Research On Corrosion Behavior Of Nickel-Aluminum Bronze Alloy And Its Surface Modification

Nickel-aluminum bronze（NAB）alloy is widely used as propeller in marine environments,due to the excellent combination of mechanical properties and corrosion resistance.Grain coarsening and composition segregation defect exist in the cast microstructure.In addition,metallurgically complex microstructures are prone to suffer selective phase corrosion,which aggravate the destruction of cavitation erosion.In this study,the microstructure of NAB alloy was regulated by heat treatment and the responding relation between microstructure and corrosion properties was established.Finally,the optimal microstructure with both static corrosion and cavitation erosion performance was found.Considering the corrosion and mechanical properties,a surface corrosion resistant layer was prepared by laser surface quenching,on the premise of ensuring the overall mechanical properties of the workpiece.The main results of this paper are as follows.（1）The selective phase corrosion of as-cast NAB alloy was determined by electrochemical factors and surface layer factors.The disparity of relative nobility of each phase in corrosive medium was the intrinsic factor of the selective corrosion,which was caused by the differences in crystal structure and chemical composition.The non-uniformity and incompleteness of the surface layer formed on each phase accelerated the corrosion rate of the preferred corrosion phases.Therefore,β′phase andαphase at boundaries with intermetallicκphases were preferentially corroded in seawater environments.In addition,the selective phase corrosion was also related to the morphology and distribution of the phase.The“α+κ_iii”eutectic area was relatively serious,since the corrosion medium was easy to penetrate along its continuous flaky tissue.（2）Cavitation is the result of electrochemical corrosion and mechanical impact.The presence of selective corrosion reduced the strength of the cast NAB alloy and accelerated the local damage.Mechanical erosion stripped the corrosion product film layer,and increased the corrosion rate.In this synergy effect,the preferential corroded phase in as-cast NAB alloy was cavitation eroded seriously,and theκphase was not eroded due to its high hardness and corrosion resistance.（3）As-cast NAB alloy was annealed,normalized,quenched and aged by adjusting the heat treatment temperature and cooling rate.βphase was reduced greatly after annealing,transformed intoαandκphases.In case of quenching,the monotonousβphase almost kept down,and transformed into acicular martensitic phase.During normalizing,someβphases were transformed into bamboo leaf shapedαphase and fineκphase,and the rest ofβphase remained asβ′phase due to rapid cooling.After aging at 450℃,extremely tinyκphase precipitated in acicularβphase.When the aging temperature reached 550℃,most of theβphase was transformed intoαphase andβ′phase with a uniform morphology.（4）The effects of phase content and distribution morphology on mechanical properties and corrosion behavior were studied.Due to the uneven phase distribution,selective phase corrosion occurred in the microstructure of casting,annealing and normalizing,which lead to the serious local damage under cavitation.The microstructure was refined and homogenized after quenching or quenching/aging at 450℃.Therefore,selective phase corrosion was eliminated.Meanwhile,nickel and aluminum were supersaturated and distributed evenly in the quenched microstructure,which in turn promoted the rapid formation of a protective corrosion production film,and reduced the corrosion rate.In addition,cavitation corrosion resistance was enhanced,which could be attributed to the improved hardness and weakened mechanical attack.However,this microstructure had a low elongation rate,and the quenching process on the workpiece will reduce its mechanical properties and cause brittle fracture.（4）Laser surface quenching technology was used to prepare modified surface layer on as-cast NAB alloy.The depth of the fine crystalline region is about 220μm,whose internal microscopic morphology was similar to the microstructure of quenching/aging at 450℃.Homogeneous distribution of constituent phases eliminated the selective phase corrosion,and reduced the corrosion rate by about 42.5%.Attributed to high hardness and homogeneous microstructure of the fine crystalline region,the synergy of mechanical erosion and electrochemical corrosion was small,and cavitation corrosion resistance of nickel-aluminum bronze alloy increased by a factor of 8.8.