Corrosion of titanium and titanium alloys

2024,08,27

Corrosion resistance of titanium

Titanium is a metal with a strong tendency to passivate, in air and oxidizing or neutral aqueous solutions can quickly generate a stable oxidizing protective film, even if the film is damaged for some reason, it can be quickly and automatically restored. Therefore, titanium has excellent corrosion resistance in oxidizing and neutral media.

Because of titanium's great passivation properties, in many cases with dissimilar metals in contact, does not accelerate corrosion, but may accelerate the corrosion of dissimilar metals. Such as in the low concentration of non-oxidizing acid, if the Pb, Sn, Cu or Monel alloy and titanium in contact with the formation of galvanic coupling, these materials corrosion accelerated, while titanium is not affected. And in hydrochloric acid, titanium and mild steel contact, due to the titanium surface produces newborn hydrogen, destroying the titanium oxide film, not only cause titanium hydrogen embrittlement, but also accelerate the corrosion of titanium, which may be due to the titanium has a high degree of activity on hydrogen.

Iron content in titanium has an impact on the corrosion resistance of certain media, the reason for the increase in iron in addition to the cause of raw materials, often welded stained iron penetration of the weld channel, so that the weld channel in the local iron content increases, when the corrosion has an uneven nature. The use of iron parts to support titanium equipment, iron staining on the iron titanium contact surface is almost inevitable in the iron stained area corrosion acceleration, especially in the presence of hydrogen. When mechanical damage occurs to the titanium oxide film on the stained surface, hydrogen penetrates into the metal, and depending on the temperature, pressure and other conditions, hydrogen diffusion occurs accordingly, which makes titanium produce varying degrees of hydrogen embrittlement. Therefore, titanium is used in medium temperature and medium pressure and hydrogen-containing systems to avoid surface iron contamination.

Titanium also has corrosion fatigue stability.

Titanium crevice corrosion resistance is better, especially Ti-0.3Mo-0.8Ni and Ti-0.2Pd alloys, so Ti-0.3Mo-0.8Ni and Ti-0.2Pd alloys are widely used in the sealing surface materials of the container equipment to solve the problem of crevice corrosion of the sealing surface of the equipment.

Total corrosion

Uniform corrosion occurs on the surface of titanium specimens or workpieces, forming a layer of corrosion products of uniform thickness, clinging to the surface of titanium, and generally does not extend inwardly with the extension of time, but there are exceptions. In many corrosive media, the corrosion performance of titanium with a protective layer of other metals (such as aluminum) as good or better. Corrosion of titanium is usually electrolytic in nature, so there is a relationship between corrosion and electrode potential and electric current. Anodic and cathodic polarization also have a strong influence on the corrosion mechanism and rate. The potential of titanium depends largely on the insulating properties of the oxide film. Therefore, the characteristics of the oxide film on the surface of titanium play a decisive role in its corrosion resistance. Any factor that improves the density of the oxide film, increases the thickness of the oxide film, and improves the insulating properties of the oxide film contributes to the corrosion resistance. Conversely, any factor that reduces the effective protective ability of the oxide film, whether mechanical or chemical, will cause a sharp decline in the corrosion resistance of titanium.

Localized corrosion

Titanium corrosion under most conditions is localized in nature, and the degree of corrosion at one point differs significantly from that at another. Crevice corrosion, pore corrosion, stress corrosion cracking are localized corrosion. Crevice corrosion occurs in the edge or folded edge and the stack near the crevice between the crevice, the crevice is too small or too large will not occur. Pore corrosion is a kind of corrosion in the open hole, in the presence of CI-, Br-, I- ions are prone to this kind of corrosion. Stress corrosion cracking is a kind of corrosion that occurs when the workpiece or specimen is under the combined effect of tensile stress and corrosive environment.

Abrasion

Specimens or workpieces in corrosive flow media corrosion form, due to the mechanical action of the fluid, corrosion accelerated, because the fluid can take away some or all of the corrosion products, exposure of new surfaces, accelerated corrosion.

Dissimilar metal contact corrosion, also known as galvanic coupling corrosion, in the corrosive environment, placed 2 different potential metal or structural components, in the case of an electrical short circuit, the potential of the low metal will produce corrosion.

H2 absorption or H2 embrittlement

Under normal conditions, titanium and titanium alloys always contain H2. If H2 is extracted from the material, when the amount extracted exceeds the solid solution limit, brittle hydrides will be formed, producing hydrogen embrittlement.

Under most conditions, the corrosion of titanium and titanium alloys is local in nature, and at the same time, the degree of corrosion at one point is very different from that at another point. Therefore, the quantitative evaluation of corrosion can only be based on a large amount of statistical material, and not on the results of a few specimens. Evaluation of corrosion is another serious problem is to what as a standard, rarely used mass loss, mostly based on the loss of strength, surface appearance changes or perforation to determine the degree of corrosion. Generally speaking, the corrosion process of titanium and titanium alloys is slow. Unless there is a complete lack of adaptation to the conditions under which they are used. In order to correctly evaluate the performance of titanium in service, it is usually necessary to carry out tests for dozens of days or even years. Titanium and titanium alloys corrode quickly at first in many cases, then slow down and often only weak corrosion occurs at the end. However, in some occasions, titanium alloys will be transformed after a period of time, and the organization and properties will change dramatically. Therefore, a short time use test is not completely reliable. There are many methods of rapid use tests, but in general, the faster the test, the less reliable the results.

Titanium belongs to one of the thermodynamically extremely unstable metals, and its standard electrode potential is -1.63 V. The surface is always covered with a thin and dense TiO2 film, and thus the stabilizing potential of titanium and titanium alloys is biased toward positive values, for example, the stabilizing potential of titanium in seawater at 25°C is about 0.09 V. The electrode potential is mostly calculated from thermodynamic data, and it may be different due to different data sources This is normal.

The surface of titanium and titanium alloys always has a thin layer of naturally occurring oxide film in the air, and its excellent corrosion resistance stems from the fact that there is always a layer of stable, strong adhesion and good protection of the oxide film on its surface. The corrosion resistance of this protective film can be expressed in the ratio of P / B. P / B value is greater than 1 to have protective, otherwise, low corrosion resistance, but not greater than 2.5, such as greater than this value, the oxide film within the compressive stress increases, which can easily cause the oxide film rupture, corrosion resistance decreases, the optimal value of 1 ~ 2.5.

Titanium in the atmosphere or aqueous solution will immediately form an oxide film, formed in the atmosphere at room temperature film thickness of 1.2nm ~ 1.6nm, and with the extension of time and growth, 70 days after the increase to 5nm, 545 days can be thickened to 8nm ~ 9nm. artificially strengthen the oxidation conditions, such as heating, oxidizing agent or anodic oxidation, etc., can accelerate the oxidation, increase the thickness of the film to improve the corrosion resistance.

The oxide film on the surface of titanium and titanium alloys is generally not a single structure, and its composition and structure are related to the generation conditions.

Usually, in the oxide film and the environment of the interface of the majority of TiO2, and in the oxide film and the metal interface may be TiO2-based, and the middle of the transition layer of different valence, or even non-chemical equivalent oxide, which indicates that the surface of titanium and titanium alloys oxide film for the complex multilayer structure. As for their formation process, it can not be simply understood as a direct reaction between Ti and O2. Some researchers and scholars have proposed a variety of formation mechanisms, Russian scholars believe that the first generation of hydride, and then in the hydride on the formation of pure oxide film.

Author:

Mr. Richard

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