What Is the Verified Sheath Thickness for 316 Stainless Steel Heaters in 25% Ammonium Nitrate Solution at 110°C with 3 Bar Pressure and No Chlorides

Oxidising Fertiliser Service: The Fundamental Trade-off

A 25% ammonium nitrate (NH₄NO₃) solution is a common intermediary in fertiliser manufacturing and as a heat transfer fluid in some industrial operations. The atmosphere is highly oxidising at 110°C and 3 bar pressure because of the presence of nitrate ion which is a cathodic inhibitor and in fact passivates 316 stainless steel. Pitting is the main worry when chloride containing solutions are used. In the case of ammonium nitrate two different failure mechanisms are to be aware of: stress corrosion cracking in concentrated nitrate solutions and general corrosion if the pH drifts outside of the stable range. Without chlorides the thickness decision is much easier. For this function, the verified sheath thickness is governed by mechanical integrity, uniform corrosion allowance, and nitrate-induced cracking resistance. Thermal efficiency is a minor but important consideration.

Electrochemical behaviour in NO3-solutions

The nitrate ion is a very strong oxidising agent and it protects the passive film on 316 stainless steel over a broad range of temperatures and concentrations. The corrosion potential in 25% ammonium nitrate at 110°C averages between +200 and +400 mV versus a silver-silver chloride reference electrode and is well within the passive region. The uniform corrosion rate in such environments is very low, in the order of 0.002-0.008 mm per year based on weight loss studies of equipment used in the fertiliser business. Uniform corrosion allowance for ten years is 0.02-0.08 mm for 1.2 mm wall heater and the sound metal is more than 1.1 mm. In this application, the thickness is not driven by uniform corrosion. The passive film is stable if the solution pH is between 4 and 9. The ammonium ion is a weak acid, hence ammonium nitrate solutions naturally have a pH in the range of 5-6, which is a stable pH range. Corrosion rate can be increased if the solution becomes contaminated with acid (from upstream process upsets) or alkali (from cleaning agents). However, such upsets are usually short term and do not require large corrosion allowance.

Risk of Nitrate Stress Corrosion Cracking

Stress corrosion cracking (SCC) is the principal concern for 316 stainless steel in hot ammonium nitrate solutions. Nitrate SCC is widely established for austenitic stainless steels and the susceptibility increases with temperature and nitrate concentration. At 110°C and 25% ammonium nitrate, the risk of SCC is moderate, but not negligible. The crucial factors for initiating SCC are tensile stress (from manufacture or pressure) and the existence of a concentrated nitrate coating on the surface. Nitrate SCC can develop over a wide potential range, whereas chloride SCC occurs at specific potentials. The crack propagation rate of nitrate SCC in 316 stainless steel at 110°C is generally 0.05-0.2 mm/year after initiation. The fundamental concern for a heater sheath is the residual tensile stresses from tube drawing and bending. In general, the lower the wall thickness, the lower the residual stresses, because less cold work is necessary to attain the final dimension. A 1.2 mm drawn tube usually contains residual hoop stresses of 50-100 MPa, whereas a 2.0 mm tube may have 80-150 MPa. The thicker wall has higher residual stress, which increases the probability of SCC initiation. Actual field experience with ammonium nitrate evaporators has shown that thin-walled (1.2-1.5 mm) 316 stainless steel heaters outlast thick-walled (2.0-2.5 mm) heaters on a consistent basis. Failure analysis has shown that thick-wall failures are caused by SCC while thin walls fail from other, non-corrosion causes after extended service.

Pressure Containment - 3 Bar

At 3 bar system pressure the hoop stress requirement is quite little. For a tube with OD = 12 mm and wall = 1.2 mm, hoop stress is 0.3 MPa x 4.8 mm / 1.2 mm = 1.2 MPa. For a wall that is 2.0 mm the stress is 0.3 MPa x 4.0 mm/2.0 mm = 0.6 MPa. Both are small compared to the 170 MPa yield strength. The thickness of the pressure containment does not need to be special. But the heater must be mechanically robust enough to resist the installation torque at the threaded connections. For reliable engagement you need a minimum thread engagement thickness of 1.0-1.2 mm. Less than 1.0 mm, the risk of thread stripping or distortion increases substantially, particularly in bigger diameter heaters.

Process Heating Needs and Thermal Efficiency

The thermal conductivity of ammonium nitrate solutions is comparable to that of water, i.e. 0.60-0.65 W/m K at 110°C. The convective heat transfer coefficient for turbulent flow is 1000-2000 W/m²·K depending on velocity. The thermal resistance of the sheath is about 0.0004 m²·K/W for a wall thickness of 1.2 mm and 0.0007 m²·K/W for a wall thickness of 2.0 mm. This difference corresponds to a sheath temperature difference of 3-4°C at a watt density of 10 W/cm² (common for circulation heaters). For a process susceptible to localised overheating, it is crucial to maintain the sheath temperature as low as possible. Ammonium nitrate can degrade exothermically above 180°C. A 1.2 mm wall heats 110°C bulk solution at 10 W/cm² to about 125-130°C at the sheath surface. A 2.0 mm wall is in operation at 128-134 °C. The decomposition temperature of ammonium nitrate is about 200 °C . Both nevertheless are much lower and the thermal difference is not safety significant . However the thinner wall is a little better for energy efficiency.

Confirmed thickness guidelines from industry experience

The fertiliser sector has a large amount of operational experience with 316 stainless steel equipment in ammonium nitrate service. The immersion and circulation heaters have proven sheath thickness range of 1.2-1.6 mm for long-term dependability. Use of thinner walls below 1.0 mm is prevented by mechanical handling problems. The thicker walls above 1.8 mm are also avoided as they demonstrate higher residual stress and a higher likelihood of SCC failure. Within the proven range, 1.4-1.5 mm is considered the best compromise, thick enough for mechanical strength and a moderate corrosion allowance, but thin enough to minimise residual stress and SCC risk. For a design life of 10 years in 25% ammonium nitrate at 110°C and 3 bar pressure with no chlorides present a wall thickness of 1.4 mm is completely adequate. Facilities with good handling standards may also use a 1.2 mm wall. The 1.6 mm wall gives an extra mechanical margin but no corrosion or SCC advantage over the 1.4 mm.

Thicker Walls That Are Actually Worse

Thicker walls are not needed in ammonium nitrate service, and may be hazardous. The increased residual stresses resulting from the manufacture of thicker tubes increase the driving force for SCC start. When a crack develops, the thicker wall has more material to fracture yet the crack growth rate is independent of thickness. A 2.0mm wall with a crack growth of 0.1mm/year will take 20 years to perforate, a 1.2mm wall will take 12 years. But the thicker wall has a larger risk of crack start. Field data from ammonium nitrate factories indicate that heaters with 2.0-2.5 mm walls fail from SCC in 3-6 years, while heaters with 1.2-1.5 mm walls from the same plant operate for 8-12 years and often fail from electrical insulation degradation, rather than sheath perforation. The proven result is that thinner walls are superior in nitrate SCC situations because of reduced residual strains.

Comparison of Verified Thickness Choices

Residual Hoop Stress Probability of SCC Initiation Thickness of SheathUniform Corrosion Allowance (10 years) Mechanical Robustness Proven Service Life in Field
1.0 mm 40-80 MPa Low 0.02-0.08 mm Marginal 8-10 years (depends on handling)
1.2 mm 50-90 MPa Low 0.02-0.08 mm Adequate 10-12 years
1.4 mm 60-100 MpaLow-Moderate 0.02-0.08 mmGood 10-12 years (better)
1.6 mm 70-110 MPa Medium 0.02-0.08 mm8-10 years Very Good
2.0 mm 90-140 MPaModerate-High 0.02-0.08 mm Excellent 5-7 years (SCC-limited)
2.5 mm 110-160 MPa High 0.02-0.08 mm Excellent 3-5 years (SCC-limited)
Conclusion: Thickness Confirmed for Long Term Reliability

316 stainless steel heaters in 25% ammonium nitrate solution at 110°C with 3 bar pressure without chlorides the proven sheath thickness range is between 1.2-1.6 mm with 1.4-1.5 mm being appropriate for most installations. From a corrosion point of view, thinner walls below 1.2 mm are acceptable, but are at danger of mechanical damage during installation and cleaning. The thicker walls above 1.6 mm contain larger residual stresses which raise the danger of nitrate stress corrosion cracking, and hence have lower service lifetimes, even with the higher amount of metal. This is contrary to the general engineering assumption that in this environment thicker is always better for corrosion resistance. This is due to the fact that the major failure mechanism (SCC) is driven by residual stress, which increases with thickness. Engineers selecting a heater for ammonium nitrate duty should specify a maximum permissible residual stress (usually 100 MPa) rather than a minimum thickness. This guarantees that the producer employs properly annealed tube with low residual stress, which is more significant than the absolute wall thickness. Most applications can be specified with 1.4-1.5 mm in solution-annealed condition to provide the confirmed combination of mechanical robustness, SCC resistance and thermal efficiency for a ten-year service life.