The most commonly specified 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 equally fail cleaning validation (CIP).
One-sentence judgement
Material 316L (not 304), contact surface Ra controlled within 0.8 μm, welds ground flush and acid-pickled and passivated, structurally free of dead corners. Only when all four are met simultaneously can it be considered "cleanable"; meeting only the Ra requirement is the most common semi-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 also has clause requirements for cleanability, absence of dead corners, and connection methods;
- EHEDG: the European hygienic engineering system, which likewise uses Ra ≤ 0.8 μm as a common benchmark, but places more emphasis on cleaning validation (cleanability) and design details: slope, drainage, no threaded connections, sealing form.
The common ground between the two is important: they are not about "how bright it is polished", but about "whether it can be cleaned clean". 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 away 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 the material flow direction → microscopically forms "interception grooves", harder to clean than a surface with the same Ra but along the flow direction;
- Weld heat tint not removed → missing passive 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 parent metal, whether post-weld acid pickling or electropolishing is done, whether the polishing line direction follows the material flow direction.
How the process meets this requirement
- Mechanical polishing: conventionally can achieve Ra 0.4–0.8 μm, lower cost, controllable line direction;
- Electropolishing (EP): can achieve Ra 0.2–0.4 μm, and the surface is chromium-enriched (thicker and more uniform passive film), commonly used in pharmaceutical and high-cleanliness applications;
- Weld treatment: grind flush with the parent metal, then acid pickling and passivation or local electropolishing; untreated post-weld seams are the most common non-conformity in this industry;
- Material: cleaning agents are mostly chlorine-containing alkaline/acidic, 316L is the lower limit; 304 will sooner or later 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, iron contamination causes 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 problems exist equally;
- Purchasing only writes "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 required), cleaning method and applicable standard. We can control the surface and weld treatment according to the drawing, and provide batch material certificates and surface inspection records.
