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 work in a somewhat passive regime in 90% formic acid at 100°C. The manner of degradation of the substance is of crucial importance for the water content of the acid. At 5% water (95% formic acid), the passive film dissolves consistently, resulting in predictable thinning at 0.10 – 0.25 mm each year. At 10% water (90% formic acid) the surface chemistry is changed by the presence of extra water, which allows for localised film disintegration and the initiation of stable pitting at rates of 0.5–1.2 mm per year. The switch from uniform attack to pitting is sharp, between 7 and 9% water, and dramatically alters the failure mode and remaining life projection for the heater.

Mechanism of the Water-Induced Transition from Uniform to Pitting Corrosion

The passive coating on titanium in formic acid is sustained by the adsorption of formate ions (HCOO-). Water molecules compete with formate for surface locations . With 5% water, formate adsorption is dominant and the film degrades in a homogeneous fashion. At 10% water, more water adsorption substitutes formate and creates local flaws in the passive layer. The chloride-like aggressiveness of high-concentration formate ions therefore promotes pit initiation at these defect sites. Once a pit has been established, the acidic, high-formate environment inside the pit encourages propagation by switching the primary mechanism from uniform thinning to rapid localised penetration.

Quantitative Corrosion Behaviour at 5% vs. 10% Water

Controlled weight loss and pit depth measurements in formic acid/water combinations at 100°C for 500 hours have demonstrated apparent differences of behaviour. At 5 % water (95 % formic acid) the uniform corrosion rate is 0.10–0.25 mm/year. The surface shows overall etching without pits deeper than 20 μm, and the mode of failure is predictable uniform thinning. The general rate decreases to 0.08-0.15 mm/year at 8% water (92% formic acid), however pitting begins after 200-400 hours exposure, with pit depths of 0.1-0.3 mm after 500 hours in a mixed mode. At 10% water (90% formic acid) the uniform rate decreases further to 0.05-0.10 mm per year but pitting is severe, commencing within 50-150 hours and forming pits 0.3-0.8 mm deep after 500 hours. At 10% water, the pit propagation rate (0.5-1.2 mm/year) is 5-10 times higher than the uniform thinning rate at 5% water.

The effect of temperature on the critical water content

The operating temperature is a function of the water level at which pitting is predominant. The switch from uniform to pitting is at about 10–12% water at 80 °C. The transition is to 7–9 % water at 100 °C. Pitting dominates even at 6% water at 110°C. Higher temperature accelerates the adsorption of water and the instability of the passive layer, therefore reducing the critical threshold of water. At 10% water content, the pit propagation rate increases from 0.2–0.5 mm/yr at 80°C to 0.5–1.2 mm/yr at 100°C.

Grade Selection Guide for Formic Acid Service and Water Content

Maximum permissible water content for titanium heat rods of Grade 2 and Grade 7 at 100°C is given in the table below, based on intended service life and prevailing 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
Beyond Water Content Control Engineering

The transition threshold is highly dependent on the titanium quality. Grade 7 (palladium stabilised) can be used at 10% water (90% formic acid) with mixed-mode corrosion. Grade 2 must be kept below 7% water to avoid pitting. Wall thickness offers a pit penetration tolerance. 2.0 mm wall with pitting at 0.6 mm/year survives 3.3 years. 1.2 mm wall collapses in 2 years. The addition of formic anhydride (1–2%) chemically binds free water, hence lowering the effective water content without changing the acid concentration. Aeration of the solution leads to homogeneous corrosion instead of pitting, by stabilising the passive film.

Knowledge-Based Specification

For a titanium heat rod in 90% formic acid at 100°C, consider using Grade 7 titanium for safe operation in 10% water with mixed-mode corrosion. Grade 2.-Reduce the water content to less than 7 % by distillation or addition of drying agents such as formic anhydride. Water content to be monitored weekly using Karl Fischer titration; corrective action if water is more than 8% for Grade 2. By regulating the water content to below the critical transition threshold, the engineer can change the corrosion mechanism from fast pitting to predictable uniform attack, permitting reliable life prediction in hot formic acid service.