Surface roughness of stainless steel: How to choose the right finish
Whether it’s a pressure transmitter, valve, pressure sensor or pipework: at first glance, stainless steel surfaces appear smooth and even. Under a microscope, however, a different picture emerges. Even polished stainless steel exhibits minute elevations and depressions. This surface roughness can significantly affect the function and service life of a component. Surface quality plays a particularly important role in chemical, food and pharmaceutical applications. Among other things, it affects cleanability, product deposits, hygiene and corrosion resistance. However, it is not always the case that the smoother the surface, the better. The key factor is that the surface is suitable for the specific application.
What appears to the naked eye as a smooth surface is, from a technical point of view, a profile consisting of minute elevations and depressions. Surface roughness describes the microscopically small irregularities on a technical surface. It is caused, amongst other things, by turning, milling, grinding, polishing or other machining processes. To assess a surface technically, its roughness is measured. One of the most important parameters here is the arithmetic mean roughness value Ra [µm].
Ra value – the arithmetic mean roughness value
The Ra value is the arithmetic mean of the magnitudes of all profile deviations along the measurement path. The arithmetic mean roughness value does not provide any information about the distribution of roughness across the measurement length, but only about the average roughness profile. The Ra value is standard on technical drawings. The lower the Ra value, the smoother the surface.
Schematic surface profile: The Ra value (Ra) describes the arithmetic mean deviation of the roughness profile from its centre line (m). The lower the Ra value, the more uniform or smoother the surface in question is.
Ball valve vs. butterfly valve: A direct comparison
Not every stainless steel surface needs to be finished to the same standard. The requirements for a pipe in general plant engineering differ from those for a surface in contact with the product in a pharmaceutical plant. Typical guide values are:
| Application | Typical reference value Ra |
|---|---|
| General mechanical and plant engineering | ≤ 3,2 µm |
| Chemical processing plants | ≤ 1,6 µm |
| Food industry – product contact | ≤ 0,8 µm |
| High-quality food and beverage processing equipment | ≤ 0,5–0,8 µm |
| Pharmaceuticals / biotechnology | ≤ 0,5 µm |
| High-purity / particularly critical applications | ≤ 0,25 µm |
| Ultra-high-purity applications such as semiconductor chemistry | ≤ 0,2 µm |
For hygienic product-contact surfaces, Ra ≤ 0.8 µm is considered the established guideline. In pharmaceutical and biotech applications, finer surface finishes are often required. The actual surface roughness required depends on the medium, the process, the material, the cleaning method and the specific function of the component.
Hygiene and cleanability: Why is surface roughness important?
Imagine a stainless steel surface in a plant that comes into contact with the product. During operation, it looks completely unremarkable. It is only during cleaning that it becomes apparent just how crucial its texture can be.
The rougher a surface is, the more likely it is that residues will become lodged in its depressions. This is particularly relevant in the food, beverage and pharmaceutical industries. A suitably finished stainless steel surface facilitates cleaning and reduces the risk of product residues or contaminants remaining. For surfaces in contact with the product, therefore, it is not just the Ra value that is decisive. The surface should also:
- be free from pores, cracks, creases, burn marks and indentations,
- not have any sharp burrs or deep grinding marks,
- be easy to clean,
- not encourage the build-up of product or microorganism deposits, and
- have adequate corrosion resistance.
Particular attention must be paid to the weld seams. Even with a base material with Ra ≤ 0.8 µm, poor-quality weld seams, discolouration, pores or burn-through marks can significantly compromise hygienic quality.
Food, pharmaceuticals or mechanical engineering: Different requirements
Grinding, polishing and electropolishing
The desired surface quality can be achieved using various machining processes. During grinding followed by polishing, surface roughness can be reduced in stages, for example:
Ra 1.6 → 0.8 → 0.5 → 0.25 µm
The specific machining process required depends, amongst other things, on the initial condition of the material and the desired final value. Electropolishing is used in particular in pharmaceutical, biotech and high-purity applications. In addition to producing a very smooth surface, it can also improve surface quality and passivation. The final requirements should always be determined on the basis of the application, the medium, cleaning in CIP and SIP processes, the corrosion load, and the applicable standards and customer specifications.
Good to know
A low surface roughness alone does not guarantee a hygienic surface. Even a surface with Ra ≤ 0.8 µm can be problematic if, for example, pores, welding defects, dead spaces or unfavourable geometries are present. In hygienic applications, therefore, it is the interplay of surface roughness, surface structure, geometry, freedom from defects, weld seam quality, dead spaces, leak-tightness and corrosion resistance that counts.
Two common mistakes when selecting
Not necessarily. An extremely smooth surface may be technically beneficial, but this is not always the case. If a finer finish is required without any specific benefit, manufacturing effort and costs increase. Therefore: Define the surface quality based on the actual application.
A component with Ra ≤ 0.8 µm is not automatically hygienic. Dead spaces, seals, weld seams, geometry and cleanability also play a key role. Therefore: Consider surface quality and hygienic design together from the outset.
Surface roughness at TRI-MATIC
TRI-MATIC AG’s pressure transmitter units for food and pharmaceutical applications come as standard with a surface roughness of Ra ≤ 0.8 µm.
Ball valves with a polished internal surface and a Ra value of ≤ 0.8 µm are also available for relevant applications. TRI-MATIC can provide the relevant surface roughness certificate for documentation purposes.
Our quick guide for practical application
- Hygiene and product contact? → Take surface quality and hygienic design into account right from the start.
- Food, beverages or pharmaceuticals? → Ra ≤ 0.8 µm is often an important guideline; depending on the application, finer surfaces may be required.
- Standard industrial application? → Do not specify unnecessarily high surface quality.
At TRI-MATIC, you can obtain individual components and receive support in the selection and design of pressure gauges, pressure transmitters, valves, pneumatic components and assemblies. This allows technical requirements such as medium, pressure, temperature, hygiene and surface quality to be taken into account jointly right from the start.
Frequently asked questions
For stainless steel surfaces in contact with products in the food and drink industry, Ra ≤ 0.8 µm is considered the established guideline. In pharmaceutical, biotech and high-purity applications, significantly finer surfaces – for example, Ra ≤ 0.5 µm or ≤ 0.25 µm – may also be required, depending on the process.
However, the Ra value alone is not the decisive factor. Surface defects, weld seams, scratches, pores, voids, material quality and cleanability also influence hygienic suitability. The surface roughness actually required therefore always depends on the medium, the process, the cleaning method and the applicable standards or customer specifications.
No. Whilst an Ra value of 1.6 µm is twice as high as an Ra value of 0.8 µm, it cannot be concluded from this that the surface is twice as poor or twice as unsuitable. Ra is a statistical parameter for the average profile deviation. The actual performance of a surface also depends on the profile shape, machining process, direction of striations and individual surface defects. An Ra value should therefore never be considered in isolation. The decisive factor is whether the surface meets the requirements of the specific application.
The answer does not begin with a number, but with the application. Five questions can help when selecting a component:
1. Does the surface come into contact with the medium?
If so, the material, surface quality and chemical resistance are particularly important.
2. Is it a hygienic process?
In food, pharmaceutical or biotechnology applications, increased requirements regarding cleanability and surface quality must be taken into account.
3. Is CIP or SIP used?
In automated cleaning and sterilisation processes, the surface and design must be appropriately engineered.
4. Does the surface have a sealing function?
If so, the roughness, sealing material and mating surface must be coordinated.
5. Is the required surface quality actually necessary from a technical point of view?
Unnecessarily complex surface treatment incurs additional costs. The technical requirement is therefore decisive, not the lowest possible Ra value.
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