Two machines cut from the same SS316 sheet can perform very differently after two years in a wet processing plant, and the difference is fabrication. This article covers how hygienic welds are made, why heat tint around a weld must be pickled out before passivation restores the protective layer, what an Ra 0.8 micron surface finish does and does not guarantee, how corner radii and sealed sections affect cleanability, and five questions to put to a stainless fabricator before ordering.
By Havmek · Howl Media Labs
Two machines can be cut from the same SS316 sheet, to the same drawing, and still perform very differently after two years in a wet seafood plant. The material is the same. The difference is fabrication: how the joints were welded, what was done to the weld afterwards, and what the surface looks like at the microscopic level.
This article walks through the three stages that decide whether stainless equipment stays hygienic: welding, post-weld treatment (pickling and passivation), and surface finish. For each one it sets out what the hygienic design standard says and what a buyer can ask to verify. Grade selection, Ra targets and CIP design are covered in our EU hygiene compliance guide; this piece picks up where that one stops, at the workshop floor.
For product-contact and product-zone surfaces in seafood processing, the working grade is SS316 (1.4401), or its low-carbon version SS316L (1.4404). EHEDG's hygienic design guidance notes that adding molybdenum to SS304 to create SS316 improves its corrosion resistance, which matters wherever brine, seawater and chloride-based cleaning chemicals are present.
The L grade is worth knowing about for welded fabrications. SS316L limits carbon to 0.03 percent, against up to 0.07 percent in standard SS316, and the lower carbon figure reduces the risk of sensitisation, the grain boundary carbide precipitation that weakens corrosion resistance in the heat-affected zone. Whichever grade is used, ask for EN 10204 3.1 certification confirming alloy composition on delivery. Fabrication can then damage a good material in three ways: welding heat, iron contamination from carbon steel tooling, and surface damage that creates crevices. The rest of this article is about preventing those.
EHEDG Document 8 is direct on the first requirement: permanent metal to metal product contact joints must be continuously welded and free of imperfections. Welds on the non-product side must also be continuous and smooth enough to allow proper cleaning. An intermittent weld or an unsealed overlap leaves a gap where product, water and detergent collect and where cleaning cannot reach.
The second requirement is protecting the weld from oxygen. The same document notes that inert gas protection of both the torch side and the opposite side of the weld may be required. On tube and pipe, this means an argon back purge: the inside of the joint is flooded with argon so the hot root of the weld never meets air. Orbital welding, where an automated head rotates around the tube, is the usual way to make this repeatable, and it records the welding parameters for each joint.
On thin sheet, which is what most frames, guards and covers are made from, distortion and burn-through are the risks. EHEDG Document 9 exists specifically for the hygienic welding of thin-walled (under 3 mm) stainless steel. The payoff of getting the weld right first time is stated plainly in Document 8: if welding is carried out properly, the need for post-weld grinding and polishing is minimised. A weld that needs heavy dressing to look acceptable was not a good weld to begin with.
The straw, blue or brown discolouration around a weld looks cosmetic. It is not. Heat tint is a chromium-depleted oxide layer, and the metal beneath it is the first part of the equipment to corrode. Chromium is what gives stainless steel its protective passive film, so a zone that has lost chromium has lost the protection.
Good purging is the first defence. One Australian food plant guidance note puts the target at keeping oxygen below 50 ppm in the purge atmosphere while the metal is hotter than about 250°C, which holds the root of a tube weld to no more than a pale straw tint. Tighter limits are common in hygienic specifications, so ask for the figure and the monitoring record, not just a statement that the weld was purged.
Where heat tint does form, passivation alone will not fix it. Passivation acids do not dissolve heat tint, so the weld zone has to be pickled or mechanically cleaned first, which removes the damaged layer. Passivation then removes free iron from the surface and lets the chromium oxide film rebuild. ASTM A967 defines the accepted passivation treatments, citric and nitric methods among them, while ASTM A380 covers the cleaning and descaling that comes before.
Stage | What it does | What goes wrong if skipped |
|---|---|---|
Purged, continuous weld | Keeps oxygen off the hot weld root and leaves no gap to trap soil | Heavy heat tint inside the joint; crevices that cleaning cannot reach |
Pickling or mechanical cleaning | Removes the chromium-depleted heat tint layer | Passivation acid cannot touch the tint; weakened metal stays in service |
Passivation (ASTM A967) | Removes free iron and rebuilds the protective chromium oxide film | Iron contamination from tooling shows up as rust staining on new equipment |
Verification | Confirms the surface is free of iron, for example by ferroxyl test | Passivation is claimed on paper but nobody can show it happened |
Verification is the point most buyers miss. ASTM A967 recognises practical tests, including the ferroxyl test, where the indicator turns blue in the presence of free iron, and a water immersion or high humidity test of 24 hours or more, checked for rust staining. A fabricator that passivates properly will have a test record to show you.
EHEDG Document 8 states that large product contact areas should have a surface finish of 0.8 µm Ra or better, and that a rougher surface is acceptable if test results show the required cleanability is achieved. The same section adds the line that matters most for buyers: it is the topography which governs the cleanability.
Ra is an average of surface height deviations. It says nothing about the shape of the surface, so two finishes with the same Ra can clean differently. This is why electropolishing needs careful wording. Document 8 describes it as rounding off peaks without necessarily improving Ra. An electropolished surface can be easier to clean and more corrosion resistant, but it should not be treated as a synonym for Ra 0.8 µm or better. ASTM B912 separately covers passivation of stainless steels using electropolishing, so electropolishing can also serve as the passivation step.
The practical question for a buyer is where the finish is measured. Plate arrives at a certified finish; the weld seam is where it is most likely to be lost. Ask what the finish is on the weld and how it was checked.
Hygiene is also built into shapes. EHEDG Document 8 says internal corners should preferably have a radius of 6 mm or larger, with 3 mm as the minimum, and that sharp corners of 90 degrees or less must be avoided. A tight internal corner is a soil trap whatever its finish.
Frames need the same discipline. The document requires that supports for piping and equipment are fabricated and installed so that no water or soils can remain on the surface or within the supports. In practice that means hollow section ends sealed by welding rather than left open, and no fabricated pockets that hold wash water. Frames and supports are also where fabrication meets system integration: the same attention to joints and drainage that applies to a freezer carries through to the conveying system it feeds, as covered in our conveyor systems buyer's guide.
BEFORE YOU ORDER
1. Grade and certificate. Is this SS316 or SS316L, and will EN 10204 3.1 certification confirm the molybdenum content of the delivered material?
2. Weld method. Are all product contact welds continuous and gas shielded on both sides, and is oxygen monitoring recorded?
3. Post-weld treatment. Are welds pickled or mechanically cleaned, then passivated to ASTM A967, and can you see the ferroxyl or equivalent test record?
4. Finish at the weld. What is the surface finish on the weld seam itself, how was it measured, and is it described by Ra alone?
5. Geometry. Are internal radii at least 3 mm (6 mm preferred), and are hollow sections sealed?
The answers are only as good as the records behind them, which is where a certified quality system earns its place. Our guide to what ISO 9001 certification means for an equipment supplier covers how to check that weld and inspection records exist before you rely on them.
Fabrication sits between choosing precision components and integrating a complete line. Get it wrong and the hygiene specification on the drawing is not the hygiene you get on the floor. For a wider view of what to ask at the specification stage, see How to Spec a Seafood Processing Line: 7 Questions to Ask Before You Buy.
Pickling removes the damaged, chromium-depleted heat tint layer around a weld using acid or mechanical cleaning. Passivation then removes free iron from the surface and allows the protective chromium oxide film to rebuild. Passivation acids do not dissolve heat tint, so pickling or mechanical cleaning has to come first. ASTM A967 defines accepted passivation treatments and ASTM A380 covers the cleaning that precedes them.
Heat tint is a chromium-depleted oxide layer. Because chromium provides stainless steel's protective film, the metal beneath heat tint corrodes first and can hold contaminants. It is controlled by purging the weld with an inert gas such as argon so oxygen cannot reach the hot metal, and removed by pickling or mechanical cleaning followed by passivation.
No. EHEDG Document 8 requires large product contact areas to be Ra 0.8 µm or better and notes that cleanability is governed by surface topography. It describes electropolishing as rounding off peaks without necessarily improving Ra. Electropolishing can help cleanability and corrosion resistance, but the Ra value should still be specified and measured separately.
EHEDG Document 8 requires permanent product contact joints to be continuously welded and free of imperfections, with welds on the non-product side also continuous and smooth enough to clean. Properly shielded welds show no more than a pale straw tint and need minimal grinding or polishing afterwards.
SS316 (1.4401), or low-carbon SS316L (1.4404) for welded fabrications, is the working grade for product contact and product zone surfaces. Its molybdenum content improves resistance to chloride from brine, seawater and cleaning chemicals compared with SS304. Request EN 10204 3.1 certification to confirm composition on delivery.
Havmek delivers end-to-end seafood processing systems built on engineering excellence and sustainable innovation. We combine India’s seafood processing expertise with Europe’s precision engineering, compliance standards, and sustainability ethos to deliver robust, scalable, and hygienic solutions. The solutions cover every stage of the seafood value chain; from grading and glazing to IQF freezing, packaging, and cold storage. All systems are designed around plant layout, throughput requirements, and export-grade quality standards with certified to ISO 9001, ISO 14001, and ISO 45001.
