Research On Pitting Corrosion Characteristics And Influencing Factors Of Titanium

Titanium is a typical passivated metal. With a stable and dense surface oxide film, it naturally has excellent corrosion resistance. Its pitting corrosion resistance is significantly better than that of ordinary stainless steel and aluminum alloy, so it is widely used in many corrosion conditions. However, as the application of titanium equipment in harsh environments such as high temperature and concentrated chloride continues to expand, cases of pitting corrosion of titanium materials have gradually emerged, bringing hidden dangers to the safe operation of the equipment. Clarifying the occurrence law and influencing elements of titanium pitting corrosion has important engineering value for the material selection, surface treatment and working condition control of titanium components.

Pitting corrosion is a type of local corrosion form unique to passivated metals. For titanium, crevice corrosion is easier to germinate than pitting corrosion itself. In many cases, crevice corrosion begins to develop in the form of pitting corrosion in the crevice area. Among the corrosion evaluation methods, electrochemical testing is a common method. By measuring the pitting corrosion potential, it is possible to quantitatively compare the tendency of the material to pitting; when the pitting corrosion potential breaks through the surface oxide film rupture potential, the passivation film will be penetrated, inducing pitting corrosion.

The pitting corrosion behavior of titanium is subject to multiple conditions such as medium environment, material composition, and surface state, and shows distinct patterns.

The composition and temperature of the medium are the primary influencing factors. In halide solutions such as chlorides and bromides, rising temperature will increase the pitting corrosion sensitivity of titanium; however, pH has a relatively limited effect on titanium's pitting corrosion resistance. Under different anionic environments, the rupture potential of titanium passivation films varies widely: the rupture potential in sulfate and phosphate systems can reach about 100 V, which is difficult to trigger pitting; the rupture potential in chloride solutions is about 8 to 10 V; bromide and iodide systems are even as low as about 1 V, so the halide environment is a high occurrence of titanium pitting. Some anions, including sulfate, nitrate, chromate, phosphate, and carbonate, can play a corrosion inhibition effect and enhance titanium's ability to resist pitting corrosion.

The internal composition and impurity phase of the material also affect the corrosion resistance level. If the iron content in titanium is too high, the resistance to pitting corrosion will decrease, and the titanium‑iron (Ti-Fe) phase formed in the material will often become the nucleation site of pitting corrosion. For example, the ruthenium oxide coated titanium anode used in the chlor-alkali industry, once the impurity iron content exceeds the standard, the matrix will undergo pitting corrosion, which will cause damage to the surface ruthenium oxide coating and cause the components to fail. For industrial pure titanium, changes in oxygen content will also change the pitting resistance: among the three grades of industrial pure titanium samples tested, the oxygen content of grade 1 is 0.0655%, grade 2 is 0.0955%, and grade 3 oxygen content is 0.2145%; under the same surface pretreatment conditions, Grade 3 industrial pure titanium has the highest pitting corrosion potential and the lowest pitting corrosion sensitivity. It can be seen that moderately increasing the oxygen content in titanium can increase its pitting corrosion potential and improve its pitting corrosion resistance.

Surface pretreatment, roughness and external contact pollution cannot be ignored. Different surface treatment processes will bring obvious differences. Samples treated with vacuum annealing and anodizing have the highest pitting corrosion potential and better pitting corrosion resistance; surfaces polished with wet sandpaper are more likely to induce pitting corrosion. In addition, surface roughening, or the surface of titanium is contaminated by friction and contact with zinc, iron, aluminum, manganese, copper and other metals, will promote the emergence of pitting corrosion.

From the point of view of the corrosion development process, titanium pitting corrosion is generally divided into three stages: nucleation, growth, and re-passivation. The premise of nucleation is that the potential is higher than the rupture potential of the passivation film; after the condition is reached, the corrosion hole begins to grow, but the area of the corrosion hole is dynamically changing, so the current value cannot directly quantitatively characterize the growth rate of the pore body. The re-passivation process can inhibit the pitting corrosion from continuing to expand. Under some working conditions, the corrosion process will stay in the growth stage, achieve self-suppression, and no longer continue to develop.

In summary, although the titanium material itself has excellent pitting resistance, there is still a risk of local pitting failure under conditions such as high temperature halide, excessive impurities, improper surface treatment, and contact pollution of dissimilar metals. In the production and processing stage of titanium alloys and pure titanium products, reasonably control the composition, optimize the surface treatment scheme such as vacuum annealing and anodizing, control the surface roughness, avoid harmful ions and dissimilar metal pollution, and screen suitable materials in combination with working conditions, which can effectively prevent and control pitting corrosion and ensure the long-term stable service of titanium equipment.