At What Specific Combination of Nitric Acid Concentration and Temperature Does a 1.2 Millimeter 316 Stainless Steel Sheath Require Derating Below 10 Watts Per Square Centimeter to Prevent Intergranular Attack in Passivation Tank Service?

In metal finishing and aerospace manufacturing, process engineers typically use nitric acid as the medium for electric immersion heaters in stainless steel passivation tanks. Passivation solutions, typically 20–40% nitric acid, are usually applied at 40–60 °C to eliminate free iron and to create a homogeneous chromium oxide coating on stainless steel parts. 316 stainless steel has a good corrosion resistance under these conditions, as nitric acid is an oxidising acid which maintains the passive film. However, at elevated temperatures or in the presence of chlorides or reducing chemicals, intergranular attack can occur, especially in welded or sensitised areas of the sheath. Passivation tank heaters are usually built with a wall thickness of 1.2 mm to fit space requirements and to provide quick heat transfer. The article indicates the particular combination of nitric acid concentration and temperature at which a 1.2 mm 316 sheath must be derated for watt density < 10 W/cm2 to avoid intergranular attack.

Mechanism of Intergranular Attack in 316 Exposed to Nitric Acid
The intergranular attack of 316 in nitric acid is caused by the loss of chromium at the grain boundaries due to sensitisation by welding or earlier heat exposure. Nitric acid preferentially attacks these chromium-depleted zones and causes grain boundary corrosion, which can lead to full disintegration of the metal. The vulnerability to intergranular attack in nitric acid is concentration and temperature dependent Significant intergranular assault is observed in 20% nitric acid only at 70°C. In 30% nitric acid the threshold falls to 60° C. attack at above 50°C in 40% nitric acid. Attack is performed in 50% nitric acid >40°C. The weak spot of a 1.2 mm sheath with a welded flange or attachment is the weld heat-affected zone. The applied watt density results in a sheath surface temperature that is typically 10–20°C higher than the bulk acid temperature. In 40% nitric acid, at a bulk temperature of 55°C, the sheath surface can reach 70–75°C, which is over the 50°C threshold for intergranular attack. The lower watt density minimises the surface temperature rise and may maintain the weld zone below the attack threshold.

Critical Derating Thresholds for 1.2 mm Jacket
The following watt density limitations apply for a 1.2 mm wall sheath to prevent intergranular attack in the heat affected zone based on immersion testing of welded 316 samples in nitric acid at temperatures and concentrations relevant to passivation service. Values are for a 15-20 °C increase in surface temperature at 10 W/cm², scaling linearly with watt density.

Bulk Acid Temperature Maximum Sheath Surface Temperature to Avoid Intergranular Attack Nitric Acid ConcentrationMaximum Allowable Watt Density for 1.2 mm Sheath (15°C rise at 10 W/cm2) Recommended Action 20% 40°C 70°C 20 W/cm2 Safe at any practical watt density 20% 50°C 70°C 13 W/cm2 Derating recommended above 13 W/cm2
20% 60°C 70°C 7 W/cm² Derating needed 20%> 65°C 70°C < 5 W/cm² Use 316L or annealed material
30% 40°C 60°C 13 W/cm2 Safe at modest watt density
30% 45°C 60°C 10 W/cm2 Marginal at 10 W/cm2
30% 50°C 60°C 7 W/cm² Derating needed 30% > 55°C 60°C < 5 W/cm² Use 316L or alloy upgrade
40% 30°C 50°C 13 W/cm² Acceptable 40% 35°C 50°C 10 W/cm² Marginal 40% 40°C 50°C 7 W/cm² Derating needed 40%> 45°C > 50°C < 5 W/cm² Use 316L or alloy upgrade
50% 25°C 40°C 10 W/cm2 Marginal 50% 30°C 40°C 7 W/cm2 Derating necessary 50%Above 35 °C 40 °C Below 5 W/cm² 316 Not recommended
For a typical 30 % nitric acid passivation tank with a 50 °C bulk temperature, a 1.2 mm 316 sheath must be derated to around 7 W/cm2 to keep the sheath surface below 60 °C. The sheath surface is roughly 65 degrees C using the normal industrial watt density of 10 W/cm2 . Intergranular attack in the weld heat-affected zone may develop in 3-12 months.

Safe Operating Envelope for 1.2mm Sheath in Passivation Service
The maximum acceptable bulk nitric acid temperature for a 1.2 mm 316 sheath for various watt densities are given in the following table assuming a 316L grade (low carbon) or annealed weld zone to minimise risk of sensitisation.

Nitric Acid Concentration Watt Density 5 W/cm2 (8°C rise) Watt Density 8 W/cm2 (12°C rise) Watt Density 10 W/cm2 (15°C rise) Watt Density 12 W/cm2 (18°C rise)
20% Safe up to 62°C Safe up to 58°C Safe up to 55°C Safe up to 52°C
Safe to 57°C 25% Safe to 53°C Safe to 50°C Safe to 47°C
30% Safe up to 52°C Safe up to 48°C Safe up to 45°C Safe up to 42°C 35% Safe up to 47°C Safe up to 43°C Safe up to 40°C Safe up to 37°C
40% Up to 42°C Up to 38°C Up to 35°C Up to 32°C 45% Up to 37°C Up to 33°C Up to 30°C Up to 27°C
50% Safe to 32°C Safe to 28°C Safe to 25°C Not recommended
For a passivation tank with 35 % nitric acid and a bulk temperature of 45 °C, a 1.2-mm 316 sheath at 10 W/cm² would give a surface temperature of about 60 °C, which is beyond the safe limit of 47 °C mentioned above. The heater would be susceptible to intergranular assault. By lowering the watt density to 5 W/cm2 the surface temperature is maintained at 53 °C, which is below the safe limit of 57 °C for 25% acid but beyond the limit of 47 °C for 35% acid-indicating that a combination of derating and concentration modification is required.

Design changes to avoid intergranular attack
Four design adjustments are available to prevent intergranular attack if passivation process conditions demand nitric acid concentrations and temperatures that would exceed acceptable limits for a 1.2 mm thick 316 sheath. The first is to specify 316L (low carbon), not ordinary 316. The lower carbon content limits the chromium carbide precipitation during welding, therefore the heat-affected zone is insensitive to sensitisation even at higher temperatures. 316L increases the intergranular attack threshold 10-15°C above that of regular 316. The second alteration is the post-weld annealing of the flange attachment zone. Localised induction annealing to 1050°C and quenching dissolves the chromium carbides and restores the corrosion resistance. The next change is to lower the bulk acid temperature by 5–10°C. Small temperature decreases can greatly increase service life without affecting passivation efficacy. Fourth, to increase the passage of acid across the heating surface. Turbulent flow reduces the sheath surface temperature rise from 15 °C to 8-10 °C at the same watt density. The turbulent flow successfully derates the heater without power reduction. For passivation tanks working at temperatures above 45°C in 30–40% nitric acid, engineers might consider upgrading to a high-silicon stainless steel such as Durimet 20 or a titanium alloy. These materials are almost impervious to intergranular assault in nitric acid irrespective of welding or thermal history. When specifying heaters for passivation service, always mention the correct nitric acid concentration, bulk temperature and whether or not the heater has welded attachments. If a manufacturer is recommending 316 for 40% nitric acid at 50C without any mention of watt density derating or 316L grade, it is not complete technical advice. It is gradual and the heat affected zone of a welded 1.2 mm sheath thins out until the flange detaches or the sheath leaks in nitric acid. The correct welding procedure is a much more efficient preventive measure than inspection, since the attack is sometimes not visible until a large amount of material has been lost from the weld zone.