Titanium - Corrosion by Acids

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The titanium metal was first commercialized in the year 1950. Since then, corrosion resistance has turned out to be a critical consideration while choosing it as an engineering structural material. Titanium has been widely accepted in many media where its engineering properties and corrosion resistance have given the corrosion and design engineer with a dependable and cost-effective material.

In this article, the erosive and corrosive effects of acids on commercially pure titanium and near commercially pure grades (Table 1) are examined.

Table 1. Titanium alloys commonly used in industry. Source: Timet

*Commercially Pure (unalloyed) titanium

Oxidizing Acids

Titanium has high resistance to oxidizing acids over a broad range of temperatures and concentrations. Standard acids in this group include nitric, perchloric, chromic, and hypochlorous (wet Cl₂) acids. These oxidizing compounds guarantee the stability of the oxide film. Low, but limited, corrosion rates from continued surface oxidation may be detected under extremely oxidizing, high-temperature conditions.

Titanium has been widely used for handling nitric acid in applications where stainless steels have to exhibit consistent resistance against intergranular attack. Titanium provides superior resistance over the entire concentration range at sub-boiling temperatures.

However, at elevated temperatures, the corrosion resistance of titanium depends more on the purity of nitric acid. In hot, high-purity solutions or vapor condensates of nitric acid, considerable normal corrosion (and trickling acid condensate attack) may take place in the 20–70 wt% range. For moderately high-temperature conditions, unalloyed grades of titanium with higher purity (that is, Grade 1) are favored for curbing quicker corrosion of weldments.

In contrast, different metallic species such as Ti, Si, Fe, Cr, or other precious metal ions (that is, Ru, Pt) in very small amounts tend to curb high-temperature corrosion of titanium in nitric acid solutions. Titanium generally shows superior performance to stainless steel alloys in high-temperature metal-contaminated nitric acid media, for example, those used in the Purex Process for U₃O₈ recovery.

Titanium’s own corrosion product, Ti⁴⁺, is a very powerful inhibitor. This is especially useful in recirculating nitric acid process streams, for example, stripper reboiler loops, where effective corrosion inhibition is achieved by maintaining steady-state levels of dissolved Ti⁴⁺.

Titanium also provides better resistance to nitric acid vapors. However, titanium is not suggested for use in red fuming nitric acid due to the risk of pyrophoric reactions.

Red Fuming Nitric Acid

In general, titanium has superior resistance to nitric acid over a broad range of temperatures and concentrations, but it should never be used with red fuming nitric acid. A pyrophoric reaction product will be formed, resulting in grave accidents.

Analysis of these accidents has revealed that the pyrophoric reaction is always preceded by a fast corrosive attack on the titanium. This attack, which is intergranular, leads to a surface residue of finely separated particles of metallic titanium. These highly pyrophoric particles can detonate in the presence of a powerful oxidizing agent like fuming nitric acid.

It has been determined that the water content of the solution should be below 1.34% and the NO₂ content more than 6% for the pyrophoric reaction to take place.

Chromic Acid

Although the data on chromic acid is not as comprehensive as data on nitric acid, the resistance of titanium to corrosion by chromic acid seems to be quite similar to that seen with nitric acid.

Reducing Acids

Titanium provides moderate resistance to reducing acids like sulfuric, hydrochloric (HCl), and phosphoric acid. There is an increase in corrosion rates with an increase in temperature and acid concentration. The grade 7 alloy provides excellent resistance to these environments, followed by grade 12, unalloyed titanium, and grade 5.

Hydrochloric Acid

Iso-corrosion data reveals that grade 2 alloy provides effective corrosion resistance to around 7% HCl at room temperature, grade 12 to around 9% HCl, and grade 7 to around 27%. There is a considerable reduction in this resistance at near-boiling temperatures. Small quantities of some multivalent metal ions in solution (for example, ferric ion) can effectively prevent the corrosion of titanium in HCl. In the presence of adequate ferric ion, grades 2, 7, and 12 exhibits similar corrosion resistance.

Other metal ions like Ni²⁺, Cu²⁺, Ti⁴⁺, and Mo⁶⁺ also passivate titanium against attack by HCl. Oxidizing agents such as sodium hypochlorite, nitric acid, chlorine, or chromate ions also have been proven as effective inhibitors. These agents have enabled titanium to be successfully utilized in a number of HCl applications.

Severe corrosion damage on titanium equipment has been caused by cleaning techniques that involve the use of pure HCl or acid incorporating amines. If sulfuric acid or HCl is used to clean titanium surfaces, sufficient ferric chloride must be incorporated to effectively prevent the corrosion of titanium.

Sulfuric Acid

Titanium exhibits resistance to corrosion by dilute solutions of pure sulfuric acid at low temperatures. Unalloyed titanium is resistant to concentrations of about 20% sulfuric acid at 32 °F (0 °C). This drops to around 5% acid at room temperature. Grade 7 alloy is resistant to approximately 45% acid at room temperature. In boiling sulfuric acid, unalloyed titanium exhibits high corrosion rates in solutions with as low as 0.5% sulfuric acid.

Grade 12 alloy has effective resistance to about 1% boiling acid. Grade 7 alloy has effective resistance in boiling sulfuric acid to about 7% concentration. Grade 5 alloy has slightly less resistance than unalloyed titanium.

The presence of some oxidizing agents or multivalent metal ions in sulfuric acid prevents corrosion of titanium in a manner similar to HCl. For example, ferric and cupric ions prevent the corrosion of unalloyed titanium in 20% sulfuric acid. Oxidizing agents such as chromic acid, nitric acid, and chlorine are also effective inhibitors.

Phosphoric Acid

Unalloyed titanium exhibits resistance to naturally aerated pure solutions of phosphoric acid up to 30% concentration at room temperature. This resistance extends to around 10% pure acid at 140 °F (60 °C) and 2% acid at 212 °F (100 °C).

Boiling solutions considerably speed up the attack. Grade 7 alloy provides considerably enhanced resistance. At room temperature, 140 °F (60 °C), and boiling temperatures, grade 7 alloy withstands concentrations of about 80%, 15%, and 6% of pure phosphoric acid, respectively. Grade 12 provides slightly better resistance to phosphoric acid compared to unalloyed titanium, but not as good as grade 7.

The presence of multivalent metal ions, such as cupric or ferric ions, or oxidizing species can prevent titanium corrosion in phosphoric acid.

Hydrofluoric Acid

Titanium undergoes quick corrosion in the presence of hydrofluoric acid even in very dilute concentrations. Therefore, it is not recommended for use with solutions of hydrofluoric acid or in fluoride-containing solutions with a pH of less than 7. Some complexing metal ions (for example, Cr⁺⁶, Al⁺³) might be effective at preventing corrosion in dilute fluoride solutions.

Sulfurous Acid

Low corrosion rates have been observed for unalloyed titanium in sulfurous acid: 0.02 milli-inches per year (mpy) (0.0005 mm/year) in 6% concentration at room temperature. Samples exposed to sulfurous acid (6% sulfur dioxide content) at 212 °F (100 °C) exhibited a corrosion rate of 0.04 mpy (0.001 mm/year).

Other Inorganic Acids

Titanium provides superior resistance to corrosion by various other inorganic acids. It is not much affected by boiling 10% solutions of boric or hydriodic acids. At room temperature, low corrosion rates are achieved on exposure to 40% hydrobromic and 50% hydriodic acid solutions.

Mixed Acids

Corrosion rates are considerably reduced by adding nitric acid to sulfuric acid or HCl. In general, titanium is resistant to corrosion by aqua regia (3 parts HCl and 1 part HNO₃) at room temperature. Grade 2, 7, and 12 alloys exhibit good corrosion rates in boiling aqua regia. Corrosion rates in mixed acids usually increase with an increase in the concentration or temperature of the reducing acid component.