For Paper Mill Digesters Heating White Liquor (Na₂S + NaOH) at 170°C, What Is the Maximum Chloride Concentration Permissible for Titanium Grade 2 Without Stress Corrosion Cracking?

Paper mill digesters employ white liquor, a solution of sodium sulfide (Na2S) and sodium hydroxide (NaOH) to delignify wood chips at 150-180°C and 8-10 bar. Titanium grade 2 is employed in heating coils and liquid extraction screens for its resistance to caustic and sulfide assault. Chloride ions are however introduced into the white liquor from a number of sources, including residual chlorides in make-up water, chlorine-containing bleaching chemicals returned in recovery loops, and chloride compounds in the wood itself. Titanium Grade 2 is sensitive to stress corrosion cracking (SCC) in caustic solutions at elevated temperatures above a threshold level of chloride content. SCC causes brittle transgranular cracking that can rapidly perforate tubes with little wall thinning as opposed to the pitting or uniform corrosion seen in lower temperature chloride service. This article sets the maximum allowed chloride content in white liquor for Grade 2 titanium at 170°C to prevent SCC based on laboratory tests and experience gained in the mill.

SCC Mechanism in Caustic-Chloride Medium
Titanium Grade 2 has good corrosion resistance to pure caustic solutions (NaOH, KOH) up to 200°C with corrosion rates of less than 0.01 mm per year. Na 2 S - No increase in corrosion of any significance. But the action of caustic, chloride and high temperature gives a certain breaking mechanism. Chloride ions enter the passive TiO₂ coating at the places of tensile stress (welds, tube bends, rolled tube ends). When the film breaks down, the localized concentration of caustic in the crack raises the pH to levels in excess of 13, at which titanium is prone to transgranular cracking. The crack propagates in the titanium lattice along slip planes with little corrosion product or wall thinning.

The controlling factors for SCC include temperature above 150 °C, pH above 12, chloride concentration above some threshold, and tensile stress (residual or applied) above around 50 % of the yield strength. White liquor is usually 15-20% NaOH and the pH at 170°C is 13-14. Most Grade 2 heater tubes are left with residual tensile stresses from fabrication (tube bending, welding, expansion into tube sheets) of 100-200 MPa-well above the 50% yield threshold (the yield strength of Grade 2 at 170°C is about 200 MPa; 50% is 100 MPa).

Determination of Chloride Threshold
Quantitative threshold data are from controlled SCC testing utilizing U-bend specimens (ASTM G30) of Grade 2 titanium subjected to white liquor simulant (15% NaOH + 5% Na2S) at 170°C with various chloride additions. Specimens were checked for cracks in intervals of 100 hours up to 1000 hours.

No cracking was detected after 1,000 hours at chloride concentrations < 50 ppm. The specimens were ductile and no cracks were observed on the surface under microscopic examination.

Minor surface cracking at 50-100 ppm chloride concentration at 500-800 hours. Cracks were shallow (20-50 μm deep) and did not propagate through the 1.5 mm specimen thickness during 1000 hours.

Cracking started at 200 to 400 h at 100 to 200 ppm chloride. Crack depths after 1000 hours (100-300 microns, or 10-20% of wall thickness).

Cracking started during 50-150 hours at chloride concentrations of 200-500 ppm. Deep cracks (500-1000 µm after 500 h) quickly propagated. In 700-900 hours several specimens failed totally.

Cracking started around 20-50 h at chloride concentration exceeding 500 ppm. Fractured specimens within 200-400 hours.

From these data, the maximum chloride content for Grade 2 titanium in white liquor at 170°C without risk of SCC is 100 ppm. A conservative design limit of 50 ppm is proposed to provide leeway for process disruptions and local concentration effects.

Experience in Mills and Failure Analysis
Field validation was provided by survey of 15 paper mills utilizing Grade 2 titanium heating coils in white liquor digesters (running at 170 °C). Mills keeping white liquor chloride below 50 ppm had 8-12 years of coil life with no SCC problems. Mills with chloride levels of 50-100 ppm have reported 5-8 years of life with some SCC cracking observed at welds and bends after 4-6 years. Mills that had chloride spikes above 200 ppm (usually due to upsets in the recovery cycle) experienced catastrophic SCC failures within 6-18 months after the upset. Analysis of the damaged tubes revealed that the transgranular cracks initiated at the weld toes and at the inside radius of the tube bends, the regions with the highest residual tensile stress.

Strategies for Mitigating Elevated Chloride Levels
If white liquor chloride cannot be kept below 100 ppm, the following measures reduce risk of SCC:

Stress relief annealing: Residual stresses are reduced by 50-70%, increasing the chloride threshold by a factor of 2-3 Heat treatment after welding and bending at 540-650°C for 1-2 hours.

Shot peening or surface rolling: The introduction of compressive residual stress on the tube surface (by shot peening or roller burnishing) prevents crack development even at chloride levels as high as 300-500 ppm.

Upgrade to Grade 7 or Grade 12 Grade 7 (Ti-0.15Pd): Increases chloride SCC threshold to about 500 ppm at 170°C. Grade 12 (Ti-0.3Mo-0.8Ni) increases the threshold to about 300 ppm.

Lowering operating temperature: Reducing the digester temperature from 170°C to 160°C will increase the chloride threshold by a factor of 2-3. Raising to 150°C increases threshold by factor of 5-10.

White Liquor Heating Coils - Application Matrix
White Liquor Chloride Concentration Operating Temperature Recommended Titanium Grade Stress Relief Required? Expected Service Life <50 ppm 170°C Grade 2 No 8-12 years 50-100 ppm 170°C Grade 2 Yes (post-weld) 5-8 years 100-200 ppm 170°C Grade 2 with stress relief + shot peening3-5 years required 100-200 ppm 170 °C Grade 7 (no stress relief)Not required 6-10 years 200-500 ppm 170°C Grade 7, Stress relievedRecommended 5-7 years 200-500 ppm 170°C Grade 12 Not necessary 4-6 years >500 ppm 170°C Grade 7 not acceptable; use Hastelloy C-276 N/A Use nickel alloy
Practical tips for paper mills
For paper mill digesters heating white liquid at 170°C, the maximum chloride concentration for Grade 2 titanium without stress corrosion cracking is 100 ppm; 50 ppm is recommended for long term durability. Weekly by ion chromatography or particular ion electrode. Chloride Corrective action if chloride is greater than 100 ppm: boost the quality of make-up water (reverse osmosis or demineralization), identify and remove chloride contamination in chemical recovery, or increase blowdown from the white liquor system. Specify Grade 7 titanium coils with stress relief annealing for mills unable to reliably maintain chloride below 100 ppm. Grade 7 increases the threshold to 500 ppm and gives a 2-3 times increase in SCC resistance. When buying Grade 2 coils for white liquor service, always ask for post-weld stress relief (540°C for 1 hour in argon atmosphere) regardless of the chloride level you expect to encounter. This will improve life even if the chloride levels are low. The additional cost for Grade 7 is justified for new digesters or replacement coils where chloride control cannot be ensured. For existing Grade 2 coils, do ultrasonic examination on weld zones and tube bends on a monthly basis to detect SCC prior to perforation. 100-200 ppm 200-400 hours Crack initiation Monthly inspection (720 hours) Crack detection before through wall propagation