Types & Causes of Stainless Steel Corrosion
2025/11/6
Stainless steel is known for its excellent corrosion resistance, but it is not completely immune to rust. Corrosion occurs when metals react with oxygen and moisture in the air, and factors such as salt, humidity, or harsh chemicals can accelerate the process. This article explains the main types of stainless steel corrosion, the causes behind it, and key ways to prevent damage.
Why Stainless Steel Resists Corrosion
Stainless steel contains higher amounts of chromium and nickel compared to other metals. When exposed to oxygen and moisture, chromium atoms form an invisible passive film only a few nanometers thick.
This passive film protects the underlying metal from oxidation.
Even if damaged, the film self-repairs by reacting immediately with oxygen in the air.
Surface treatments like chrome plating and nickel plating also rely on this property to prevent rust.
Common Causes of Stainless Steel Corrosion
While stainless steel is more resistant than many metals, certain conditions can still damage or weaken its passive layer:
Environmental Conditions: Exposure to water, humidity, acidity, etc.
Surface Contamination: Dust, dirt, or other contaminants can accumulate over time on the surface and interfere with the protective film.
Harsh Cleaning Chemicals: Harsh chemicals may damage the passive layer.
Poor Design: Avoid designs with gaps, crevices, or areas where water can pool, which encourages localized corrosion.
- Why Stainless Steel Resists Corrosion
- Common Causes of Stainless Steel Corrosion
- Uniform Corrosion
- Pitting Corrosion
- Crevice Corrosion
- Intergranular Corrosion
- Galvanic Corrosion (Dissimilar Metal Contact Corrosion)
- Material Selection Based on Environment
- Use Near the Ocean or with Seawater
- Materials Resistant to Everyday Water
- Nitric Acid
- Sulfuric Acid
- Hydrochloric Acid
- Alkali
- Influence of Added Elements on Corrosion Resistance
- Chromium (Cr)
- Molybdenum (Mo)
- Carbon and Nitrogen (C, N)
- Sulfur (S)
- Niobium and Titanium (Nb, Ti)
- Copper (Cu)
- Key Takeaway
Uniform Corrosion
Uniform corrosion occurs when the entire surface of the metal corrodes evenly over time. Most outdoor corrosion falls into this category, where rust progresses slowly in environments with low oxidizing power.
Pitting Corrosion
Pitting corrosion is a localized form of corrosion that attacks small, specific spots on the surface. A common example is guardrails along coastlines, where chloride from sea spray causes pinpoint rusting. In high-chloride environments, there may not be enough chromium to maintain the passive film, allowing corrosion to penetrate at those points.
Crevice Corrosion
Crevice corrosion also occurs in localized areas but specifically inside narrow gaps or joints between components. Oxygen supply is limited in these spaces, preventing the passive film from reforming. This makes crevice corrosion one of the most frequent types of stainless steel corrosion in seawater environments.
Intergranular Corrosion
Intergranular corrosion happens due to heat from welding or heat treatment. At elevated temperatures, chromium and carbon can combine to form chromium carbides. This process reduces the amount of chromium available for the passive film, leaving the surrounding areas vulnerable to corrosion.
Galvanic Corrosion (Dissimilar Metal Contact Corrosion)
Galvanic corrosion occurs when two different metals come into contact in water. One metal becomes the positive pole (anode) and corrodes, while the other becomes the negative pole (cathode) and is protected. The corrosion rate depends on the surface area ratio between the two metals.
Material Name | Material Characteristics |
|---|---|
SUS304L (18Cr-8Ni Low C) | Improved intergranular corrosion resistance (reduced carbon content). Like SUS304, it's a general-purpose material. |
SUS316 (18Cr-12Ni-2.5Mo) | Improved corrosion resistance and acid resistance (increased Ni and added Mo). It's an enhanced version of SUS304 and is used in applications requiring pitting corrosion resistance, such as coastal areas. |
SUS316L | Improved intergranular corrosion resistance due to reduced carbon content. Used in roofing, exterior materials, water pipes, sewage pipes, and water heaters. |
SUS316Ti (18Cr-12Ni-2.5Mo-Ti) | Improved intergranular corrosion resistance (added Ti). |
SUS316J1 (18Cr-12Ni-2Mo-2Cu) | Improved acid resistance (added Cu). It has better pitting corrosion resistance than SUS316 and is suitable for environments using sulfuric acid. |
SUS317 (18Cr-12Ni-3.5Mo) | Has superior pitting corrosion resistance to SUS316. |
SUS317L | Improved intergranular corrosion resistance. Used as a material for dyeing equipment. |
SUS317J1 (18Cr-16Ni-5Mo with increased Ni, Mo content) | An enhanced version of SUS317L. Suitable for environments where SUS317L cannot withstand. |
SUS317J3L (Increased Ni, Mo, added N) | Has superior pitting corrosion resistance to SUS317. |
SUS309S (22Cr-12Ni) | Improved oxidation resistance (increased Cr, Ni content). While its corrosion resistance is better than SUS304, its primary use is as a heat-resistant steel. Used in jet engine parts and various high-temperature environments. |
SUS310S (25Cr-20Ni) | Has improved oxidation resistance compared to SUS309S, and its primary use is as a heat-resistant steel. |
SUS312L (20Cr-18Ni-6Mo-0.7Cu-0.2N-Low C) | Contains a high amount of Mo and can handle various acidic environments. Used in seawater equipment, exterior building materials in salt-damaged areas, and sulfuric acid plants. |
SUS321 (18Cr-8Ni-Ti) | Further improved intergranular corrosion resistance (added Ti). Used in jet engine parts, various industrial machinery, etc. |
SUS347 (18Cr-8Ni-Nb) | Further improved intergranular corrosion resistance (added Nb, Ta). |
SUSXM15J1 (18Cr-13Ni-4Si) | Improved stress corrosion cracking resistance. Can handle environments containing chloride ions. |
SUS836L (22Cr-25Ni-6Mo-0.2N-Low C) | Has superior pitting corrosion resistance to SUS317. |
SUS890L (21Cr-24.5Ni-4.5Mo-1.5Cu-Reduced C) | Has strong properties against seawater environments. |
SUS420J1 (13Cr-0.2C) | Often used as a material for knives. Has strong stress corrosion cracking resistance. |
SUS410 (13Cr) | Improved corrosion resistance (reduced carbon content). A general-purpose metal that can be hardened, used for screws, bolts, etc. |
SUS410J1 (13Cr-Mo) | Improved corrosion resistance of SUS410 by adding Mo |
SUS430 (18Cr) | Improved corrosion resistance (increased Cr). A general-purpose ferritic metal. |
SUS431 (16Cr-2Ni) | Has superior corrosion resistance to SUS410 and SUS430. |
SUS630 (17Cr-4Ni-4Cu-Nb) | Improved corrosion resistance of SUS431. Reduced carbon content, increased Ni. Provides precipitation hardening. |
SUS444 (19Cr-2Mo-Ti, Nb, Zr-Reduced C, N) | Enhanced corrosion resistance by adding a large amount of Mo, for stress corrosion cracking resistance. |
SUS447J1 (30Cr-2Mo-Reduced C, N) | Strong against acetic acid environments, and possesses stress corrosion cracking resistance and pitting corrosion resistance. |
SUS329J1 (25Cr-4.5Ni-2Mo) | Improved pitting corrosion resistance (increased Cr, Ni, Mo content). Used in water gates, seawater pumps, and flue gas desulfurization equipment. |
SUS329J2 (22Cr-5Ni-3Mo-N-Reduced C) | Improved pitting corrosion resistance (increased Ni, Mo content). Used in aqueduct bridges, receiving tanks, chemical tankers, chemical plants, and salt production plants. |
SUS329J3 (25Cr-6Ni-3Mo N-Reduced C) | Improved pitting corrosion resistance (increased Cr, Ni, Mo content). |
Material Selection Based on Environment
Use Near the Ocean or with Seawater
SUS304 and SUS316 provide moderate corrosion resistance, but harsher seaside environments require higher-performance grades such as SUS312L, SUS836L, SUS890L, and SUS329J4L. Alloys with 25Cr-7Ni-3Mo offer good pitting resistance. In seawater environments, it's important to consider whether chlorides and biological buildup can be regularly cleaned and removed.
Materials Resistant to Everyday Water
SUS430: Common in low-cost kitchenware and home appliances.
SUS316L: Used in water pipes, sewage lines, heaters, and high-temperature food equipment (e.g., noodle tanks). Its acid resistance also makes it suitable for seasoning storage.
Nitric Acid
At room temperature and concentrations of about 20%, nearly all stainless steels can withstand nitric acid.
At concentrations of 65% or higher, and at boiling temperatures, only SUS304 and SUS316 offer sufficient resistance.
Ferritic grades such as SUS430, and martensitic grades such as SUS410 and SUS420J1, are not suitable.
Sulfuric Acid
Up to ~10% concentration at room temperature: Stainless steels equal to or better than SUS316 can resist corrosion.
At boiling temperatures: SUS316 cannot withstand even 5% concentration.
Materials alloyed with molybdenum or with a combination of molybdenum and copper offer better corrosion resistance.
Hydrochloric Acid
Hydrochloric acid is highly aggressive to stainless steel and should generally be avoided. Alternative materials other than stainless steel are recommended for these environments.
Alkali
High-nickel stainless steels perform well in alkaline environments. For caustic soda (sodium hydroxide) at room temperature up to 50% concentration, most stainless steels are suitable. However, at higher concentrations, the likelihood of corrosion increases significantly.
Influence of Added Elements on Corrosion Resistance
Chromium (Cr)
Chromium is the main component that forms the passive film, and increasing its content enhances corrosion resistance. Stainless steel requires a minimum of 12% chromium content.
Example: SUS304 (18% chromium, 8% nickel).
Molybdenum (Mo)
Molybdenum strengthens the passive film.
Example: SUS316 (18% chromium, 8% nickel, 2% molybdenum).
Carbon and Nitrogen (C, N)
Improves pitting and crevice corrosion resistance.
Example: SUS836L (22% chromium, 25% nickel, 6% molybdenum, low carbon, with nitrogen).
Sulfur (S)
Improves machinability but lowers corrosion resistance.
Example: SUS303 (18% chromium, 8% nickel, 0.3% sulfur).
Niobium and Titanium (Nb, Ti)
Improves corrosion resistance of welded areas.
Examples: SUS347 (niobium), SUS321 (titanium).
Copper (Cu)
Enhances acid resistance.
Example: SUS312L (20% chromium, 18% nickel, 6% molybdenum, 0.7% copper, low carbon).
Key Takeaway
Selecting the appropriate stainless steel depends on the usage environment and application.
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