Potential mapping and corrosion of steel in concrete

The results of potential mapping surveys of bridge decks of the Swiss highways, using a new eight-wheel electrode measurement system, are reported here. A comparison of the state of corrosion of the rebars (obtained by visual inspection after removing the concrete cover), including the potential fields in addition to theoretical considerations, clearly demonstrates that an absolute potential value (that is, - 350 mV copper sulfate electrode (CSE) as proposed in ASTM Test for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete (C 876-80) for the identification of active corrosion of steel in concrete does not exist. Combining the local potential gradient on the surface with information on the electrical resistivity of the concrete (measured with the four-point method or a-c impedance) allows determination of a rough estimate of the corrosion rate in a single macrocell. Impedance spectroscopy, well suited for studying corrosion mechanisms in the laboratory, was found to be too time consuming and difficult to interpret in field measurements. Instead, a very rapid new technique-using galvanostatic pulse measurements-was tested successfully on site. It gives clear, unambiguous results on the corrosion state of the rebars, when half-cell potential measurements are uncertain

INVESTIGATIONS INTO THE ACTIVE PASSIVE TRANSITION OF 304 STAINLESS-STEEL IN ORGANIC MEDIA CONTAINING WATER AND HYDROGEN-CHLORIDE PART 1. IR DROP MEASUREMENT BY A COMPUTER-ASSISTED INTERRUPTER TECHNIQUE

An interrupter technique with computer-assisted calculation of the ohmic potential drop UΩ is described. It allows to measure UΩ while the working electrode is under potentiostatic control. As an additional result the differential double layer capacity CD is calculated, which includes some informations about the state of the working electrode surface. Experiments with the presented interrupter technique show, that just using a Luggin capillary is absolutely insufficient to avoid IR drop, especially in low conductive media: steady state current-potential curves will yield erroneous results of the kinetic parameters of the electrode reactions if the IR drop is not carefully eliminated. The described interrupter technique gives the same results for the ohmic resistance RΩ as eg AC techniques or potential pulse measurements. The value of RΩ measured is always associated with the primary current distribution

A study of the potentials achieved during mechanical abrasion and the repassivaton rate of titanium and Ti6Al4V in inorganic buffer solutions and bovine serum

Titanium alloys in orthopaedic implants are susceptible to mechanical disruption of the passive film (fretting corrosion). To study this effect, open-circuit potential (ocp) measurements before, during and after mechanical disruption of the passive film in a tribo-electrochemical cell on commercial pure titanium and Ti6Al4V alloy in inorganic buffer solutions in the pH range from 2.0 to 12.0 and calf bovine serum at pH 4.0 and 7.0 are reported. Additionally, the effect of pH, electrolyte and sample composition on the repassivation rate has been investigated. The potentials achieved during the abrasion of Ti6Al4V are the same as those characterizing pure titanium, which indicates that the corrosion current of both materials in the active state is due to the oxidation of titanium. However, commercial pure titanium displays a tendency to repassivate faster than Ti6Al4V in inorganic buffer solutions thanks to the lower critical current density and the higher catalytic activity towards the hydrogen evolution reaction observed on the pure metal in comparison with the alloy. Proteinaceous solutions like bovine serum, significantly slow down the anodic dissolution and the cathodic reactions both on titanium and the alloy. However, the repassivation rate of the Ti6Al4V is not affected by serum, while that of cp titanium significantly decreases both at pH 4.0 and 7.0

INVESTIGATION INTO THE ACTIVE PASSIVE TRANSITION OF 304 STAINLESS-STEEL IN ORGANIC MEDIA CONTAINING WATER PART 2. INFLUENCE OF WATER ON THE ACTIVE DISSOLUTION AND PASSIVATION

The influence of water on the corrosion and passivation of type 304 stainless steel has been studied in deaerated ethanolic solutions containing hydrogen chloride. The potentiostatic polarisation curves show that both, critical current density for passivation and passivation potential, strongly depend on the water content of the solution. A passivation model based on the assumption of a reversible primary passivation potential explains this influence of water and allows the determination of other kinetic parameters. The propounded passivation model also explains the result that the part of “passivated” surface at the maximum current density is the same for all water contents examined. The dominating influence of the water content is shown more clearly when measurements from methanolic and propanolic solutions are concerned. The passivation potential only depends on the water content and not on the type of alcohol

Effect of metalloids on the passivity of amorphous Fe-Cr alloys

The passivation and the stability of passive films of amorphous FeCr(B, P, C) alloys has been investigated by polarization measurements, impedance spectroscopy and potential decay measurements. The results show that phosphorus facilitates the active-passive transition by forming a porous iron phosphate pre-passive layer on the alloy surface in the active range of the dissolution. This layer blocks the active sites of the surface and accelerates the cathodic H2 evolution reaction. The formation of the passivating chromium oxide layer takes place in the pores of this layer. Phosphorus subsequently improves the resistance of the passive film against localized attack in Cl−-containing solutions. The effect of the incorporated phosphates in the passive film is discussed with respect to the bipolar fixed-charge-induced passivity model. The phosphates make the passive film cation selective and thus hinder the penetration of the chlorides into the film. The oxidized boron species cannot change the ion selectivity of the film; instead of this they deleteriously affect the stability of the passive film