Crevice corrosion is a localized corrosion phenomenon that occurs in a close gap position, the gap can be generated by the structure (such as flange connection surface or gasket surface, pipe and tube plate expansion joints and bolts or rivets connection surface, etc.), or it can be scaling or deposits with its under the cover caused by the surface. Early that titanium in seawater, salt spray does not occur at all crevice corrosion, and later in the high temperature chloride medium (such as seawater heat exchanger), wet chlorine gas (such as wet chlorine gas tube condenser), oxidizer corrosion inhibition of hydrochloric acid solution, formic acid and oxalic acid solution and other media, have been issued one after another. Crevice corrosion damage of the equipment.
Titanium crevice corrosion and ambient temperature, chloride type and concentration, pH value and the size and geometry of the crevice and many other factors. In addition, titanium and PTFE, titanium and asbestos and other non-metallic composition of the crevice, than titanium and titanium composition of the crevice more sensitive to crevice corrosion.
Comprehensive domestic and foreign research and industrial practice, titanium crevice corrosion has the following characteristics and laws.
① The occurrence of crevice corrosion has a gestation period, and the length of the gestation period is related to many factors, such as the ambient temperature, the type and concentration of chloride, the concentration of oxidizing agent, the material in contact with titanium, the pH value of the solution, and the size and geometry of the crevice. Titanium in sodium chloride solution, the higher the concentration of chloride ions, the higher the temperature, the lower the pH value, then the shorter the gestation period of crevice corrosion, that is, the stronger the sensitivity of crevice corrosion.
② The composition and pH value of the solution in the crevice are completely different from the body solution. Generally speaking, the gap in the lower concentration of oxygen, chloride ions and hydrogen ions in higher concentrations (pH lower than the body solution), the pH value in the gap can fall to <1, the gap in the electrode potential becomes more negative, thus making the gap in the titanium in the active state. Laboratory electrochemical measurements show that the crevice corrosion potentials of various halide ions are in the order of CI- ＜ Br- ＜ I-, i.e., titanium in chloride has the greatest crevice corrosion sensitivity, which is the opposite of titanium's pitting corrosion sensitivity.
(iii) The crevice corrosion of titanium is usually localized on the crevice surface, and generally does not occur comprehensively on the entire crevice surface. Crevice corrosion after the end of the incubation period, that is, once the "nucleation" is due to the autocatalytic mechanism makes the corrosion of rapid development, ultimately leading to local perforation and destruction.
The process of titanium crevice corrosion is often accompanied by hydrogen absorption, and even the presence of needle-like hydrides in titanium can be observed using a metallurgical microscope. With the increase of hydrogen absorption, the hydride on the surface continues to increase, making the corrosion accelerated across the board. At the same time, hydrogen continuously penetrates into the metal, the internal hydride precipitation may become the crack source of stress corrosion cracking, resulting in cracking under external stress.
⑤ After years of research, the physical image of the crevice corrosion process of titanium has become clearer. Briefly, it is divided into two stages: the incubation period and the active dissolution period.
At the beginning of the gestation period, the same reaction is carried out inside and outside the crevice. Its cathodic reaction consumes the oxygen in the crevice solution, and when the crevice is depleted of oxygen, the cathodic reaction is carried out only outside the crevice, and the anodic reaction - anodic dissolution of titanium - is mainly carried out inside the crevice. As the titanium ions in the crevice continue to increase, chloride ions continue to migrate into the crevice in order to maintain the charge balance of positive and negative ions in the crevice. At the same time, titanium ions accumulate in the crevice and undergo hydrolysis, generating white corrosion products, i.e., titanium hydroxide, titanium hydroxide dehydration after the white corrosion products were identified as TiO2 . The hydrolysis reaction makes the pH value in the crevice decrease, further destroying the passivation of titanium. Therefore, once the incubation period of crevice corrosion is over, its development is very rapid, which is called "autocatalytic effect".
(6) Among the "geometric factors" of crevice corrosion of titanium, there are factors such as crevice length, crevice width and the ratio of the area inside and outside the crevice. These values generally need to be determined experimentally for specific systems and are not theoretically predictable. Tests tell us that the tendency of crevice corrosion of narrow seams is much larger than that of wide seams, and the general width of the seam is below 0.5mm.
(7) In order to improve the corrosion resistance of titanium in reducing inorganic acids and to reduce the sensitivity of crevice corrosion, titanium alloys are generally used, such as Ti-Pd alloys, Ti-Ni-Mo alloys are superior to the performance of industrially pure titanium, especially Ti-Pd alloys. Titanium surface crevice position palladium plating, thermal oxidation or anodic oxidation and other surface treatment techniques can improve the performance of titanium crevice corrosion resistance.
