At What Specific Combination of Caustic Concentration and Sheath Surface Temperature Does a 1.2 Millimeter 316 Stainless Steel Sheath Experience Caustic Stress Corrosion Cracking in Pulp and Paper Industry Digester Heater Service?

The use of process engineers responsible for electric immersion heaters in pulp and paper digesters, the combination of sodium hydroxide and sodium sulphide at elevated temperatures produces an environment in which caustic stress corrosion cracking is the major failure cause. Caustic cracking creates brittle intergranular cracking which can permeate a 1.2 mm 316 sheath in a matter of weeks with little obvious warning, unlike pitting or ordinary corrosion. The kraft pulping process employs white liquor with 15–20 % NaOH and 5–10 % Na₂S at temperatures of 150–170 °C. These heaters generally have a wall thickness of 1.2 mm because of the limited space in digester vessels. The article identifies the particular combination of caustic concentration and sheath surface temperature at which a 1.2-mm 316 sheath will develop caustic stress corrosion cracking in 1000 hours of continuous service.

Mechanism of Caustic Stress Corrosion Cracking in 316
Caustic stress corrosion cracking of 316 stainless steel requires three conditions: a caustic concentration over a threshold value, temperature above the cracking threshold and tensile stress. The swaging method used to compact MgO insulation creates large residual tensile strains on the inner surface of the sheath-typically 150-250 MPa for a 1.2 mm wall. These residual stresses are the source of the tensile stress required for cracking. The caustic concentration for cracking in 316 at 150°C is about 5% NaOH. The threshold temperature is decreased to 130°C at 10% NaOH. Cracking at 15% NaOH is above 110 °C. Cracking at 20% NaOH over 100°C. The conditions in white liquor (15–20% NaOH, 150–170°C) are well above the cracking threshold. Cracks in a 1.2-mm 316 sheath normally initiate at the inner surface where the residual stresses are highest after 100–500 h exposure. Once established, cracks propagate at rates of 0.1–1.0 mm per week, penetrating the 1.2-mm wall within 1–3 months.

Sheath 1.2 Critical Cracking Thresholds for 1.2
Based on laboratory testing of swaged 316 sheath samples in white liquor simulant and field failure analysis from pulp mill digesters, the following combinations of caustic concentration and sheath surface temperature result in caustic stress corrosion cracking within 1,000 hours.

Sodium Hydroxide Concentration (Weight %) Sodium Sulphide Concentration (Weight %) Critical Temperature for Cracking Onset Time to Crack Initiation at 150°C Sheath Surface Time to Through-Wall Crack (1.2 mm) at 150°C 5 – 8% 2 – 4% 140°C 800 – 1,200 hours 1,200 – 2,000 hours 8 – 10% 4 – 6% 130°C 500 – 800 hours 800 – 1,500 hours 10 – 12% 5 – 7% 120°C 300 – 500 hours 500 – 1,000 hours
12 – 15% 6 – 8% 115°C 200 – 400 hours 400 – 800 hours 15 – 18% 7 – 9% 110°C 150 – 300 hours 300 – 600 hours
18 – 20% 8 – 10% 100°C 100 – 200 hours 200 – 400 hours
More than 20% More than 10% Less than 100°C 50 – 150 hours 100 – 300 hours
For a kraft digester working with 16% NaOH at 155°C bulk temperature, the sheath surface temperature is roughly 155–160°C (assuming a small temperature rise due to low watt density in big digesters). This is over the 110°C cracking threshold and would generate cracks in 150–300 hours. A 1.2 mm sheath would likely perforate after 1-2 months of continuous operation. Field experience shows that white liquor service sheaths generally break in 3-6 months with cracking the predominant mode of failure.

Safe Operating Envelope for 1.2mm Sheath in Caustic Service
The table below lists maximum safe caustic concentration for a 1.2 mm 316 sheath at different sheath surface temperatures to avoid caustic stress corrosion cracking throughout a 1,000 hour service life. Values assume residual tensile stresses after swaging, which cannot be reduced without post-swage annealing.

Sheath Surface Temperature Maximum Safe NaOH Concentration to Prevent Cracking (No Sulfide) Maximum Safe NaOH Concentration with 5% Na2S Expected Time to Cracking at Maximum Safe ConcentrationRecommended Action 80°C 20% 15% 2,000 – 3,000 hours Marginal for continuous service 90°C 15% 12% 1,500 – 2,500 hours Limited service life 100°C 12% 10% 1,000 – 2,000 hours 316 not recommended 110°C 10% 8% 800 – 1,500 hours Use annealed or alloy upgrade 120°C 8% 6% 500 – 1,000 hours Alloy upgrade required 130°C 6% 4% 300 – 800 hours Alloy upgrade mandatory 140°C 4% 2% 200 – 500 hours 316 unsuitable
Above 150°C Not safe Not safeUnder 200 hoursUse Nickel Alloy
The safe operating envelope for a 1.2 mm 316 sheath does not exist for pulp and paper digester service with 16% NaOH at 155°C. The cracking will come in weeks or months for sure. Engineers will need to choose a more durable alloy.

Design changes to prevent caustic cracking
When 316 sheath is to be used in caustic service, three design adjustments can prolong life, but none make 316 dependable for kraft digester conditions. The initial step is to anneal post-swage at 1050°C to eliminate residual tensile tensions. Annealing reduces the inner surface residual stress from 150–250 MPa to below 20 MPa, removing the tensile component required for cracking. Annealing a 1.2 mm 316 sheath in 16% NaOH at 150°C can extend its life to 3–6 months, rather than 1–2 months. The second alteration is to shot peen or otherwise compressively treat the outside surface. The compressive layer resists crack propagation but does not alleviate the tensile stresses on the inner surface. The final change is to operate at the lowest possible sheath surface temperature by reducing the watt density and boosting the circulation. A 10°C decrease in surface temperature doubles the time to crack initiation. These changes are not enough for most pulp and paper applications. Alloy 625 or Alloy C-276 is the typical material for white liquor heater sheaths. These nickel-based alloys resist caustic stress corrosion cracking at all concentrations and temperatures seen in digesters. A 1.6 mm Alloy 625 sheath will normally endure 5-10 years in white spirits service as vs 2-6 months for 316. The higher initial cost is compensated for by the savings in downtime and replacement labour. Engineers selecting heaters for digester operation should not accept 316 for any application where caustic concentration is above 10% at temperatures exceeding 100°C. The cost of a rapid cracking failure in a pressurised digester including product loss, vessel cooldown, and replacement is significantly greater than the incremental cost of a high nickel alloy. If a manufacturer is recommending 316 for white spirits service and not talking about caustic stress corrosion cracking they are not providing proper engineering counsel. Cracks in a 1.2 mm sheath develop quickly and without any apparent evidence until the sheath leaks. The only sure solution is prevention by correct choice of alloy.