The most commonly written requirement when purchasing food, beverage, and dairy equipment 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 also 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 and passivate, and have 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 has a clear upper limit for contact surface roughness (commonly used value Ra ≤ 0.8 μm), and has clause requirements for 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, sealing forms.
The common ground between the two is important: they are not about "how shiny it is ground," but about "whether it can be cleaned." 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 is perpendicular to the material flow direction → microscopically forms "interception grooves," making it harder to clean than a surface with the same Ra but along the flow direction;
- Weld oxidation color not removed → missing passivation film, pitting corrosion occurs even if Ra meets the standard.
In actual acceptance, besides Ra, one should also check: whether there are visible pits/scratches, whether welds are ground flush with the base material, whether pickling or electropolishing is done after welding, and whether the polishing grain direction follows the material flow direction.
How to Apply Processes to Meet This Requirement
- Mechanical polishing: Conventionally can achieve Ra 0.4–0.8 μm, lower cost, controllable grain direction;
- Electropolishing (EP): Can achieve Ra 0.2–0.4 μm, and the surface is chromium-rich (passivation film is thicker and more uniform), commonly used in pharmaceutical and high-cleanliness applications;
- Weld treatment: Grind flush with base material, then pickle and passivate or perform local electrolysis; untreated welds after welding are the most common nonconformity in this industry;
- Material: Cleaning agents are mostly chlorine-containing alkaline/acidic; 316L is the lower limit; 304 will eventually develop rust spots in frequent CIP applications.
Common Pitfalls
- Only sampling and measuring Ra, not inspecting 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, which are 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 problems exist equally;
- Purchasing only specifies "food-grade 316L," without specifying Ra upper limit, polishing method, and weld treatment requirements, making it impossible to judge upon arrival;
- Ignoring gasket and seal materials — seals are often the source of cleaning dead corners.
For food/pharmaceutical equipment, tell us the 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.
