Study On The Influence Of Passive Film On Different Corrosion Forms Of Titanium Alloys

The excellent corrosion resistance of titanium alloys is attributed to the dense passive film on the surface,which makes them known as "Marine Metal".However,the service conditions in marine environment are extremely harsh,and titanium alloys are vulnerable to corrosion damage.The corrosion damage of titanium alloys is closely related to the failure of passive film,such as,crevice corrosion caused by dissolution of passive film in acidic occlude environment,corrosion damage caused by scratching of passive film on the surface,and the passive film also plays an important role on the galvanic corrosion of titanium coupled with dissimilar metals.At present,the effect mechanism of passive film on the corrosion process of titanium alloy is not very clear.Therefore,the effect of microstructure and corrosive medium on corrosion behavior of titanium alloys were compared by electrochemical measurements and surface morphology observation,and the crevice corrosion of TC4 with different Fe content in the NaCl solution containing few F-was investigated.Meantime,the dynamic evolution progress of passive film in scratched area was studied using in situ scratch test in corrosive medium,and the galvanic corrosion behavior of titanium alloy coupled with dissimilar metals was studied from the point of view of charge transfer on the passive film.The results indicate that compared with TC4-L(low Fe content)and TC4-H(high Fe content),the ZTi60 with basket weave micro structure shows the worst corrosion resistance,which is attributed to the lamellae structure of α phases.Meanwhile,TC4-H shows excellent corrosion resistance in NaCl solution containing Br-or H+;in the NaCl solution containing F-,the passivation ability of TC4-H gradually decreases with the increase in F-concentrations;When F-and H+coexist,the anodic dissolution of TC4-H is accelerated by the synergistic effect of F-and H+,while the addition of a certain amount of Fe3+would result in the inhibition of corrosion of titanium alloy in the acidic F-by changing the cathodic reduction reaction and the complexation effect with F’.Inside the crevice,the more oxygen vacancies Vo in the passive film are induced with the decrease in the dissolved oxygen content,and these vacancies could be occupied by F-.The binding interaction between F-and Ti4 +in the lattice of film would increase and the fluorine-titanium complex[TiF6]2-is formed,and then the crevice corrosion is initiated.The crevice corrosion propagates along the edge of crevice to inside the crevice as the corrosion activation area increases,while it declines with the accumulation of corrosion products inside the occluded crevice.In addition,the TC4-H shows worse crevice corrosion resistance than the TC4-L.With the increasing Fe content of TC4 alloys,the enrichment of Fe in β phases increases,resulting in the increasing Volta potential difference between the β and α phases,and the Fe doping on the passive film results in the worse compactness and passivation ability of the passive film.Regarding to the scratched area of ZTi60 passive film,there is a competitive mechanism between F-and dissolved oxygen in the repassivation process of passive film.At the film/electrolyte interface,the oxygen vacancies of the passive film are consumed competitively by F-and dissolved oxygen,and the effect would be affected by the concentration of F-and the dissolved oxygen content in the electrolyte.Only when the concentration of F-is over than the critical value,the repassivation ability of the passive film in the scratching area would be weaker than the damage ability of F-to the film,resulting in the corrosion damage of the passive film.Meantime,the value of critical F-concentration is positively correlated with the dissolved oxygen content,and the decrease of dissolved oxygen content will result in the decrease of the value of critical F-concentration.When titanium alloys are coupled with dissimilar metals,there may be a risk of galvanic corrosion only when their potential difference is greater than 400 mV.It can be attributed to the low electron donor density of passive film,which inhibits the electron charge transfer process at the film/solution interface,causing the slow cathodic reaction rate on ZTi60.Therefore,a larger driving force of potential difference is needed to induce the galvanic corrosion.Furthermore,the galvanic effect of ZTi60-Al 2024 can be reduced using the micro-arc oxidation treated on the ZTi60 surface or an organic coating coated on Al 2024 surface.Based on the above investigations,the effect of physical-chemical process at the metal/passive film/solution interface on the corrosion process of titanium alloys is clarified,and it would provide a basis for the safe application of titanium alloy in the marine environment,and also provide a theoretical guidance for the selection of materials which need to couple with titanium in marine engineering equipment.