The most commonly written requirement in food, beverage, and dairy equipment procurement is "contact surface Ra ≤ 0.8 μm." This is correct, but it is only the entry line: a tank that meets Ra but has weld depressions will still fail cleaning validation (CIP).
One-Sentence Verdict
Use 316L material (not 304), control contact surface Ra within 0.8 μm, grind welds flat and pickle/passivate, and ensure no dead corners in the structure. Only when all four are met simultaneously can it be considered "cleanable"; meeting only the Ra requirement is the most common half-finished product.
What the Two Systems Require
- 3-A Sanitary Standards: The US dairy/food equipment system, which sets a clear upper limit on contact surface roughness (commonly Ra ≤ 0.8 μm) and has clauses on cleanability, absence of dead corners, and connection methods;
- EHEDG: The European hygienic engineering system, which also uses Ra ≤ 0.8 μm as a common baseline but places more emphasis on cleaning validation (cleanability) and design details: slope, drainage, no threaded connections, seal types.
The common ground between the two is important: they are not about "how shiny it is ground" but about "how clean it can be washed." Ra is the means; cleaning validation is the end.
Why Ra Is Not Enough
Ra is the arithmetic mean deviation of the profile — it averages out a deep pit and a wavy area. Therefore, the following can occur:
- Overall Ra 0.6 μm passes, but there is a 0.3 mm pit at the weld root → material accumulation, bacterial growth;
- Polishing direction perpendicular to material flow → microscopically forms "interception grooves," making it harder to clean than a surface with the same Ra but parallel to flow;
- Weld heat tint not removed → missing passive film, leading to pitting even if Ra meets the standard.
In actual acceptance, besides Ra, one should also check: presence of visible pits/scratches, whether welds are ground flush with the base metal, whether post-weld pickling or electropolishing is done, and whether polishing lines follow the material flow direction.
How to Achieve This Requirement in Processing
- Mechanical polishing: Typically achieves Ra 0.4–0.8 μm, lower cost, controllable line direction;
- Electropolishing (EP): Can achieve Ra 0.2–0.4 μm, with a chromium-rich surface (thicker and more uniform passive film), commonly used in pharmaceutical and high-cleanliness applications;
- Weld treatment: Grind flush with base metal, then pickle/passivate or local electropolish; untreated welds after welding are the most common nonconformity in this industry;
- Material: Cleaning agents are mostly chlorine-containing alkaline/acidic; 316L is the minimum; 304 will eventually develop rust spots under frequent CIP.
Common Pitfalls
- Only sampling and measuring Ra, without checking welds and dead corners — audits look at structure and cleaning results;
- Using carbon steel tools to grind welds, causing iron contamination and local rust spots, misjudged as "wrong material";
- Using 2B sheet directly as food grade: 2B is a rolled surface; although Ra is low, it has waviness, and weld issues remain the same;
- Procurement only specifies "food-grade 316L" without stating Ra upper limit, polishing method, and weld treatment requirements, making it impossible to judge upon delivery;
- Ignoring gasket and seal materials — seals are often the source of cleaning dead corners.
For food/pharmaceutical equipment, tell us your contact surface requirements (Ra upper limit, polishing method, whether electropolishing is needed), cleaning method, and applicable standards. We can control surface and weld treatment according to drawings and provide batch material certificates and surface inspection records.
