September_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 2 0 2 6 compared with either the initial num- ber or the number remaining after the same amount of time on another sur- face known not to possess antimicrobi- al properties. This is then stated in the literature as a measure of antimicrobi- al activity. REPORTS OF SUCCESS Over the past 20 years or so, there have been numerous efforts toward the development of antimicrobial stainless steels. For example, Bahmani-Oskooee et al. [3] added 1 to 5 wt% Cu into 410 martensitic stainless steel. After solu- tion treatment at 1100°C to dissolve all alloying elements followed by oil quenching, these steel samples were aged at 500°C for 0.25 to 4 hours. Trans- mission electron microscopy shows that precipitates of fcc Cu on the order of 10-30 nm in diameter formed after the aging treatment. It was discovered that the addition of 3 wt% Cu along with aging at 500°C for 2 hours was suf- ficient to deactivate all exposed bacte- ria ( E. coli and S. aureus ) in 24 hours. In another study, Xi et al. [4] added 2.5 and 3.5 wt% Cu into 316L stainless steel. After solution treatment at 1100°C for 30 minutes and water quenching, the steel samples were aged at 700°C. The researchers found that after aging for 6 hours, 95% and 99% of E. coli in con- tact with the Cu-containing steel sam- ples were inactivated within 24 hours. In other research, Yang and Lu [5] added 3.8 wt% Cu to 316L and explored its antimicrobial properties against four different types of bacteria after an ag- ing treatment of 700°C for 6 hours. The team observed the formation of fcc Cu precipitates with diameters in the range of 10-20 nm. E. coli bacteria were deac- tivated the fastest, with complete inac- tivation in about 10 hours. In another study, Hong and Koo [6] started with 304 stainless steel and added from 1.5 to 5.5 wt% Cu. After solution treatment at 1050°C, samples were aged at 700° and 800°C for 0.5 to 4 hours. They observed formation of Cu precipitates with diam- eters on the order of 100 nm after aging at 800°C for 2 hours. Steel samples with 3.5 wt% Cu or higher and after aging at temperatures ≥ 700°C for 30 minutes were found to deactivate all bacteria ( S. aureus ) within 24 hours. Note that Cu-containing stainless steels exhib- it only weak antimicrobial activity af- ter solution treatment [4] , indicating the importance of having Cu-rich precipi- tates. Several patents on antimicrobial stainless steels developed using similar strategies also exist. BUT IS IT REALLY SUCCESS? As noted earlier, most studies of antimicrobial stainless steel report re- sults of bacteria inactivation on steel samples after 24 hours of incubation time. Such a measure is too impre- cise to differentiate antimicrobial per- formance among different alloys and aging treatments and provides little in- formation on inactivation kinetics. Yang and Lu [5] measured E. coli inactivation as a function of time, giving 2.5 hours for the half-life on stainless steel 317L plus 3.8 wt% Cu. In other research, Chai et al. [7] showed that the half-lives of E. coli and S. aureus on 317L plus 4.5 wt% Cu are both about 6 hours. In contrast, the half-life of SARS-CoV-2 (the virus caus- ing COVID-19) is 0.77 hours on pure Cu [8] , and the half-life of E. coli on pure Cu is 0.25 hours [9] . Based on the previous discussion of these results, the half-life of these microbes on Cu-containing stainless steels is significantly longer than that on pure Cu. One possibility can be in- ferred from studies of the antimicrobi- al activity of stainless steels and other samples with coatings containing dif- ferent Cu concentrations. Zhang et al. [10] found that the half-life of E. coli on 304 stainless steel coated with Cu-Ni al- loy containing 90% Cu had a half-life of about 0.5 hours, increasing to about 2 hours when the coating contained 2.5% Cu. Efforts to increase the surface Cu concentration by increasing aging temperature to 800°C and aging time to 400 hours of 304 stainless steel plus 3.8 wt% Cu, followed by pickling, do not seem to yield antimicrobial properties any closer to Cu [11] . PRELIMINARY STUDIES As noted in the previous discus- sion, it is not enough to just add Cu to stainless steels and expect things to work as well as Cu. The authors have been experimenting with Cu-precipita- tion-strengthened ferritic steels for the past 20 years as a new class of weath- ering steels for civil infrastructure ap- plications, with markedly improved low-temperature toughness, weldabil- ity, and weathering resistance. These steels are approved as ASTM A710 Grade B bridge steels. They are low-car- bon ferritic steels designed to maximize the number density of nanometer-size Cu precipitates. A typical alloy has the following composition in wt%: 0.06C-1.3Cu-0.9Ni-0.5Mn-0.4Si-0.06Nb- 0.1Ti, balance Fe, with 500 MPa yield strength, 30% elongation to failure, and Charpy impact fracture energy of 160 J at -80°C. This excellent combination of strength and toughness is primarily due to the precipitation of coherent bcc nanometer-size Cu precipitates [12] . Steels with improved strength based on ASTM A710 Grade B have since been developed, mainly by adding Ni and Al to promote the co-precipitation of Cu and NiAl, producing a new se- ries of low-carbon ferritic steels (desig- nated CF series) with yield strength up to 1600 MPa [13,14] . It dawned on the au- thors that A710 Grade B and these CF steels may possess potent antimicrobi- al activity due to the high number den- sity of nanometer-size Cu precipitates present in these alloys. As an example, Fig. 1 shows the atom probe tomogra- phy image obtained from one steel in this series after solution treatment at 950°C, followed by water quenching and aging at 500°C for 2 hours. Note the high density of Cu-con- taining precipitates, with an average radius of 2.0 nm and interprecipitate distance of ≈ 12 nm. Antimicrobial per- formance of two polished Cu-contain- ing steel samples were explored using RESEARCHERS FOUND THAT AFTER AGING FOR 6 HOURS, 95% AND 99% OF E. COLI IN CONTACT WITH THE CU-CONTAINING STEEL SAMPLES WERE INACTIVATED WITHIN 24 HOURS.