The Anticorrosive Application Of Impressed Current On Chain Structure And Its Effect On Corrosion Behavior Of Mooring Chain Steel

In recent years,the development of marine engineering equipment and technology has continued to advance,and human exploitation of marine resources has moved from shallow to deep waters.Offshore engineering mostly uses fixed offshore platforms,which are rarely used in the deep sea due to their high construction costs and low safety.Floating platforms are used for offshore drilling,production,storage,wind power and fishing equipment in deep sea waters,and they mainly rely on mooring chains for positioning and fixing at sea,except for power positioning.Corrosion is the primary problem facing mooring chains in service,as seawater is a naturally strong electrolyte solution,and in addition,environmental loads such as wind and waves combined with corrosion promote fracture.The design life of a typical offshore platform is 20-30 years,so it’s important to study the appropriate corrosion prevention measures for mooring chain and their electrochemical corrosion behavior in seawater to provide data reference for mooring chain corrosion protection,which is important to prevent mooring chain breakage and ensure operation of floating platform.Impressed current cathodic protection(ICCP)has been widely used on ship hulls,oil pipelines and pipe rack platforms because of its advantages of long protection cycle,environment friendly and adjustable current according to environmental changes.However,good electrical continuity is a prerequisite for the application of ICCP protected structures.Perhaps due to the fact of the contact surface between chain rings is positively small,ICCP is rarely applied to flexible chain structure.But it is still worth trying to apply ICCP to chain structures considering mooring chain anti-corrosion requirements and its unique advantages.In addition,the corrosion of mooring chain is generally accompanied by electrochemical changes,and both external loading and impressed current can affect their electrochemical behavior.Electrochemical measurement can sensitively monitor the changes of corrosion,so it can be used to investigate the corrosion state of mooring chain close to service.The specific research contents are as follows:(1)Study on the influence of electrical continuity on the potential distribution and effect of chain structure ICCP in seawater.Through actual potential measurement and weight loss method,combined with COMSOL numerical simulation software,the potential distribution of chain under different electrical continuity was studied to verify the feasibility of applying ICCP to chain;the potential distribution of chain under different corrosion time applied current and the effect of auxiliary anode distance on the potential distribution were studied by the COMSOL.The results indicated that the potential distribution of physical contact between the chain ring alone cannot reach the required range of protection potential,the maximum difference of potential under improvement of electrical continuity didn’t exceed 12 mV,the potential distribution was uniform,the corrosion rate of the chain structure under ICCP for 90 days was 0.00305 g/m2h,the corrosion protection effect was obvious,proved that the ICCP of the chain structure was feasible;the experimental and simulation results were in good agreement,the error was less than 7.98%;the potential of chain structure negative shifted firstly and then positive shift with the corrosion time extension;the larger the diameter of the chain link,the greater the critical conductivity of the chain structure;chain structure potential overall positive shift with the auxiliary anode distance increased,the highest potential of the adjacent contact part,the lowest end potential.The new experimental exploration of the mooring chain preservative method was carried out by above research.(2)Electrochemical corrosion behavior of mooring chain steel under constant stress and cathodic protection.The effects of constant stress,applied current and the combined effect of both on the electrochemical corrosion of R4 mooring chain steel were investigated by electrochemical tests and characterization methods.The results indicated that the open circuit potential and linear polarization resistance(LPR)of R4 steel increased under stress,and the stress inhibited the electrochemical corrosion reaction,but the inhibition was caused the formation of corrosion products;the composition of corrosion products wasn’t the influencing factor of loading stress in inhibiting electrochemical corrosion,and its composition were mainly FeOOH under different stresses;the LPR and charge transfer resistance(Rct)increased firstly and then decreased with the negative shift of cathodic protection potential;calcium and magnesium deposition layer thickness increased,but the uniformity decreased,the main composition was Mg(OH)2;according to Rct determined the optimal cathodic protection potential range of R4 steel at about-0.75 V;stress and cathodic protection on R4 steel electrochemical corrosion behavior was different from the two alone,the constant stress synergistically strengthen the inhibition of electrochemical corrosion at the appropriate cathodic protection potential;while at over protection(-1.1 V)the constant stress weaked the inhibition.The research provided data reference for the application of mooring chains in marine engineering and the development of protective measures.