​​​​​​​Marine New Materials: Marine Metal—Titanium Alloy

Materials used in marine engineering must possess high strength, resistance to seawater hydrothermal corrosion, sulfide corrosion, microbial attachment, and exhibit high toughness. Titanium, being lightweight, strong, and corrosion-resistant-especially immune to the corrosive effects of saltwater, seawater, and the marine atmospheric environment-is an excellent lightweight structural material. Known as the "marine metal," titanium is a critical strategic material. It is widely used in marine engineering and is particularly suitable for lightweight offshore equipment, making it one of the key new materials in the field. Therefore, fully utilizing titanium and titanium alloys as marine materials will significantly contribute to the development of national marine strategies.

Applications of Titanium Alloys in Marine Engineering

1. Applications in Naval Ships

The application of titanium alloys in shipbuilding began in the 1960s, approximately a decade later than in the aerospace industry. The United States, Russia, Japan, and China were among the first countries to research titanium applications in naval vessels.

A. Hull Structural Materials
Titanium hulls, compared to traditional materials such as fiber-reinforced plastics, aluminum alloys, and steel, are lighter, allow for increased payload capacity, have longer service lives, require minimal maintenance, and are resistant to marine biofouling. For instance, titanium fishing vessels built by Japan's Nippon Steel, Toho Technology, and Eto Shipbuilding feature hulls, decks, and structural components entirely made of titanium. The "Titan Fast" speedboat, built by Nissei Industry, is approximately 12 meters long with a streamlined hull that minimizes drag.

B. Pumps, Valves, Piping, and Other Components
On naval vessels, pumps, valves, and pipes often operate under harsh conditions. Traditional copper or stainless-steel piping lasts only 2–5 years. Titanium offers superior corrosion and erosion resistance, good yield strength, and low density, making it ideal for thin-walled, small-diameter pipes and fittings. Titanium piping systems significantly reduce weight, extend service life, and enhance reliability. For example, titanium condenser tubes weigh about half as much as B30 copper-nickel tubes. Military use has shown that titanium alloy piping systems have excellent mechanical strength and corrosion resistance, with service lifespans exceeding 120,000 hours (over 40 years), far outlasting copper or stainless-steel components.

C. Power Propulsion Systems
Using titanium alloys for propellers and shafts improves propulsion speed and lifespan. The U.S. Navy employs titanium propellers on various vessels, including 1,500 mm-diameter detachable supercavitating four-blade propellers for hydrofoils. Titanium alloys are also used in water jet propulsion systems, such as the Ti-6Al-4V system on Japan's "PT-10" torpedo boat, which reduced shaft weight by 600 kg. Russia's nuclear-powered icebreakers also feature titanium steam engines. Titanium propulsion systems reduce induced currents and avoid triggering magnetic mines, a problem with copper alloys.

China began propeller research in the 1960s and, by 1972, developed titanium alloy propellers for hydrofoil boats. Today, China can manufacture fixed titanium alloy propellers up to 1,200 mm in diameter and weighing 130 kg. On Type 25 torpedo boats, titanium alloys replaced AK-27 steel and copper alloys, reducing weight by 30–40%, extending service life, eliminating the need for coatings, easing biofouling removal, and simplifying maintenance.

Sonar Domes
Sonar fairings are streamlined covers that reduce hydrodynamic noise and protect sonar equipment. These must exhibit excellent acoustic transparency. Currently, Chinese naval vessels use stainless steel or fiberglass-reinforced plastic sonar domes. While Russia once used fiberglass, it now primarily employs titanium alloy domes. These are used on vessels such as the Russian "Kursk," "Minsk," and "Kiev" aircraft carriers.

2. Deep-Sea Submersibles

As part of China's "863 Program," China Shipbuilding Industry Corporation's 702 Institute developed a 7,000-meter submersible made with special titanium alloys. Measuring 8 meters long, 3.4 meters high, and 3 meters wide, it can withstand 710 tons of pressure and integrates cutting-edge technologies. The manned spherical cabin, capable of withstanding 700 atmospheres of pressure, supports a life-support system comparable to those used in space missions.

3. Condensers

According to UNESCO, global ocean energy capacity reaches 76.6 billion kW. In coastal and nuclear power plants, condensers are vital components using seawater as a coolant. Traditional steel and copper alloy condensers suffer from poor seawater corrosion resistance. Titanium's durability and corrosion resistance make it ideal for these applications. Approximately 3–4% of the world's thermal and hydro power plants use titanium condensers, and 30% of nuclear power plants do. In China, plants such as Taizhou, Zhenhai, Qinshan, and Daya Bay use all-titanium condensers due to their excellent corrosion resistance, long service life, high heat transfer efficiency, safety, and reduced maintenance needs.

4. Nuclear Submarines

Russia leads in titanium alloy nuclear submarine technology. It was the first to use titanium alloy for pressure hulls, beginning in the 1960s. The world's first all-titanium nuclear submarine, K-162, launched in 1968, has operated over 30 years without incident. The ALFA-class submarines, constructed in the 1970s–1980s, used approximately 3,000 tons of titanium each and could dive to 914 meters. Russia's Typhoon-class submarines feature double hulls made from about 9,000 tons of titanium, offering non-magnetic properties, deep-diving capabilities, high speed, low noise, and reduced maintenance.

The Southwest Research Institute (SWRI) in the U.S. developed a manned deep-sea submersible using ELITi-64 titanium alloy. The 2.1-meter-diameter spherical cabin accommodates three people and can dive to depths of 6,500 meters.

5. Deep-Sea Stations

Deep-sea mobile space stations, often called the "Tiangong" of the ocean, are used for marine scientific exploration. Since the 1960s, the U.S. and Soviet Union have advanced deep-sea station systems. Russia's 2000 design targeted Arctic Ocean oil extraction. China proposed the concept in the 1990s for peaceful marine resource development. These stations rely heavily on titanium alloy materials. The "13th Five-Year Plan" and "2030 Major Science and Technology Innovation Projects" emphasize deep-sea station construction, estimating over 4,000 tons of titanium materials per main station.

6. Seawater Desalination

Seawater desalination is crucial in arid regions like the Middle East. The most reliable and widely used method is multi-stage flash (MSF) distillation, which includes components such as seawater heaters, heat recovery units, and various condensers. Originally made with copper alloys, these components are now commonly made with titanium due to its corrosion resistance, especially in high-temperature, high-salinity, and chlorinated environments. Titanium is now the preferred material for heat exchangers in desalination systems.

With rapid growth in coastal petrochemical and power industries, seawater is increasingly replacing freshwater as a cooling medium. Due to seawater's corrosiveness, traditional carbon steel or stainless-steel tubing rapidly degrades, causing equipment failure, frequent shutdowns, and economic losses. Upgrading to titanium tubing significantly extends heat exchanger life and improves efficiency.

7. Offshore Drilling Platforms

Titanium alloys are ideal for marine drilling systems due to their high strength, low density, excellent corrosion resistance, and toughness. Components such as risers, drill pipes, and stress joints benefit from titanium or titanium-steel composites, improving performance and reducing costs.

(1) Marine Drilling Risers
Titanium risers are lightweight, damage-tolerant, and easily inspected. First extensively used in the North Sea, most practical applications use stainless steel/titanium or composite/titanium hybrid risers due to cost considerations.

(2) Drill Pipes
In short-radius drilling (under 18 m), conventional stainless steel drill pipes suffer from early fatigue. U.S.-based RTI developed titanium drill pipes combining Grade 5 titanium with Cr-Mo steel joints, reducing sticking and wear while maintaining toughness. In 1999, 10 short-radius wells were successfully drilled using 73 mm titanium pipes. Later, pipes as small as 63.5 mm drilled even tighter-radius wells. Titanium's non-magnetic nature also benefits exploration. Titanium extends drill depth vertically to 9.1 km (vs. 6.1 km for steel) and horizontally to 9.1 km. Larger diameter pipes reduce lifting force and torque by 30–40% and overcome hydraulic transmission limitations.