The corrosion resistance of titanium

Titanium is a metal with high chemical reactivity, showing particularly excellent corrosion resistance to most corrosive media. Titanium has a strong affinity for oxygen, and when exposed to the atmosphere or any oxygen-containing medium, its surface immediately forms a hard and tight protective oxide film. This film is very stable and, if subjected to mechanical damage, it will immediately reform (as long as there is a certain amount of oxygen present).

There are several special forms of corrosion for titanium, such as high-temperature corrosion, stress corrosion, pitting corrosion, crevice corrosion, galvanic corrosion and corrosion fatigue, but the application of titanium in marine valves is mainly galvanic corrosion.

Galvanic corrosion occurs when two different metals form an electric couple in an electrolyte, and the corrosion of the metal acting as the anode can be accelerated. The amount of corrosion depends on the electrode potential difference between the materials, as well as the area ratio of the anode and cathode and the polarization characteristics of both. Titanium is different from most materials; it is in a passive state in many media and shows a potential similar to that of passive 18-8 stainless steel. The potential of titanium in seawater is - 0.10V (relative to a saturated calomel electrode). The potential difference between titanium and monel alloys, Hastelloy C, 18-8 stainless steel, and aluminum bronze tubes and plates is very small when in contact with them, and no galvanic corrosion occurs. However, in seawater, the potential difference between titanium and aluminum, zinc, and carbon steel is small, and these metals are not corroded. Nevertheless, their corrosion rates are smaller than those caused by contact with stainless steel. The corrosion degree of these metals varies with the ratio of the surface area of the contacting metal to that of titanium. When the ratio of the surface area of the contacting metal to titanium is 1:10 (that is, the area ratio of the anode to the cathode is 1:10), the contacting metal will be quickly corroded. On the contrary, when the ratio of the surface area of the contacting metal to titanium is 10:1, the corrosion rate is significantly reduced. The corrosion amount of titanium in contact with certain metals in seawater is shown in Figure 1. Similar situations also exist in low-concentration reducing acids (such as sulfuric acid or hydrochloric acid). Moreover, titanium has a strong polarization tendency, meaning that in an electric couple with titanium as the cathode, only a small current is generated. Due to the presence of an oxide film on the surface of titanium, its corrosion rate usually does not increase when in contact with other metals. Therefore, attention should be paid to the comparison of galvanic corrosion in other materials used in marine titanium valves.