Mechanisms Of Deep-Sea Corrosion Damage To Fe-Based Alloys And Surface Diamond-Like Films

"To the deep sea"has become a major development goal of our country and has attracted much attention.However,the problem of material corrosion failure is becoming more and more prominent in this process.At present,researches on corrosion failure of Marine materials are mostly concentrated in shallow sea environment,while there are few related researches in deep sea environment,and the relevant data are mostly mastered by some foreign countries and organizations.Domestic researches on deep-sea corrosion are few and basically in the initial stage at present.The deep sea conditions are obviously different from the shallow sea conditions,and the existing research results can not be directly applied to the deep sea conditions.The deep sea corrosion research is of great significance for the deep sea material selection and material protection,and can provide practical experience and theoretical basis for the development of the deep sea cause.In this thesis,Diamond-like Carbon（DLC）films were prepared on the surface of316L stainless steel and an ultra high strength steel produced by Anshan Iron and Steel by plasma enhanced chemical vapor deposition technology.In order to study the influence of different hydrostatic pressure on corrosion,the exposure experiment and in situ electrochemical test under different hydrostatic pressure were completed by using a four electrode corrosion test autoclave.In order to study the effect of long-term exposure at high hydrostatic pressure on corrosion behavior,480 h exposure experiment at 10 MPa and in situ electrochemical test were completed.In order to study the influence of over-negative cathodic protection potential on corrosion behavior of ultra high strength steel,electrochemical hydrogen charging was carried out on ultra high strength steel.The hydrogen concentration in the electrode was calculated by potentiostatic oxidation measurement in a single electrolytic cell,and the corrosion performance was characterized by polarization and AC impedance test.The main work progress achieved is as follows:Compared with the normal pressure environment,the passivation film at 5 MPa and 10 MPa is denser and the defect density is lower.The donor concentration of passivation film at 0 MPa is the highest,which is 4.75×10¹⁹/cm³,while the lowest is3.37×10¹⁹/cm³ at 5 MPa.High pressure can increase the chloride ion activity and the pitting sensitivity of 316L stainless steel,and this effect is most obvious when the hydrostatic pressure is 10 MPa.Long time exposure will increase the pitting sensitivity of 316L stainless steel,along with pitting initiation and repair.The pitting sensitivity of 316L stainless steel surface DLC film at 10 MPa is greater than that at 0 MPa.At 5 MPa and 10 MPa,Fe₃O₄ oxide with good protective effect is formed in ultra high strength steel,which reduces the corrosion rate.The corrosion current densities at5 MPa and 10 MPa are 2.651×10^-6 A/cm² and 3.751×10^-6 A/cm²,respectively,which are lower than 7.015×10^-5 A/cm² at 0 MPa.The corrosion potential of ultra high strength steel decreases from-648 m V to-772 m V,the corrosion current density decreases from 1.915×10^-6 A/cm² to 1.140×10^-6 A/cm²,and the charge transfer resistance increases after 480 h exposure at 10 MPa.480 h corrosion products contain more Fe₃O₄ than 48 h corrosion products.DLC film makes ultra high strength steel from uniform corrosion to local corrosion.The corrosion tendency and rate of DLC films on ultra high strength steel surface are not different under different pressures.Compared with 48 h after exposure to 10 MPa for 480 h,the self-corrosion potential,corrosion current density and pitting corrosion of DLC films on ultra high strength steel surface decreased significantly.The corrosion products generated in the process of exposure will cause tensile stress of DLC films,which will eventually cause the destruction of DLC films and lose the protective effect on the matrix.The hydrogen content in steel under different hydrogen charging current densities was measured by single cell oxidation,and the hydrogen concentration was calculated.When the hydrogen charging current density was 400 m A/cm²,the hydrogen concentration in ultra high strength steel was 1.613×10^-6 mol/cm³,which was significantly higher than the 1.444×10^-6 mol/cm³ when the hydrogen charging current density was 200 m A/cm².Electrochemical hydrogen charging increases the corrosion sensitivity and corrosion rate of ultra high strength steel.When the hydrogen filling current density changes from 200 m A/cm² to 400 m A/cm²,only the corrosion current density increases,indicating that within the scope of this experiment,the increase of hydrogen filling current density can only lead to the increase of corrosion rate,while the corrosion sensitivity does not change significantly.