Electrochemistry of passive metals modified by manganese oxides deposited by Leptothrix discophora: two-step model verified by ToF-SIMS Xianming Shi a , Recep Avci b , Zbigniew Lewandowski a,c, * a Department of Civil Engineering, Montana State University, Bozeman, MT 59717-3980, USA b Department of Physics, Image and Chemical Analysis Laboratory, Montana State University, Bozeman, MT 59717-3980, USA c Center for Biofilm Engineering, EPS Building, Room 314, Montana State University, Bozeman, MT 59717-3980, USA Received 5 February 2001; accepted 31 May 2001 Abstract We have applied time-of-flight secondary ion mass spectroscopy (ToF-SIMS) to study microbially induced ennoblement of 316L stainless steel and Ti–6Al–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 (Mn 2þ ) is oxidized to manganese oxyhydroxide, MnOOH; then the MnOOH is further oxidized to manganese dioxide, MnO 2 . Both reactions contribute to an increase in the open circuit po- tential and hence to the ennoblement of the material. Manganese cycling at a surface of passive metals produces renewable cathodic reactants, manganese oxyhydroxide and man- ganese dioxide, and endangers the material integrity. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Time-of-flight secondary ion mass spectroscopy; Manganese-oxidizing bacteria; Ennoblement; Biomineralization www.elsevier.com/locate/corsci Corrosion Science 44 (2002) 1027–1045 * Corresponding author. Address: Center for Biofilm Engineering, EPS Building, Room 314, Montana State University, Bozeman, MT 59717-3980, USA. Tel.: +1-406-994-5915; fax: +1-406-994-6098. E-mail address: zl@erc.montana.edu (Z. Lewandowski). 0010-938X/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S0010-938X(01)00104-4