Browsing by Author "Yurt, Nurdan"

Now showing 1 - 6 of 6

Biofouling and corrosion of stainless steels in natural waters
(2002) Lewandowski, Zbigniew; Avci, Recep; Geiser, Michael Joseph; Braughton, K. R.; Yurt, Nurdan
The noble shift in corrosion potential to values between +300 and +400 mVSCE and the accompanying increase in cathodic current density and polarization slope at mild cathodic potentials that develop during microbial colonization of passive metals, are collectively known as ennoblement. This phenomenon is of concern as the noble shift in the corrosion potential may lead to pitting corrosion. We have demonstrated, by growing pure cultures of manganese oxidizing bacteria (MOB) Leptothrix discophora SP-6 under well defined conditions, that microbial deposition of manganese oxides causes ennoblement of 316L stainless steel (SS). Exposing 316L corrosion coupons in lakes and streams supported this conclusion; the rate and extent of ennoblement were positively correlated with the rates of deposition and the amounts of biomineralized manganese oxides deposited on the surfaces of the SS corrosion coupons. X-ray photoelectron spectroscopy (XPS) analyses of the deposits from the ennobled coupons revealed a mixture of manganese oxides, as expected. Many natural waters can support growth of MOB. When manganese-oxidizing biofilms accumulate on surfaces of passive metals there is a potential for manganese redox cycling on the metal surface. This process is initiated by depositing minute amounts of manganese oxides on the metal surface. These microbially deposited manganese oxides are then reduced by the electrons derived from anodic dissolution of the metal; the metal is corroding and the manganese oxides are reduced to divalent manganese ions. However, since the manganese ions are liberated within the manganese-oxidizing biofilm, the manganese ions are immediately reoxidized, and the cycle continues.
Effect of biomineralized manganese on pitting corrosion of type 304l stainless steel
(2001-11) Olesen, Bo H.; Yurt, Nurdan; Lewandowski, Zbigniew
During the past few years, biomineralized manganese has been shown to cause ennoblement of various stainless steels to open circuit potentials of 3 mV/SCE. We have demonstrated that ennoblement, caused by biologically deposited manganese minerals, along with a relatively low stainless steel pitting potential, caused by the presence of chloride, is sufficient to initiate and drive active pitting corrosion. Stainless steel samples (type 304L), chemically or microbiologically ennobled with manganese dioxide, were exposed to a 0.35% w/v NaCl solution; an environment otherwise not corrosive against the 304L stainless steel. In the first case, steel samples were ennobled by electroplating the sample with a thin film of manganese dioxide, except for a small anodic area. In the latter case, the manganese dioxide was deposited on the steel within biofilms of the manganese oxidizing bacterium Leptothrix discophora SP-6. After 24 h exposure to the chloride solution the samples were investigated by atomic force microscopy (AFM). Both types of ennobled samples were found severely pitted, whereas reference samples (w/o manganese minerals) had remained intact.
Leptothrix discophora SP-6 : effects of biofilms on passive film chemistry of 316L stainless steel and modeling of growth
(Montana State University - Bozeman, College of Engineering, 2002) Yurt, Nurdan
Multiple substrate growth kinetics of leptothrix discophora sp-6
(2002-10) Yurt, Nurdan; Sears, John; Lewandowski, Zbigniew
The growth parameter of Leptothrix discophora SP-6 were quantified on the basis of the steady-state concentrations and utilization rates of pyruvate, dissolved oxygen, and concentration of microorganisms in a chemostat operated at 25°C, pH 7.2, and an agitation rate of 350 rpm. The results showed that the microbial growth was limited by both pyruvate and dissolved oxygen. A combined growth kinetics model using Monod growth kinetics for pyruvate and Tessier growth kinetics for oxygen showed the best correlation with the experimental data when analyzed using an interactive multiple substrate model. The growth kinetics parameters and the respective confidence limits, estimated using the Monte Carlo simulation, were µmax = 0.576 ± 0.021 h-1, KsMp = 38.81 ± 4.24 mg L-1, KsTo = 0.39 ± 0.04 mg L-1, YX/p = 0.150 (mg microorganism mg-1 pyruvate), YX/o = 1.24 (mg microorganism mg-1 oxygen), the maintenance factors for pyruvate and oxygen were mp = 0.129 (mg pyruvate consumed mg -1 microorganism h-1 ) and mo = 0.076 (mg oxygen consumed mg-1 microorganism h-1), respectively.
Passive film chemistry on 316l stainless steel ennobled by biomineralized manganese
(2002-04) Yurt, Nurdan; Avci, Recep; Lewandowski, Zbigniew; Sears, Joe
The effect of ennoblement on chemistry of passive films on 316L stainless steel (SS) was quantified using surface-sensitive analytical techniques. Under well-defined laboratory conditions, SS coupons were ennobled to ~ +350Vsce by biofilms of manganese-oxidizing bacterium Leptothrix discophra SP-6. Ennobled coupons were analyzed by x-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (TofSIMS). From the XPS depth profiles of Fe, Cr, O, Ni, C and Mn, we evaluated thickness of the passive layers before and after ennoblement, while the TofSIMS depth profiles were used to evaluate spatial distribution of Mn, Cr, Fe and Ni on the surface. Because the ennobled coupons were covered with biomineralized deposits, sputtering was used to remove these deposits under ultrahigh vacuum (UHV) conditions before probing the chemistry of the underlying passive layers. The main conclusion of the paper is that oxide layers on the ennobled coupons are significantly thinner that those on the pre-ennobled coupons, which may, hypothetically, contribute to their susceptibility to localized corrosion.
Quantifying selected growth parameters of leptothrix discophora sp-6 in biofilms from oxygen concentration profiles
(2003-10) Yurt, Nurdan; Beyenal, Haluk; Sears, Joe; Lewandowski, Zbigniew
It is a dubious but common practice to use growth parameters measured in suspended cultures to predict substrate concentration profiles in biofilms. To obtain biofilm biokinetic parameters that apply to biofilms, a reliable method is needed that allows the computation of biokinetic parameters from substrate concentration profiles measured directly in biofilms. We have developed such a method and demonstrated its utility by evaluating biokinetic parameters from oxygen concentration profiles measured in biofilms of Leptothrix discophora SP-6 grown on a membrane, which was placed on top of an agar plate by fitting the data to Monod or Tessier growth kinetics, including maintenance substrate consumptions. We found that the Monod model represented the growth of L. discophora SP-6 biofilms marginally better than the Tessier model. The Monod half saturation coefficient was 0.333 ± "0.077 mg/l.