Gap corrosion is a localized corrosion phenomenon that occurs in tight gaps,which can be caused by structural factors such as flange connection surfaces or gasket surfaces,expansion joints between pipes and pipe plates,and connection surfaces of bolts or rivets,or by scaling or deposits on the underlying surface.In the early days,it was believed that titanium did not undergo crevice corrosion in seawater and salt spray.Later,it was found to undergo secondary corrosion in high-temperature chloride media(such as seawater heat exchangers),wet chlorine gas(such as wet chlorine gas tubular condensers),hydrochloric acid solutions with oxidant corrosion inhibitors,formic acid and oxalic acid solutions,and other media Corrosion and damage of gaps in production equipment.

The crevice corrosion of titanium is related to many factors such as environmental temperature,chloride type and concentration,pH value,and the size and geometry of the crevices.In addition,the gaps between titanium and non-metallic materials such as polytetrafluoroethylene and asbestos are more sensitive to crevice corrosion than the gaps between titanium and titanium.

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

①The occurrence of crevice corrosion has a incubation period,which is related to many factors such as environmental 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.The higher the concentration of chloride ions,the higher the temperature,and the lower the pH value of titanium in sodium chloride solution,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 those of the bulk solution.Generally speaking,the oxygen concentration in the gap is low,while the chloride and hydrogen ion concentrations are high(pH value lower than the bulk solution).The pH value in the gap can drop to<1,and the electrode potential in the gap becomes more negative,thereby making the titanium in the gap in an active state.Laboratory electrochemical measurements show that the order of crevice corrosion potential of various halide ions is CI-<Br-<I-,indicating that titanium has the highest crevice corrosion sensitivity in chloride,which is opposite to the point corrosion sensitivity of titanium.

③The 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,that is,once"nucleation"occurs,the corrosion develops rapidly due to the self catalytic mechanism,ultimately leading to local perforation and damage.

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

⑤After years of research,the physical image of the crevice corrosion process of titanium has 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 inside and outside the gaps.The cathodic reaction consumes oxygen in the gap solution.When oxygen is depleted in the gap,the cathodic reaction only occurs outside the gap,while the anodic reaction inside the gap mainly occurs-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 gaps and undergo hydrolysis reactions,producing white corrosion products,namely titanium hydroxide.The white corrosion product after dehydration of titanium hydroxide is 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 is over,its development is very rapid,which is called"self catalytic effect".

⑥In 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 obtained through theoretical predictions.The experiment tells us that the corrosion tendency of narrow gaps is much greater than that of wide gaps,and the width of gaps is generally below 0.5mm.

⑦In 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 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 improve the resistance of titanium to crevice corrosion