Unalloyed titanium (commercially pure titanium, CP-Ti) is widely recognized for its excellent corrosion resistance across various aggressive environments, and among its standard grades, Grade 2 (CP-Ti Grade 2, also designated as ASTM B265 Grade 2) is universally regarded as the most balanced and outstanding option for corrosion resistance in most industrial scenarios, while Grade 7 (a modified unalloyed grade with palladium addition) excels in highly reducing and chloride-containing environments that challenge conventional pure titanium grades.
All unalloyed titanium grades rely on the formation of a spontaneous, adherent, and self-healing titanium dioxide (TiO₂) passive film for corrosion protection. This film is impermeable to most corrosive media, chemically stable over a wide pH range (typically pH 1–14), and can rapidly repair itself if damaged in oxygen-containing environments. The purity and trace element content of each grade directly affect the uniformity and stability of this passive film, thus determining the grade's corrosion performance in specific scenarios.
Grade 1 (ASTM B265 Grade 1)
Grade 1 is the most ductile and lowest-strength unalloyed titanium grade, with the highest purity (minimum 99.5% Ti, with extremely low iron and oxygen content). It exhibits good corrosion resistance in mild corrosive environments such as dilute acids (e.g., 10% H₂SO₄ at room temperature), freshwater, and atmospheric conditions. However, its passive film is less stable in concentrated oxidizing acids or high-chloride media, and it is not recommended for use in reducing environments (e.g., hot dilute hydrochloric acid) due to limited film-repairing capability.
Grade 2 (ASTM B265 Grade 2)
Grade 2 is the most widely used unalloyed titanium grade, with slightly higher oxygen and iron content than Grade 1 (oxygen: 0.18–0.25%, iron: ≤0.30%) while maintaining high purity (≥99.2% Ti). Its corrosion resistance is superior to Grade 1 in most aggressive environments for the following reasons:
Broad media compatibility: It resists corrosion in organic acids (acetic acid, formic acid), inorganic oxidizing acids (nitric acid up to 65% concentration at boiling temperature), saltwater (seawater, brine), and most alkaline solutions (sodium hydroxide up to 50% concentration).
Chloride pitting resistance: It has excellent resistance to pitting and crevice corrosion in chloride-containing environments (e.g., seawater with chloride concentrations up to 35,000 ppm) at temperatures below 120°C, which is critical for marine and offshore applications.
Stability in oxidizing-reducing mixed media: It performs well in environments with alternating oxidizing and reducing conditions (e.g., chemical process streams containing both nitric acid and chloride ions), where the passive film remains intact without breakdown.
Grade 3 (ASTM B265 Grade 3)
Grade 3 has higher oxygen and iron content (oxygen: 0.25–0.35%, iron: ≤0.30%) than Grade 2, resulting in higher mechanical strength but slightly reduced corrosion resistance. While it retains good performance in neutral and weakly oxidizing media, its passive film is more prone to damage in concentrated chloride solutions or hot dilute sulfuric acid, making it less suitable for highly corrosive applications compared to Grade 2.
Grade 4 (ASTM B265 Grade 4)
Grade 4 has the highest oxygen content (0.35–0.45%) and strength among standard unalloyed titanium grades, but its corrosion resistance is the lowest among CP-Ti grades. Its passive film is less uniform due to higher interstitial element content, and it is susceptible to localized corrosion in high-chloride or reducing environments, limiting its use to corrosion-moderate, high-strength-required scenarios.
Grade 7 (ASTM B338 Grade 7, Ti-Pd Alloy)
Though often categorized as a "modified unalloyed titanium" (with 0.12–0.25% palladium addition), Grade 7 addresses the key limitation of conventional CP-Ti grades in reducing environments. Palladium acts as a cathodic stabilizer, promoting the reduction of hydrogen ions and maintaining the passivity of the TiO₂ film even in deaerated hot dilute hydrochloric acid, sulfuric acid, or formic acid (media where Grade 2 would suffer general corrosion). In highly reducing, chloride-rich environments (e.g., chemical plants handling acidic chloride streams), Grade 7's corrosion resistance surpasses all standard unalloyed grades, though it is not a "pure titanium" grade in the strict sense due to the palladium alloying element.
For strictly unalloyed (commercially pure) titanium grades, Grade 2 offers the most outstanding and versatile corrosion resistance across the widest range of industrial corrosive environments, balancing corrosion performance, mechanical ductility, and cost-effectiveness, which is why it is the go-to choice for marine engineering, chemical processing, and biomedical applications (e.g., implantable devices). For specialized reducing environments where standard CP-Ti fails, Grade 7 (Ti-Pd) is the optimal alternative, despite containing a small amount of palladium alloying.
