The word on the enquiry, and what it leaves out

A dairy, brewery or ready-meal plant rarely sends an enquiry that says hygienic. It sends a drawing with a grade in the title block, a finish note, a thickness, and a line about washdown. The grade decides whether the steel survives the cleaning chemistry; the surface and the joint decide whether anything survives on the steel. The 304 grade page explains where the reputation came from, an 18/8 austenitic that deep draws into sinks and saucepans, and a food plant then asks that grade to work in a room washed with chlorinated water twice a shift.

Finish is a process sequence, not a shine

The 304/304L coil listing sets out the finishes in the order a mill makes them. 2B is the cold rolled route: heat treat, pickle or an equivalent, then a final light skin pass for lustre. BA leaves a bright anneal without the pickling. No.3 and No.4 are polished with No.100 to No.120 and No.150 to No.180 grit under JIS R6001. No.1 is hot rolled, heat treated and pickled, and its market is chemical tank and pipe.

The same table lists 2B for food industry and kitchen utensils, which is accurate and incomplete. 2B is a rolled surface and rolling leaves a profile: typically 0.4 to 1.0 micrometre Ra, against 0.1 to 0.5 for bright annealed strip. Where a customer writes 0.8 micrometre for food contact, or 0.4 for pharmaceutical duty, those figures come from hygienic design practice, not from a mill guarantee. Specify the finish on the face that touches product: the 316 and 316L page lists 2B, BA, No.4, 8K, HL, satin and mirror, and the mill will not guess which face is the product side.

Where 304 runs out, and why the answer is not always 316

The 316/316L sheet data sheet states the case: the molybdenum addition improves general corrosion and pitting and crevice resistance over 304, the alloy handles hot organic and fatty acids in food and pharmaceutical work to minimise product contamination, and it resists boiling 20 per cent phosphoric acid. The warning inside the same paragraph gets skipped. In strongly oxidising acids such as nitric acid, molybdenum-bearing grades are less resistant than 304, so a CIP regime built on nitric acid and a tank specified in 316 because 316 feels safe are two decisions pulling apart. 316L rather than 316 belongs where a heat-affected zone would sensitise: not the base metal, the strip beside the weld. The 316L coil page lists food processing and medical industry first, at 7.93 g/cm3 with a plus or minus one per cent thickness tolerance.

Brine moves the ladder. Fifteen per cent sodium chloride at ambient temperature is mild; the same brine in a plate pack, at velocity, pits 316L in time. 2205 duplex pipe runs from 1/2 to 12 inch nominal bore in schedules 20 to 80 with a 550 MPa yield and 800 MPa tensile, and the 2205 plate page quotes pitting and crevice resistance superior to 316L and 317L in almost all media at roughly twice the austenitic yield. Twice the yield is also a thinner wall for the same tank, which changes weld volume, weight and cooling rate at every seam, and the coil form is stocked as duplex coil for shells.

Where a plant wants the molybdenum without the duplex, 317LMN tube and 317LMN sheet carry 4.0 to 5.0 per cent molybdenum and 0.10 to 0.20 per cent nitrogen, and the 317LMN grade page makes the case for staying fully austenitic in acidic chloride service, since the low carbon keeps it free of grain-boundary carbide as welded. 254 SMO goes further, at 6.0 to 6.5 per cent molybdenum and nearly twice the strength of the 300 series. 904L brings 4 to 5 per cent molybdenum, 1.2 to 2 per cent copper and a 0.02 per cent carbon ceiling that holds the welded condition against intergranular attack, which is why it appears in phosphoric and sulphuric duty. C-276 covers the hypochlorite and chlorine dioxide a sanitation cycle can introduce, and Alloy 825 plate, stocked from 4.8 to 50.8 mm, names food process equipment beside acids and alkalis.

416HT earns its place for a different reason: a free-machining martensitic with resistance to natural food acids and a frank note that it is less resistant than the austenitics, so it belongs on valve stems and machined fittings, not product contact plate.

The joint decides more than the grade does

A hygienic vessel fails at a weld far more often than in the plate. Heat tint comes first: the oxidised zone beside a TIG bead has had its chromium consumed, and pitting in a chloride wash starts there. Geometry comes second, because a lap joint or a stiffener welded on one side leaves a crevice no CIP cycle reaches. The surface treatment service covers the tint with rust removal by sandblasting, shot blasting and pickling, then polishing where roughness matters, and the plate welding service is where the seam sequence is decided. What the plant has to add is order: remove the tint, passivate, then verify. Pickling alone does not open a crevice.

Frames repeat the argument in a lighter section. Square stainless tube runs from 10x10 to 100x100 mm in 1 to 12 metre lengths and makes a clean, drainable frame for a filler or conveyor, but only if the tube ends are closed and the feet sealed. An open end on a washdown floor is a cup, and what drains out of it a year later is why some plants insist on sealed sections or heavy-wall legs with welded caps. The frame also has to survive the wash chemistry: caustic soda attacks zinc, so a galvanized plate bracket or a Z275 coil panel in a caustic-washed room corrodes at the cut edge and every fixing long before the stainless beside it shows anything.

What to put on the order

Name the grade and the L suffix wherever welding is involved. Name the finish and which face it applies to, with a roughness figure if the customer has one. Take the thickness from the design rather than the last job: cold rolled sheet runs 0.3 to 4 mm and hot rolled to 100 mm, so 4 mm and 5 mm shells are both stock items and the difference is a decision. Say whether the plate is sheared or formed at the mill or in your shop, because bend radius and rolling direction have to agree with the finish, and whether it ships with protective film. For the line itself, 304/304L pipe is listed from 3 to 2200 mm outside diameter with walls of 0.08 to 170 mm, from a sample valve to a transfer main.

Then add the item nobody draws: the cleaning regime. Temperature, caustic concentration, acid type, chloride content of the wash water, and how often the line is opened. A supplier who knows the CIP chemistry can say whether 316L is enough or whether the brine tank should be duplex in S32205 grade. A supplier told only "food grade" will quote 304 with a 2B finish, and both sides will believe the specification is complete.