CORROSIVITY AND PASSIVITY

More effort has probably been devoted to study of the corrosion and passivation properties of Fe-Cr-Ni alloys, e.g. stainless steel and other transition-metal alloys, than to any other metallic system [2.42, 2.44, 2.55, 2.56]. The type of spectral information obtainable from an Fe-Cr alloy of technical origin, carrying an oxide and contaminant film after corrosion, is shown schematically in Fig. 2.13 [2.57]. 

Impingement attack (sometimes termed erosion corrosion) is a result of the combined effect of flow and corrosion on a metal surface and it occurs when metal is removed from the surface under conditions where passivation is insufficiently rapid. It is a function of flow, corrosion and passivation. 

Whereas lowering the potential results in a decrease in, the converse applies when the potential is raised. However, this increase in activity is again limited by the formation of a solid phase. Thus curve e of Fig. 1.15 (top) gives the equilibrium between Fe(OH)3 and Fe at any predetermined activity of the latter in the range 10 — 10". At flpe2+ = 10 g-ion/l, E= [ 1-06-t-(-6 X 0-059)] - 0-177pH which defines the boundary between corrosion and passivity at high potentials (equation 1.19). 

The effect of anions on the zones of corrosion and passivation can be exemplified by a comparison of the Pb-H20 and Pb-H20-S04 equilibrium diagrams (see Section 4.3, Figs. 4.13 and 4.14) and it can be seen that in the presence of SOl the corrosion zone corresponding with stability of... 

Fig. 1.34 Corrosion and passivation of Fe-18Cr-SNi stainl s steel. Potentiosiatic anodic curve JKLM, hydrogen evolution reaction, curve Hl low concentration of dissolved oxygen, curve t> FG, high concentration of dissolved oxygen, curve AflC (Section 3...

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. ... 

Potentiostatic methods, being capable of detecting differences in corrosion and passivation behaviour of various parts of a heterogeneous surface, have been applied to the electrochemical determination of grain boundary corrosion... 

It was learned that pitting-type metal and semiconductor corrosion is attended by the generation of noise seen in the form of dynamic irregularities in the changes of the anodic potential and current density. Thus, electrochemical noise studies were applied to the corrosion and passivation of metals and to their activation by external chemical (activating additives in the electrolyte) or electrochemical (anodic or cathodic polarization) agents. 

Titanium Carbide. Carbides of transition metals are known for their hardness, wear resistance and also for their high electrical conductivity, which makes them attractive as a refractory coating material for cutting tools or bearings. Only little work has been done on the electrochemical stability of transition metal carbides with the exception of TiC, where a corrosion and passivation mechanism was suggested by Hintermann et al. [119,120]. This mechanism was confirmed on amorphous TiC produced by metal-... 

Pioneering work was done in this field by Schwabe [34, 35]. Substantial and very informative radiotracer adsorption studies carried out by him and his coworkers using Pt, Ni, and Fe sheets and evaporated films, using labeled anions in order to interpret the role of anions in the corrosion and passivation processes [34, 35]. 

Protection of iron from corrosion by zinc or tin coatings is based on a very small corrosion current density at large areas of these metals so that the whole electrode is brought to potentials where iron (in small areas, for instance pores) is protected and cannot dissolve. It is beyond the scope of the present chapter to discuss corrosion and passivation in detail and the reader is referred to specialized bibliography [142—143]. 

Fig. 2 distinguishes the domains of immunity, corrosion and passivity. At low pH corrosion is postulated due to an increased solubility of Cu oxides, whereas at high pH protective oxides should form due to their insolubility. These predictions are confirmed by the electrochemical investigations. The potentials of oxide formation as taken from potentiodynamic polarization curves [10] fit well to the predictions of the thermodynamic data if one takes the average value of the corresponding anodic and cathodic peaks, which show a certain hysteresis or irreversibility due to kinetic effects. There are also other metals that obey the predictions of potential-pH diagrams like e.g. Ag, Al, Zn. 

In situ methods permit the examination of the surface in its electrolytic environment with application of the electrode potential of choice. Usually they are favored for the study of surface layers. Spectroscopic methods working in the ultra high vacuum (UHV) are a valuable alternative. Their detailed information about the chemical composition of surface films makes them an almost inevitable tool for electrochemical research and corrosion studies. Methods like X-ray Photoelectron Spectroscopy (XPS), UV Photoelectron Spectroscopy (UPS), Auger Electron Spectroscopy (AES) and the Ion Spectroscopies as Ion Scattering Spectroscopy (ISS) and Rutherford Backscattering (RBS) have been applied to metal surfaces to study corrosion and passivity. 

The thermodynamic information is normally summarized in a Pourbaix diagram7. These diagrams are constructed from the relevant standard electrode potential values and equilibrium constants and show, for a given metal and as a function of pH, which is the most stable species at a particular potential and pH value. The ionic activity in solution affects the position of the boundaries between immunity, corrosion, and passivation zones. Normally ionic activity values of 10 6 are employed for boundary definition above this value corrosion is assumed to occur. Pourbaix diagrams for many metals are to be found in Ref. 7. 

In this review results from two surface science methods are presented. Electron Spectroscopy for Chemical Analysis (ESCA or XPS) is a widely used method for the study of organic and polymeric surfaces, metal corrosion and passivation studies and metallization of polymers (la). However, one major accent of our work has been the development of complementary ion beam methods for polymer surface analysis. Of the techniques deriving from ion beam interactions, Secondary Ion Mass Spectrometry (SIMS), used as a surface analytical method, has many advantages over electron spectroscopies. Such benefits include superior elemental sensitivity with a ppm to ppb detection limit, the ability to detect molecular secondary ions which are directly related to the molecular structure, surface compositional sensitivity due in part to the matrix sensitivity of secondary emission, and mass spectrometric isotopic sensitivity. The major difficulties which limit routine analysis with SIMS include sample damage due to sputtering, a poor understanding of the relationship between matrix dependent secondary emission and molecular surface composition, and difficulty in obtaining reproducible, accurate quantitative molecular information. Thus, we have worked to overcome the limitations for quantitation, and the present work will report the results of these studies. 

THE BASIC ELECTROCHEMICAL concepts and ideas underlying, the phenomena of metal dissolution are reviewed. The emphasis is on the electrochemistry of metallic corrosion in aqueous solutions. Hie role of oxidation potentials as a measure of the "driving force" is discussed and the energetic factors which determine the relative electrode potential are described. It is shown that a consideration of electrochemical kinetics, in terms of current-voltage characteristics, allows an electrochemical classification of metals and leads to the modern views of the electrochemical mechanism of corrosion and passivity. 

Fig. 2D Schematic HE plots for a system undergoing corrosion and passivation. Active dissolution region A-B. Passive region C-D. Transpassive region where pitting occurs D-E.

Fig. 17M Schematic representation of the corrosion and passivation of iron in sulfuric acid. The primary passivation potential and the corresponding critical current density for corrosion i are shown. Breakdown of the passive film occurs at potentials more positive than E. ...

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