For a Titanium Heat Rod Used to Reheat 90% Formic Acid at 100°C, How Does the Water Content (5% vs. 10%) Shift the Corrosion Mechanism from Uniform Attack to Pitting?

Titanium heaters are operating in a near-passive regime at 100 deg C in 90% formic acid. The mode of the material deterioration is decisively determined by the water content in the acid. At 5% water (95% formic acid) the passive film dissolves uniformly and predictable thinning occurs at 0.10–0.25 mm per year. At 10 % water (90 % formic acid), the added water affects the surface chemistry, causing local breakup of the film and commencing stable pitting at rates of 0.5–1.2 mm/year. The change from uniform attack to pitting is very abrupt, between 7 and 9 percent water, and impacts the heater's failure mode and the forecast of remaining life.

Water-Induced Transition Mechanism from Uniform to Pitting Corrosion

The passive film on titanium in formic acid is based on the adsorption of HCOO- (formate ions). Water molecules battle with the formate for the surface locations. At 5% water, formate adsorption prevails and the film dissolves uniformly. At 10% water, the higher water adsorption leads to the displacement of formate and local defects in the passive layer. Then the chloride- like aggressiveness of concentrated formate ions leads to the initiation of the pit at these defect sites. When a pit initiates, the acidic and high formate condition within the pit further speeds propagation. The dominating mechanism then changes from uniform thinning to rapid localized penetration.

Quantitative Corrosion Behavior at 5% vs 10% Water Content

Controlled weight loss and pit depth measurements have established distinct behaviors in formic acid/water mixtures at 100°C for 500 h. At 5 % water (95 % formic acid) the uniform corrosion rate is 0.10–0.25 mm/year. Surface displays widespread etching with no pits deeper than 20 µm. Failure mode is predicted uniform thinning. At 8% water (92% formic acid) the uniform rate lowers to 0.08–0.15 mm per year, but pitting begins after 200–400 hours, with pit depths of 0.1–0.3 mm after 500 hours in a mixed mode. With 10% water (90% formic acid) the uniform rate reduces further to 0.05–0.10 mm per year, but pitting is severe, beginning in 50–150 hours and giving pits 0.3–0.8 mm deep after 500 hours. At 10% water the pit propagation rate (0.5-1.2 mm per year) is 5-10 times higher than the uniform thinning rate at 5% water.

Temperature Dependence of Critical Water Content

The water content at which pitting becomes dominant is dependent on the operating temperature. The shift from uniform to pitting is roughly 10-12 % water at 80°C. At 100 C the transition shifts to 7-9 % water. Even at 6% water pitting dominates at 110°C. At higher temperatures, water adsorption and passive film instability are faster. Therefore, the critical water threshold decreases. At a fixed water content of 10 %, increasing the temperature from 80 °C to 100 °C increases the pit propagation rate from 0.2–0.5 mm yr −1 to 0.5–1.2 mm yr −1 .

Formic Acid Service Water Content and Grade Selection Guide

The following chart shows the maximum permissible water content for Grade 2 and Grade 7 titanium heat rods at 100°C depending on expected service life and dominant corrosion mechanism.

Desired Service Life (years) Max Water (%) for Grade 2 Max Water (%) for Grade 7 Dominant Mechanism Expected Penetration Rate (mm/year) >5 <6 <10 Uniform 0.05–0.15 3–5 6–7 10–12 Mixed 0.15–0.30 1–3 7–8 12–14 Pitting (marginal) 0.30–0.60 <1 >8 >14 Pitting (severe) >0.60
Engineering other than controlling water content

The titanium grade changes the transition threshold quite a lot. At 10% water (90% formic acid) grade 7 (palladium stabilized) can be used with mixed mode corrosion. Grade 2 should be used with water content below 7% to prevent pitting. Wall thickness offers a pit penetration tolerance, a 2.0 mm wall with pitting at 0.6 mm per year survives 3.3 years, while a 1.2 mm wall fails in 2 years. Addition of formic anhydride (1–2%) chemically binds free water, reducing effective water content without changing acid concentration. Aeration of the solution promotes homogeneous corrosion rather than pitting by stabilizing the passive film.

Specifying with Knowlege

For a titanium heat rod in 90% formic acid at 100°C. Choose Grade 7 titanium if you want to securely operate at 10% water with mixed-mode corrosion. Grade 2 must have a water content reduced to less than 7% by distillation or by the addition of drying agents such as formic anhydride. Water content – weekly by Karl Fischer titration. If > 8% for Grade 2 take corrective measures. The engineer's approach is to maintain the water content below the critical transition threshold, shifting the corrosion mechanism from rapid pitting to a predictable uniform attack, and thus permitting accurate life prediction in hot formic acid service.