Electrochemical measurements corrosion inhibitors

The methods of measuring corrosion rates in the course of testing corrosion inhibitors are conventional weight loss, electrochemical techniques such as linear polarization resistance, potentiodynamic polarization, AC impedance, and electrochemical potential or current noise. 

Simple but pedagogically useful theories of electrode kinetics are presented in Chapter 3. This permits discussion of models for anodic and cathodic reactions at the metal/environment interface and for diffusion of species to and from the interface. Mathematical models of these theories lead to so-called kinetic parameters whose values govern the rate of the interface reaction. The range of values that these parameters can have and some of the variables that can influence the values are emphasized since these will relate to understanding the influence of such factors as surface conditions (roughness, corrosion product films, etc.), corrosion inhibitors and accelerators, and fluid velocity on corrosion rates. This chapter also introduces electrochemical measurements to determine values of the kinetic parameters. 

The concepts in Chapters 2 and 3 are used in Chapter 4 to discuss the corrosion of so-called active metals. Chapter 5 continues with application to active/passive type alloys. Initial emphasis in Chapter 4 is placed on how the coupling of cathodic and anodic reactions establishes a mixed electrode or surface of corrosion cells. Emphasis is placed on how the corrosion rate is established by the kinetic parameters associated with both the anodic and cathodic reactions and by the physical variables such as anode/cathode area ratios, surface films, and fluid velocity. Polarization curves are used extensively to show how these variables determine the corrosion current density and corrosion potential and, conversely, to show how electrochemical measurements can provide information on the nature of a given corroding system. Polarization curves are also used to illustrate how corrosion rates are influenced by inhibitors, galvanic coupling, and external currents. 

The proof of the importance of iron sulfide in the interaction between the corrosion inhibitor and the metal surface was brought in 1970 when it was shown (3U) that corrosion inhibition was considerably enhanced and prolonged when the corrosion inhibitor was adsorbed on a presulfided specimen rather than on the nonsulfided surface. These measurements were made electrochemically and gave support to the practical well-known fact that periodic "filming" could inhibit the corrosion for relatively long periods of tdme. 

The relationship between a corrosion product layer and inhibitors has also been discussed, among others, by Lorenz [66], and Lorenz and Mansfeld [67]. These authors point out that in many practical systems, for instance aerated water and carbon steel, an interaction occurs between, in this case, iron oxide and the inhibitor to the point where the inhibitor is not only adsorbed on the oxide surface, but actually incorporated into the three-dimensional oxide layer. Clearly, three-dimensional or interphase inhibition caimot be achieved in short tests or by filming procedures. Furthermore, measuring techniques have to take into consideration the altered chemical and electrochemical conditions across such bulk interphase layers. It is unfortunate that this aspect of corrosion and corrosion inhibition has not received more attention, and it is suggested that this lack of attention has seriously held back all aspects of corrosion inhibitor applications and monitoring of effectiveness. 

The electrochemical studies of the corrosion inhibition process of Al-Mg-Si alloy in seawater using three selected natural products as corrosion inhibitors show that the corrosion rate of the alloy significantly reduced upon the addition of studied inhibitors. PP measurement reveals that the studied inhibitors can be classified as mixed-type inhibitors without changing the anodic and cathodic reaction mechanisms. The inhibitors inhibit both anodic metal dissolution and also cathodic hydrogen evolution reactions. 

Abstract This chapter provides an overview of major corrosion testing and analysis techniques and their applications in corrosion inhibitor research, with a particular focus on electrochemical evaluation of corrosion protection by rare earth metal (REM) compounds. Attempts are made to discuss fundamental issues in inhibitor test design such as limitations in corrosion measurement techniques and challenges that may lead to the reporting of inaccurate corrosion rates and patterns. 

Successful inhibitor tests require suitable corrosion measurement and analysis techniques that are able to correctly record and interpret corrosion rate data. Many testing and monitoring techniques that were developed initially for the diagnosis and prediction of corrosion have been successful employed in laboratory and field corrosion inhibitor testing and research. These techniques include the use of corrosion coupons, solution analysis, electrical resistance probe, polarization resistance, electrochemical impedance spectroscopy and many other physical, electrical and electrochemical methods. 

Variously designed weight-loss coupon, electrochemical and surface analytical techniques have been utilized in REM-based corrosion inhibitors and conversion coatings research. In particular, electrochemical techniques including EIS and polarization measurements have been widely used to evaluate corrosion inhibition by REM compounds under various environmental conditions. Relatively less attention has been paid to the evaluation of localized corrosion inhibition by REM-based compounds, probably because of methodological difficulties and complexities in making accurate localized corrosion rate measurements. Recently developed techniques such as the scanning probe techniques, electrochemical noise analysis and the wire beam electrode are expected to be useful tools in further REM inhibitor research. 

Standard corrosion testing such as weight loss. Unear polarization resistance (LPR) and cyclic potentiodynamic polarization (CPP) were used to assess each compound s inhibition efficiency for steel in 0.01 M NaCl solution. Cerium and lanthanum combined with cinnamate and substituted cinnamates produced the most effective corrosion inhibition for steel. As an example, lanthanum 4-nitrocirmamate achieved 92% inhibition for mild steel itmnersed for 7 days in sodium chloride solution. Cerium 4-methoxycinnamate was also effective as a corrosion inhibitor. Electrochemical measurements showed the rare earth cinnamate compounds to be mixed inhibitors, with an initial suppression of anodic processes, followed by a decrease in cathodic processes after longer exposure times. 

Electrochemical Testing. Potentlodynamlc polarization measurements provided a sensitive means of evaluating the inhibitors with respect to environmental (Cl ) corrosion protection. The results obtained from anodlcally polarizing polished 7075-T6 A1 samples are presented in Fig. 9. For the control electrolyte (O.IN Na2S0, 0.002N KCl, no inhibitor), pitting was observed almost immediately on the surface, and the aluminum showed no evidence of passivation. The addition of NTMP to the solution did not appear to protect the metal... 

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