Surface finish is one of the most important characteristics of stainless steel products, affecting corrosion resistance, appearance, cleanability, and functional performance. Understanding the surface finish standards — including Ra (arithmetic average roughness), Rz (maximum height), and N grade designations — is essential for engineers, architects, and procurement professionals specifying stainless steel in 2026.
What Is Surface Finish in Stainless Steel?
Surface finish refers to the texture, roughness, and reflectivity of a stainless steel surface. It is created during manufacturing through processes such as rolling, grinding, polishing, and pickling. The finish affects not only the visual appearance but also the functional performance of the material in service.
Ra — Arithmetic Average Roughness
Ra (Arithmetic Average Roughness) is the most commonly used parameter for quantifying surface roughness. It represents the arithmetic mean of the absolute values of the profile deviations from the mean line over the evaluation length. Ra is measured in micrometers (μm):
- Ra 0.1 μm: Mirror finish (BA, No.8)
- Ra 0.2-0.5 μm: Fine polished (No.4, satin)
- Ra 0.5-1.0 μm: Standard mill finish (2B)
- Ra 1.0-3.2 μm: Rough mill finish (No.1)
- Ra 3.2-12.5 μm: As-cast or heavy ground surfaces
Rz — Maximum Height of Profile
Rz (Maximum Height of the Profile) measures the vertical distance between the highest peak and the deepest valley within five sampling lengths. It provides a more comprehensive view of surface texture than Ra alone.
| Finish | Ra (μm) | Rz (μm) |
|---|---|---|
| No.8 Mirror | ≤ 0.1 | ≤ 0.5 |
| BA (Bright Annealed) | 0.1 – 0.3 | 0.5 – 1.5 |
| No.4 Satin | 0.2 – 0.5 | 1.0 – 3.0 |
| 2B | 0.3 – 0.8 | 2.0 – 5.0 |
| 2D | 0.5 – 1.5 | 3.0 – 10.0 |
| No.1 Hot Rolled | 1.5 – 5.0 | 8.0 – 25.0 |
N Grade System (ISO 1302)
The N grade system classifies surface roughness into 12 grades (N1 through N12), each corresponding to a range of Ra values:
| N Grade | Ra Range (μm) | Typical Application |
|---|---|---|
| N1 | 0.025 – 0.05 | Optical mirrors, precision gauges |
| N2 | 0.05 – 0.1 | Mirror finish stainless |
| N3 | 0.1 – 0.2 | Bright annealed, fine polish |
| N4 | 0.2 – 0.4 | Satin finish, food processing |
| N5 | 0.4 – 0.8 | Standard 2B finish |
| N6 | 0.8 – 1.6 | General mill finish |
| N7 | 1.6 – 3.2 | Rough machined surface |
Standard Stainless Steel Finishes
No.1 (Hot Rolled Annealed & Pickled): Rough, matte grey surface. Ra 1.5-5.0 μm. Used for industrial applications where appearance is not critical.
2B (Cold Rolled, Bright): The most common standard finish. Smooth, slightly reflective with Ra 0.3-0.8 μm. Used for food processing equipment, chemical tanks, and architectural panels.
BA (Bright Annealed): Highly reflective mirror-like surface. Ra 0.1-0.3 μm. Used for decorative trim, elevator panels, and kitchen appliances.
No.4 (Satin/Brushed): Directional polishing with fine, consistent lines. Ra 0.2-0.5 μm. The most popular architectural and decorative finish.
No.8 (Mirror): True mirror finish with no visible grinding lines. Ra ≤ 0.1 μm. Used for decorative applications, optical components, and clean rooms.
Surface Finish and Corrosion Resistance
Smoother surfaces (lower Ra) provide better corrosion resistance because fewer surface defects reduce initiation sites for pitting and crevice corrosion, and polished surfaces promote formation of a more uniform passive oxide layer. Research has shown that surfaces with Ra below 0.5 μm exhibit significantly improved resistance to chloride-induced pitting in marine environments.
Measurement Methods
- Contact profilometry: A diamond stylus traces across the surface. Most common method. Accuracy ±5%. Standards: ISO 4287, ASME B46.1.
- Optical profilometry: Uses white light or laser interferometry. Non-contact. Better for soft or delicate surfaces.
- Comparators: Visual and tactile comparison to reference specimens. Quick but subjective (ISO 25178-7).
Summary
Understanding stainless steel surface finish standards — Ra, Rz, and N grades — is essential for selecting the right material for each application. Whether specifying No.4 satin for architectural panels, 2B for food processing equipment, or No.8 mirror for decorative applications, proper surface finish specification ensures both aesthetic and functional requirements are met.
CoreMetal Steel supplies stainless steel sheet, plate, and coil in all standard finishes (2B, BA, No.4, No.8) with verified Ra values and mill test certificates.