martensitic stainless steel. The main alloying element of this kind of stainless steel is chromium, which does not contain nickel. The overall corrosion resistance is not as good as that of austenitic stainless steel, which is magnetic.
The chromium content of 304 stainless steel is at least 18%, and the nickel content is at least 8%, so it is also called 18 / 8 stainless steel. This stainless steel has excellent oxidation resistance and corrosion resistance. It is the most widely used and common austenitic stainless steel. Compared with 316 stainless steel, 304 stainless steel has lower cost.
303 is added sulfur or selenium on the basis of 304. The addition of sulfur or selenium improves the machinability, and its composition contains at least 17% chromium, 8% nickel and 0.15% sulfur or selenium. The addition of sulfur slightly reduces corrosion resistance, but increases machinability, which is why it is used in fasteners, bushings, bearings and other smaller parts.
316 stainless steel contains at least 16% chromium, 10% nickel and 2% molybdenum. The main difference between 304 and 316 stainless steel is that 316 stainless steel contains molybdenum.
The addition of molybdenum improves the corrosion resistance because molybdenum is a transition metal with high corrosion resistance. It also makes 316 stainless steel have high heat resistance up to 871 ℃. 316 stainless steel is preferred if the environment contains a large number of corrosive elements or in underwater applications.
410 stainless steel is one of the most commonly used martensitic stainless steels. It contains about 12% Cr, so it has medium corrosion resistance. Unlike austenite (such as AISI304 and AISI316), martensite is magnetic and can be hardened by heat treatment such as carbon steel and low alloy steel.
420 stainless steel is basically the high carbon version of 410, which can provide higher strength, certain wear resistance, corrosion resistance and high hardness. This kind of stainless steel is used to make parts resistant to atmosphere, steam, water and oxidizing acid, such as bearings, instruments, household appliances, guide rails, etc.
440C is a high carbon and high chromium martensitic stainless steel. 440C has high strength, good hardness, corrosion resistance, wear resistance and dimensional stability at high temperature. 440 stainless steel can be quenched and tempered in the same way as alloy steel. 440C is difficult to machine because of its high annealing hardness and abrasive chromium carbide.
440 and 440C stainless steel are used for bearing parts working in corrosive environment. They can also be used to manufacture high-quality tools, such as medical scalpels, surgical and dental instruments, springs, tableware, cams, ball bearings, measuring tools, etc.

Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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