Characteristics and Laws of Titanium Crevice Corrosion

Characteristics and Laws of Titanium Crevice Corrosion

Crevice corrosion is a localized corrosion phenomenon that occurs in close gaps. The gaps can be structurally generated (such as flange connection surfaces or gasket surfaces, expansion joints between tubes and tube sheets, and connection surfaces of bolts or rivets, etc.), It can also be caused by scaling or deposits and the underlying coverage. In the early days, it was believed that crevice corrosion of titanium did not occur in seawater and salt spray at all. Later, in high-temperature chloride media (such as seawater heat exchangers), in wet chlorine gas (such as wet chlorine gas tube condenser), oxidant corrosion-inhibited hydrochloric acid solution , Formic acid and oxalic acid solutions and other media, have occurred successively. Crevice corrosion damage of equipment.

Crevice corrosion of titanium is related to many factors such as ambient temperature, chloride species and concentration, pH, and crevice size and geometry. In addition, the crevice composed of non-metals such as titanium and PTFE, titanium and asbestos is more sensitive to crevice corrosion than the crevice composed of titanium and titanium.

Based on the research and industrial practice at home and abroad, the crevice corrosion of titanium has the following characteristics and laws:

①The occurrence of crevice corrosion has a gestation period, and the length of the gestation period is related to many factors, such as ambient temperature, chloride type and concentration, oxidant concentration, materials in contact with titanium, pH value of the solution, and the size and geometry of the crevice, etc. . In the sodium chloride solution of titanium, the higher the chloride ion concentration, the higher the temperature, and the lower the pH value, the shorter the incubation period of crevice corrosion, that is, the stronger the sensitivity of crevice corrosion.

②The composition and pH value of the solution in the gap are completely different from the bulk solution. Generally speaking, the oxygen concentration in the gap is lower, the chloride ion and hydrogen ion concentration is higher (the pH value is lower than the bulk solution), the pH value in the gap can drop to <1, the electrode potential in the gap becomes more negative, Thus, the titanium in the gap is in an active state. Laboratory electrochemical measurements show that the order of crevice corrosion potential of various halide ions is: CI- < Br- < I-, that is, the crevice corrosion sensitivity of titanium in chloride is the largest, which is exactly the same as the pitting corrosion sensitivity of titanium. on the contrary.

③The crevice corrosion of titanium is usually localized on the crevice surface, and generally does not corrode in the entire crevice surface. After the incubation period of crevice corrosion is over, that is, once "nucleated", the corrosion develops rapidly due to the autocatalytic mechanism, which eventually leads to local perforation and destruction.

④The occurrence of titanium crevice corrosion is often accompanied by hydrogen absorption, and even the existence of needle-like hydrides in titanium can be observed using a metallographic microscope. As the amount of hydrogen absorption increases, the hydrides on the surface continue to increase, which accelerates the overall corrosion. At the same time, hydrogen continuously penetrates into the metal, and the internal hydride precipitation may become the crack source of stress corrosion cracking, resulting in cracking under the action of external stress.

⑤ After years of research, the physical image of the crevice corrosion process of titanium has been relatively clear. Briefly, it is divided into two phases: the incubation period and the active dissolution period.

At the beginning of the gestation period, the same reaction takes place inside and outside the crevice. The cathodic reaction consumes the oxygen in the crevice solution. When the oxygen in the crevice is depleted, the cathodic reaction only proceeds outside the crevice, and the anodic reaction is mainly carried out in the crevice - the anodic dissolution of titanium. With the continuous increase of titanium ions in the gap, in order to maintain the charge balance of positive and negative ions in the gap, chloride ions continue to migrate into the gap. At the same time, titanium ions accumulate in the crevices and undergo a hydrolysis reaction to generate a white corrosion product, namely titanium hydroxide. The white corrosion product after the dehydration of titanium hydroxide is identified as TiO2. The hydrolysis reaction reduces the pH value in the crevice, which further destroys the passivation of titanium. Therefore, once the incubation period of crevice corrosion is over, its development is very rapid, which is the so-called "autocatalysis".

⑥In the "geometric factor" of crevice corrosion of titanium, factors such as the length of the crevice, the width of the crevice and the ratio of the area inside and outside the crevice are included. These values ​​generally need to be determined experimentally for a specific system, and cannot be obtained by theoretical prediction. The test tells us that the crevice corrosion tendency of the narrow seam is much larger than that of the wide seam, and the width of the general seam is less than 0.5mm.

⑦ In order to improve the corrosion resistance of titanium in reducing inorganic acids and reduce the sensitivity of crevice corrosion, titanium alloys, such as Ti-Pd alloys, Ti-Ni-Mo alloys are generally superior to industrial pure titanium, especially Ti -Pd alloy. Surface treatment techniques such as palladium plating, thermal oxidation or anodizing at the crevice position on the titanium surface can improve the crevice corrosion resistance of titanium.