D. Festy

In situ detection and characterization of biofilm in waters by electrochemical methods

Abstract Biofilm is considered with respect to mass transport as an inert porous layer. The biofilm is developed in fresh water and in natural seawater on a disk electrode in gold. Its thickness is determined by considering the limiting diffusion current measured on this electrode for different rotation speeds. The same thickness is obtained when two different electrochemical tracers are used: Fe(CN) 6 3− and oxygen which validates the technique and permits the use of oxygen naturally present in natural water as an electrochemical tracer. The dependence of the thickness on the rotation speed of the electrode shows the elastic behavior of the biofilm.

Modified transparent SnO2 electrodes as efficient and stable cathodes for oxygen reduction

The aim of this work is to develop an efficient and stable transparent electrode for the reduction of dissolved oxygen, to be used for characterising biofilm formation in seawater and scale deposit, both by optical observations and electrochemical experiments. It was achieved by depositing a transparent conductive tin dioxide film on a glass substrate and modifying the surface by a very small gold charge. A reduction kinetics of dissolved oxygen as fast as that for a massive gold electrode has been obtained with a few monolayer only equivalent gold deposit via a simple chemical treatment. The long term stability has been checked, opening the way for practical applications. Electrochemical impedance spectroscopy was used to analyse the working conditions of the modified electrode. It is shown that SnO2 is operating in the cathodic regime under weak depletion conditions with a surface-state mediated charge transfer.

Characterization of deposits by direct observation and by electrochemical methods on a conductive transparent electrode. Application to biofilm and scale deposit under cathodic protection

Abstract A new device is proposed where the electrode is a transparent electrode in modified SnO 2 . With this new electrode the reduction of oxygen occurs in a way similar to a gold electrode. Simultaneously the oxygen current is recorded versus time and picture of the interface itself is taken with a digital camera through the electrode. An image analysis is performed to determine the coated area and the number of objects by surface unit. Under cathodic protection, the reduction of oxygen induces an increase of the interfacial pH and then formation of scale deposit. The usual chronoamperometric analysis was completed by an in situ observation of the scale deposit. In natural water the biofilm development was studied by electrochemical method to determine the biofilm thickness and this analysis is completed by a direct observation through the electrode. A good correlation between the two techniques was found.

Influence of the hydrodynamics on the biofilm formation by mass transport analysis

Biofilm are formed wherever there is some water in our environment: pipes, pipelines, tap water systems, air conditioning systems... Furthermore, the ecological and economical consequences are very important: energy losses, bacterial contamination, material deterioration. The aim of this work is to develop a new method to detect and monitor the biofilm formation. This method can also determine some mechanical properties of the biofilm. An application of this method is a realization of a biofilm sensor. Biofilm is considered as an inert porous layer with respect to mass transport. In our experiment, the biofilm is grown on a gold electrode in natural seawater. Its thickness is determined by considering the oxygen diffusion limiting current measured for different rotation speeds on this electrode. Two different incubators are used during the biofilm development: one, with a laminar flow, permits the rotation of the electrode during the biofilm formation, and for the second, a tube is used under turbulent conditions during the biofilm formation. This experiment allows us to characterize the mechanical behavior (thickness, elasticity, rigidity) of the biofilm in function of different conditions of development.

EHD and steady state measurements for characterization of transport properties in a gel layer

Mass-transfer to a rotating disk electrode has been studied in the case of an interface covered by porous inert layers of thickness δ f through which species are transported by molecular diffusion with a diffusivity D f . In this study, the porous layer consists of a gel deposited on a gold electrode and the species used as tracers are Fe(CN) 6 3- and oxygen. The method is based on the analysis of tracer reduction current under total mass transport control (diffusion plateau of the species). The tracer concentration gradient is distributed between the porous layer where mass transport is only determined by molecular diffusion and the electrolyte solution where mass transport is governed by convective diffuson. Two techniques are used here namely electrohydrodynamical impedance (EHD) and a steady flow technique. From the proposed physical model, the diffusion time (δ f 2 /D f ) is obtained by regression of the impedance data while the steady state current data yields the diffusion rate (D f /δ f ) through the porous layer. By combining the EHD and steady state data, a separate determination of δ f and D f is available and allows the calculation of the porosity of the film. A comparison between the two tracers on the same layer has been made.

Characterization of Biofilms Formed on Gold in Natural Seawater by Oxygen Diffusion Analysis

Abstract Studies using a gelatin layer showed the feasibility of using combined steady-state and electrohydrodynamic (EHD) impedance techniques to determine the diffusion layer thickness (δf), diff...

Aging of Type 316L Stainless Steel in Seawater: Relationship Between Open-Circuit Potential, Exposure Time, and Pitting Potential

Abstract The pit generation rate (g) on stainless steel (SS) in natural seawater was analyzed according to the stochastic theory. Under the experimental conditions with statistical measurements tak...

The effect of hydrostatic pressure upon the hydrogen embrittlement of a HSLA steel

Abstract The steels used in sea water needs to be protected against corrosion. The cathodic protection methods is widely used, coupled or not to coating technics. The use of High Strength steels (HSLA) cathodically protected in sea water is faced to the problem of hydrogen embrittlement. This phenomen is controlled by applied protection potential and sea water physico-chemical parameters (temperature, pH, dissolved oxygen contents, etc). The exploitation of ocean is going deeper, so the effect of pressure has to be take in account. Previous works on pressure influence upon hydrogen permeation in steels and theorical approachs concluded on a possible effects of pressure upon the hydrogen embrittlement of high strength steels. Performing tests in a “Corrosion Tank” facility, we studied the behaviour of HSLA steel smooth and Compact Tensile specimens cathodically protected in natural sea water. The results show that an increase of hydrostatic pressure decreases the load threshold of smooth specimens and incr...