In a Titanium Electric Heater Used to Maintain 110°C in a 60% Sorbitol Solution, How Does the Reducing Sugar Content (0.1% vs. 1%) Catalyze the Complexation of Titanium Ions and Accelerate General Corrosion?
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Sorbitol solutions at a concentration of 60 % and a temperature of 110 °C are employed as humectants and sweeteners in pharmaceutical and food applications. Sorbitol, obtained from starch hydrolysis, is usually contaminated with reducing sugars such as glucose and fructose. The rate of titanium corrosion in 0.1% reducing sugar is 0.01–0.03 mm/year. The rate rises to 0.08–0.15 mm/year at 1.0% decreasing sugar. The catalysis of complexation of titanium ions by reducing sugars occurs via the creation of stable titanium-sugar complexes. The compounds hinder the formation of a protective passive coating resulting in a uniform accelerated corrosion.
Dextrose-Accelerated Corrosion Mechanism
Titanium ions (Ti⁴⁺) have a considerable affinity for hydroxyl and carbonyl moieties. Reducing sugars have several vicinal hydroxyl groups in cis configuration which act as bidentate ligands. The reaction is Ti4+ + 3C6H12O6 --> [Ti(C6H12O6)3]4+ This soluble complex extracts Ti4+ from the metal surface immediately upon its formation, thus preventing the formation of a protective TiO2 layer. The corrosion is homogeneous and diffusion controlled. The complexation rate and the steady-state corrosion rate rise with the increasing concentration of reducing sugar.
Quantitative corrosion rate versus reducing sugar content
The following has been set for controlled testing of Grade 2 titanium in 60% sorbitol with varied content of reducing sugar at 110oC for 1000 hours. Titanium release from 0% reducing sugar (pure sorbitol) is 0.5–1.0 ppm and corrosion rate is 0.005–0.010 mm/year with shining surface. At 0.1% reducing sugar Ti release is 2–5 ppm, corrosion rate 0.01–0.03 mm/yr and light etching is observed. At 0.5% reducing sugar, Ti release is 10-20 ppm and corrosion rate is 0.05-0.10 mm/year. Uniform etching is observed. The Ti release at 1.0% reducing sugar is 20-40 ppm, corrosion rate of 0.08-0.15 mm/year and uniform matte surface. The Ti release is 40-80 ppm, the corrosion rate is 0.15-0.25 mm per year and severe etching occurs at 2.0% reducing sugar.
Effect of Temperature and Sorbitol Concentration on the Rate of Complexation
The rate of complexation is Arrhenius-like with activation energy of 50 kJ mol−1. At 80 °C and 1 % reducing sugar, the corrosion rate is 0.03-0.06 mm per year. The rate at 100 °C is 0.06–0.12 mm/year. At 110°C the rate is 0.08–0.15 mm per year. The rate is 0.12–0.22 mm/year at 120°C The rate also depends on sorbitol content, and at 50% sorbitol with 1% reducing sugar and 110°C the rate is 0.06-0.12 mm/year. 60% sorbitol, the rate is 0.08-0.15 mm per year. At 70% sorbitol, the rate is 0.10–0.20 mm/year. The higher percentage of sorbitol improves the viscosity, while the higher sugar content controls the corrosion rate.
Guide to Sugar Reduction for Sorbitol Heaters
The following table gives the maximum permitted reducing sugar content for Grade 2 and Grade 7 titanium heaters in 60% sorbitol at 110°C as a function of the intended service life and acceptable corrosion rate.
Required Service Life (years) Maximum Reducing Sugar (%) for Grade 2 Maximum Reducing Sugar (%) for Grade 7 Predicted Corrosion Rate (mm/year)Recommended Action >10 <0.2 <0.5 <0.03 None 5-10 0.2-0.5 0.5-1.0 0.03-0.08Annual monitoring 3–5 0.5–1.0 1.0–2.0 0.08–0.16Use Grade 7 1-3 1.0-1.5 2.0-3.0 0.15-0.25 Lower temperature to 100°C <1 >1.5 >3.0 >0.25 Purify sorbitol
Engineering Beyond Sugar Control
The grade of titanium has a large effect on the resistance to complexation with sugar. Grade 7 (palladium stabilized) has 2-3 times lower corrosion rates than Grade 2 at the same reducing sugar concentration. Wall thickness offers a corrosion allowance; 2.0 mm wall with 0.10 mm/year corrosion is for 20 years. The sorbitol should be made from glucose of high purity to reduce the reducing sugar concentration. After manufacture, the sorbitol solution is hydrogenated to lower remaining reducing sugars. Chemical reduction of reducing sugars by addition of 0.1% sodium borohydride. A nitrogen blanket decreases oxygen ingress . Oxygen ingress slows the corrosion reaction .
Specifying with confidence
60% Sorbitol, 110 °C, titanium electric heater Maximum amount of reducing sugars: 0.2% for grade 2 or 0.5% for grade 7 Require the sorbitol provider to certify the sorbitol decreasing sugar content. For existing heaters, determine the reducing sugar content weekly with Fehling's solution or HPLC. If the reducing sugar is above the limit, reduce the reducing sugar with 0.1% sodium borohydride, or reduce the temperature to 100°C. The engineer uses control of the reducing sugar level below the critical level to prevent complexation of titanium ions and to maintain a low uniform corrosion rate in hot sorbitol service.
