Browsing by Author "Shi, X."

Now showing 1 - 4 of 4

Comparative study in chemistry of microbially and electrochemically induced pitting of 316l stainless steel
(2003-11) Shi, X.; Avci, Recep; Geiser, Michael Joseph; Lewandowski, Zbigniew
Ennoblement of stainless steel (SS) by microbially deposited manganese oxides can lead to pitting corrosion at low chloride concentrations, causing unexpected material failures. We exposed 316L SS to manganese oxidizing bacteria Leptothrix discophoraunder well-defined laboratory conditions, and then placed the ennobled coupons in a 0.5 M sodium chloride solution until pitting developed. Using time-of-flight secondary ion mass spectroscopy we demonstrated that the pits and their immediate vicinity associated with microbial influenced corrosion had different chemical signatures than those associated with electrochemically induced pitting, suggesting a possibility that the microorganisms were directly involved in pit initiation. Based on the differences in the chemical signatures we were able to distinguish the microbially induced pits from those induced by anodic polarization.
Electrochemistry of passive metals modified by manganse oxides deposited by leptothrix discophora: two-step model verified by tof-sims
(2002-05) Shi, X.; Avci, Recep; Lewandowski, Zbigniew
We have applied time-of-flight secondary ion mass spectroscopy (ToF-SIMS) to study microbially induced ennoblement of 316L stainless steel and Ti-A1-4V surfaces exposed to manganese-oxidizing bacteria Leptothrix discophora SP-6. Our observations indicate that manganese biomineralization occurs in two steps: first, the divalent manganese (Mn2+) is oxidized to manganese dioxide, MnO2. Both reactions contribute to an increase in the open circuit potential and hence to the ennoblement of the material. Manganese cycling at a surface of passive metals produces renewable cathodic reactants, manganese oxyhydroxide and manganese dioxide, and endangers the material integrity.
Microbially deposited manganese and iron oxides on passive metals - their chemistry and consequences for material performance
(2002-09) Shi, X.; Avci, Recep; Lewandowski, Zbigniew
The open-circuit potential (OCP) values of Type 316L (UNS S31603) stainless steel and Ti-6Al-4V (UNS R56400) corrosion coupons, exposed to fresh river water, were ennobled to as high as 365 mV vs saturated calomel electrode (SCE) and 400 mVsce respectively. With microchemical imaging capabilities and high-detection sensitivity, a surface analysis technique based on time-of-flight secondary ion mass spectroscopy (ToF-SIMS) was developed to identify the oxidation states and distribution of biominerals on the ennobled metal surfaces. ToF-SIMS spectra of the microbial deposits compared to spectra of different manganese and iron mineral standards indicated that the biominerals on the metal surfaces are a mixture of ferric oxide (Fe2O3), manganese oxide (Mn3O4), and manganese oxyhydroxide (MnOOH) on fully ennobled coupons, and a mixture of iron oxide (Fe3O4), Fe2O3, Mn3O4, and manganese(III) oxide (Mn2O3) on partially ennobled coupons. Biomineralized manganese and iron oxides on the Type 316L stainless steel surfaces, regardless of the oxidation states, endanger the material integrity in a similar manner, as evidenced by the elevated OCP and increased cathodic current density upon mild polarization.
Microbially deposited manganese and iron oxides on passive metals - their chemistry, distribution, and consequences for material performance
(2002-09) Shi, X.; Avci, Recep; Lewandowski, Zbigniew
316L stainless steel and Ti-6A1-4V corrosion coupons exposed to fresh river water ennobled the open circuit potential (OCP) to values of +365 mVsce and +400 mVsce , respectively. ToF-SIMS spectra indicated that the biominerals on the metal surfaces area a mixture of Fe2O3, Mn3O4 on partially ennobled coupons. Biomineralized manganese and iron oxides on the 316L stainless steel surfaces, regardless of the oxidation states, endanger the material integrity in a similar manner, as evidenced by the elevated OCP and increased cathodic current density upon mild polarization.