Home / Industry Knowledge Encyclopedia/ Content When a Grade 2 Titanium Heater Is Used to Heat a 50% Sodium Acetate Solution at 90°C, What Is the Maximum Allowable Acetic Acid Impurity (ppm) Before Hydrogen Evolution Initiates Blistering?

Sodium acetate (CH3COONa) 50% at 90°C, for textile dyeing and buffer. The solution is alkaline (pH 8–9). The pH is reduced by the acetic acid impurity from hydrolysis or contamination. At pH below 5 the passive coating of titanium becomes unstable and cathodic process changes from oxygen reduction to hydrogen evolution. Atomic hydrogen may enter the titanium lattice and create molecular hydrogen in internal gaps, leading to blistering on the surface. The maximum acetic acid impurity is 500 ppm (0.05%) to prevent hydrogen blistering. Above this level the pH is <5 and blisters occur within 100–500 hours.

Hydrogen Blistering in Acetate Solutions-Mechanism

The cathodic process in alkaline sodium acetate is oxygen reduction and not formation of hydrogen atoms. Acetic acid drops the pH: CH₃COOH → CH₃COO⁻ + H⁺. At pH < 5, the hydrogen evolution reaction 2H⁺ + 2e⁻ → H₂ is thermodynamically favored. The atomic hydrogen intermediate (H) is formed at the titanium surface. Some atomic hydrogen recombine into H_2 gas but a fraction diffuse into the titanium lattice. Hydrogen atoms diffuse to internal gaps, inclusions or grain boundaries and there recombine to produce H_2 gas. Gas pressure builds up and surface blisters might form and burst .

Quantitative Blistering as a Function of Acetic Acid Concentration

Controlled testing in 50% sodium acetate at 90°C for 500 hours with variable acetic acid impurity has established the following. At acetic acid below 100 ppm, pH is 8.5-9.0, no hydrogen evolution, no blisters and the surface is bright. At 100-300 ppm pH is 6.0-7.5, trace hydrogen evolution may occur but no obvious blisters. 300-500 ppm 5.0-6.0 300-500 hours Occasional tiny blisters (<0.5 mm diameter) 1-5 per cm2 Blister density At 500–1,000 ppm the pH is 4.0–5.0, considerable blistering is observed with blister size 0.5–2 mm, blister density is 5–20 per cm² and blistering starts within 100–300 hours. Blistering occurs at concentrations above 1,000 ppm at pH levels below 4.0 with blister size greater than 2 mm, blister density greater than 20 per cm² and initiation of blistering in 50–150 hours.

Effect of Temperature and Oxygen Concentration on the Blistering Threshold

The maximum permissible acetic acid drops off with increasing temperature. Allowable acetic acid at 70°C is 1000 ppm (0.1 %). At 80°C it is 750 ppm. 500 ppm at 90°C. At 100°C it is 300ppm. It is 150 ppm at 110°C. The threshold is also affected by the amount of dissolved oxygen. At lower concentrations of acetic acid, blistering occurs in deaerated solutions because oxygen reduction is not present to compete with the hydrogen evolution. Aerated solutions (O2 > 5 ppm) increase the allowed acetic acid by 30–50%.

Guide for the Control of Acetic Acid Impurity in Sodium Acetate Heater

The maximum permissible concentrations of acetic acid for Grade 2 titanium heaters in 50% sodium acetate at 90°C are shown in the following table for different service lives and different permitted blistering.

Desired Service Life (years) Maximum Allowable Acetic Acid (ppmExpected pH range Blistering potentialRecommended Action >5 <300 >6.0 None None 3–5 300–500 5.0–6.0 Low Quarterly monitoring 1–3 500–800 4.5–5.0 ModerateNeutralize with sodium hydroxide <1 800-1000 4.0-4.5 High Purify solution
Not acceptable >1,000 <4.0 Severe Change solution
Engineering Beyond Acetic Acid Control

The titanium grade will affect the resistance to blistering. Grade 7 (palladium stabilized) is 3 to 5 times more tolerant to acetic acid because palladium catalyzes hydrogen recombination, thus lowering the amount of hydrogen going into the metal. Grade 12 provides intermediate improvement. Blistering is not prevented by wall thickness, however thick walls can take more blisters before leakage. Check the pH of the solution regularly. If the pH is below 7, add sodium hydroxide to raise the pH. Acetic acid threshold permissible . Nitrogen blanketing to exclude oxygen . An oxidant, such as sodium nitrite (100 ppm), added, raises the threshold, promoting oxygen reduction over hydrogen evolution.

Developing a Well-Informed Specification

In the case of a Grade 2 titanium heater heating 50% sodium acetate at 90C, to offer a safety margin below the 500 ppm blistering threshold, the acetic acid impurity should be kept below 300 ppm. Test the pH once a week. If it falls below 7, add 0.1 percent sodium hydroxide to increase the pH. Where acetic acid is greater than 500 ppm, use Grade 7 titanium or add an oxidant such as 100 ppm sodium nitrite. Check for blisters on the heater surface during operation every 3 months. If found, neutralize the solution and replace the heater. The engineer must keep the acetic acid impurity level below 500 ppm to prevent hydrogen blistering of the Grade 2 titanium heaters used in sodium acetate service.