Uniform corrosion electrochemical tests

Electrochemical tests are often preferred to mass loss studies as a method of measuring uniform corrosion. Such tests are preferred because in theory they (1) provide a realtime measurement of the metaUic corrosion rate, (2) can provide time-corrosion rate data on a single coupon, and (3) are rapid to perform. Disadvantages of the electrochemical tests include the requirements for comparatively expensive equipment (versus mass loss tests) and higher levels of technical expertise for data analysis. Fmthermore, the data reduction requires the use of conversion-"constants," factors applied to the results of the electrical measurements to convert the data to a corrosion rate. These constants ... 

Use and Uimitations of Electrochemical Techniques A major caution must be noted as to the general, indiscriminate use of all electrochemical tests, especially the use of AC and EIS test techniques, for the study of corrosion systems. AC and EIS techniques are apphcable for the evaluation of very thin films or deposits that are uniform, constant, and stable—for example, thin-film protective coatings. Sometimes, researchers do not recognize the dynamic nature of some passive films, corrosion produc ts, or deposits from other sources nor do they even consider the possibility of a change in the surface conditions during the course of their experiment. As an example, it is note-... 

The first question that might be of interest is to determine if the material passivates or undergoes uniform active corrosion in the relevant environment. If the form of corrosion is active corrosion, then the corrosion rate needs to be measured, and a determination can be made if there is sufficient material to survive the lifetime requirements. Corrosion rate, r (units of thickness loss per unit time), is related to a corrosion current density, i corr (A cm ), which is the outcome of most electrochemical tests, by way of Faraday s law ... 

Immersion tests provide no information about reaction mechanisms and often they require relatively long exposure times. Electrochemical tests do not have these drawbacks and they are therefore widely used in practice. In the following electrochemical polarization methods are presented that provide information on the rate of uniform corrosion under conditions where the rate is controlled by charge-transfer. Other electrochemical test methods will be presented in subsequent chapters. 

The impedance method consists in measuring the response of an electrode to a sinusoidal potential modulation of small amplitude (typically 5-10 mV) at different frequencies. The ac modulation is superimposed either onto an applied anodic or cathodic potential or onto the corrosion potential. Another possibility is to modulate the current and to measure the potential. Impedance measurements as a function of modulation frequency are commonly referred to in the literature as electrochemical impedance spectroscopy, abbreviated EIS. Among the different transient methods discussed in this chapter, EIS is most widely used in corrosion studies. It serves for the measurement of uniform corrosion rates, fortheelucidationofreactionmechanisms, for the characterization of surface films and for testing of coatings. 

Electrochemical techniques measure electrochemical potentials and currents that are fundamentally related to the thermodynamics and kinetics of corrosion reactions. They are often used to measure the rates of uniform corrosion, to determine the tendency of localized corrosion, to study a wide range of corrosion-related phenomena such as passivation, galvanic corrosion and sensitization effects. Electrochemical corrosion testing and monitoring can be performed in a diverse range of environments in the laboratory or in the field, in a pipeline or in an autoclave. For instance, they have been successfully employed to monitor corrosion in multiphase oil/water conditions with as little as 1-2% water " ... 

Although important contributions in the use of electrical measurements in testing have been made by numerous workers it is appropriate here to refer to the work of Stern and his co-workerswho have developed the important concept of linear polarisation, which led to a rapid electrochemical method for determining corrosion rates, both in the laboratory and in plant. Pourbaix and his co-workers on the basis of a purely thermodynamic approach to corrosion constructed potential-pH diagrams for the majority of metal-HjO systems, and by means of a combined thermodynamic and kinetic approach developed a method of predicting the conditions under which a metal will (a) corrode uniformly, (b) pit, (c) passivate or (d) remain immune. Laboratory tests for crevice corrosion and pitting, in which electrochemical measurements are used, are discussed later. 

As an example of SAMs, eorrosion inhibition by self-assembled films formed by adipic acid (AA) molecules on caibon steel surfaces is discussed below. SAMs of AA were formed on iron oxide/caibon steel surfaces by the immersion coating method. The metal was immersed in an aqueous solution containing 60 ppm of Cl (to initiate the corrosion process and the formation of iron oxide) in the absence and presenee of adipic acid. The formation, uniformity, ordering and bonding of the monolayers accompUshed by the immersion method have been evaluated by FTIR and AFM. The electrochemical properties of the unmodified and modified caibon steel surfaces were characterized by polarization study and EIS analysis to test the abiUty of the monolayer to reduce the corrosion of the surface. 

The electrochemical process of corrosion is complex, and the corrosion rate depends on myriad physical and chemical parameters. Different degrees of corrosive attack are often observed at different locations on the same part seemingly exposed to the same corrosive environment. Figure 7.6 shows typical uniform general corrosion on a steel tube. Most areas show relatively imiform corrosion, while an unusually severe corrosive attack at one location has penetrated through the tube thickness. Multiple corrosion mechanisms can be activated therefore, different failure modes predominate in accelerated and natural tests. This might require the use of different corrosion models in corrosion rate calculation and introduce different confidence levels in... 

---