W przypadku tytanowego pręta grzejnego używanego do podgrzewania 90% kwasu mrówkowego w temperaturze 100 stopni, w jaki sposób zawartość wody (5% w porównaniu z. 10%) zmienia mechanizm korozji z jednolitego ataku na wżery?

90% formic acid (HCOOH) at 100°C is a reducing environment which somewhat passivates titanium. The corrosion behaviour is strongly affected by the water content in the acid. At 5% water (95% formic acid) the solution is highly reducing and titanium experiences uniform corrosion at a rate of 0.1-0.3 mm per year. At 10% water (90% formic acid), the addition of water changes the solution chemistry, increases the hydronium ion (H3O+) concentration and changes the stability of the passive film. Pitting becomes the dominating mechanism with pit propagation rates of 0.5–1.5 mm per year, 5 times quicker than the uniform attack. The change is abrupt between 7% and 9% water content. Mechanism of Transition from Uniform to Pitting Corrosion Induced by Water The formate ions (HCOO-) adsorbed on the titanium, and contributed to the stability of the passive film in formic acid. Water competes with formate for adsorption sites on the surface. At 5% water, formate adsorption is the predominant effect and the film degrades uniformly. The water adsorption is increased to lead to localized film breakdown at defects such as grain boundaries or intermetallic particles at 10% water. After a pit starts, the formate ions (which tend to act like chlorides in concentrated form) are highly aggressive and accumulate in the pit, thus promoting propagation. The changeover takes place when the surface water coverage hits a crucial value of about 30 %. Corrosion Rates and Mechanisms Quantitative as a Function of Water Content Controlled testing in formic acid/water mixtures at 100°C for 500 h has shown distinct corrosion behaviors of Grade 2 titanium at different water concentrations. At 2% water (98% formic acid) the uniform corrosion rate is 0.20-0.40 mm per year with no pitting and a uniformly scratched surface . The uniform rate drops to 0.10 - 0.25 mm per year at 5% water (95% formic acid), and occasional shallow pits under 20 µm deep develop in the transition zone. The uniform rate is 0.08-0.15 mm/year at 8 % water (92% formic acid), pitting begins after 200-400 h, and pit depth is 0.1-0.3 mm at 500 h. With water content increased to 10% (90% formic acid), the uniform rate drops further to 0.05–0.10 mm/year but rapid pitting is observed within 50–150 hours with pit depths of 0.3–0.8 mm after 500 hours thus pitting is the dominant failure mechanism. At 15% water (85% formic acid), the uniform rate is 0.03–0.08 mm/yr but serious pitting occurs in 20–80 hr, with pit depths of 0.5–1.2 mm after 500 hr. Effect of temperature and formic acid concentration on the transition water content The water content at which the change from uniform corrosion to pitting takes place depends on the temperature. Uniform corrosion is common with a rate of 0.05-0.10 mm/year at 80°C in 95% formic acid (5% water). The uniform corrosion rate at 100°C in 95% formic acid is 0.10 to 0.25 mm/year. Mixed corrosion is observed in the form of pit propagation at a rate of 0.15–0.30 mm per year at 100°C with 92% formic acid (8% water). At 80°C, 90% formic acid (10% water) causes pitting at rates of 0.20–0.50 mm/year. 90% formic acid at 100°C gives 0.50-1.20 mm/year for pitting corrosion. Pitting by 85% formic acid (15% water) at 100°C is 1.00–2.00 mm/year. The essential water content for transition to pitting decreases with an increase in the temperature. Water Content Control Manual for Formic Acid The following table recommends maximum permitted water contents in formic acid for Grade 2 and Grade 7 titanium heaters at 100°C, based on intended service life and acceptable corrosion mechanism. Desired Service Life (years) Operating Temperature (deg C) Grade 2 Maximum Allowable Water (%) Maximum Allowable Water (%): Grade 7 Dominant Mechanism >5 80 <8 <12 Uniform >3 100 <6 <10 Uniform 2-3 100 6-7 10-12 Mixed (acceptable) 1–2 100 7–8 12–14 Pitting (marginal for Grade 7) <1 100 >8 >14 Inżynieria w trudnych warunkach wżerowych Zarządzanie zawartością wody w przeszłości Zawartość wody przejściowej jest silnie zależna od gatunku tytanu. Stopień 7 (stabilizowany palladem) przenosi jednolite przejście w wżery do 10–12% wody, umożliwiając bezpieczną pracę przy 90% kwasie mrówkowym (10% wody). Stopień 12 oznacza poprawę pośrednią. Grubość ścianki nie zapobiega wżerom, ale zapewnia naddatek na korozję. Ściana o grubości 2,0 mm z wżerami 0,5 mm rocznie przetrwa 4 lata. Ważne jest, aby kwas mrówkowy był czysty. Obecność zanieczyszczeń, takich jak chlorki (<10 ppm) or sulfates (<50 ppm) reduces the transition water content by 2–3 %. Aeration (dissolved oxygen) causes uniform corrosion rather than pitting. Deaeration causes the transition to occur at lower water content. A Detailed Specification Select Grade 7 titanium to be in the uniform or mixed regime for a titanium heat rod in 90% formic acid (10% water) 100°C. Grade 2 decrease the water content to below 7% by distillation or by use of anhydrous formic acid. For current Grade 2 heaters in 90% formic acid add 1-2% formic anhydride or acetic anhydride to chemically bind water and keep free water below 6%. Monitor water content by Karl Fischer titration weekly during operation; add drying agent or dilute with anhydrous acid if water reaches 7% for Grade 2 or 10% for Grade 7. By maintaining the water content below the critical transition level , the engineer changes the corrosion mechanism from rapid pitting to uniform attack thus extending the heater life in hot formic acid service.