Potentiostats corrosion studies

The determination of polarisation curves of metals by means of constant potential devices has contributed greatly to the knowledge of corrosion processes and passivity. In addition to the use of the potentiostat in studying a variety of mechanisms involved in corrosion and passivity, it has been applied to alloy development, since it is an important tool in the accelerated testing of corrosion resistance. Dissolution under controlled potentials can also be a precise method for metallographic etching or in studies of the selective corrosion of various phases. The technique can be used for establishing optimum conditions of anodic and cathodic protection. Two of the more recent papers have touched on limitations in its application and differences between potentiostatic tests and exposure to chemical solutions. ... 

Instruments very suitable for corrosion work are readily available, with several different models produced commercially. Although most, if not all, of the available potentiostats are properly designed, it should be kept in mind that corrosion studies require the instrument to have a low internal resistance and to react quickly to changes of potential of the working electrode. 

Construction of Apparatus. The schematic of the apparatus for supercritical corrosion studies is shown in Figure 1. The important components include a type 396-89 Simplex Minipump which can accurately meter (between 46 and 460 ml/hr) a wide variety of solvents at pressures up to 6000 psi (about 400 atm) an EG G Model 362 Scanning Potentiostat the electrochemical cell an IBM PC computer with interface hardware for electrochemical potential and current, temperature, and pressure measurement and control and a 316 stainless steel reactor, which holds the supercritical fluid for the measurements. The alloy was selected for excellent corrosion resistance properties and relatively low cost when compared with other exotic alloys such as Hastelloy C. 

Repassivation processes have become an important subject in stress corrosion studies and also in other forms of corrosion, e.g., pitting corrosion and corrosion fatigue. A range of scratching and straining electrode techniques have been employed. While it is not possible to go into detail, the results have to be examined in relation to the techniques employed, e.g., has repassivation started before the scratching or straining has stopped It is important also to know whether the current measured under potentiostatic conditions is a complete anode current or the difference between an anode current and a cathode current (most commonly due to H" " ion reduction). Typical repassivation rates correspond to an equation (2 1 ) of the type ... 

THE POTENTIOSTAT AND CORROSION STUDIES Rotating Disc-ring Electrodes... 

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