A nitrogen complex is four liquids, not one

An ammonia plant, a nitric acid plant, an ammonium nitrate neutraliser and a urea unit are drawn as one block on a site plan and bought as one project. Metallurgically they have almost nothing in common. Ammonia is stored as a cold liquid at minus 33 C. Nitric acid runs at 55 to 65 per cent and 60 to 80 C. Ammonium nitrate is handled as a melt at 130 to 140 C, and the phosphoric acid on an NPK line is a different chemistry again.

The grade map follows the liquid, and the first rule is not the one most buyers expect.

Ammonia is carbon steel with a cold clause

Anhydrous ammonia boils at minus 33 C at atmospheric pressure, and carbon steel is the correct material for the tank, the pump casing and most of the pipe. What matters is not the grade but the impact test: the steel has to show toughness at the design metal temperature, which is a Charpy requirement rather than a chemistry one. A106 seamless pipe and A53 welded pipe are both normal here, and both are ordered with a low-temperature test when the line can go cold.

The second clause is stress relief. Carbon steel in ammonia service is exposed to a recognised cracking mechanism, controlled by stress relieving the welds and by holding the product inside its specified water content. Neither clause is expensive to design in. Both are expensive to retrofit.

Nitric acid rewards the leaner grade

Nitric acid at 60 per cent and 60 C is a strongly oxidising medium, and that changes which alloy wins. 304L passivates in it and is the standard material for absorption towers, bleaching columns and storage tanks. 316L is the reflex choice for anything called acid, and here the reflex is wrong: in a strongly oxidising acid such as nitric, the molybdenum-free grade resists it better than the molybdenum-bearing one, not worse.

The leaner grade also welds better in the field. 304L pipe carries 0.03 per cent carbon maximum, so a joint that cannot be solution annealed still holds its intergranular resistance, and the range runs from 3 to 2,200 mm outside diameter with 0.08 to 170 mm wall. That covers the tower internals and the transfer main in one specification, which is worth more on site than a marginal corrosion gain.

The chloride arrives with the raw material

Ammonium nitrate is made by neutralising nitric acid with ammonia, and the neutraliser runs at 130 to 140 C at close to neutral pH. In that window chloride is the controlling impurity. It comes in with the nitric acid, with the process water and, on an NPK line, with phosphoric acid and recycled dust. A few hundred milligrams per litre is enough to start stress corrosion cracking in an austenitic grade that is otherwise comfortable in the duty.

The answer is a chloride limit on the raw materials plus a grade that survives an upset. 316L, with molybdenum at 2 to 2.5 per cent, is the common choice for the neutraliser and the evaporator, not because it resists nitric acid better than 304L but because it handles the chloride that 304L cannot. The purchase specification should carry the chloride limit, and the incoming certificate should be checked against it rather than filed.

Phosphoric acid moves the order to 904L and 825

Wet-process phosphoric acid is not a pure acid. It carries chloride and fluoride from the rock, and the concentration train runs at about 80 C and 30 per cent P2O5, which is enough to take 316L out of the picture. 904L is the first stop: nickel at 24 to 26 per cent, molybdenum at 4 to 5 per cent, copper between 1.2 and 2 per cent and carbon at 0.02 per cent, with sulphuric and phosphoric acid named on its application list and intergranular resistance held in the welded condition.

Where the stream is a mixture and oxidising and reducing conditions alternate, Alloy 825 is the broader answer. It is a nickel-iron-chromium alloy with molybdenum, copper and titanium, built for both oxidising and reducing media, resistant to chloride stress corrosion and pitting, and stocked in plate between 4.8 and 50.8 mm thick. The choice between them is usually made by the fluoride number rather than by the acid number.

Fluoride is where the stainless ladder ends

The fluoride in phosphoric acid is the reason part of the plant is not stainless at all. Hastelloy C-276, with 15 to 17 per cent molybdenum and 14.5 to 16.5 per cent chromium, is among the few materials that withstand wet chlorine, hypochlorite and chlorine dioxide, and it is used where the acid is both reducing and contaminated. It is not a vessel material in this duty. It appears as nozzles, dip pipes and liner plate, and the shell stays 904L.

The building sees the product too

Ammonium nitrate is hygroscopic, and a bagging shed after ten years shows it. Dust settles on the structure, pulls moisture from the air and holds a concentrated salt solution against galvanized steel and painted plate. Z275 galvanized coil, at 0.12 to 3 mm and Z60 to Z275 coating, is the right envelope for the cladding, but the frame behind it needs a coating system that tolerates nitrate, and the handrails are better in 304 square tube than in galvanized pipe.

Q345 plate, 0.3 mm to 200 mm thick with 6,000 tonnes a month behind it, covers the structural steel, the tank saddles and the pipe supports, and it is cheap enough to specify generously. The mistake is leaving it bare in a shed where the floor is washed every shift.

What goes on the enquiry

Five lines, one per stream: carbon steel with an impact test for ammonia, 304L for nitric acid, 316L for the neutraliser and evaporator, 904L or 825 for phosphoric, and C-276 for the fluoride-rich nozzles. Add the certificate to EN 10204 3.1, an intergranular corrosion test to ASTM A262 where the grade is welded, and a welding procedure that ends with pickling and passivation rather than a wire brush.

The grade map is not a preference. On a nitrogen site it is the part of the specification that the process chemistry actually decides, and it is settled long before the first enquiry reaches a mill.