| Stainless steel is widely used in the food industry,marine,aerospace,petrochemical and other fields based on its excellent corrosion resistance,mechanical and processing properties,which is preferred to apply in control valves and piston pumps as a dynamic sealing material.However,due to the frequent movement of the sealing mechanism,coupled with the severe corrosive environment,stainless steels are damaged by tribo-corrosion that causes seal failure,resulting in safety accidents.Although austenitic stainless steel has good corrosion resistance,it contains nickel elements,resulting in higher costs.At the same time,nickel as the main component of superalloy,its strategic importance is very important.In addition to the shortage of nickel resources,the development of nickel-free corrosion resistant stainless steel has become a hot spot.The Cr26Mo1 ultra-pure high-chromium ferritic stainless steel was developed by Institute of Metal Research,Chinese Academy of Sciences,as a nickelfree stainless steel with better corrosion resistance.It is a valuable topic to study whether Cr26Mol has the advantage of tribocorrosion resistance compared with the general austenitic stainless steels.In this paper,the commercial AISI 304 austenitic stainless steel and Cr26Mo1 ultra-pure high chromium ferrite stainless steel were selected as the research materials.The tribo-corrosion behaviors of stainless steels in different solutions were systematically studied by forming friction pairs with different rubbing materials.Based on the static corrosion behavior of stainless steels,a tiny electrode was prepared to study the effect of friction conditions on the corrosion behavior of stainless steels under the condition of limited mass transfer.Experiments were designed to study the tribocorrosion behavior of stainless steel in corrosive media,and the interaction between corrosion and wear was quantitatively analyzed to illustrate the correlations of the friction pair material,corrosive environment,wear conditions to the tribocorrosion behavior of stainless steel.The main conclusions obtained are as follows.(1)In 3.5%NaCl solution,the cathodic reaction is dominated by oxygen reduction reaction(ORR),the purification process makes Cr26Mo1 have better corrosion resistance than AISI 304.Friction makes stainless steel change from passivation state to activation state,which promotes corrosion rate significantly.In 0.5 mol/L H2SO4 solution,hydrogen reduction reaction(HRR)dominates the process,thus high Cr content makes the corrosion resistance of Cr26Mo1 weaker than AISI 304.With or without friction,stainless steel is always in the activation state,indicating the promotion effect of friction on corrosion is not remarkable.With the load increasing,the role of friction changes from the mechanical removal of passivation film to promoting the formation of friction oxide film,thus its promotion to corrosion is weakened.(2)When the mass transfer between friction surfaces is hindered,the damage of the stainless steels is slight,which is dominated by uniform corrosion.The corrosion behavior of the stainless steels during wear process was related to its static corrosion performance.Micro-cutting,micro-ploughing and micro-spalling occur on the surface of stainless steels,and damage of the materials mainly attributes to plastic removal mechanisms.Good mechanical properties are beneficial to frictional corrosion resistance.While rubbed with hard materials(Al2O3),the stainless steel is badly damaged and with localized corrosion.Pits,furrows and spalling are formed on the surface,damage of the materials is controlled by brittle removal mechanisms,in which there is an obvious promoting effect between corrosion and wear.(3)In the 3.5%NaCl solution,intermittent sliding wear have a great effect on the open circuit potential(OCP)of stainless steels,which gradually shifts positively with increasing scratch interval and gradually increases the fluctuation range.Wear promotes corrosion because friction destroys the passivation film on the surface of stainless steel and forms pits.Due to the lubricating effect of Cl-,the friction coefficient and shear stress between the friction pairs are reduced,thus corrosion inhibits wear.For stainless steels there is a significant negative interaction between wear and corrosion in the NaCl solution.The friction frequency decreases with increase of the scratch interval,thus the total damage to the stainless steels decreases.The tribocorrosion resistance of Cr26Mo1 is always inferior to that of AISI 304.(4)In 0.5 mol/L H2SO4 solution,subsurface grains in stainless steels under the action of friction undergo severe plastic deformation,resulting in grain boundaries increasing,grain refinement,geometry necessary dislocation(GND)density increasing,which make wear have a significant role in promoting corrosion process.Corrosion can also accelerate wear.On the one hand,the enrichment of hydrogen atoms in the abrasion defects induces material embrittlement.On the other hand,the microhardness of the subsurface layer decreases,lowering wear resistant of the materials.In the normal service condition there is a positive interaction between wear and corrosion,which accounted for a large proportion of tribo-corrosion.Cr26Mo1 has better tribocorrosion resistance than AISI 304.(5)The subsurface structure of stainless steel during wear process is closely related to its crystal structure and second phase.A large amount of second phase can strongly inhibit the formation of subsurface deformation layers.Under shear stress,Cr26Mo1 and AISI 304 form high strain gradients through GND and crystal rotation,which further develop to low angle grain boundary(LAGB)and form submicron crystal structures near the surface layer.The ferrite submicron grains have a meritocratic orientation,and their slip plane direction coincides with the direction of the stress plane constituted by the load and shear forces.While austenitic submicron grains generate stress-induced martensitic transformation,which can significantly improve the wear resistance of the material. |
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