Stainless steel is generally “non-rusting” in the atmosphere. The so-called “non-rusting” is a relative concept, and its corrosion resistance is conditional. Under certain conditions, stainless steel can also undergo corrosion.
Currently, there is no stainless steel that has non-rusting and corrosion resistance in all corrosive environments. The more serious forms of corrosion that are prone to occur in stainless steel fasteners include local corrosion, which mainly includes stress corrosion cracking, pitting, intergranular corrosion, crevice corrosion, and fatigue corrosion.
The influence of environmental media
Certain types of corrosion usually occur under specific environmental conditions. Pitting corrosion is prone to occur in media with special ions; crevice corrosion may happen in crevices where solutions are stagnant or within shielded surfaces, at the junctions of metals or between metals and non-metals, and at points of contact with rivets, bolts, gaskets, valve seats, loose surface deposits, and marine organisms; the necessary conditions for stress corrosion cracking to occur include tensile stress (whether residual stress, applied stress, or both) and the presence of specific corrosive media, etc.
The corrosion resistance of stainless steel fasteners is affected by the environment. Take pitting corrosion as an example. The composition, concentration, pressure, temperature, and pH value of the environmental medium all have an impact on the corrosion resistance of stainless steel fasteners.
Stainless steel is prone to pitting corrosion in media containing halogen anions such as CI-, Br-, and I-. Generally, it is believed that pitting corrosion can only occur when the halogen concentration reaches a certain level. The pitting potential of stainless steel fasteners is related to the concentration of halogen elements, etc.
If there are anions such as OH-, SO42- in the medium, they will have a corrosion-inhibiting effect on the pitting corrosion of stainless steel fasteners, and the effect decreases in the following order: OH- > NO3- > AC- > SO42- > CIO4-. At the same time, the temperature, pH value, and flow rate of the medium all affect the pitting corrosion of stainless steel fasteners.
As the temperature rises, the pitting potential of stainless steel fasteners decreases, making pitting corrosion more likely to occur. When pH > 10, the pitting potential increases, and the effect is small when pH < 10. Generally, as the flow rate of the medium increases, the tendency of pitting corrosion decreases. For stainless steel fasteners, a flow rate of about 1 m/s is beneficial to reduce pitting corrosion. If the flow rate is too high, erosion corrosion will occur.
The Influence of Chemical Composition
The corrosion resistance of stainless steel fasteners in certain environments is related to their passivation performance. When steel is in a passivated state, a dense oxide film can form on the surface, which hinders the corrosion process and temporarily stops corrosion.
Chromium is the most fundamental element in stainless steel and is also a necessary element for enhancing the stability of the steel’s passive film. When the chromium content reaches 12%, the alloy can achieve complete self-passivation.
The self-passivation ability of the alloy to a certain extent determines the corrosion resistance of stainless steel. Therefore, the chromium content in austenitic stainless steel should not be less than 12%. However, currently, the chromium content in many SUS304 on the market is less than 12%.
Nickel is an element that enhances the corrosion resistance of steel, and this effect is more pronounced in non-oxidizing sulfuric acid. When nickel is added to chromium stainless steel, it can improve its corrosion resistance in sulfuric acid, acetic acid, oxalic acid, and neutral salts (especially sulfates).
Manganese can also enhance 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.
Molybdenum can enhance the passivation ability of stainless steel and expand the range of passivation media, such as its application in hot sulfuric acid, dilute hydrochloric acid, phosphoric acid and organic acids.
In stainless steel containing molybdenum, a molybdenum-containing passivation film can be formed, which has high stability in many strong corrosive media and can also prevent the damage of the film by chloride ions.
Silicon can enhance the corrosion resistance of steel in hydrochloric acid, sulfuric acid and concentrated nitric acid. Generally, 2% to 4% silicon is added to stainless steel to improve its corrosion resistance in the above-mentioned media.
The Impact of Organizational Structure
According to the standard GB/T20878-2007 “Steel grades and chemical compositions of stainless steels and heat resisting steels”, there are 143 grades of stainless steel. Among them, there are 18 grades of ferritic stainless steel, 38 grades of martensitic stainless steel, 66 grades of austenitic stainless steel, 10 grades of precipitation hardening stainless steel, and 11 grades of austenitic-ferritic (duplex) stainless steel.
Generally speaking, among stainless steels with comparable chromium content, austenitic stainless steel has the best corrosion resistance, followed by ferritic stainless steel, and martensitic stainless steel has the poorest corrosion resistance.
1. Austenitic Stainless Steel
Austenitic stainless steel has excellent resistance to general corrosion in many media, but it is most sensitive to intergranular corrosion and stress corrosion.
An important factor contributing to intergranular corrosion is the material’s own microstructure, that is, the chemical composition differences between grains and grain boundaries of metals or alloys themselves, the structure of grain boundaries, the characteristics of solid solution of elements, the precipitation and dissolution process, solid solution diffusion and other metallurgical issues, which lead to electrochemical inhomogeneity and make metals prone to intergranular corrosion.
2. Ferritic Stainless Steel
Ferritic stainless steel also has a tendency for intergranular corrosion, but compared with carbon-containing and chromium-nickel austenitic stainless steel, ferritic stainless steel with general passivation is more prone to sensitization and thus has a higher tendency for intergranular corrosion.
Ferritic stainless steel has good resistance to stress corrosion cracking in chloride media, much stronger than that of austenitic stainless steel, but it is not absolutely immune. Cracks often originate from intergranular corrosion and pitting.
3. Austenitic-ferritic (duplex) stainless steel
Austenitic-ferritic (duplex) stainless steel combines the characteristics of austenitic steel and ferritic steel. Generally speaking, the corrosion resistance of duplex stainless steel is roughly the same as that of high-chromium ferritic stainless steel or chromium-nickel austenitic stainless steel with comparable chromium and molybdenum content, and is affected by the ratio of the microstructure.
However, compared with austenitic stainless steel, duplex stainless steel has higher resistance to intergranular corrosion and stress corrosion cracking.
4. Martensitic Stainless Steel
Martensitic stainless steel has poorer corrosion resistance than austenitic and ferritic stainless steels. Its main advantage is that it can be strengthened by heat treatment and is suitable for fasteners that have high requirements for strength, hardness, wear resistance, etc., and also have certain corrosion resistance.
5. Precipitation Hardening Stainless Steel
Precipitation-hardening stainless steel has the characteristics of high strength and good corrosion resistance. Its corrosion resistance is not only related to the composition but also closely related to heat treatment.
The precipitation of fine phases and aging reactions are both detrimental to the corrosion resistance. The high strength feature of this type of stainless steel may also lead to hydrogen embrittlement or stress corrosion during use. This must be noted.
In conclusion, various microstructures, phases and chemical composition contents in stainless steel fasteners, such as sulfides, δ-ferrite phase, σ-phase, αˊ-phase, precipitated phases in precipitation-hardened stainless steel, sensitized grain boundaries and welds, may all have an impact on the pitting corrosion resistance of stainless steel fasteners.
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