When Heating Monochloroacetic Acid at 120°C, Why Is Titanium Grade 7 Unsuitable Above 110°C Despite Its Palladium Content?

Monochloroacetic acid (MCA, ClCH2COOH) is an intermediate chemical used in the synthesis of carboxymethylcellulose, surfactants and medicines. MCA is a powerful organic acid (pKa 2.85) with a chlorine substituent, so more aggressive than acetic acid. Heating MCA is frequently performed with Palladium in Titanium Grade 7 (Ti-0.15Pd) as it has a positive effect on lowering acids. However, the monochloroacetic acid (MCA) has a particular difficulty, the chlorine atom is labile at temperatures above 110°C and the monochloroacetic acid decomposes to generate hydrochloric acid and other chlorinated species. The dechlorination reaction on the titanium surface is accelerated by the presence of palladium content of Grade 7 which, while protective in pure reducing acids, leads to the localized formation of concentrated HCl at the metal contact. This self-catalyzing reaction leads to rapid pitting and eventual tube perforation. This article discusses the mechanism of decomposition and establishes the maximum safe operating temperature for grade 7 in MCA service.

Monochloroacetic Acid Dechlorination Mechanism
Monochloroacetic acid undergoes thermal decomposition at > 110 °C by ClCH₂COOH + H₂O → HOCH₂COOH + HCl and ClCH₂COOH → CH₂O + CO + HCl. The reaction is heterogenous and catalyzed by metal surfaces, especially those with strong catalytic activity for carbon-halogen bond cleavage. Platinum group metals, including palladium, are among the most active catalysts for dechlorination processes.

Palladium islands (0.12–0.25% Pd) on a Grade 7 titanium surface are active sites for the decomposition of MCA. Each palladium site catalyzes the release of chloride ion (Cl⁻) which instantly combines with available hydrogen ions to create HCl. The generated HCl concentrates at the metal surface creating a localized environment of high chloride concentration and low pH - ideal conditions for titanium pitting. Once a pit is initiated, the titanium surface within the pit provides more catalytic sites for further MCA decomposition. The pit grows autocatalytically.

Grade 2 titanium (no palladium) is a far less effective catalyst for dechlorination of MCA. On Grade 2 surfaces the reaction is sluggish and the HCl formed is diffused into the bulk solution before it can induce localized attack. The Grade 2 in MCA at 120°C shows uniform deterioration (0.1-0.2 mm/year) but not the fast pitting observed on the Grade 7.

Grade 7 Failure – Temperature Ranges
Corrosion behavior of Grade 7 titanium specimens in 100% monochloroacetic acid at temperatures of The corrosion behavior of Grade 7 titanium specimens in 100% monochloroacetic acid at temperatures of

Grade 7 is passive at 100°C with a corrosion rate less than 0.02 mm/year. The dechlorination reaction is sluggish and any HCl formed diffuses away from the surface. After 1,000 hours, no pitting was noticed.

Grade 7 shows a little pitting initiation at 500-800 hrs at 110°C. Maximum pit depth is 0.1-0.2 mm. The surface is somewhat discoloured by palladium catalysed breakdown. Low rate of overall rusting.

At 115°C pitting begins in 100-200 h and pit depths reach 0.5-1.0 mm after 500 h. Some pits are autocatalytic, and will continue propagating, even after the temperature is lowered. Grade 2 was evaluated in side-by-side reveals uniform etching but no pitting.

Grade 7 pits begin at 120°C within 20-50 hours. A 1.65 mm wall specimen is perforated by several through-wall pits after 200 h. The surface is covered with a dark deposit of decomposition products of carbon and titanium chlorides. Grade 2 demonstrates homogeneous corrosion of 0.15 mm/year without perforation.

Grade 7 fails catastrophically at 130°C in 50 hours. The corrosion rate increases to 0.4-0.6 mm/year for grade 2, which again is not good for long term usage.

Thus, while palladium is useful in other reducing acids, the palladium catalyzed dechlorination makes Grade 7 unsuitable for monochloroacetic acid above 110°C due to fast pitting. Grade 2 performs better in MCA above 110°C as there is no catalyst to generate HCl.

Comparison of Titanium Grades in MCA Service
Temperature Behavior in 2nd Grade Behavior in 7th Grade Material Recommended
80-100°C Uniform corrosion <0.02 mm/year, great Uniform corrosion <0.01 mm/year, superb Either Grade (Grade 2 more economical)
100-110°C Uniform corrosion 0.02-0.05 mm/year Minor pitting after 500+ hours Grade 2 preferred 110-115°C Uniform corrosion 0.05-0.10 mm/year Pitting starts in 200-400 hours Grade 2 only (Grade 7 unsuitable)
115-120 °CGeneral corrosion 0.10-0.20 mm/yrFast pitting, perforation in 200-500 hr Grade 2 (marginal); hastelloy may be considered120°C General corrosion>0.20 mm/yearNon-usable (>100 hr perforation)Hastelloy C-276, zirconium
MCA Heating Application Matrix
MCA Service Condition Temperature Suggested Heater MaterialLife expectancy (1.65 mm wall)Comments
MCA storage tank heating (intermittent) 80-100°C Grade 2 titanium >10 years Use Grade 2 not Grade 7 MCA distillation (continuous) 105-110°CGrade 2 titanium 5-8 years Uniform thinning allowed
MCA distillation (higher temp) 110-115°C Grade 2 titanium (heavy wall 2.5 mm) 3-5 yrs Annual wall loss inspection
MCA reactor (115-120°C) 115-120°C Hastelloy C-276 5-7 years Titanium (any grade) not recommended
MCA with water content >5% (reduces decomposition temperature) 100-110°C Grade 2 Titanium 4-6 yearsWater speeds acid formation
Anhydrous MCA (no water, decomposition suppressed) 110-120 o C Grade 7 (re-evaluate) Variable Test coupon recommended
MCA Service Mitigation for Grade 7
If Grade 7 is already installed in MCA service operating above 110°C, the following may extend life:

Keep the operating temperature at 110 °C or below. Reducing by 5°C usually doubles the remaining life.

Add a corrosion inhibitor (0.1-0.5% sodium nitrate or chromate) to passivate the palladium sites. This is not necessarily consistent with the processes farther down the line.

Buff the surface to remove the visible islands of palladium. Mechanical polishing to Ra <0.2 μm decreases the number of active catalytic sites.

Replace with Grade 2 at next scheduled outage.

Titanium grade 7 is not suited for heating monochloroacetic acid at 120°C, even though it contains palladium. The palladium catalyzes dechlorination of MCA, producing concentrated HCl at the metal surface, resulting in fast pitting. Grade 2 titanium without the palladium catalyst does well at 110-120°C with uniform corrosion and not pitting, but its corrosion rate is significant over 115°C. For service above 115 °C, suitable materials are Hastelloy C-276 or zirconium. When specifying heaters for MCA operation, always mention the maximum operating temperature and water content and request a corrosion test coupon in the specified MCA grade to identify the optimum material. If the MCA is over 110°C don't just go for Grade 7 since it has a good reputation in other reducing acids. Grade 2 is the safer bet.