At What Specific Combination of Caustic Concentration and Sheath Surface Temperature Does a 1.6 Millimeter 316 Stainless Steel Sheath Transition from Passive to Active Corrosion in Sodium Hydroxide Service for CIP Systems?

Process engineers building electric immersion heaters for clean-in-place systems for food processing, pharmaceutical manufacturing and beverage production use sodium hydroxide as the principal cleaning chemical. Typical CIP solutions consist of 1-3% NaOH at 70-85 °C. In this situation, the passive film forms, and generally, the corrosion resistance of 316 stainless steel is good. However, at higher concentrations or higher surface temperatures from heater watt density the passive film can breakdown and active corrosion can occur with rates surpassing 1 millimetre per year. Typical wall thicknesses of CIP heaters are 1.6 mm, giving a reasonable compromise between corrosion allowance and thermal responsiveness. The paper establishes the caustic concentration and sheath surface temperature where a 1.6 mm 316 sheath changes from passive to active corrosion in sodium hydroxide service.

316 Transition in Sodium Hydroxide from Passive to Active
Sodium hydroxide is unique among alkaline solutions in that the corrosion behaviour of 316 stainless steel exhibits a clear passive-active transition which is dependent on both concentration and temperature. In dilute solutions below 5% NaOH and temperatures below 80oC, 316 stays passive with corrosion rates less than 0.05 mm per year . As concentration grows or temperature increases, the passive film becomes unstable. At 10% NaOH the transition temperature is about 90°C. It decreases to 75C 20% NaOH. At 30% NaOH the value drops to 60 °C. At 50% NaOH, over 40°C the passive range vanishes completely. The sheath surface temperature is typically 15–30°C higher than the bulk solution temperature for an immersion heater due to the applied watt density. A CIP system with 3% NaOH and 80°C bulk temperature with a heater of 8 W/cm² provides a sheath surface temperature of 95-105°C. The passive coating on 316 can break down at this surface temperature and active corrosion occurs at rates of 0.5–2 mm per year. Active corrosion will puncture a 1.6 mm sheath in 1–3 years, however passive operation will give 10+ years of service.

Critical Transition Thresholds for 1.6 mm Sheath
Electrochemical testing of 316 samples in sodium hydroxide solutions at CIP service related temperatures has demonstrated that the following combinations of bulk caustic concentration and sheath surface temperature will cause the passive-to-active transition for a 1.6 mm wall. Values are for standard swaged surface polish and a typical watt density of 6–8 W/cm².

Sodium Hydroxide Concentration (Weight %) Passive to Active Transition Temperature Minimum Sheath Surface Temperature for Active Corrosion Safe Bulk Temperature for 1.6 mm Sheath at 8 W/cm2 (25 °C rise) Expected Corrosion Rate in Active Regime
1 – 2% >100°C 100°C 75°C 0.3 – 0.7 mm/yr
2 – 3% 95 – 100°C 95 – 100°C Up to 70 – 75°C 0.5 – 1.0 mm/year
3 – 5% 85 – 95°C 85 – 95°C Up to 60 – 70°C 0.8 – 1.5 mm/year
5 – 7% 75 – 85°C 75 – 85°C Up to 50 – 60°C 1.0 – 2.0 mm/yr
7 – 10% 65 – 75°C 65 – 75°C Up to 40 – 50°C 1.5 – 3.0 mm/year 
10 – 15% 55 – 65°C 55 – 65°C Up to 30 – 40°C 2.0 – 4.0 mm/year
> 15% < 55 degrees CBelow 55°C Not safe at any useful temperature>3.0 mm/yr
For a CIP system with a bulk temperature of 80°C and 2% NaOH utilising a 1.6 mm sheath at 8 W/cm2, the sheath surface temperature is around 105°C, higher than the transition threshold of 100°C. Active corrosion will occur and the sheath may perforate in 1-2 years. Lowering the watt density to 4 W/cm², decreases the surface temperature rise to 12–15°C while the sheath temperature stays at 92–95°C, which is below the 100°C threshold. This change allows for 5-10 years of service life.

1.6 Millimetre Sheath Safe Operating Envelope In Caustic CIP Service
The following table gives the maximum permissible bulk sodium hydroxide concentration for a 1.6 mm 316 sheath at various bulk temperatures and watt densities. For certain concentration values, the sheath surface temperature is kept at least 5°C below the passive-to-active transition temperature.

Bulk Solution Temperature Watt Density 4 W/cm2 (12°C increase) Watt Density 6 W/cm2 (18°C rise) Watt Density 8 W/cm2 (25°C rise) Watt Density 10 W/cm2 (32°C rise) 60°C Safe to 10% NaOH Safe to 7% NaOH Safe up to 5 % NaOH Safe to 3% NaOH 65°C Safe to 8% NaOH Per cent safe to 5 NaOH Safe to 3% NaOH Safe to 2% NaOH 70°C Safe to 6% NaOH Safe up to 4 % NaOH Safe up to 2% NaOH Not safe over 1% 75oC Not safe above 5% NaOH max 3%, safe NaOH Safe up to 1.5% NaOH Not safe above 1% 80°C Safe up to 4% NaOH safe up to 2% NaOH Not safe above 1.5% Not safe 85°C Safe up to 3% NaOH Up to 1.5% NaOH safe Not safe Not safe
90°C Safe up to 2% NaOHNot safe above 1% Not safe Not safe
For a typical CIP cycle at 80°C with 2% NaOH, a 1.6 mm 316 sheath is safe only at watt densities of 4 W/cm2 or less. At an industrial watt density of 8 W/cm² the heater will be in the active corrosion regime and a restricted service life of 1–3 years should be predicted rather than the often assumed 5–10 years.

Passive Operation Design Changes
Three design modifications can be made to preserve passive operation without changing the sheath material under CIP process conditions that would otherwise force a 1.6 mm 316 sheath into active corrosion (e.g., caustic concentrations or temperatures): The first method is to increase the heated length of the element to reduce the watt density. Doubling the length (1 m to 2 m) reduces the sheath surface temperature rise from 25 °C to 12 °C and returns to passive functioning when the watt density is reduced from 8 W/cm2 to 4 W/cm2 . The second adjustment is to improve circulation over the heater surface. For the same watt density, the surface temperature rise can be reduced to 8–10°C at a velocity of 2–3 m/s using a CIP circulation pump. The final change is the addition of a corrosion inhibitor to the caustic solution. Silicates or phosphates at 100 to 200 ppm can stabilise the passive film on 316 and increase the transition temperature by 10 to 20°C. But these changes probably won't be enough for CIP systems that require 5% NaOH at 80oC. In these instances the engineer should specify the use of a nickel-based alloy such as Alloy 600 or Alloy 825, which are passive in caustic solutions up to 50% concentration at temperatures over 100°C. Cost of upgrade from 316 to Alloy 825 is typically 2-3 times greater, but service life increases from 1-2 years to 10-15 years. Always tell the supplier when specifying heaters for CIP service: the maximum concentration of caustic, the bulk temperature and the expected cycle time. When a manufacturer suggests 316 for 5% NaOH at 85 deg C without any consideration of passive-active transition or derating for watt density, that is not appropriate technical advice. The change from passive to active corrosion in sodium hydroxide is abrupt and a function of temperature. A 316 sheath of 1.6 mm thickness operating just below the threshold may persist for ten years; one operating just beyond it may fail in 18 months. Accurate thermal analysis of the sheath surface temperature is necessary for reliable specification.