Corrosion Resistance of Corrosion-Resistant Ti - Knowledge

1. Tolerance to Hydrochloric Acid (HCl)

Hydrochloric acid is a strong non-oxidizing acid that challenges titanium alloys by dissolving their passive film, with performance dependent on concentration, temperature, and alloy composition:

Commercially Pure Titanium (CP Ti, Grades 1–4)

At room temperature (RT), CP Ti resists dilute HCl (≤5% concentration) with a corrosion rate <0.1 mm/year, as the passive film remains intact. However, concentrations >10% at RT cause rapid film dissolution, leading to corrosion rates exceeding 1 mm/year.

At temperatures >40°C, even dilute HCl (3–5%) degrades the passive film. For example, CP Ti in 5% HCl at 60°C has a corrosion rate of ~2.5 mm/year, making it unsuitable for long-term service.

Pd-Alloyed Titanium (Ti-0.2Pd, Gr.7)

Pd acts as a cathodic stabilizer, enabling the alloy to maintain passivation in non-oxidizing HCl. It tolerates 10–15% HCl at RT (corrosion rate <0.05 mm/year) and 5% HCl at 80°C (rate <0.1 mm/year). Above 15% HCl at RT or 10% HCl at 60°C, passivation fails, and corrosion accelerates.

High-Molybdenum Alloys (Ti-32Mo, Ti-15Mo)

These β-phase alloys have superior resistance to concentrated HCl. Ti-32Mo withstands 20% HCl at RT (rate <0.01 mm/year) and 10% HCl at 100°C (rate <0.2 mm/year), making it ideal for chemical processing equipment handling mid-concentration HCl.

Key Limitation: All titanium alloys fail in concentrated, hot HCl (e.g., 37% HCl at 80°C) without oxidizing inhibitors (e.g., nitric acid, ferric chloride), as the film cannot self-repair in strongly reducing conditions.

2. Tolerance to Sulfuric Acid (H₂SO₄)

Sulfuric acid is a highly aggressive non-oxidizing acid, and titanium alloy performance hinges on the balance between concentration, temperature, and oxidizing additives:

CP Ti

At RT, CP Ti resists only very dilute H₂SO₄ (≤2% concentration) (corrosion rate <0.05 mm/year). Concentrations >5% at RT cause film breakdown, with rates exceeding 3 mm/year. At 60°C, even 1% H₂SO₄ leads to measurable corrosion (~0.5 mm/year).

Ti-0.2Pd (Gr.7)

The alloy extends tolerance to 5–10% H₂SO₄ at RT (rate <0.1 mm/year) and 2% H₂SO₄ at 80°C. Adding small amounts of oxidizing agents (e.g., 0.5% HNO₃ or 0.1% Fe³⁺) stabilizes the passive film, allowing service in 20% H₂SO₄ at RT.

Ti-Mo-Ni Alloys (Ti-15Mo-5Ni, Gr.12)

This alloy tolerates 15% H₂SO₄ at RT (rate <0.08 mm/year) and 5% H₂SO₄ at 100°C. It is widely used in sulfuric acid-based chemical pickling and fertilizer production equipment.

Ti-32Mo

The highest-performing alloy for H₂SO₄, it resists 30% H₂SO₄ at RT (rate <0.02 mm/year) and 15% H₂SO₄ at 100°C. It is the material of choice for concentrated sulfuric acid handling systems where other titanium alloys fail.

Critical Note: In hot, concentrated H₂SO₄ (e.g., 98% at 100°C), even specialized titanium alloys corrode rapidly, and materials like Hastelloy C or glass-lined steel are preferred.

3. Tolerance to Chlor-Alkali Solutions (NaOH/KOH + NaCl)

Chlor-alkali environments (e.g., caustic soda production, brine electrolysis) combine alkaline media and high chloride concentrations, but titanium alloys exhibit excellent stability here due to their resistance to both alkaline corrosion and chloride-induced pitting:

CP Ti and Gr.5 (Ti-6Al-4V)For sodium hydroxide (NaOH) solutions:

At RT, they tolerate concentrations up to 50% NaOH (corrosion rate <0.01 mm/year). At 100°C, they resist 30% NaOH (rate <0.1 mm/year); above 40% NaOH at 120°C, the passive film degrades slightly (rate ~0.3 mm/year), but remains acceptable for short-term service.

For chloride-containing caustic solutions (e.g., 30% NaOH + 5% NaCl):

They maintain corrosion rates <0.05 mm/year at 80°C, with no pitting or stress corrosion cracking (SCC). This makes them standard for chlor-alkali electrolyzer components (e.g., cathode baskets, cell liners).

High-Purity CP Ti (Gr.1)

For ultra-high-concentration caustic (e.g., 70% NaOH at 150°C), Gr.1 outperforms other grades with a corrosion rate <0.2 mm/year, as its low interstitial content (O, N, C) reduces alkaline embrittlement risk.

Key Advantage: Unlike stainless steels, titanium alloys do not suffer from chloride pitting or SCC in chlor-alkali media, even at elevated temperatures and high chloride loads.

4. Tolerance to Seawater and Marine Environments

Seawater is a complex medium containing high chloride ions (~3.5% NaCl), dissolved oxygen, and biofouling organisms, but titanium alloys deliver unmatched long-term corrosion resistance:

All Titanium Alloys (CP Ti, Gr.5, Gr.7)

At ambient to elevated temperatures (up to 120°C), all grades exhibit corrosion rates <0.001 mm/year-negligible for practical purposes. The passive film resists chloride-induced pitting, crevice corrosion, and intergranular corrosion, even in stagnant or low-oxygen seawater (e.g., deep-sea environments).

For tidal/splash zones (cyclic wet-dry exposure), titanium alloys avoid the corrosion-fatigue failures common in carbon steel or copper-nickel alloys. Gr.5 (Ti-6Al-4V) additionally retains high mechanical strength, making it ideal for offshore risers and ship propeller shafts.

In biofouling-prone seawater, while marine organisms may attach to the surface, they do not penetrate the passive film or cause under-deposit corrosion (unlike stainless steels). Periodic cleaning removes fouling without damaging the alloy.

Extreme Marine Conditions (e.g., seawater + H₂S, high-pressure deep-sea)

Ti-0.2Pd (Gr.7) and Ti-32Mo resist hydrogen embrittlement from H₂S and maintain corrosion resistance at pressures up to 100 MPa (10,000 m depth), suitable for subsea oil/gas equipment.

Long-Term Performance: Titanium alloy components have operated in natural seawater for over 30 years with no measurable corrosion or degradation, far exceeding the service life of traditional marine alloys.

Summary of Corrosion-Resistant Titanium Alloy Tolerance Ranges

Corrosive Medium	Alloy Grade	Concentration/Temperature Limit (Long-Term Service)	Corrosion Rate (Typical)
Hydrochloric Acid (HCl)	CP Ti	≤5% at RT	<0.1 mm/year
	Ti-0.2Pd (Gr.7)	10–15% at RT; 5% at 80°C	<0.05 mm/year
	Ti-32Mo	20% at RT; 10% at 100°C	<0.2 mm/year
Sulfuric Acid (H₂SO₄)	CP Ti	≤2% at RT	<0.05 mm/year
	Ti-0.2Pd (Gr.7)	5–10% at RT (with oxidizers: 20% at RT)	<0.1 mm/year
	Ti-32Mo	30% at RT; 15% at 100°C	<0.02 mm/year
Chlor-Alkali (NaOH+NaCl)	CP Ti/Gr.5	50% NaOH at RT; 30% NaOH at 100°C (with 5% NaCl)	<0.1 mm/year
Seawater	All Grades	Up to 120°C, 100 MPa pressure	<0.001 mm/year