In Waste Incinerator Wet Scrubbers Handling Hydrochloric Acid Vapors, Which Titanium Alloy (Ti-0.15Pd vs Ti-0.3Mo-0.8Ni) Offers Superior Crevice Corrosion Resistance?

Waste incinerator wet scrubbers sit at the interface between high-temperature combustion gases and corrosive liquid absorption systems. The vapors of hydrochloric acid from burning chlorinated plastics and other wastes are condensed to a scrubber liquor of 5-15% HCl at 60-80oC. Titanium alloys are often selected for scrubber internals such spray headers, mist eliminators, and reheater tubes. The most common failure mode is crevice corrosion at tube-to-tubesheet junctions, under gaskets and under solid deposits. There are two titanium alloys that are contenders for this service, Grade 7 (Ti-0.15Pd) and Grade 12 (Ti-0.3Mo-0.8Ni) and they offer different methods of crevice corrosion resistance. This paper evaluates the performance of the alloys in hydrochloric acid scrubber conditions and determines which alloy provides better protection against crevice attack.

Mechanism of Crevice Corrosion in HCl Scrubbers
In a crevice – usually a 0.1-0.5 mm gap between a titanium tube and a PTFE gasket or between a tube and a rolled tube sheet – the chemistry is very different from bulk scrubber liquid. Cathodic processes quickly exhaust oxygen in the crevice. In the absence of oxygen titanium is anodically dissolved with formation of Ti³⁺ and Ti⁴⁺ ions which hydrolyze to HCl steadily reducing the crevice pH. In a bulk scrubber with 10% HCl, the pH can be below 0 at a pH of approximately 0.5. The combination of high chloride content, low pH and limited mass transport results in an autocatalytic environment that leads to fast crevice corrosion once it has commenced.

The resistance of an alloy to this attack is defined by the critical crevice temperature (CCT) and critical crevice potential (CCP). CCT is the maximum temperature at which a standardized crevice assembly (ASTM G78) can be immersed for 72 hours without evident crevice attack. The CCP is the threshold beyond which no crevice start occurs at a particular temperature and chloride concentration.

Grade 7: Palladium Stabilized Passivation
Grade 7 titanium contains 0.12-0.25% palladium, a noble metal which is an excellent cathodic site for hydrogen evolution and oxygen reduction. In the oxygen-starved crevice environment, palladium catalyzes the reduction of hydrogen ions (2H+ + 2e- → H2) at a lower overpotential than titanium itself. This cathodic reaction adjusts the crevice potential in the noble direction and hence helps to sustain the passive film even under low pH, oxygen starved conditions.

Grade 7 has a CCT in 10% HCl at 70 deg C of about 65 deg C - i.e. crevice attack does not start below 65 deg C under standard test conditions. Crevice attack is during 100-200 hours at 75°C. Grade 7 is at or slightly below its CCT for a scrubber running at 60-70°C, providing a tiny but functional safety margin.

Grade 12: Mo-Ni Power-Up
Grade 12 titanium has 0.2-0.4% molybdenum and 0.6-0.9% nickel. In contrast to palladium, which alters the cathodic process, molybdenum is enriched in the passive film as molybdate species (MoO₄²⁻) that impede chloride ion penetration and reduce the rate of anodic dissolution. Nickel increases the stability of the passive film in reducing acids and aids repassivation after mechanical damage to the film. This combination results in a more resilient passive film resistant to breakdown even in aggressive crevice chemistries.

Grade 12 has a CCT of around 80°C in 10% HCl at 70°C, which is much higher than Grade 7. Grade 12 shows slight crevice attack after 500 hours at 85°C. The enhanced performance is attributed to the fact that the molybdenum-nickel addition leads to an increase in intrinsic stability of the passive film, rather than noble metal catalysis, which can be hampered when hydrogen evolution is the predominant cathodic reaction.

Incinerator Scrubber Performance Comparison
Data from waste incinerator scrubbers running at 65-70°C with 8-12% HCl showed clear differences between the two alloys. Grade 7 tubes removed after 5 years service show shallow crevice attack (0.1-0.3 mm deep) at rolled tube joints and under gaskets. Some of the Grade 7 installations have experienced through-crevice perforation at 7-8 years. Under the same conditions, Grade 12 tubes show no significant crevice attack after 5 years, and only minimal surface etching (0.05 mm) after 8-10 years.

The advantage of Grade 12 is particularly obvious in scrubbers which have temperature problems. 75-80°C exposure for long durations causes attack in Grade 7 crevices which may grow to 0.5 mm depth in months. Grade 12 is relatively unaffected at these temperatures.

Incinerator Scrubber Titanium Alloy Application Matrix
Scrubber Operating Condition HCl Concentration (Recommended Alloy Temperature RangeAnticipated Crevice-free Life
Consecutive operation, stable temperature 5-10% 55-65°C Grade 7 5-7 years
Continuous working, consistent temperature 10-15% 55-65°C Grade 12 8-10 years
Variable operating Temperature upsets to 75 °C 5 to 10 % 60 to 75 °C Grade 12 6 to 8 years
High chlorine waste, often excursions10-15% 65-80°C Grade 12 (min) 5-7 years
Any service where heavy solid deposits are presentAny Any Grade 12 Deposits increase crevice risk; prefer Grade 12
New build, 10 years design life 5-15% 60-70°C Grade 12 10+ years
Conclusion on Incinerator Scrubber Specification
In waste incinerator wet scrubbers where hydrochloric acid vapors are expected to be at 60-80°C, Grade 12 titanium (Ti-0.3Mo-0.8Ni) is preferable to Grade 7 (Ti-0.15Pd) from the standpoint of crevice corrosion resistance. The anodic inhibition from the molybdenum-nickel addition improves the stability of the passive film and results in a critical crevice temperature approximately 15°C higher than Grade 7. Typical scrubber service is at 65-70°C with 10% HCl. Grade 12 will give 8-10 years of crevice-free service but Grade 7 will begin to exhibit attack after 5-7 years. Grade 12: The preferred alloy for new installations or replacements requiring 10-year life, but costs a little more (about 15-20% more than Grade 7). Grade 7 is still suitable for the lower temperature scrubbers (55-65°C) or if initial capital cost is the main limitation, but Grade 12 should be specified where the scrubber temperature can be greater than 65°C or where the chloride content surpasses 10%. When you buy, ask for ASTM B338 Grade 12 tubes with pickled surfaces and welded (not rolled) tube-to-tubesheet joints to eliminate crevices at the tube-to-tubesheet connections and help prolong service life.