Stainless steel is an alloy material including elements such as iron, chromium, and nickel, chromium is one of the most important elements of stainless steel, which can form a layer of dense chromium oxide film, thus preventing the rust phenomenon produced by the oxidation of iron. The corrosion resistance of stainless steel mainly depends on the formation of chromium-rich oxide film on its surface, which can prevent oxygen atoms from continuing to penetrate the metal, thus protecting stainless steel from corrosion.
Why Does Stainless Steel Rust?
In general, stainless steel is not susceptible to rust, but may still rust under certain conditions. The rust resistance of stainless steel depends mainly on its alloy composition, especially chromium and nickel. These elements react with oxygen to form a dense oxide film, or passivation film, which enhances the corrosion resistance of stainless steel. However, stainless steel can still rust if it is not properly used or maintained, or subjected to a harsh environment, such as prolonged exposure to salt water. This is because the chloride ions in the salt water will react with the stainless steel, leading to corrosion and rust.
Stainless steel relies on the surface formation of a very thin and strong, fine and stable chromium-rich oxide film (protective film), preventing the continued penetration of oxygen atoms, continuing to oxidize and obtaining the ability to resist rust and corrosion.
Factors Cause Stainless Steel to Rust
Once the environmental factors or other conditions destroy the oxide film, stainless steel will rust! When this protective film is constantly destroyed, air or liquid oxygen atoms will continue to penetrate, or metal iron atoms continue to precipitate out, the formation of loose iron oxide, the metal surface is also subject to constant corrosion.
Damage to The Chromium Oxide Film
- Surface damage: the corrosion resistance of stainless steel relies heavily on the chromium oxide film formed on its surface, which protects it from corrosion. However, if the surface is subjected to mechanical damage, scratches or abrasion, the chromium oxide film may be damaged, resulting in exposed metal surfaces susceptible to corrosion.
- Acid and alkali environment: strong acid or alkali environment can destroy the passivation film on the surface of stainless steel, resulting in direct exposure of the metal to corrosive media, which accelerates corrosion.
Chlorine-containing Environment
- Chlorine ions: the corrosive effect of chlorine ions on stainless steel is particularly obvious, especially in the marine environment or chlorine-containing cleaning agents. Stainless steel is susceptible to pitting or stress corrosion cracking in chlorine-containing environments, leading to rust and damage.
- Saltwater erosion: Chlorine ions in salt water attack the passivation film of stainless steel, which in turn triggers corrosion, especially in coastal applications.
High-Temperature Environment
- Heat treatment or welding: in high-temperature environments, stainless steel may undergo excessive carbonization, leading to the destruction of the passivation film. Especially in the welding process, high temperatures will make the oxides generated on the surface of stainless steel difficult to remove, increasing the risk of rust.
- High-temperature oxidation: stainless steel used for a long time at high temperatures may experience oxidation reactions, forming a thick oxide layer, which affects its corrosion resistance.
Improper Cleaning and Maintenance
- Improper Cleaning: When acidic or alkaline cleaners are used to clean stainless steel, the residual chemicals may lead to corrosion if the cleaner is not completely rinsed off.
- Lack of maintenance: Long-term lack of maintenance and cleaning, and surface accumulation of dirt and contaminants may also affect the corrosion resistance of stainless steel.
Material Quality Problems
- Improper alloy composition: the corrosion resistance of stainless steel is closely related to its alloy composition. If the content of chromium, nickel and other elements is insufficient, it may lead to the instability of its passivation film, thus increasing the risk of rust.
- Manufacturing defects: In the production process, the quality control of stainless steel is not strict, which may lead to defects within the material, such as inclusions or uneven distribution of alloy, and these defects may also lead to rust.
Environmental Factors
- Humidity and temperature: high humidity and changing temperature conditions can accelerate the corrosion process of stainless steel. In particular, pitting corrosion may occur on stainless steel surfaces in environments where condensation forms.
- Deposition of pollutants: airborne pollutants (e.g., sulfides, oxides of nitrogen, etc.) deposited on the surface of stainless steel can form a corrosive medium and increase the risk of rusting.
Electrochemical Corrosion
- Galvanic coupling corrosion: when stainless steel is in contact with other dissimilar metals, galvanic coupling corrosion may be formed, and this corrosion phenomenon will accelerate the rusting process of stainless steel, especially in the electrolyte solution is more obvious.
Types of Stainless Steel
Usually, according to the metallurgical organization, common stainless steels are divided into five categories, these five types of basic metallurgical organization.
1. Austenitic Stainless Steel
The matrix-to-face-centered cubic crystal structure of the austenitic organization (CY phase) is dominated by non-magnetic, mainly through cold working to make it strengthened (and may lead to a certain degree of magnetism) of stainless steel. American Iron and Steel Institute to 200 and 300-series numerical labels, such as 304.
2. Ferritic Stainless Steel
Matrix to the body-centered cubic crystal structure of ferrite organization ((a phase) is dominated by magnetic, and generally can not be hardened by heat treatment, but cold working can make it slightly strengthened stainless steel. American Iron and Steel Institute to 430 and 446 for the label.
3. Martensitic Stainless Steel
The matrix is the martensitic organization (body-centered cubic or cubic), magnetic, through heat treatment, can adjust its mechanical properties of stainless steel. The American Iron and Steel Institute to 410, 420 and 440 digital labelling. Martensite at high temperatures with austenitic organization, when cooled to room temperature at an appropriate rate, the austenitic organization can be transformed into martensite (i.e. hardened).
4. Duplex Stainless Steel
The matrix has both austenite and ferrite two-phase organization, of which the content of the lesser phase matrix is generally greater than 15%, magnetic, can be strengthened by cold working stainless steel, 329 is a typical duplex stainless steel. Compared with austenitic stainless steel, duplex steel’s high strength, resistance to intergranular corrosion and resistance to chloride stress corrosion and pitting corrosion are significantly improved.
5. Precipitation Hardening Stainless Steel
The matrix is austenitic or martensitic organization and can be hardened by precipitation hardening treatment to make it hardened stainless steel. The American Iron and Steel Institute to 600 series of digital labels, such as 630, that is 17-4PH.
In general, in addition to alloys, the corrosion resistance of austenitic stainless steel is superior, in a less corrosive environment, you can use ferritic stainless steel, in mildly corrosive environments, if the material is required to have high strength or high hardness, you can use martensitic stainless steel and precipitation hardening stainless steel.
How To Prevent Stainless Steel From Rusting?
- Surface passivation: enhances the corrosion resistance of stainless steel by forming a thin but strong protective oxide film on its surface. The passivation process usually involves cleaning the surface of the stainless steel to remove oxides and contaminants, and then immersing it in a passivation solution, such as nitric acid, oxalic acid, etc.
- Plating or coating: Depositing a protective metal layer (e.g. nickel, zinc, or chromium) on the surface of stainless steel through plating, or using anti-rust coatings (e.g. epoxy, polyurethane coatings) to form a protective layer on the surface of stainless steel, isolating the surface of stainless steel from direct contact with the external environment.
- Regular cleaning and maintenance: clean the stainless steel surface regularly to remove dirt, grease and other contaminants to prevent localized corrosion. Use neutral cleaners and soft cloths when cleaning, and avoid using chlorine-containing cleaners or wire brushes, which may cause damage to the stainless steel surface.
- Control environmental conditions: reduce direct contact between stainless steel and salty, acidic or alkaline substances, especially in marine or chemical environments. Improve ventilation to reduce moisture build-up, thereby reducing the incidence of corrosion.
- Use of suitable alloys: select the appropriate alloy according to the specific requirements of the use environment. For example, 304 stainless steel is suitable for most general environments, while 316 stainless steel performs better in marine and chemical environments1.
- Avoid contact with dissimilar metals: contact between stainless steel and other metals (e.g. copper, aluminum) may initiate galvanic corrosion. Direct contact between stainless steel and dissimilar metals should be avoided as much as possible, or measures such as the use of insulating gaskets should be taken to prevent galvanic reactions.
- Application of corrosion inhibitors: In some environments, corrosion inhibitors (e.g., rust preventive oils, and antioxidants) can be used to improve the rust resistance of stainless steel. These corrosion inhibitors can usually form a protective film on the surface of stainless steel, reducing the risk of corrosion1.
- Heat treatment: the corrosion resistance of stainless steel is improved by heat treatment processes such as solid solution treatment. During heat treatment, stainless steel is heated to a high temperature and then rapidly cooled to form a more stable organizational structure.
How To Remove Stainless Steel Rust?
Physical Cleaning
Physical cleaning is the direct removal of rust by mechanical means. This method uses tools and equipment and is suitable for heavy rust stains.
Sandpaper or abrasive block sanding: use fine sandpaper (e.g. 320 mesh to 600 mesh) to gently sand the rust stains and gradually polish it to a smooth surface. Be careful not to use too rough sandpaper when sanding, so as not to cause scratches on the stainless steel surface.
Wire brush or metal brush: for more stubborn rust spots, you can use a wire brush to clean. Be sure to rinse with water after cleaning to remove the bristles and rust flakes.
Pickling Treatment
Acid washing is an effective chemical method, usually using a dilute acid solution to remove rust and oxides.
Choose the right acid: you can use dilute sulfuric acid, dilute hydrochloric acid, or a special stainless steel pickling solution. When used, the appropriate acid should be selected according to the guidance of professionals or the advice of professionals.
Operation steps: Immerse the stainless steel parts in the acid bath for a few minutes, observe the rust removal, then remove and rinse thoroughly with water. Passivation is required after treatment to increase corrosion resistance.
Chemical Cleaning Aagent
There are a variety of special stainless steel cleaners or rust removers on the market, which can effectively remove rust from the stainless steel surface.
Choose the right product: these cleaners usually contain detergent ingredients that can break down rust spots. According to the product instructions, apply the cleaner and allow a short natural reaction before rinsing off with water.
Electrolytic Polishing
Electrolytic polishing is an efficient method of removing rust with the help of electrochemical reactions, especially suitable for large areas or complex shapes of stainless steel surfaces.
Steps: Place the stainless steel workpiece in a tank containing electrolyte and electrify it to strip off the surface metal to achieve the effect of removing rust. This method can maintain the luster of the stainless steel surface.
Advantages: electrolytic polishing can simultaneously remove rust and increase the surface smoothness, with a better aesthetic effect.
FAQ
Placing stainless steel in water does not cause it to rust. The passive chromium oxide layer of stainless steel is still protected in water. As water is a relatively mild solution, it will not adversely affect the chemical structure of stainless steel, and therefore will not destroy its surface passive chromium oxide layer. However, it should be noted that stainless steel in heavily polluted water, such as corrosive seawater, will not have complete corrosion protection.
304 stainless steel will rust under certain conditions, although 304 stainless steel has good corrosion resistance, it is easy to corrode and rust in certain environments, such as contact with environments containing many chloride ions (such as salt, soil, sweat, seawater, etc.), or after the passivation layer on its surface is destroyed. In addition, if the solid solution treatment of 304 stainless steel is not in place, it will also lead to its poor corrosion resistance, which is more prone to rusting.
316 stainless steel may corrode and rust under certain specific conditions, such as exposure to high chloride ion concentrations (e.g., seawater, salt spray environments, etc.), In addition, if the surface is scratched or damaged, localized rusting may result. Overall, 316 stainless steel has good corrosion resistance in most environments, but protection is still required in some special environments.
Stainless steel jewelry will not rust. Stainless steel jewelry has a very tough texture and corrosion-resistant properties, and will not blacken like silver jewelry, nor will it be easily allergic to copper jewelry or toxic due to the presence of lead.1 Stainless steel jewelry can resist corrosion in the air or in chemically corrosive mediums, and it does not need to undergo surface treatments such as color plating to give play to its inherent corrosion-resistant properties. Although stainless steel jewelry is not susceptible to rust, it may still rust under certain specific conditions, such as when it comes into contact with strong acids or media containing corrosive substances. In addition, the protective film on the surface of stainless steel if damaged, such as adhesions, organic juices, acid and alkali salts accumulation of substances, may also lead to rusting.