Study On Corrosion And Hydrogen Permeation And Stress Corrosion Cracking Of High-strength Steel In The Tidal Zone

Low alloy high-strength steels are widely used in offshore engineering equipment such as wind power and oil platforms due to their excellent mechanical properties.However,in harsh marine environments,especially in alternating dry/wet marine environments,the corrosion and hydrogen permeation of high strength steels expose them to a high risk of hydrogen embrittlement（HE）and stress corrosion cracking（SCC）.The tidal zone is a typical dry/wet alternating marine corrosion zone.At present,there are few studies on the hydrogen permeation behavior and mechanism of high-strength steel in the tidal zone,and the SCC behavior and mechanism of high-strength steel in the tidal zone is not fully understood.Therefore,based on the actual marine exposure experiment,the corrosion,hydrogen permeation and SCC behaviors and mechanisms of AISI 4135 low alloy high-strength steel under natural corrosion conditions（including independent hanging conditions and electrically connected conditions）and sacrificial anodic protection conditions were studied,and the relationship between corrosion,hydrogen permeation and SCC was established.The corrosion behavior and mechanism of AISI 4135 steel in the tidal zone under independent hanging conditions were investigated by electrochemical corrosion test and confocal laser scanning microscopy（CLSM）etc.The results showed that the corrosion rate of steel is determined by the dry/wet time ratio of steel in the tidal zone,and the higher the dry/wet time ratio,the higher the corrosion rate.The oxidation–reduction–reoxidation reaction cycles of corrosion products under thin liquid film states accelerates the corrosion of steel,resulting in the corrosion rate of steel under thin liquid film states being much greater than that of steel under seawater immersion states,so a higher dry/wet time ratio will lead to a higher corrosion rate.The corrosion morphology shows that the corrosion form of steel in the tidal zone is mainly pitting corrosion,and the higher the dry/wet time ratio,the more serious the pitting corrosion.The hydrogen permeation behavior and mechanism of AISI 4135 steel in the tidal zone under independent hanging conditions were studied through a 3-month electrochemical hydrogen permeation test.The results showed that the steel placed at the high tidal level（HTL）had the highest hydrogen content,with a value of 5.60×10^–2wt ppm,followed by the steel placed at the full-immersion zone（FIZ）and the low tidal level（LTL）.The steel placed at the middle tidal level（MTL）had the lowest hydrogen content,with a value of 1.72×10^–2 wt ppm.Under natural corrosion conditions,the hydrogen produced by the hydrolysis of corrosion products is the main source of the diffused hydrogen in steel,and the acidification of the electrolyte solution at the interface caused by hydrolysis of corrosion products leads to the positive shift of the hydrogen evolution potential,thereby promoting hydrogen permeation.The SCC behavior and mechanism of AISI 4135 steel in the tidal zone under independent hanging conditions were studied through the slow strain rate tensile test（SSRT）of AISI 4135 steel after 3-month exposure in the actual marine environment.The results showed that the elongation loss（I_δ）of steel placed at the HTL reached 28.2%,indicating high sensitivity to SCC,and HE and pitting were the main factors leading to its high SCC sensitivity.However,steel placed at the MTL,LTL and FIZ had relatively low SCC sensitivity,and their SCC was mainly controlled by microvoid coalescence（MVC）and was less affected by pitting and HE.Similarly,the corrosion,hydrogen permeation and SCC behaviors and mechanisms of AISI 4135 steel in the tital zone under electrically connected conditions were also studied by electrochemical corrosion test,electrochemical hydrogen permeation test and SSRT respectively.The results of electrochemical corrosion test showed that the corrosion rate of steel placed at different positions from high to low was:FIZ,LTL,HTL and MTL.For steel in the tidal zone,the final corrosion state is determined by the macroscopic oxygen concentration battery cotrolled corrosion in the seawater immersion states and the free corrosion in the thin liquid film states.The corrosion morphology of the steel showed that the steel at the HTL had obvious pitting characteristics,but with the decrease of the dry/wet time ratio,the steel had a tendency to develop from pitting corrosion to uniform corrosion.The results of the electrochemical hydrogen permeation test lasting for one year in the tidal zone showed that steel placed at the HTL had the highest hydrogen content,with a value of 2.72×10^–2 wt ppm,followed by the steel placed at the LTL and the MTL,and steel in the FIZ had the lowest hydrogen content,with a value of only 4.8×10^–3 wt ppm.The hydrolysis of corrosion products and the excitation of hydrogen evolution reaction by the cathodic protection effect brought by macro-oxygen concentration cells are the main reasons for the high hydrogen content of steel in the tidal zone.The SSRT results of AISI 4135 steel after one-year exposure showed that steel placed at the HTL had the highest SCC sensitivity,and its I_δreached 39.9%.The sensitivity of steel placed at the MTL and LTL to SCC decreased sequentially,and the steel in the FIZ had the lowest SCC sensitivity,with an I_δof 27.9%.HE and pitting jointly control the SCC process of the steel placed at the HTL and MTL,while the SCC of steel placed at the LTL is determined by higher corrosion loss and HE,and the SCC of steel placed in the FIZ is dominated by corrosion loss and is less affected by HE.Similarly,the corrosion,hydrogen permeation and SCC behaviors and mechanisms of AISI 4135 steel in the tital zone under sacrificial anodic protection conditions were also studied by electrochemical corrosion test,electrochemical hydrogen permeation test and SSRT respectively.The results of electrochemical corrosion test showed that steel placed at the HTL had the highest corrosion rate,on the contrary,the corrosion rate of steel placed at the LTL and FIZ was very low.The dry/wet time ratio of the steel in the tidal zone determines how long AISI 4135 steel is subjected to cathodic protection,thereby determining the corrosion rate of the steel.The steel placed at the HTL had obvious pitting characteristics,while the corrosion of steel placed at other locations was almost uniform corrosion.The results of the electrochemical hydrogen permeation test lasting for one year in the tidal zone showed that steel in the FIZ had the highest hydrogen content due to receiving continuous cathodic protection,followed by the steel placed at the MTL and HTL.Severe biofouling at the LTL inhibits the hydrogen permeation of steel under cathodic protection,thus resulting in a low hydrogen content in steel.The SSRT results of AISI4135 steel after one-year exposure in tidal zone showed that steel placed at the HTL had the highest SCC sensitivity,and its I_δwas as high as 47.0%,followed by steel placed at the MTL and FIZ.The steel placed at the LTL had the lowest SCC sensitivity,and its I_δwas 11.8%.HE and pitting jointly control the SCC process of steel placed at the HTL,while HE is the main SCC mechanism of steel placed at the MTL,LTL and FIZ.In addition,the study of the factors affecting hydrogen permeation of AISI 4135steel in the tidal zone shows that the calcareous deposits layer formed under cathodic protection conditions can promote hydrogen permeation to a certain extent by inhibiting hydrogen recombination and escape,while the application of petrolatum tape cover technology can significantly inhibit the permeation of hydrogen into the steel matrix in the marine environment.