A short review of antibacterial Cu-bearing stainless steel:antibacterial mechanisms,corrosion resistance,and novel preparation techniques

Cu-bearing stainless steel is widely used in the fields of food,medical and household sanitary equipment because of its surface finish and corrosion resistance.However,the growth of bacteria on stainless steel leads to the formation of biofilms,which causes corrosion.Therefore,the antibacterial property of stainless steel is a worthy research topic.Reviews of breakthroughs in the field of corrosion resistance and antimicrobial properties are still lacking.Herein,due to the scarcity of publications on the antibacterial mechanisms and processing methods of antibacterial Cu-bearing stainless steel,we review the current state of relevant research and progress.The toxicity of Cu,corrosion resistance mechanism of stainless steel,and antibacterial mechanism and preparation method of antibacterial stainless steel are reported.In addition,alloying,surface modification and other methods are found to have limitations in balancing the toxicity and antibacterial properties of copper and the relationship between the antibacterial properties and corrosion resistance of Cu-bearing stainless steel.A new preparation method of antibacterial stainless steel associated with selective laser melting(SLM)is proposed.SLM is becoming a powerful additive manufacturing technology that can be used to manufacture customized and complex metals.The research status of SLM applied in antibacterial stainless steel preparation is described.Finally,the future research direction of Cu-bearing antibacterial stainless steel is discussed.

Effect of Heat Treatment on Microstructure and Mechanical Behavior of Cu-Bearing 316L Stainless Steel Produced by Selective Laser Melting

Cu-bearing stainless steels(SSs)with high strength,excellent plasticity,and effective antimicrobial properties hold significant potential for applications in the marine industry.In this study,Cu-bearing SSs with copper ranging from 0 to 6.0 wt%were successfully prepared using selective laser melting(SLM)technology.For the Cu-bearing SSs with different copper contents,the effect of heat treatment on the microstructural and mechanical behaviors was studied systematically.Microstructural observations revealed that the subgrain size of Cu-bearing SSs increased with heat treatment at 500℃ and 700℃ for 6 h.Furthermore,the tensile strength and elongation increased after the heat treatment temperature due to the combined effect of dislocations,twins,andε-Cu precipitated phases.Notably,after heat treatment at 700℃,the SLM4.5Cu sample exhibited an abnormal rise in tensile strength and elongation.This finding suggests that the diffusion strengthening caused byε-Cu precipitates exceeded the stacking fault energy.Consequently,the tensile strength and elongation reached 693.32 MPa and 56.94%,respectively.This work provides an efficient approach for preparing Cu-bearing SSs with exceptional strength and plasticity.

Enhanced resistance of 2205 Cu-bearing duplex stainless steel towards microbiologically influenced corrosion by marine aerobic Pseudomonas aeruginosa biofilms

An antibacterial 2205-Cu duplex stainless steel （DSS） was shown to inhibit the formation and growth of corrosive marine biofilms by direct contact with copper-rich phases and the release of Cu^2＋ ions from the 2205-Cu DSS surface. In this work, the microbiologically influenced corrosion （MIC） resistance of 2205- Cu DS5 in the presence of the corrosive marine bacterium Pseudornonos aeruginosa was investigated. The addition of copper improved the mechanical properties such as the yield strength, the tensile strength and the hardness of 2205 DSS. Electrochemical test results from linear polarization resistance （LPR）, electrochemical impedance spectroscopy （EI5） and critical pitting temperature （CPT） measurements showed that 2205-Cu DSS possessed a larger polarization resistance （Rp）, charge transfer resistance （Rct） and CPT values, indicating the excellent MIC resistance of2205-Cu DSS against the corrosive P. aeruginosa biofilm. The live]dead staining results and the SEM images of biofilm confirmed the strong antibacterial ability of 2205-Cu DSS. The largest pit depth of 2205-Cu DSS was considerably smaller than that of 2205 DSS after 14d in the presence ofP. aeruginosa （2.2 μm vs 12.5 μm）. 2205-Cu DSS possessed a superior MIC resistance to regular 2205 DSS in the presence of aerobic P. aeruginosa.

Antibacterial mechanism of Cu-bearing 430 ferritic stainless steel

Copper(Cu)-bearing stainless steel has testified its effectiveness to reduce the risk of bacterial infections.However,its antibacterial mechanism is still controversial.Therefore,three 430 ferritic stainless steels with different Cu contents are selected to conduct deeper research by the way of bacterial inactivation from two aspects of material and biology.Hereinto,electrochemical and antibacterial results show that the increase in Cu content simultaneously improves the corrosion resistance and antibacterial property of 430 stainless steel.In addition,it is found that Escherichia coli(E.coli)on the surface 430 Cu-bearing stainless steel by the dry method of inoculation possesses a rapid inactivation ability.X-ray photoelectron spectroscopy(XPS)aids with ion chelation experiments prove that Cu(Ⅰ)plays a more crucial role in the contact-killing efficiency than Cu(Ⅱ),resulting from more production of reactive oxygen species(ROS).

Dissolution and repair of passive film on Cu-bearing 304L stainless steels immersed in H_2SO_4 solution

The antibacterial Cu-bearing 304 L stainless steel is a new kind of structural and functional integrated metal material. In this work, evolution behavior of passive film of different heat treated Cu-bearing 304 L stainless steel immersed in 0.5 M H2SO4 solution was investigated by using electrochemical measurements, atomic force microscopy（AFM） observation and X-ray photoelectron spectroscopy（XPS） analysis.The results show that the solution and aging treated samples have the similar polarization behaviors. The passive film impedance experiences an initial decrease within 7 days followed by a subsequent increase,while the defect density of passive film presents the opposite trend. Meanwhile, the evolution of surface morphology and the estimated thickness of the passive film confirm that it experiences initial dissolution and follow-up repair. Furthermore, the Cr^3＋ content in passive film undergoes sequential reduction to increase, however the variation tendency of Cu^2＋ content is just opposite, indicating that the content variation of Cr and Cu in passive film reflects the competitive process of film dissolution and repair. In addition, compared with solution treated samples, aged samples have a bigger icorrvalue and the rougher passive film. This indicates that the passive film of solution treated steel is more compact and stable.

Antibacterial Performance of Cu-Bearing Stainless Steel against Staphylococcus aureus and Pseudomonas aeruginosa in Whole Milk

Pathogen microorganisms exist in various environments such as dairy processing facilities. They are not easily eliminated, and significantly raise the risk of bacterial contamination. The inhibition ability of a novel type 304 Cu-bearing stainless steel （304CUSS） with nano-sized Cu-rich precipitates against Staph-ylococcus aureus （S. aureus） and Pseudomonas aeruginosa （P. aeruginosa） added whole milk was investigated in this study. The results showed that after 24 h contact, the inhibition rates of the 304CUSS against S. aureus and P. aeruginosa added whole milk reached 99.2% ± 0.3% and 99.3% ± 0.2%, respectively, in contrast with the 304SS. In the plain whole milk, the inhibition rate of the 304CUSS also reached 66.9% ± 2.0% compared with the 304SS. The results demonstrated that the 304CUSS killed majority of the planktonic bacteria, and inhibited sessile bacteria adherence to the steel surface in the whole milk with and without bacteria addition, significantly reducing the bacterial growth rate. These research outcomes explicitly show an application potential of this novel antibacterial stainless steel in the dairy related food industry.

Antibacterial behavior and related mechanisms of martensitic Cu-bearing stainless steel evaluated by a mixed infection model of Escherichia coli and Staphylococcus aureus in vitro

Escherichia coli(E.coli)and Staphylococcus aureus(S.aureus)are the most typical pathogenic bacteria with a significantly high risk of bio-contamination,widely existing in hospital and public places.Recent studies on antibacterial materials and the related mechanisms have attracted more interests of researchers.However,the antibacterial behavior of materials is usually evaluated separately on the single bacterial strain,which is far from the practical condition.Actually,the interaction between the polymicrobial communities can promote the growing profile of bacteria,which may weaken the antibacterial effect of materials.In this work,a 420 copper-bearing martensitic stainless steel(420 CuSS)was studied with respect to its antibacterial activity and the underlying mechanism in a co-culturing infection model using both E.coli and S.au reus.Observed via plating and counting colony forming units(CFU),Cu releasing,and material characterization,420 CuSS was proved to present excellent antibacterial performance against the mixed bacteria with an approximately 99.4%of antibacterial rate.In addition,420 CuSS could effectively inhibit the biofilm formation on its surfaces,resulting from a synergistic antibacterial effect of Cu ions,Fe ions,reactive oxygen species(ROS),and proton consumption of bacteria.

Hot Deformation Behavior and Processing Map of a Cu-Bearing 2205 Duplex Stainless Steel

The hot deformation behavior and processing map of Cu-bearing 2205 duplex stainless steel(2205-Cu DSS)were investigated at temperatures of 950-1150℃ and strain rates of 0.01-10 s^-1.The effects of Cu addition and different deformation parameters on deformation behavior were,respectively,characterized by analyzing flow curves,constitutive equations and microstructures.The results indicated that the shapes of flow curves strongly depended on the volume fraction of two phases.When deformed at low strain rate,DRV in ferrite was prompted with increase in the temperature and was further developed to continuous DRX.At high strain rate,flow localization preferentially occurred in ferrite at low deformation temperature due to the strain partitioning and relatively less fraction of ferrite.The activation energy for 2205-Cu DSS was 452 kJ/mol and was found to connect with the variation of strain,strain rate and deformation temperature.The optimum hot deformation parameters for 2205-Cu DSS were obtained in the temperature range of 1100-1150℃ and strain rate range of 0.1-1 s^-1 with a peak power dissipation efficiency of 41%.Flow localization was the main way to lead to flow instability.Meanwhile,the Cu-rich precipitates were generated within a few ferrite grains when deformed at temperature lower than 1000℃.The interaction between dislocations and Cu-rich precipitates at high strain rate,as well as the limited DRV in ferrite and DRX in austenite,contributed to the complex microstructure and flow behavior.

Antibacterial Performance of a Cu-bearing Stainless Steel against Microorganisms in Tap Water

Tap water is one of the most commonly used water resources in our daily life. However, the increasing water contamination and the health risk caused by pathogenic bacteria, such as Staphylococcus aureus and Escherichia coli have attracted more attention. The mutualism of different pathogenic bacteria may diminish antibacterial effect of antibacterial agents. It was found that materials used for making pipe and tap played one of the most important roles in promoting bacterial growth. This paper is to report the performance of an innovative type 304 Cu-bearing stainless steel(304Cu SS) against microbes in tap water. The investigation methodologies involved were means of heterotrophic plate count, contact angle measurements, scanning electron microscopy for observing the cell and subtract surface morphology,atomic absorption spectrometry for copper ions release study, and confocal laser scanning microscopy used for examining live/dead bacteria on normal 304 stainless steel and 304 Cu SS. It was found that the surface free energy varied after being immersed in tap water with polar component and Cu ions release.The results showed 304 Cu SS could effectively kill most of the planktonic bacteria(max 95.9% antibacterial rate), and consequently inhibit bacterial biofilms formation on the surface, contributing to the reduction of pathogenic risk to the surrounding environments.

HREM Observation of Age-Precipitated Particles in Practical Cu-bearing Ultra-Low Carbon Steels

Morphology,distribution and crystal structures of age-precipitated particles in practical Cu-bearing steels with ultra-low carbon were studied by high resolution electron microscopy（HREM）.The age-precipitation behavior was explored when poly-microalloying elements Cu,Nb,Cr,Mo and Ni were used and ε-Cu particles coexisted with carbides（carbonitrides） in steels.The fine dispersed ε-Cu particles and carbides of microalloying element Nb,Cr,Mo being about 10 nm only in size were observed in peak aging specimens and over-aging specimens.The twin structures and stacking faults in the particles and the transient structures formed in the early stage of the particle precipitation were also observed.The crystal structure of three types of age-precipitation particles was firstly identified by lattice streaks in HREM images of the single particles.The experimental method identifying crystal structure of fine and dispersed particles using HREM images of single particle was proposed.

In vitro study on infectious ureteral encrustation resistance of Cu-bearing stainless steel

Cu-bearing stainless steel has been found to have obvious inhibition performance against encrustation in vitro. This study was aiming to further investigate the inhibitory effect of a Cu-bearing stainless steel（316 L-Cu SS） on the infectious encrustation based on its antimicrobial activity. The encrustation in presence of bacteria, antibacterial performance, urease production and Ca and Mg precipitation were examined by scanning electron microscopy, antibacterial assay, enzyme-linked immunosorbent assay and inductively coupled plasma-mass spectrometry, respectively. It was found that 316 L-Cu SS could inhibit the formation of bacterial biofilm due to the release of Cu^（2＋） ions and then decrease the urease amount splitting by bacteria, which produced a neutral environment with pH around 7. However, more encrustations coupled with bacterial biofilms on the surface of comparison stainless steel（316 L SS） with an alkaline environment were recorded. It can thus be seen that the 316 L-Cu SS highlights prominent superiority against encrustation in the presence of microorganisms.

Effect of modifying matrix microstructures and nanosized precipitates on strengthening mechanisms and ductile-to-brittle-transitiontemperature in a 1000 MPa Ni-Cr-Mo-Cu steel

A superior combination of yield strength(1001 MPa)and-20℃ impact toughness(166 J)was obtained in Nb-V-Timicroalloyed Ni-Cr-Mo-Cu steel treated by direct quenching and tempering route(DQT).The tested steels treated by DQT route and re-austenitization and tempering route(QT)were compared with each other in terms of mechanical properties and microstructures characterized by optical microscopy,transmission electron microscopy,X-ray diffraction,electron back-scattered diffraction method and so on.Strength and Vickers hardness of the tested steel treated by the above two routes vary with isothermal aging temperature(400-600℃),shown as under-aged state,peak-aged state and overaged state.All DQT specimens show higher strength and Vickers hardness than QT specimens with the same aging condition.Furthermore,the largest difference of yield strength between DQT and QT specimens was shown in DQT600 and QT600 specimens.DQT600 or QT600 specimens refers to direct quenched(DQ)or quenched(Q)specimens isothermally aged at 600℃.The main disparities in quenched microstructure between DQ and Q specimens are mainly in morphology of prior austenite grains,dislocation density of martensite matrix and solution amount of Nb and Mo elements dissolving in martensite matrix,which play key roles in affecting microstructure and mechanical properties of DQT and QT specimens.Higher dislocation density of matrix and finer average diameter of both MC(M is any combination of Nb,Mo and V)and Cu-rich particles were shown in DQT600 specimens than in QT600 specimens.Strengthening from dislocations and nanosized MC and Cu-rich particles mainly leads to the largest difference of yield strength between DQT600 and QT600 specimens.In addition,strong dislocation strengthening and precipitation strengthening in DQT600 specimen also elevated its ductile-to-brittle-transition-temperature,compared with QT600 specimen.

New strategy to delay food spoilage:Application of new food contact material with antibacterial function

Frequent food poisoning and food-borne diseases outbreaking in recent years have caused people to attach great attention to food safety,especially the food contact materials that are essential in the food industrial chains and daily lives,ensuring their clean sanitation are of great importance in blocking microbial contamination and spread of food-borne pathogens.Stainless steel(SS)is one of the most accepted and widely used food contact material,and the Cu-bearing SS possesses excellent antibacterial performance and maintains the original mechanical properties of SS,maybe making it a better substitute for the conventional SS in the food area.Taking advantages of bactericidal and antifouling properties of Cubearing SS,this study simulated a variety of food contact scenarios,explored a new strategy for food preservation and food safety by using Cu-bearing SS as a food contact material.The results showed that the Cu-bearing SS could not only delay the spoilage of different foods by inhibiting the activity of microorganisms in foods,but also reduce the expressions of spoilage traits of bacteria as well as the formation of biofilms by quenching the quorum-sensing signals,and further creating a good bacteriostatic atmosphere for the contacted food and its surrounding environment.In addition,the remarkable antifouling property of Cu-bearing SS would give the material a self-cleaning feature for food applications,which can avoid secondary contamination of food as a source of contamination.This study well demonstrates that the Cu-bearing SS has broad application potentials and prospects in the food area.