The characteristics and laws of titanium crevice corrosion

Gap corrosion is a localized corrosion phenomenon that occurs in tight gaps, which can be caused by the structure (such as flange connection surfaces or gasket surfaces, pipe to pipe plate expansion joints, and bolt or rivet connection surfaces), or by scaling or sediment on the covering surface below it. In the early days, it was believed that titanium did not undergo crevice corrosion in seawater or salt mist. Later, in high-temperature chloride media (such as sea water heat exchangers), wet chlorine gas (such as wet chlorine gas tubular condensers), hydrochloric acid solutions with oxidant corrosion inhibition, formic acid and oxalic acid solutions, and other media, crevice corrosion damage occurred in equipment.

The crevice corrosion of titanium is related to many factors such as environmental temperature, types and concentrations of chlorides, pH value, and the size and geometric shape of the crevices. In addition, gaps composed of non-metallic materials such as titanium and polytetrafluoroethylene, titanium and asbestos are more sensitive to crevice corrosion than gaps composed of titanium and titanium.

Based on research and industrial practice both domestically and internationally, the crevice corrosion of titanium exhibits the following characteristics and patterns.

① The occurrence of crevice corrosion has a incubation period, and the length of the incubation period is related to many factors, such as environmental temperature, chloride type and concentration, oxidant concentration, material in contact with titanium, pH value of the solution, and the size and geometric shape of the crevice. In sodium chloride solution, the higher the concentration of chloride ions, the higher the temperature, and the lower the pH value of titanium, the shorter the incubation period of crevice corrosion, which means the stronger the sensitivity of crevice corrosion.

② The composition and pH value of the solution in the gap are completely different from the bulk solution. Generally speaking, when the oxygen concentration in the gap is low, and the concentration of chloride and hydrogen ions is high (with a pH value lower than the bulk solution), the pH value in the gap can decrease to<1, and the electrode potential in the gap becomes more negative, resulting in the titanium in the gap being in an active state. Laboratory electrochemical measurements indicate that the order of crevice corrosion potentials for various halide ions is CI -<Br -<I -, indicating that titanium has the highest sensitivity to crevice corrosion in chloride, which is exactly opposite to its sensitivity to pitting corrosion.

③ The crevice corrosion of titanium usually occurs locally on the crevice surface, and generally does not occur comprehensively on the entire crevice surface. After the incubation period of crevice corrosion ends, once it nucleates, the corrosion rapidly develops due to the self catalytic mechanism, ultimately leading to local perforation and damage.

④ The occurrence of titanium crevice corrosion is often accompanied by hydrogen absorption, and even the presence of needle like hydrides in titanium materials can be observed using a metallographic microscope. With the increase of hydrogen absorption, the amount of hydride on the surface continuously increases, leading to an overall acceleration of corrosion. At the same time, hydrogen continuously penetrates into the interior of the metal, and the precipitation of internal hydrides may become a source of stress corrosion cracking, leading to cracking under external stress.

⑤ After years of research, the physical images of the crevice corrosion process of titanium have become relatively clear. Simply put, it is divided into two stages: incubation period and active dissolution period.

At the beginning of the gestation period, the same reaction occurs both inside and outside the gap. The cathodic reaction consumes the oxygen in the crevice solution. When the oxygen in the crevice is poor, the cathodic reaction only occurs outside the crevice, and the main anodic reaction inside the crevice is 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 continuously migrate into the gap. At the same time, titanium ions accumulate in the gaps and undergo hydrolysis reactions, generating white corrosion products, namely titanium hydroxide. After dehydration of titanium hydroxide, the white corrosion products are identified as TiO2. The hydrolysis reaction causes a decrease in pH value within the gap, further disrupting the passivation of titanium. So once the incubation period of crevice corrosion ends, its development is very rapid, which is called "self catalysis".

⑥ In the "geometric factors" of titanium crevice corrosion, factors such as crevice length, crevice width, and the ratio of the area inside and outside the crevice are included. These values generally need to be determined through specific system experiments and cannot be predicted theoretically. Experiments have shown us that the corrosion tendency of narrow gaps is much greater than that of wide gaps, with a general gap width of less than 0.5mm.

⑦ In order to improve the corrosion resistance of titanium in reducing inorganic acids and reduce its sensitivity to crevice corrosion, titanium alloys such as Ti Pd alloy and Ti Ni Mo alloy are generally used, which have superior performance than industrial pure titanium, especially Ti Pd alloy. Surface treatment techniques such as palladium plating, thermal oxidation, or anodizing can all improve the corrosion resistance of titanium in crevices