Titanium corrosion resistance, mechanism

The basic corrosion behaviour of stainless steels is dependent upon the type and quantity of alloying. Chromium is the universally present element but nickel, molybdenum, copper, nitrogen, vanadium, tungsten, titanium and niobium are also used for a variety of reasons. However, all elements can affect metallurgy, and thus mechanical and physical properties, so sometimes desirable corrosion resisting aspects may involve acceptance of less than ideal mechanical properties and vice versa. 

Zirconium and hafnium are two refractory elements which are closely, related to titanium (see Encycl Vol 9, T227-R) but which in addition to having desirable corrosion resistance and mechanical properties, have many significant ordnance applications. Hf free Zr moreover is possessed of a low neutron capture crossection, and is therefore of value as a reactor material. In as much as Hf is chemically similar to Zr and as in nature it is always found in association with Zr, it will be discussed as part of the Zr technology, except where its special properties... 

The production of corrosion-resistant materials hy alloying is well established, hut the mechanisms are noi lull) understood. It is known, of course, that elements like chromium, mckcl. titanium, and aluminum depend for their corrosion resistance upon a tenacious surface oxide layer (passive film). Alloying elements added for the purpose of passivation must be in solid solution. The potential of ion implantation is promising because restrictions deriving from equilibrium phase diagrams frequently do not applv li e., concentrations of elements beyond tile limits of equilibrium solid solubility might he incorporated). This can lead to heretofore unknown alloyed surfact-s which are very corrosion resistant... 

Because hafnium has a high absorption cross-section for thermal neutrons (almost 600 times that of zirconium), has excellent mechanical properties, and is extremely corrosion resistant, it is used to make the control rods of nuclear reactors. It is also applied in vacuum lines as a getter —a material that combines with and removes trace gases from vacuum tubes. Hafnium has been used as an alloying agent for iron, titanium, niobium, and other metals. Finely divided hafnium is pyrophoric and can ignite spontaneously in air. 

Next-generation metallic biomaterials include porous titanium alloys and porous CoCrMo with elastic moduli that more closely mimic that of human bone nickel-titanium alloys with shape-memory properties for dental braces and medical staples rare earth magnets such as the NdFeB family for dental fixatives and titanium alloys or stainless steel coated with hydroxyapatite for improved bioactivity for bone replacement. The corrosion resistance, biocompatibility, and mechanical properties of many of these materials still must be optimized for example, the toxicity and carcinogenic nature of nickel released from NiTi alloys is a concern. ... 

Titanium aluminide alloys based on Ti3 A1 and TiAl are of interest as construction material for high temperature components particularly in aerospace industry. Good mechanical properties can be attained with alloys consisting of y-TiAl with 3 to 15 vol% a2-Ti3Al. The disadvantages are the low ductility and the inadequate oxidation resistance at service temperatures of 700-900°C [1]. A fundamental understanding of the oxidation behaviour is necessary in order to improve the corrosion resistance. The formation of the oxides on the alloy surface depends on the temperature, the oxygen partial pressure of the corrosive atmosphere, and the thermodynamic activities of Ti and A1 in the alloys. 

Alloy steel pipe composition has various elements, with total concentration between 1.0% and 50% by weight, which enhances the mechanical properties and corrosion resistance. These steels can be grouped under low-alloy steels. Along with economic growth, the demand of alloy steel pipes and tubes for industrial use has increased enormously. The most common alloying elements are nickel, chromium, silicon, vanadium, and molybdeniun. Special pipe steels also contain very small amounts of aluminum, cobalt, tungsten, titanium, and zirconium. Alloy steel has different properties on the basis of its composition. Alloy steel tubes cater to domestic and industrial requirements, such as gas drilling, offshore projects, refineries, and petrochemical plants. 

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