Titanium is a highly corrosion-resistant structural metal that has a high strength-to-weight ratio.
Its density of 4501 kg/ m3(281 lbm/ ft3) is midway between aluminum, 2691 kg/ m3 (168 lbm/ft3), and iron, 7897 kg/m3 (493 lbm/ft3).
Chemically pure titanium is a soft metal that is impractical to use in mechanical structures. Commercially pure titanium, however, is strengthened by adding relatively small amounts of oxygen (0.1% to 0.4%) and nitrogen (0.01% to 0.025%), as well as small amounts of iron, carbon, and hydrogen.
The corrosion resistance of titanium is the result of a stable, protective, self-healing, strongly adherent oxide film on the metal surface that is formed under oxidizing, neutral, or naturally occurring conditions.
The film, however, does not form under strong reducing conditions, so rapid attack results. In slightly reducing environments or in environments that form complexions with titanium, tendencies to corrode depend upon the presence of metal ion inhibitors, alloying elements, temperature, arid other variables.
Titanium is one of the most “noble materials” in the galvanic series of metals and this means that when coupled to other metals it will almost always act as a cathode in the galvanic cell there will be no galvanic corrosion on titanium.
In the passive state titanium is the cathode in a galvanic couple. In reducing environments in which the oxide film breaks down, titanium becomes the anode in a galvanic couple. In seawater there is no appreciable galvanic corrosion on titanium when is coupled to austenitic 18/8 stainless steel, Hastelloy, or Monel.
Titanium is cathodic to aluminum, carbon steel, zinc, and magnesium in seawater. The rate of corrosion of these less noble metals depends upon the ratio of the surface areas of the two metals in the galvanic couple.
Titanium is also one of the most corrosion resistant metals, in fact it is used in very corrosive service, like hot seawater, where other corrosion resistant materials like super duplex stainless steel or nickel alloys will fail.
When a metal is coupled to Titanium special precautions shall be foreseen in order to prevent Titanium galvanic corrosion; this includes:
ref: Material Deterioration Prevention and Control Guide MIL-HDBK-735