The chemical that plain steel handles

Caustic soda is a genuinely aggressive chemical with an unusual property: at moderate temperatures plain carbon steel is one of the best materials for it, and the expensive alloy ladder is often unnecessary. A dense magnetite film forms on the steel surface and protects it, which is why a carbon steel tank can store 50 percent sodium hydroxide for decades while far milder chemicals eat through stainless. The material pages make the practical case: Q235 plate, A36 and Q345 are stocked from 0.3 to 200 mm in widths of 1000, 1250 and 1500 mm, and A106 seamless pressure pipe and A53 pipe cover the transfer lines. The question is not whether carbon steel works. It is where it stops working, and the answer is temperature plus stress.

Where carbon steel stops: the stress corrosion cliff

Above roughly 50 degrees Celsius in general practice, unrelieved carbon steel weldments in caustic service become susceptible to caustic stress corrosion cracking, the old name for which is caustic embrittlement. The failure is intergranular, it starts at welds and cold-formed regions where residual stress is highest, and it produces cracks rather than thinning, which means a wall thickness measurement will not see it coming. The mitigation is thermal, not chemical: post-weld heat treatment of the vessel and of field welds, a stress relief that removes the driving force before the equipment enters service. The plate welding page carries fabrication and weld repair work, and the honest enquiry for hot caustic service names the stress relief requirement rather than leaving it to be inferred. Concentration raises the stakes as well, because hot concentrated liquor is the demanding combination even though dilute caustic is close to harmless on carbon steel.

Watch the zinc and the neighbours

Two metals that otherwise live happily in a chemical plant misbehave around alkali. Zinc is amphoteric, and strong sodium hydroxide dissolves it, so the Z275 galvanized coil that protects roof sheet and cable trays across the rest of the site is the wrong skin for anything that collects caustic condensate, and hot dip galvanized handrails above a caustic tank should be specified with that chemistry in mind. The same amphoteric behaviour rules out aluminium in wet caustic. Neither fact changes the general use of the hot dip galvanized H-beam on tank farm structures, but it does decide which members sit inside the splash zone rather than beside it.

Stainless in the middle of the ladder

Austenitic stainless takes dilute and moderate caustic comfortably, and it is the default where product purity or chloride contamination of the liquor matters. The 304/304L coil and 304 plate listing carry the general purpose grades, the 316/316L coil adds molybdenum where chlorides ride along with the caustic, and stainless pipe and 304/304L pipe cover the transfer side. Where concentration and temperature both climb, the lean duplex grades hold the line: the 2205 page describes yield strength about twice that of austenitic grades with a service window of minus 50 to plus 600 degrees Fahrenheit, available as S31803 seamless pipe, and LDX 2101 offers 65 ksi minimum yield at 21.5 percent chromium and only 1.5 percent nickel. The duplex coil listing stocks 0.2 to 60 mm in widths to 2000 mm with a one tonne minimum and seven to fifteen working days after deposit.

Where nickel earns its price

Hot concentrated caustic is classic nickel territory, and the ladder within the nickel alloys maps directly onto temperature. Monel 400, at 63 percent nickel minimum, is published with high resistance to alkalis and excellent resistance to neutral and alkaline salts, with good mechanical properties from sub-zero temperatures up to 1020 degrees Fahrenheit. Alloy 825 is listed for alkalis including sodium and potassium hydroxide alongside its acid resistance, with plate stocked from 4.8 to 50.8 mm, though the same page warns that exposure above about 1000 degrees Fahrenheit changes the microstructure and lowers ductility and impact strength. Alloy 600 is the caustic specialist of the group, its bulletin naming resistance to caustic corrosion directly and its approval running across ASME Code Sections I, III and VIII, which matters when the equipment is a code vessel. Alloy 625 states the broadest claim, that in alkaline environments, salt water, fresh water and neutral salts almost no attack occurs, with strength from cryogenic to 2000 degrees Fahrenheit. On the acid side of the same plants C-276 handles the oxidising chlorides that caustic systems often neighbour, and 904L carries the sulphuric and phosphoric duties between them.

The titanium exception

Titanium sits on the list for a different reason. The titanium rod page describes the passive TiO2 film that withstands dilute acids, alkaline solutions including 50 percent sodium hydroxide, and oxidants, with Grade 2 at 345 MPa minimum tensile and a seawater rate below 0.005 mm per year. It is rarely the volume choice for caustic storage, but for internal hardware, heat transfer surfaces and fasteners inside mixed liquor systems it outlasts everything cheaper.

High temperature neighbours

Where the same plant runs evaporators and steam systems, the adjacent duties pull in the heat-resistant grades. 310S is published for continuous service to 2000 degrees Fahrenheit and is stocked as 310S pipe; the stabilised grades take the intermediate band, with 321 titanium stabilised for 1000 to 1600 degrees Fahrenheit and 347H pipe at 515 MPa tensile and 205 MPa yield. For tank farm structures the steel structure page lists the welded H sections and purlins, and shop services cover cutting, bending, shearing, punching and surface treatment, so a caustic tank shell or a full bund and pipework package is ordinary procurement rather than a special order.

The five lines that decide the material

Concentration, metal temperature, chloride content of the liquor, the stress state of the weldments, and whether the equipment is a code vessel. Carbon steel with stress relief covers the classic storage and transfer range; 304L and 316L cover clean and dilute duties; lean duplex covers strength at modest cost; the nickel alloys cover hot concentrated liquor, with Monel 400, 825, 600 and 625 each documented on their own pages; and titanium covers the hardware that must never be the first thing replaced. Quote those five lines in the enquiry and the vendor's answer becomes a grade, not a guess.