• Valérie, Debout
• Emilie, Malard
• Hervé, Gueune
• Jean-François, Ghiglione
• Christelle, Caplat
• Florence, Mazeas
• Stéphanie, Salaun
• Compère, C.

Abstract

Base nickel alloy (i.e. A625 alloy Ni base 22% Cr, 9% Mo, Nb) and austenoferritic stainless steel of duplex type are widely used for sea water piping system since 80ies. These alloys are specifically resistant to all forms of uniform corrosion. Nevertheless, they can be susceptible to localized corrosion for instance crevice corrosion. This phenomenon can occur in all oceans and seas in the world and at all seasons and has been widely observed [1]. It is characterized by open circuit potential increase and cathodic current increase that lead to corrosion initiation on  anodic surfaces (inside the crevice) , and propagation maintained  by cathodic current on  cathodic surfaces (outside the crevice).  The objective of this study was to better understand the biofilm role in crevice corrosion phenomenon of A 625. A multidisciplinary characterization has been carried out in controlled conditions that lead to “active biofilms” and “non-active biofilms”. Bacteria diversity, phytoplankton diversity, chemical composition (lipids, carbohydrates, amino acids, hydrogen peroxide, mineral elements, metallic elements) and spatial organization have been analyzed. The first step of the study was mainly to condition surfaces to established a so called “active biofilm” (with a high cathodic current) and “non active biofilm” (open circuit potential up to 300 mVvsSCE and low cathodic current).  Almost 500 metallic specimens have been conditioned and analyzed. A multidisciplinary characterization has pointed out variations between “active biofilm" and “non active biofilm”, essentially:  - A higher concentration of microorganisms associated to a higher surface coverage  - An association of the microorganisms in aggregate  - A lower diversity of bacteria communities -  A higher concentration of total lipids and carbohydrates - A higher concentrations of certain mineral elements or metallic elements, such as Ca, Si, Fe, Al, S, - A higher aminopeptidase enzymatic activity  in ”active  biofilm” compared to “ non-active biofilm “.   Globally, results have highlighted the selection of a bacterial population which is correlated with the intensity of the cathodic current of A625. Pyrosequencing analysis has allowed to identify a bacterial population affiliated to Halomonas genus.

Topics

• impedance spectroscopy
• mineral
• surface
• nickel
• stainless steel
• Hydrogen
• chemical composition
• uniform corrosion
• crevice corrosion
• nickel alloy