Study Of Corrosion Behavior Of TC4 Titanium Alloy And Diamond-like Carbon Films In Deep Sea Environment

Because of the abundant materials,the deep-sea area has become the focus of development and research.Safe serving of marine equipment in the deep sea area is the core issue of deep sea development.Titanium alloy has become the backbone of the ocean and deep sea area because of its excellent properties.Titanium alloys are prone to corrosion failure during service due to the influence of corrosive medium,excessive cathodic protection and stress.Therefore,corrosion behavior and failure mechanism of titanium alloy in deep sea are studied,and the deep sea corrosion principle of titanium alloy is clarified to provide an important guarantee for the long-term safe service of deep-sea engineering equipment.In this thesis,the corrosion behavior of TC4 titanium alloy and diamond-like carbon coating（TC4-DLC）in simulated marine and deep sea environment was studied by a self-built high-pressure reactor to simulate the deep sea environment.And the effect of cathode hydrogen charging on the corrosion behavior of TC4 was also studied.The surface product composition after hydrogen charging was characterized by XRD and GIXRD.SEM was used to characterize the micromorphologies of the samples after hydrogen charging and immersion.Electrochemical workstation was used to characterize the electrochemical behavior of samples during immersion.The results are as follows:The corrosion behavior of TC4 and TC4-DLC under atmospheric pressure was studied.The results showed that TC4 and TC4-DLC have excellent corrosion resistance in simulated seawater.After soaking for 60 days,no obvious corrosion products were found on the surface of TC4 and TC4-DLC,and the weight loss rate was very low,both of which remained at the order of 10^-7 g/cm²/a.The polarization resistance and corrosion current densities of TC4 were 4.51×10⁶Ω·cm² and 6.29×10^-7A/cm²,respectively.However,the numerical values of TC4-DLC were 8.51×10⁶Ω·cm² and 9.09×10^-8 A/cm²,respectively.The use of DLC increased the polarization resistance of TC4 and decreased the corrosion current density.The results showed that DLC provided good protection for TC4 matrix.The corrosion behavior of TC4 after hydrogen charging was studied.The results showed that after hydrogen charging,the surface passivation film gradually transformed into flocculent titanium hydride layer Ti H_x,which mainly distributed at the junction ofαandβphases.And the longer the hydrogen charging time,the more titanium hydride,the greater the thickness of the film.The electrochemical results showed that the corrosion current density increased to 8.14×10^-6 A/cm²,the polarization resistance decreased by 6.32×10⁴Ω·cm²,and the carrier density increased from the initial 3.95×10¹⁹ cm^-3 to 9.84×10¹⁹ cm^-3.The corrosion behavior of TC4 and TC4-DLC under high hydrostatic pressure was studied.The results showed that the corrosion process was accelerated by hydrostatic pressure:the charge-transfer resistance decreased to 2.51×10⁵Ω·cm²,and the corrosion current density increases to 5.42×10^-7 A/cm².TC4-DLC also presented the same change trend,but it was 1～2 orders of magnitude better than the matrix.The reason is that the hydrostatic pressure led to the enhancement of ion activity and permeability,changed the content of Ti O₂ and related elements in the film layer,and also changed the size of pores in the DLC film layer,weakening the film base binding force.The corrosion behavior of TC4 and TC4-DLC cathodes after hydrogen charging was studied in deep sea.The results showed that the corrosion resistance decreased obviously after hydrogen charging in deep sea.The TC4 film resistance and charge transfer resistance decreased from 1.56×10⁵Ω·cm² and 6.12×10⁴Ω·cm² to 9.14×10³Ω·cm² and 7.26×10³Ω·cm²,and the corrosion current density also increased to2.23×10^-5 A/cm².The presence of DLC prevented the invasion of hydrogen and slowed down the damage of hydrogen to the passivated film.Moreover,under high hydrostatic pressure,the film resistance,charge transfer resistance and corrosion current density were 1.22×10⁴Ω·cm²,4.82×10⁴Ω·cm² and 8.26×10^-7 A/cm²,respectively,which still provided good protection for the matrix.