Corrosion of Titanium Alloys: Part One

---

Understanding Titanium's Superior Corrosion Resistance

Titanium has earned recognition for its exceptional performance characteristics in severe environments, with specialized applications spanning the chemical process industry, energy sector, desalination facilities, and military operations. The metal's outstanding reputation stems from its unique ability to withstand corrosive conditions that would rapidly degrade other materials.

Industrial Applications and Environmental Performance

Chemical Process and Energy Industries

Titanium has established itself as the material of choice in some of the most challenging industrial environments. In the chemical process industry (CPI), energy sector (including geothermal applications), pulp and paper manufacturing, and desalination facilities, titanium components consistently outperform alternative materials. Multi-stage flash desalination units, refineries, and utility steam condensers rely heavily on titanium's corrosion resistance to maintain operational efficiency and reduce maintenance costs.

Marine and Military Applications

The past decade has witnessed a significant increase in titanium usage for military applications, particularly in naval environments where seawater exposure presents ongoing challenges. Titanium serves critical functions in armor systems, protective linings, ballast tanks, fire-main systems, and general service water piping systems. Additionally, the material provides essential protection in flue gas desulfurization (FGD) linings, where corrosive conditions would quickly compromise lesser materials.

Seawater Environment Applications

Within seawater environments, titanium's versatility becomes particularly evident. Heat exchanger seawater cooling systems, both shell-and-tube and plate-frame configurations, benefit from titanium's resistance to marine corrosion. Service water lube oil coolers and general heat exchanger systems in both shipboard and land-based installations demonstrate titanium's reliability across diverse operating conditions.

The material's applications extend to hot water heater units, refrigeration systems, air chillers, and air-conditioning systems. Titanium products encompass a comprehensive range including sheet materials for heat exchanger shells and baffles, plate materials for tube sheets and vessels, tubes for seawater cooling, pipe systems, fittings (elbows, tees, reducers), fasteners, flanges, pumps, and valves for seawater service applications, fire protection systems, and drainage infrastructure.

Corrosion Resistance Mechanisms

Protective Oxide Film Formation

The exceptional corrosion resistance of titanium results from a stable, protective, strongly adherent oxide film that forms instantaneously when fresh surfaces contact air or moisture. This oxide film, measuring 12-16 angstroms thick upon initial formation, provides immediate protection against corrosive attack. The film continues growing slowly over time, reaching 50 angstroms after 70 days and 80-90 angstroms after 545 days of exposure.

The titanium oxide film demonstrates remarkable stability and resists attack from most substances, with hydrofluoric acid being the most notable exception. Due to titanium's strong affinity for oxygen, the material can heal ruptures in the protective film almost instantly in any environment containing trace amounts of moisture or oxygen.

Critical Environmental Considerations

While titanium excels in most environments, anhydrous conditions lacking oxygen sources should be avoided. Under such conditions, the protective film may not regenerate if damaged, potentially compromising the material's corrosion resistance.

Specific Corrosion Challenges and Solutions

Pitting and Stress Corrosion Immunity

Titanium's immunity to both pitting corrosion and stress corrosion cracking in chloride solutions represents one of its most valuable properties. This resistance proves particularly important in environments where stainless steels would experience rapid degradation. Consequently, titanium finds extensive use in chemical industry vessels, heat transfer tubes in steam turbine condensers, and multi-stage flashing desalination plants.

Crevice Corrosion Considerations

Despite its excellent general corrosion resistance, titanium can experience crevice corrosion in hot chloride solutions. Since titanium commonly operates under such conditions, this form of corrosion presents practical challenges that require careful consideration during design and operation.

Extensive research has investigated environmental factors affecting crevice corrosion, including chloride ion concentration, temperature, and solution pH. This research has led to the development of several prevention methods and resistant materials.

Advanced Alloy Development

The most popular enhanced titanium alloy is titanium-0.15% palladium (Ti-0.15Pd), which provides superior crevice corrosion resistance. Currently, researchers are investigating titanium-0.3% molybdenum-0.8% nickel (Ti-0.3Mo-0.8Ni) as a lower-cost alternative. Additionally, surface treatment applications using palladium and titanium oxide mixtures (PdO/TiO₂) are being implemented in industrial equipment through simple surface treatment processes.

Erosion-Corrosion Resistance

Critical Velocity Performance

The protective oxide films of most metals become vulnerable to removal above critical water velocities, leading to accelerated erosion-corrosion attack. Some metals experience this phenomenon at velocities as low as 2-3 ft/s. In contrast, titanium's critical velocity in seawater exceeds 90 ft/s, demonstrating exceptional resistance to erosion-corrosion.

Numerous corrosion-erosion tests have consistently shown titanium's outstanding resistance to this form of attack, making it ideal for high-velocity fluid applications.

General Corrosion Testing Methods

Testing Approaches

General corrosion rates for titanium alloys can be determined through multiple methods including weight loss data analysis, dimensional change measurements, and electrochemical testing. Electrochemical polarization testing often supplements weight loss testing to provide comprehensive corrosion assessment.

Polarization testing can identify whether an alloy maintains true passivity or exists in a metastable condition, information that weight loss tests alone cannot reliably provide.

Equivalent Weight Calculations

Corrosion rates (mm/yr) can be calculated from electrochemical measurements following ASTM G5 standards using the formula:

Corrosion rate = (0.0033)(i_corr)(EW)/d

Where i_corr represents the measured corrosion current (in milliamps per square centimeter), d represents alloy density (in grams per cubic centimeter), and EW represents the equivalent weight for titanium. The equivalent weight for titanium approximates 16 under reducing acid conditions and 12 under oxidizing conditions. The icorr value is typically determined through Tafel slope extrapolation or linear polarization methods.

Performance in Water Environments

Fresh Water and Steam Applications

Titanium demonstrates complete resistance to all forms of corrosive attack by fresh water and steam at temperatures reaching 600°F (316°C). The material exhibits extremely low corrosion rates and typically experiences slight weight gain during exposure. While titanium surfaces may acquire a tarnished appearance in hot water or steam, they remain completely free of corrosion damage.

Natural river waters often contain manganese, which deposits as manganese dioxide on heat exchanger surfaces. This deposition proves harmful to both austenitic stainless steels and copper alloys, promoting pitting corrosion. Chlorination treatments used for slime control result in severe pitting and crevice corrosion on stainless steel surfaces. Titanium's immunity to these forms of corrosion makes it an ideal material for handling all natural water applications.

Seawater Corrosion Resistance

Titanium resists seawater corrosion at temperatures up to 500°F (260°C). Titanium tubing exposed to seawater for many years at depths exceeding one mile shows no measurable corrosion, demonstrating the material's exceptional long-term performance. The material has provided over twenty-five years of trouble-free seawater service across the chemical, oil refining, and desalination industries.

Pitting and crevice corrosion remain completely absent even when marine deposits form on titanium surfaces. The presence of sulfides in seawater does not affect titanium's corrosion resistance. Exposure to marine atmospheres, splash zones, or tidal zones does not cause corrosion damage to titanium components.

Figure 1: Marine Titanium Heat exchanger coil titanium coil