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General Overview of Stainless Steel Ⅱ
2023-05-31This document aims to provide a basic understanding of stainless steel, focusing on its use as a construction material.
For more specialized information or further assistance, please feel free to contact us, and we will be happy to help.
○ Corrosion Resistance to Various Chemicals
1) Chemical Solutions
Stainless steel has high corrosion resistance to nitric acid and is widely used in the processing equipment for nitric acid and other chemicals containing nitric acid.
2) Cleaning Solutions
Cleaning solutions used to remove dirt or rust from bathroom tiles or exterior and interior building materials can cause discoloration or rust on stainless steel if left on the surface. Therefore, thorough rinsing with water after cleaning is essential.
4. Types of Corrosion
○ General Corrosion: Corrosion that occurs uniformly across the entire surface of a material exposed to various corrosive environments.
○ Localized Corrosion: Corrosion that occurs in specific areas, such as intergranular corrosion, pitting, or stress corrosion cracking.
Classification |
Cause |
Prevention |
|
General Corrosion |
Corrosion that occurs when the entire metal surface exposed to a corrosive environment is uniformly eroded due to electrochemical or chemical reactions. |
- Selection of appropriate material grade for the environment · Marine atmosphere: 316, 316L · High-temperature environment: 321 - Various surface treatments: Mirror finishing, etc. |
|
Localized Corrosion | Intergranular Corrosion | Austenitic steels, when heated in the temperature range of 450–850°C, precipitate chromium carbides at the grain boundaries. As a result, chromium becomes depleted around the grain boundaries, causing the passive film to be eroded and corrosion to progress. · Influence of composition: High carbon content · Precipitation of carbides in the welded heat-affected zone |
- Solution annealing · Fully re-dissolve precipitated carbides at high temperatures, followed by rapid cooling / reduce carbon content (below 0.03%) - Selection of stabilized alloys · Ti, Nb alloyed steels |
| Crevice Corrosion (Pitting) | It is the type of corrosion most sensitive to chlorine, where the passive film is locally destroyed by chlorine, and the affected area continues to dissolve preferentially. · The reaction accelerates as the temperature increases. · When contaminants are present on the product surface, the chlorine concentration effect is enhanced. |
- Prevent contact with chlorine environments - Surface treatment: Polishing (e.g., mirror finish) - Perform heat treatment on welds - Use materials resistant to chlorine environments: Mo-alloyed steels (316, 316L) |
|
| Stress Corrosion Cracking (SCC) | It occurs only in austenitic steels in an environment where tensile stress and chlorine coexist. · The higher the chlorine concentration, the more likely it is to occur in a short period. · The higher the temperature, the greater the likelihood of occurrence. · The likelihood increases when residual stress is concentrated. |
- Remove residual stresses and stress concentration areas after processing - Perform stress relief heat treatment - Prevent contact with chloride environments - Use materials resistant to stress corrosion: Ni-alloyed steels |
|
| Galvanic Corrosion | It is a phenomenon where, when dissimilar metals come into contact, the relatively more anodic metal corrodes first. |
Prevent corrosion first by contacting materials that are electrochemically similar. |
|
※ Sensitization : Carbon has a tendency to combine easily with chromium. When heated to high temperatures, chromium carbide (Cr23C6) forms at the grain boundaries.
As a result, the areas around the chromium carbides become depleted of chromium, and depending on the environment, these chromium-depleted areas may undergo selective corrosion, leading to intergranular corrosion.
The formation of chromium carbides at the grain boundaries, which creates susceptibility to intergranular corrosion, is referred to as "sensitization."
5. Maintaining Stainless Steel Properties
○ Design Considerations
1)Selection of Stainless Steel Grades Based on Environment
A. Rural Areas: Areas with no airborne salt particles and no air pollution
- STS 304 is suitable
- STS 430 can also be used if frequent cleaning is required
B. Urban Areas: Residential, commercial, and light industrial areas with air pollution from automobile exhaust
- STS 316 is suitable
- STS 304 can also be used if frequent cleaning is required
C. Industrial Areas: Areas with air pollution from heavy industries
- STS 316 is suitable
- STS 304 can also be used if frequent cleaning is required
- In locations with harsh corrosive environments, special high-corrosion-resistant stainless steels should be used.
D. Coastal Areas: Areas with airborne salt particles
- STS 316 is suitable
- STS 304 can also be used if frequent cleaning is required
- In locations with harsh corrosive environments, special high-corrosion-resistant stainless steels should be used.
2) Selection of Stainless Steel Grades Based on Building Structure and Area
The degree of corrosion in stainless steel can vary depending on the part of the building.
This is because certain areas of the building may be more prone to the attachment and accumulation of contaminants that destroy the passive film, or because some areas may be difficult to clean with rainwater.
Whether or not rainwater can clean the surface is an important factor in determining the corrosion resistance of stainless steel.
3) Structural Considerations
In addition to selecting appropriate materials based on the environment and area, the following structural considerations help avoid conditions that promote rust formation and make post-construction cleaning easier.
A. The design of stainless steel components should have simple shapes to facilitate easy cleaning of dirt and rust.
B. Protruding parts should be designed to prevent the splashing of rainwater or the flow of dirty water, ensuring the lower parts of the building remain clean.
C. Horizontal surfaces and gaps should be designed to allow for proper condensation drainage and water runoff.
D. Joints should be designed to prevent capillary action and avoid the accumulation of debris.
E. Fasteners such as screws and rivets used in joints should be made of stainless steel.
F. Fasteners, clamps, and reinforcements used on the underside of stainless steel components should be treated with corrosion-resistant coatings or finishes for preservation.
