The main difference between 304 and 316 stainless steel lies in their chemical composition, which directly impacts their properties, particularly corrosion resistance.
Both are austenitic stainless steels, meaning they share a similar crystalline structure and are generally non-magnetic in their annealed state. They both contain significant amounts of chromium (typically 18%) and nickel (typically 8-10%). This composition gives them good general corrosion resistance, formability, and weldability.
The Key Difference: Molybdenum
The crucial distinction is that 316 stainless steel contains molybdenum (typically 2-3%), while 304 stainless steel generally does not (or only contains trace amounts).
Impact of Molybdenum in 316 SS:
- Enhanced Corrosion Resistance: Molybdenum significantly increases resistance to pitting and crevice corrosion, especially in environments containing chlorides (like seawater, de-icing salts, and certain industrial chemicals).
- Improved Resistance to Acids: It offers better resistance to many acids, such as sulfuric acid and acetic acid, compared to 304.
- Slightly Increased Strength at High Temperatures: Molybdenum provides a minor improvement in strength and creep resistance at elevated temperatures.
Chemical Composition Comparison (Typical %):
| Element | Type 304 | Type 316 |
|---|---|---|
| Chromium (Cr) | 18.0 – 20.0 | 16.0 – 18.0 |
| Nickel (Ni) | 8.0 – 10.5 | 10.0 – 14.0 |
| Molybdenum (Mo) | Trace | 2.0 – 3.0 |
| Carbon (C) Max | 0.08 | 0.08 |
| Manganese (Mn) Max | 2.00 | 2.00 |
| Silicon (Si) Max | 0.75 | 0.75 |
Summary of Differences:
- Corrosion Resistance: 316 is significantly more resistant to chlorides and many industrial chemicals than 304. 304 offers good resistance to atmospheric corrosion and many organic/inorganic chemicals but is susceptible to pitting in chloride environments.
- Cost: 316 is generally more expensive than 304 due to the addition of molybdenum and slightly higher nickel content.
- Applications:
- 304: Widely used for kitchen equipment, food processing, architectural trim, storage tanks, automotive trim, general-purpose stainless steel wire.
- 316: Preferred for marine applications, chemical processing equipment, pharmaceutical equipment, medical implants, coastal architecture, and environments with high salt exposure.
- Mechanical Properties & Formability: Largely similar, though 316 might be slightly less formable in some operations.
In essence, choose 304 for general-purpose applications with good corrosion resistance. Choose 316 when superior resistance to pitting corrosion, particularly from chlorides, is required.
