At What Specific Combination of Organic Acid Concentration and Sheath Surface Temperature Does a 1.6 Millimeter 316 Stainless Steel Sheath Transition from Passive to Active Corrosion in Acetic Acid Service for Pharmaceutical Reactors?

Process engineers developing electric immersion heaters for pharmaceutical reactors and food processing vessels often utilise acetic acid as solvent and reaction medium. Organic acids (e.g. acetic) exhibit complicated corrosion behaviour on 316 stainless steel, in contrast to mineral acids, with passive regions at low concentrations and active corrosion at intermediate concentrations. The passive-active transition can be shifted considerably by the presence of trace chlorides or oxygen. Typical pharmaceutical service would be a wall thickness of 1.6 mm to allow for corrosion allowance and cleanability. This paper reports the combination of acetic acid concentration and sheath surface temperature at which a 1.6 mm 316 sheath changes from passive to active corrosion in pharmaceutical reactor service.

Corrosion of 316 in Acetic Acid
316 stainless steel corrosion by acetic acid has a particular pattern. In dilute liquids at concentrations < 10 % and temperatures < 80 °C, 316 stays passive with corrosion rates < 0.05 mm per year. In the concentration range of 10%–50% the passive film is unstable and the corrosion rates can be 0.5–2.0 mm/year, depending on the temperature and oxygen content. Above 50% concentration acetic acid is reducing and 316 passivates again with rates below 0.1 mm/year. The presence of oxygen or air in the system moves the transition to greater concentrations by maintaining oxidising conditions. Acetic acid concentrations in pharmaceutical reactors range from dilute wash solutions to concentrated reaction medium. The temperature of the sheath surface determines if the heater is in the passive or active regime. Due to watt density this temperature is normally 10-25°C higher than the bulk acid temperature.

1.6 Millimetre Sheath Critical Transition Limits for
The following concentration and sheath surface temperature combinations cause the passive to active transition from immersion testing of 316 samples in acetic acid with regulated oxygen content (typical of pharmaceutical reactors with air headspace).

Acetic acid concentration (wt %) Transition temperature from passive to active (in air)Deaerated Passive-to-Active Transition TemperatureMaximum permissible sheath surface temperature for passive operation Expected corrosion rate in active regime
< 5% > 100°C > 90°C 95°C .3 -.6 mm/year 5 - 10% 90 - 100°C 80 - 90°C 85°C .5 - 1.0 mm/year
10 – 15% 80 – 90°C 65 – 75°C 75°C 0.8 – 1.5 mm/year 15 – 20% 70 – 80°C 55 – 65°C 65°C 1.0 – 2.0 mm/year 20 – 30% 60 – 70°C 45 – 55°C 55°C 1.5 – 2.5 mm/year
30-40% 50-60°C 35-45°C 45°C 1.5-2.5 mm/year 40-50% 45-55°C 30-40°C 40°C 1.0-2.0 mm/year
50 – 60% 55 – 65°C 40 – 50°C 50°C 0.5 – 1.0 mm/yr
Above 60% Above 80 °C Above 65 °C 75 °C 0.2 – 0.5 mm/year
For a pharmaceutical reactor with 15% acetic acid at 60°C bulk temperature with a 1.6 mm sheath at 8 W/cm 2 (20°C increase) the sheath surface temperature is 80°C. This is over the permissible limit of 75 °C for 15% acid and the heater would be in the active corrosion regime with predicted rates of 1 - 2 mm per year. "1-2 years to perforate 1.6 mm sheath." Lowering the watt density to 4 W/cm 2 (10°C rise) maintains the sheath surface at 70°C, below the 75°C threshold, and returns to passive operation, providing 10+ years life.

Safe Operating Envelope for 1.6 mm Sheath in Acetic Acid Service
The table below shows the maximum safe bulk acetic acid temperature for a 1.6 mm 316 sheath at a range of watt densities for aerated circumstances typical of pharmaceutical reactors with air headspace.

Acetic Acid Concentration Watt Density 4W/cm² (10°C rise) Watt Density 6W/cm² (15°C rise) Watt Density 8W/cm² (20°C rise) Watt Density 10W/cm² (25°C rise)
<5% Safe to 85°C Safe to 80°C Safe to 75°C Safe to 70°C
5 – 10% Safe up to 80°C Safe up to 75°C Safe up to 70°C Safe up to 65°C
10 – 15% Safe up to 75°C Safe up to 70°C Safe up to 65°C Safe up to 60°C 15 – 20% Safe up to 65°C Safe up to 60°C Safe up to 55°C Safe up to 50°C 20 – 30% Safe up to 55°C Safe up to 50°C Safe up to 45°C Safe up to 40°C 30 – 40% Safe up to 45°C Safe up to 40°C Safe up to 35°C Safe up to 30°C 40 – 50% Safe up to 40°C Safe up to 35°C Safe up to 30°C Safe up to 25°C
50 – 60 % Safe to 50°C Safe to 45°C Safe to 40°C Safe to 35°C
>60% Safe up to 70 °C Safe up to 65 °C Safe up to 60 °C Safe up to 55 °C
A sheath of 1.6 mm at 6 W/cm² (surface 70°C) is over the permissible limit of 55°C for a pharmaceutical reactor with 20% acetic acid at 55°C bulk temperature. The heater must be derated to 4 W/cm2 (surface 65° C) or less. Even 10% acetic acid at 60oC bulk results in a surface temperature of 85oC at 10 W/cm2, which is beyond the safe limit of 70oC for that concentration.

Design Improvements for Continued Passive Operation
For acetic acid process conditions, the concentrations and temperatures are close to the passive-active boundary, and three design adjustments are proposed to retain passive operation without modifying the sheath material. The first is to assure there is enough aeration. Oxygen from air or from sparging moves the passive-active transition to higher temperatures by 10-15 °C. A reactor under nitrogen blanketing (deaerated) will have far less corrosion resistance than a reactor with air headspace. The second adjustment is to decrease the watt density, either by expanding the heated length or by using several heaters. A 20% reduction in watt density could lead to an 8-12°C sheath surface temperature reduction, from active to passive. The third alteration is adding a corrosion inhibitor. Small amounts (50-200 ppm) of oxidising chemicals, such as hydrogen peroxide or nitric acid, can stabilise the passive film in acetic acid, increasing the transition temperature by 10-20°C. 316 is an average selection for pharmaceutical reactors requiring acetic acid concentrations above 30% at temperatures above 50°C independent of watt density. Under these conditions, engineers should switch to a higher alloy, such as Alloy 20 or Alloy 825, that maintains passive behaviour in acetic acid at boiling temperatures at all concentrations. The cost difference is usually 30-50% more than 316, but service life increases from 1-2 years to 10-15 years. Specify the exact concentration, bulk temperature, aeration conditions, and predicted watt density when ordering heaters for acetic acid service and supply this information to the supplier. A manufacturer that suggests 316 for 25% acetic acid at 65°C without considering the passive-active transition is not giving complete engineering information. The shift from passive to active corrosion in acetic acid is concentration dependent and sudden. A 316 sheath 1.6 mm run just below threshold may endure 10 years, whereas one run just over threshold may perforate in 18 months. For reliable specification, accurate calculation of sheath surface temperature from watt density and bulk conditions is required.