Salt arrives by the tonne and never leaves the liquor

A dyehouse buys salt the way a foundry buys sand: in bulk, against a recipe that changes with the shade. Reactive dyes need electrolyte to drive the molecule onto the fibre, and the concentration is not a trace. Most recipes sit between 50 and 80 grams per litre, and a 1,000 kg jet machine running at a liquor ratio of 1:8 holds 8,000 litres. That is 400 to 640 kg of salt in one batch, six or eight times a day, three hundred days a year.

The salt is sodium chloride or sodium sulphate, chosen on cost and dye class rather than on the metallurgy of the machine. From the steel's side the two are not interchangeable. One delivers chloride, the other sulphate, and only the chloride attacks an austenitic stainless at temperature.

At 130 C the bath becomes a stress corrosion test

Polyester is dyed at 130 C under 3 to 4 bar in a vessel that stays sealed for the whole cycle. Sodium chloride at 60 g/L puts roughly 36 g/L of chloride into the liquor, which is a different duty from a coastal wash-down. The surfaces are hot, the liquor is stagnant in the gaps, and no fresh oxygen reaches a damaged passive film.

316L is what most machines are built from, and its 2 to 2.5 per cent molybdenum buys pitting resistance that 304 does not have. It does not buy immunity. Cracks begin where a gasket traps liquor, at a weld with heat tint still on it, or under a dye deposit baked onto the shell.

The machine is zoned, not built from one grade

Machines that last are specified in zones. Shell and main coil are 316L in 2B, smooth enough to clean between shades and ductile enough to roll. The heat exchanger is a separate argument. Pipework carrying the hottest, most concentrated liquor is often 317LMN, whose molybdenum sits between 4 and 5 per cent and nitrogen between 0.10 and 0.20 per cent, placing it ahead of 316L and 317L in acidic chloride service. The frame and outer panels can be 304, in square tube and No.4 sheet, because they see condensate rather than liquor.

That zoning is also a purchasing decision. One mill order for three grades is three certificates, three heat numbers and three sets of mechanicals, and the machine builder who ignores it ends up with a spare coil of 317LMN on the shelf for two years.

The first leak is almost always the heat exchanger

A plate pack is a stack of crevices by design: process liquor at 130 C on one side, cooling water at 30 C on the other, gaps of a few millimetres and a gasket at every plate. Scale forms on the cooling side, chloride concentrates underneath it, and a plate perforates from a pit nobody could see. 254 SMO is the usual step up, with molybdenum between 6.0 and 6.5 per cent, nitrogen between 0.18 and 0.20 per cent and a yield strength of 310 MPa; high-halide tanks and pipelines are on its application list.

Titanium is the other answer where the cooling water is brackish or the plates run thin. Grade 2 sits at 345 MPa and a density of 4.51 g/cm3, and it corrodes at less than 0.005 mm a year in seawater. It is not universal: fluoride in the water is outside its range, and a dry chlorine atmosphere is a different problem again.

Mercerising is a separate room with a separate problem

Caustic mercerising runs 20 to 30 per cent sodium hydroxide, cold in the common process and up to about 70 C in the hot version, and 316L handles it without complaint. What corrodes in that room is the rinse water and the chloride that arrives with the caustic, not the alkali. The structural steel deserves more attention than it gets: caustic stress corrosion cracking of carbon steel is a real failure mode, and it is answered by stress relief and inspection rather than by a stainless upgrade.

Welds, heat tint and the finish a dyer can see

Sheet is laser cut, rolled and welded, and the weld is purged. Heat tint is removed by pickling, not by a wire brush, because a tinted weld is a chromium-depleted surface sitting in hot chloride and it will pit before the parent metal. Internal surfaces are 2B and external panels No.4, and the difference is not cosmetic: a smooth wall releases the last shade at the next fill, a rough one holds pigment and gives it back.

Gaskets deserve the same discipline. A compressed joint is a crevice, and a joint that has been opened and re-tightened twice is a worse crevice. Spiral wound gaskets with a PTFE facing are normal on the liquor side; the plate pack gaskets are usually EPDM, which sets the top temperature of the exchanger as firmly as the alloy does.

Cleaning with hypochlorite is the habit that shortens a 316L machine most reliably. Where a plant cannot give it up, the wetted parts that see the dosing point move to Hastelloy C-276, which is one of the few alloys listed for hypochlorite and chlorine dioxide. Putting the whole machine in C-276 is not a serious option; putting the dosing spool and the first metre of pipe in it is.

The effluent is a second order with different rules

Spent liquor carries sulphate, chloride, colour and COD to the effluent plant, where equalisation and pH correction matter more than alloy selection. The tank that receives it is a chloride duty with solids in it, and 2205 duplex pipe is the usual step where 316L has already failed once. Water reuse changes the picture. Every recycle loop raises the chloride, and a machine that was safe on fresh water is not automatically safe on the same water after two passes through the works. That is a specification change, not a maintenance item, and it belongs in the next order rather than in the maintenance log.

What goes on the enquiry

Write the grade by zone rather than by machine: 316L coil for the shell, 317LMN tube for the hot liquor, 254 SMO or titanium for the exchanger, 304 for the frame. Add the finish, the thickness and its tolerance, the edge condition, and whether the certificate to EN 10204 3.1 covers chemistry and mechanicals. A 316L coil order at 0.2 to 60 mm thick and up to 2,000 mm wide is a stock item, one tonne minimum and one to two weeks of lead time. The exchanger plates are the long lead, and they are the part the production plan actually depends on.