For Catalytic Reactors Heating Acetic Acid with Trace Formic Acid, What Surface Treatment (Thermal Oxidation vs. Noble Metal Coating) Best Protects Titanium Heaters?

Trace amounts of formic acid, a reducing organic acid that aggressively attacks titanium passive films, are common contaminants in acetic acid service in catalytic reactors. The combination of acetic acid (usually 80-99%) with 0.1-2% formic acid at temperatures of 100-150°C provides an environment in which commercially pure titanium Grade 2 is corroded at rates of 0.2-0.8 mm per year, which is unacceptably high for long-term heater service. Surface treatments can help. Thermal oxidation builds a thick, crystalline TiO 2 (rutile) layer. Noble metal coatings (palladium or platinum) promote passive film preservation. This article compares these two surface treatment approaches with quantification of their corrosion protection performance in formic-acid contaminated acetic-acid.

Mechanism of titanium corrosion in formic-acetic acid mixtures
Among organic acids, formic acid (HCOOH) is unique in its ability to chemically decrease titanium dioxide. The reaction is TiO2 + 2HCOOH → Ti(HCOO)4 + 2H2O. The titanium formate complex is soluble, thus what happens is the passive film dissolves, not just breaks down. In 100 % pure acetic acid at 120°C the attack on titanium by the acid is negligible-corrosion rates are < 0.01 mm per year. However, the presence of only 0.1 % formic acid will boost the corrosion rate to 0.05-0.10 mm/yr. At 1% rates of formic acid are 0.3-0.5 mm/year. at 2% formic acid rates are over 1.0 mm/yr drilling through a 1.65 mm wall tube in 1-2 years.

The attack is widespread and homogeneous, not isolated pitting. This sets formic acid corrosion apart from chloride-induced corrosion and surface treatments providing a barrier layer or catalytic repassivation may be potentially useful.

Treatment via thermal oxidation
Titanium is thermally oxidized by heating the finished heater in air or oxygen at a temperature of 500 to 700 °C for 2 to 10 hours. This treatment leads to conversion of the native amorphous TiO2 film (usually 5-10 nm) into crystalline rutile layer of 0.5-5.0 µm thickness. The rutile layer is denser, tougher and more chemically resistant than the naturally passive film. In acetic acid containing formic acid, the thicker oxide forms a barrier that must be consumed before attack on the underlying titanium can occur.

Thermal oxidation of Grade 2 titanium at 600°C for 4 hours to generate a 2-3 µm thick rutile layer results in a corrosion rate in 99% acetic acid + 1% formic acid at 120°C of 0.06-0.10 mm per year, compared to 0.4-0.5 mm per year for untreated material (a 5-8 fold reduction). The benefit persists until the thermal oxide is broken. If the oxide layer is 2 µm thick and it dissolves at a rate of ~0.08 mm per year, then the protective effect will last for 2-3 years, and then the underlying titanium will corrode at the untreated rate.

The drawback of the thermal oxidation is that the oxide can only be grown to a certain thickness. Once it's gotten through, the protection is gone. Thicker oxides (5–10 µm) need higher treatment temperatures (700–800°C), which can degrade mechanical qualities of the titanium substrate and deform thin-walled tubes.

Noble Metal Coating (Paladium)
Another protection mechanism is provided by palladium coating deposited by electroplating, electroless deposition or physical vapour deposition (usual thickness 0.5-2.0 µm). Palladium is noble and affords an excellent cathodic site for the reduction of any oxidizing specie that may be present or for hydrogen ion reduction in the absence of oxidants. The solution is reducing in formic acid including acetic acid and palladium facilitates the reduction of H + to H 2 . This shifts the corrosion potential of the titanium substrate into the passive area. The effect is similar to that of palladium alloyed throughout the Grade 7 titanium, but concentrated on the surface.

Corrosion testing of palladium-coated Grade 2 (1 µm Pd, electroplated) in 99% acetic acid + 1% formic acid at 120°C indicates a corrosion rate of <0.01 mm per year-essentially total protection. Even with 2% formic acid the pace is still less than .02 mm/yr. The coating does not have to be complete across the entire surface; isolated palladium islands (even as little as 0.5% surface coverage) can cathodically shield the surrounding titanium by galvanic coupling.

Palladium coating is vulnerable to mechanical damage. Any scratch or abrasion eliminates the noble metal, and the titanium that is exposed by the scratch may not be protected if the area of damage is vast compared to the palladium that remains. In catalytic reactors the coating can be damaged for heaters if mechanical abrasion from catalyst particles or agitators is possible.

Performance Summary in Comparison
Protection Method Thickness Corrosion Rate in 1% Formic Acid at 120°C Mechanism of Protection Vulnerability
Untreated Grade 2 N/A 0.4-0.5 mm/yearNative oxide layerNone (intrinsic)
Thermal oxidation (600°C, 4 h)2-3 µm oxide 0.06-0.10 mm every yearBarrier layer Limited finite oxide thickness reduces lifetime to 2-3 years
Thermal oxidation (750°C, 8h) 5-8 µm oxide 0.03-0.05 mm/year Barrier layer Risk of embrittlement, deformation of substrate
Palladium coating (electroplated) 1 µm <0.01 mm/year Cathodic modification Mechanical damage to coating
Palladium coating (thick, 5 µm) <0.01 mm/year 5 µmCathodic modification Higher cost, probable adhesion problems
Application Matrix for Selection of Surface Treatment for Heaters
Reactor Condition Formic Acid ConcentrationRecommended Surface Treatment Operating TemperatureExpected Life of Heater (1.65 mm wall)
Formic acid, pure <0.05% 100-120°C None (Grade 2 standard) >10 years
Trace level of formic contamination 0.05-0.2% 100-120°CNone or light thermal oxidation 5-8 yrs
Continuous operation with low formic 0.2-0.5% 100-130°C Thermal oxidation (2-3 micron) 3-5 years
Moderate formic, clean service 0.5-1.0% 120-140°C Palladium plating (1-2 μm) 8-10 years
1.0-2.0% 120-150°C Palladium coating (thick, 5 µm) 5-7 years High formic, abrasive service (catalyst present)
Any formic with mechanical abrasion Any Any Palladium plating + regular inspection Variable
Summary of Heater Specifications for the Catalytic Reactor
Thermal oxidation of catalytic reactors for heating acetic acid containing traces of formic acid is less protective than palladium coating of titanium heaters. At 1% formic acid and 120°C, Grade 2 covered with palladium decreases the corrosion rate from 0.4-0.5 mm per year (untreated) to less than 0.01 mm per year, extending heater lifetime from 1-2 years to 8-10 years. Thermal oxidation increases performance 5-8 times but only provides 2-4 years protection before the oxide layer is consumed. Palladium coating is a cathodic modification and will protect the entire titanium surface even if the coating is not continuous. Thermal oxidation is a finite barrier layer and will eventually fail . For applications where there is little or minimal mechanical wear, a 1-2 µm palladium coating on Grade 2 is recommended. A thicker coating (5 µm) or solid Grade 7 titanium (which contains bulk palladium) should be specified where catalyst particles or agitators may scratch the surface. When ordering, request documentation on coating thickness, adhesion test results (tape test according to ASTM D3359) and data on corrosion tests in the specific acid mixture.