Stainless Steel Classification
1. According to chemical composition can be divided into: chromium stainless steel, chromium-nickel stainless steel, chromium-manganese stainless steel, chromium-nickel-molybdenum stainless steel, ultra-low carbon stainless steel, high molybdenum stainless steel , high purity stainless steel, etc.
2. According to the metallographic structure can be divided into: martensitic stainless steel, ferritic stainless steel, austenitic stainless steel, austenitic ferritic stainless steel, etc.
3. According to the performance characteristics and uses of steel : such as nitric acid resistant stainless steel (nitric acid grade), sulfuric acid resistant stainless steel, corrosion resistant stainless steel, stress resistant stainless steel, stainless steel high strength, etc.
4. According to the functional characteristics of the steel: such as low-temperature stainless steel, non-magnetic stainless steel, free-cutting stainless steel, superplastic stainless steel, etc.
The development process of stainless steel grades is shown in the figure below:
Effect of alloying elements on the microstructure and properties of stainless steel
Note:口– strong effect, 
—— Moderate action, ▲ – weak action
Ways to improve the corrosion resistance of stainless steel
(1) To obtain a stable anodic polarization curve of the passivation zone for a specific medium, ensure that the stainless steel is prepared properly.
(2) Increasing the electrode potential of the stainless steel substrate and at the same time reducing the electromotive force of the corrosive galvanic cell can help improve its corrosion resistance.
(3) Improving the single-phase structure of steel and reducing the number of microbatteries can improve its corrosion resistance.
(4) To form a stable protective film on the surface of steel, the addition of elements such as silicon, aluminum and chromium can help create a dense protective film in many corrosion and oxidation situations, thereby increasing the corrosion resistance of steel.
(5) Eliminating or reducing various irregular phenomena in steel is also a vital step in increasing its corrosion resistance.
Adding alloying elements to steel is the main method used to improve its corrosion resistance.
The addition of different alloying elements can work in one or several ways simultaneously to improve the corrosion resistance of steel.
Effect of alloying elements on iron electrode polarization and potential
The type and content of alloying elements have a direct impact on the corrosion resistance of stainless steel. The main function of alloying elements is to influence the polarization performance of iron and electrode potential.
1. Effect of alloying elements on the polarization properties of iron
The anodic polarization process of commonly used metals such as Fe, Cr, Ni and Ti follows a unique polarization pattern.
After passing the anode, the anode potential increases and the anode current (corrosion rate) changes accordingly, almost with the same pattern.
The typical shape of the polarization curve is shown in the figure below.
As the anode polarization potential increases, the corrosion current does not decrease uniformly. Instead, it first increases, then decreases to a minimum and maintains this current through a certain stage of potential rise before increasing again.
This polarization curve is called anodic polarization curve with activation and passivation transition. It is divided into three regions: activation region (A), passivation region (B) and superpassivation region (T).
Fig. Anodic polarization curve of activated and passivated transition metals
Polarization plays a significant role in improving the corrosion resistance of metals. Factors that improve anodic or cathodic polarization can increase corrosion resistance, while depolarizing factors can reduce it.
Different alloying elements have varying effects on the polarization properties of iron. Elements that expand the passivation zone, which reduces the ECP and P zone potential and increases the Er point potential, can improve the corrosion resistance of steel. On the other hand, all elements that improve the passivation performance by making the ICP and I1 points move to the left can reduce the corrosion current and improve the corrosion resistance.
Elements that increase the Er point potential tend to reduce pitting corrosion because when the potential fluctuates close to the superpassivation potential and the Er point potential is low, it can lead to local breakdown of the passivation film, resulting in pitting corrosion .
Among the alloying elements commonly used in steel, Cr can significantly improve the passivation performance of pure iron, increase the potential of Ecp, Ep and Er points, and shift the position of Icp and I1 points to the left. Therefore, it is the most effective element to increase the corrosion resistance of iron.
In addition to Cr, alloying elements such as Ni, Si, Mo, etc. They can also improve the passivation performance and expand the passivation zone to varying degrees.
Mo, for example, not only improves the passivation performance of iron, but also increases the Er point potential, which improves the pitting corrosion resistance of iron.
2. Influence on the potential of the iron electrode
In general, the electrode potential of a solid metal solution is lower than that of other compounds. Therefore, during the corrosion process, the metal solid solution is more likely to corrode as the anode.
One way to increase the corrosion resistance of iron is to increase the electrode potential. Studies have shown that adding Cr to iron to form a solid solution can significantly increase the electrode potential of the resulting material, as illustrated in the figure below.
By raising the electrode potential of a material, its corrosion resistance can be markedly increased.
Fig. Effect of chromium on electrode potential of FeCr alloy
Due to the good effect of chromium on iron passivation and electrode potential, chromium has become the main alloying element of various stainless steels.
Effect of alloying elements on corrosion resistance and matrix structure of stainless steel
The stainless steel matrix structure is crucial to achieving the desired mechanical and process properties, as well as ensuring excellent corrosion resistance.
Two types of stainless steels, single-phase ferritic steel and single-phase austenitic steel, exhibit superior corrosion resistance.
The effect of alloying elements on the matrix structure depends mainly on whether they act as ferrite stabilizers (α) or austenite stabilizers (γ).
When the stabilizing element is dominant, single-phase α stainless steel can be obtained; otherwise, single-phase γ stainless steel is obtained.
1. Effect of alloying elements on the corrosion resistance of stainless steel
1. Chrome
Chromium is the primary element that determines the corrosion resistance of stainless steel. When the chromium content (atomic ratio) reaches between 1/8 and 2/8, the iron electrode potential jumps, leading to an improvement in the corrosion resistance of the steel. Chromium is also a stabilizing element that helps increase the overall durability of the material.
One reason for this is that chromium oxide is relatively dense and can form a protective film that resists corrosion.
2. Carbon and nitrogen
Carbon plays an essential role in the production of stainless steel, as it strongly stabilizes austenite, with a stabilizing capacity about 30 times greater than that of nickel. Furthermore, carbon is the main element used to strengthen stainless steel. However, carbon can also form a series of carbides with chromium, which can significantly impact the corrosion resistance of stainless steel. Additionally, carbon can worsen the processing and welding properties of stainless steel and cause ferritic stainless steel to become brittle.
Therefore, it is crucial to carefully control and apply carbon during the production and development of stainless steel. The combination of carbon and chromium has a significant effect on the formation of stainless steel structures, as shown in the figure below.
The figure shows that when the carbon content is low and the chromium content is high, a ferrite structure is obtained, while a martensite structure is obtained when the carbon content is high and the chromium content is low.
In chromium stainless steel, an increase in carbon content will lead to the formation of martensite when the chromium content is below 17%. On the other hand, a low carbon content and 13% chromium will result in the formation of ferritic stainless steel.
As the chromium content increases from 13% to 27%, the stabilization capacity of the ferrite increases, which in turn causes an increase in the carbon content (from 0.05% to 0.2%). Despite the increase in carbon content, the ferrite matrix can still be maintained.
Fig. effect of carbon and chromium on the microstructure of stainless steel
3. Nickel
Nickel is one of the three important elements in stainless steel as it can improve the corrosion resistance of the material. As a stabilizing element of the γ phase, nickel is the main component required to obtain single-phase austenite and promote its formation in stainless steel.
One of the main benefits of nickel is that it can effectively reduce the Ms point, keeping austenite stable at very low temperatures (-50℃) without undergoing martensitic transformation. However, increasing the nickel content will reduce the solubility of carbon and nitrogen in austenitic steel, thereby increasing the tendency of these compounds to desolvate and precipitate.
As the nickel content increases, the critical carbon content for intergranular corrosion decreases, making the steel more susceptible to this type of corrosion. However, the effect of nickel on the pitting corrosion resistance and crevice corrosion resistance of austenitic stainless steel is not significant.
In addition to its corrosion resistance benefits, nickel can also improve the high-temperature oxidation resistance of austenitic stainless steel. This is mainly due to nickel's ability to improve the composition, structure and properties of the chromium oxide film. However, it is important to note that the presence of nickel can reduce the steel's resistance to vulcanization at high temperatures.
4. Manganese
Manganese is a relatively weak austenite-forming element, but it plays a crucial role in stabilizing the austenite structure.
In austenitic stainless steel, manganese partially replaces nickel and 2% Mn is equivalent to 1% Ni.
Manganese can also increase the corrosion resistance of chromium stainless steel in organic acids such as acetic acid, formic acid and glycolic acid, and is more effective than nickel.
However, when the chromium content in steel exceeds 14%, the addition of manganese alone cannot result in a single austenite structure.
Because austenitic stainless steel has better corrosion resistance when the chromium content is above 17%, the industry mainly employs Fe-Cr-Mn-Ni-N steel such as 12Cr18Mn9Ni5N as a substitute for nickel-containing alloys. The amount of nickel-free Fe-Cr-Mn-N austenitic stainless steel used is relatively less.
5. Nitrogen
In the early stages, nitrogen was mainly used in Cr-Mn-N and Cr-Mn-Ni-N austenitic stainless steels to save Ni. However, in recent years, nitrogen has become an essential alloying element of CrNi austenitic stainless steel.
Adding nitrogen to austenitic stainless steel can stabilize the austenitic structure, improve strength and increase corrosion resistance, especially for local corrosion such as intergranular corrosion, pitting corrosion and crevice corrosion.
In ordinary low-carbon and ultra-low-carbon austenitic stainless steel, intergranular corrosion resistance can be improved. Nitrogen affects the chromium carbide precipitation process during the sensitization treatment, increasing the chromium concentration at the grain boundary.
In high-purity austenitic stainless steel, where there is no chromium carbide precipitation, nitrogen increases the stability of the passive film and reduces the average corrosion rate. Although chromium nitride precipitates in steel with high nitrogen content, the precipitation rate of chromium nitride is slow. Therefore, sensitization treatment will not cause intergranular chromium deficiency, having little effect on intergranular corrosion.
Nitrogen can also inhibit phosphorus segregation at the grain boundary and improve the intergranular corrosion resistance of steel.
At present, nitrogen-containing austenitic stainless steel mainly has high strength and corrosion resistance. It can be divided into three types: nitrogen control type, medium nitrogen type and high nitrogen type.
Nitrogen control type involves adding 0.05% ~ 0.10% N to ultra-low carbon Cr Ni austenitic stainless steel (C ≤ 0.02% ~ 0.03%) to improve strength, optimize strength intergranular corrosion and increase the stress corrosion resistance of steel.
The medium nitrogen type contains 0.10% ~ 0.50% N and is melted and poured under normal atmospheric pressure. On the other hand, the nitrogen content of high nitrogen type is more than 0.40%.
It is generally melted and cast under conditions of increasing pressure. This type of steel is mainly used in the solid solution state or semi-cold working state, as it has high strength and corrosion resistance.
At present, austenitic steel with high nitrogen content and nitrogen content ranging from 0.8% to 1.0% has been successfully applied in practical applications and started industrial production.
6. Titanium, niobium, molybdenum and rare earth elements
Titanium and niobium are elements that can strongly form carbides, which can react preferentially with carbon than with chromium, thus preventing intergranular corrosion and improving the corrosion resistance of steel.
When adding titanium and niobium to steel, it is important to maintain a certain proportion with the carbon content.
Molybdenum, on the other hand, can increase the passivation capacity of stainless steel and expand the range of passivation media. This means it can withstand hot sulfuric acid, dilute hydrochloric acid, phosphoric acid and organic acids. The passivation film created with molybdenum is highly stable in various media and is less likely to dissolve.
Molybdenum-containing stainless steel is resistant to pitting corrosion, as it can protect the passive film from damage caused by Cl-.
When rare earth elements such as Ce, La and Y are added to stainless steel, they may dissolve slightly into the matrix. This process helps to purify the grain boundary, modify inclusions, homogenize the structure and reduce precipitate precipitation and grain boundary segregation. This leads to an improvement in the corrosion resistance and mechanical properties of the steel.
2. Effect of alloying elements on the microstructure of stainless steel
The influence of alloying elements on the matrix structure of stainless steel can be classified into two categories:
- Ferrite-forming elements such as chromium, platinum, silicon, titanium, niobium, etc.
- Austenite-forming elements such as carbon, nitrogen, nickel, manganese, copper, etc.
When these elements with different functions are added to steel simultaneously, the microstructure of stainless steel depends on their comprehensive effects.
To simplify the treatment, the effect of ferrite-forming elements is converted into the effect of chromium, known as chromium equivalent (Cr), while the effect of austenite-forming elements is converted into nickel equivalent (Ni).
Based on the chromium equivalent (Cr) and the nickel equivalent (Ni), a diagram is created to represent the actual composition of the steel and the resulting structural state, as shown in the following figure.
Fig. stainless steel structure diagram
The figure illustrates that 12Cr18Ni9 steel belongs to the family of austenitic stainless steels and is located in the a-phase zone.
On the other hand, Cr28 stainless steel is classified as ferritic stainless steel and can be found in the ferritic phase zone.
Meanwhile, 30Cr13 stainless steel falls into the martensitic stainless steel category and is situated in the martensitic phase zone.
To obtain a single-phase austenite structure, a specific balance of alloying elements is required. Otherwise, some ferrite structure will appear in the steel, resulting in a multiphase structure.
Effect of alloy composition and microstructure on the mechanical properties of stainless steel
1. Stainless steel reinforcement mechanism
Stainless steel strengthening is achieved through several mechanisms, including solid solution strengthening, phase transformation strengthening, second phase strengthening, grain refinement strengthening, precipitation strengthening, and substructure strengthening.
The figure below illustrates the contribution of these mechanisms to the yield strength in 8% ~ 10% Ni austenitic stainless steel.
As depicted in the figure, chromium, silicon, and carbon provide solid solution strengthening to the matrix, resulting in a several-fold increase in the yield stress of the austenitic matrix.
Another strengthening mechanism is the existence of α ferrite as a second phase, together with grain size refinement and precipitation of precipitates, which significantly increases the strength of austenite.
The figure highlights that in austenitic stainless steel, solid solution strengthening is a crucial mechanism, and grain refinement contributes most to overall strength.
Fig. Factors affecting the strength of austenitic stainless steel
2. Strength and plasticity of various stainless steels
The properties of different stainless steels vary depending on their composition and structure.
See the figure below for a comparison of the strength and plasticity of various stainless steels.
Fig. comparison of strength and plasticity of various stainless steels and pure iron
Among all stainless steels, austenitic stainless steel has the best ductility, while precipitation hardened stainless steel has the highest strength.
Martensitic stainless steel has good general mechanical properties, characterized by high strength and some degree of ductility.
Duplex stainless steel, which is a combination of ferritic and austenitic stainless steels, has higher strength and better ductility.
Ferritic stainless steel and austenitic stainless steel have similar strength properties, but the ductility of the latter is much higher than that of other types of stainless steel. (For comparison purposes, the pure iron curve is also included in the figure).
Effect of corrosive medium on corrosion resistance of stainless steel
The corrosion resistance of metal is not only determined by its material, but also by the type, concentration, temperature, pressure and other environmental conditions of the corrosive medium.
In practical applications, the oxidizing capacity of the corrosive medium has a greater impact on metal corrosion. Therefore, when selecting stainless steel grades for specific work environments, it is important to consider the characteristics of the corrosive medium.
In weakly corrosive media such as atmosphere, water and steam, the corrosion resistance of stainless steel can be guaranteed as long as the Cr content of the solid solution in the stainless steel matrix is greater than 13%. This makes it suitable for use in components such as water compressor valves, steam generator turbine blades and steam piping.
However, in oxidizing media such as nitric acid, NO3- ions have a strong oxidizing capacity. This results in the formation of an oxide film on the surface of the stainless steel with a short passivation time, thus compromising its corrosion resistance.
The H+ in the acid acts as a cathodic depolarizer. As the H+ concentration increases, the depolarization of the cathode strengthens and the chromium content required for passivation also increases. Therefore, only the oxide film containing high chromium content has good stability in nitric acid.
In boiling nitric acid, 12Cr13 stainless steel is not corrosion resistant. However, Cr17 and Cr30 steels with chromium content of 17% to 30% are resistant to corrosion in nitric acid with a concentration of 0% to 65%.
In non-oxidizing media such as dilute sulfuric acid, hydrochloric acid and organic acid, the oxygen content of these corrosive media is low and the passivation time needs to be extended. When the oxygen content in the medium is low to a certain extent, stainless steel cannot be passivated. For example, in dilute sulfuric acid, the SO42- in the medium is not an oxidant, and the dissolved oxygen content in the medium is relatively low, making it unable to passivate steel. Consequently, the corrosion rate of chromium stainless steel is even faster than that of carbon steel.
Therefore, general Cr stainless steel or Cr Ni stainless steel are difficult to reach the passivation state and are not resistant to corrosion when working in this kind of medium. To improve the passivation ability of steel, elements such as molybdenum, copper and others need to be added.
Hydrochloric acid is a non-oxidizing acid known to cause corrosion in stainless steel. To prevent corrosion, a Ni-Mo alloy is required to form a stable protective film on the alloy surface.
In strong organic acids, the passivation of chromium and chromium-nickel stainless steel is difficult due to the low oxygen content in the medium and the presence of H+. Adding Mo, Cu, Mn and other elements to steel can improve its passivation ability. Therefore, Cr-Mn stainless steel is considered the best option.
To make the steel corrosion-resistant and easy to passivate, a certain amount of Mo and Cu is added to the steel.
In Cl--containing media, the oxide film on the surface of stainless steel is easily destroyed, leading to pitting corrosion of the steel. As a result, seawater is highly corrosive to stainless steel.
It is important to note that no stainless steel can resist corrosion from all types of media. Therefore, the selection of stainless steel should be based on the specific corrosion environment and the characteristics of various types of stainless steel.
