Study On Several Galvanic Corrosion In Simulating Deep Sea Hydrostatic Pressure Condition

With the exploration and exploitation of deep-sea resources,more and more engineering equipment must work in the deep ocean environment.The harsh deep-sea environment makes the equipment and devices suffer great corrosion.There are many occasions that multiple metal materials have to be connected with each other in the sea water,which can induce galvanic corrosion.Galvanic corrosion may greatly increase the corrosion rate of materials and thus threaten the service life and safety of deep-sea equipment.However,in present,studies on corrosion behavior of materials in deep sea environment mostly focus on the self-corrosion behavior and are quite lack in the galvanic corrosion issue,which has become an urgent topic.The hydrostatic pressure is the most significant environmental characteristic factor of deep-sea environment,which can greatly affect the corrosion behavior of metals.Hence,to study the galvanic corrosion between common materials used in deep sea environment is of great significance for the long term service of deep-sea equipment.Based on the above background,this work selected several common marine galvanic couple:B10 copper-nickel alloy/Ti6A14V alloy,B10 copper-nickel alloy/316L stainless steel,Al-Zn-In-Mg-Ti sacrificial anode/980 low alloy steel to conduct the research work.The simulating deep sea environmental system built by the laboratory was used.Then the experiments were carried out by using traditional mass loss weight experiment,polarization curves,EIS,zero-ammeter experiment(ZRA),in conjunction with some analysis technology like XRD,XPS,SEM/EDS and finite element analysis etc.The corrosion behavior of the three selected couples and the effect of environmental factors were studied systematically.At the begining,the corrosion behavior of B10 in 3.5%NaCl solution under hydrostatic pressure+alternating wet and dry condition was studied,which provided some basic datas for the further study of the galvanic corrosion behavior of B10 under high hydrostatic pressure.The results show that:The corrosion of B10 during the investigated period can be divided into two stages.The corrosion at the early stage was controlled by the adsorption of Cl' on the surface of the alloy,while the corrosion at the later stage was controlled by the migration of Cu+ and electron through the corrosion products and the total corrosion process was oxidation reaction.Hydrostatic pressure accelerated the corrosion rate of the alloy in the whole immersion time.At the early stage,increasing hydrostatic pressure can enhance the adsorption and the migration rate of Cl-to the surface of the alloy,which resulted in a higher corrosion rate.At the later stage,the corrosion rate of the B10 was limited by the protectiveness of the corrosion product.However,the high hydrostatic pressure can promote the generation of defects and change the component of corrosion products,which weakened the protectiveness of the corrosion product and hence accelerated the corrosion rate of the B10 alloy.Afterwards,based on the study above,the influence of hydrostatic pressure on the corrosion of B10 coupled to Ti6A14V in 3.5%NaCl solution was studied.The results show that:The B10 was always the anode part of the couple and went through an increased corrosion rate at different studied condition.The galvanic current density and the natural corrosion rate of B10 in the couple at high hydrostatic pressure was bigger compared with that at the common pressure,which resulted in a much higher total corrosion rate of the alloy.Further studies show that the growth rate of pits on the surface of the B10 coupled to Ti6A14V can be significantly promoted by high hydrostatic pressure,which explained the high corrosion rate of B10 under the effect of both high hydrostatic pressure and coupling.Furthermore,the effect of hydrostatic pressure on the galvanic corrosion combined with the effect of temperature was carried out.The relevant experiments were carried out on the B10/316L couple in 3.5%NaCl solution and the results show that:B10 was always the anode part in the pair and went through an increased corrosion rate at all the studied conditions.The self-corrosion rate of B10 alloy decreased with decreasing temperature and increased with increasing hydrostatic pressure,for the decreasing temperature can reduce the corrosion activity and mass transfer rate and the increasing hydrostatic pressure can increasing the anodic dissolution rate.The galvanic current density decreased with decreasing temperature and increasing hydrostatic pressure,for the increasing hydrostatic pressure can reduce the galvanic effect and increase the interface resistance of 316L and the decreasing temperature could reduce the galvanic effect significantly and increased the interface resistance of both B10 and 316L stainless steel.The influence of temperature on the self-corrosion rate of B10 alloy and the galvanic current density follows the Arrhenius function.The influence of hydrostatic pressure on the self-corrosion rate of B10 and the galvanic current density follows the exponential function.Finally,the influence of hydrostatic pressure on the performance of Al-Zn-In-Mg-Ti sacrificial anode which is serviced actively in shallow water was studied;the basic anode demand to meet the cathodic protection design at different hydrostatic pressure was calculated.The results show that the self-corrosion rate of Al-Zn-In-Mg-Ti would increase with increasing hydrostatic pressure.The main reason is that increasing hydrostatic pressure would accelerate the growth of pits both in parallel and vertical direction and significantly promote the intergranular corrosion of the anode,resulting in increasing grain exfoliation with increasing hydrostatic pressure.The increasing hydrostatic pressure could increase the mass loss during the self-discharging experiment and thus decreased the anode efficiency.When designed a cathodic protection system at the same criterion,the total anode demand with hydrostatic pressure follows the exponential function.