Galvanic anodes surface films

The assessment for nonalloyed ferrous materials (e.g., mild steel, cast iron) can also be applied generally to hot-dipped galvanized steel. Surface films of corrosion products act favorably in limiting corrosion of the zinc. This strongly retards the development of anodic areas. Surface film formation can also be assessed from the sum of rating numbers [3, 14]. 

Pure aluminum cannot be used as an anode material on account of its easy passivatability. For galvanic anodes, aluminum alloys are employed that contain activating alloying elements that hinder or prevent the formation of surface films. These are usually up to 8% Zn and/or 5% Mg. In addition, metals such as Cd, Ga, In, Hg and T1 are added as so-called lattice expanders, these maintain the longterm activity of the anode. Activation naturally also encourages self-corrosion of the anode. In order to optimize the current yield, so-called lattice contractors are added that include Mn, Si and Ti. 

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. 

Another factor that alters the galvanic position of some metals is the tendency, especially in oxidizing environments, to form specific surface films. These films shift the measured potential in the noble direction. In this state, the metal is said to be passive (see Chapter 6). Hence, chromium, although normally near zinc in the EMF Series, behaves galvanically more like silver in many air-saturated aqueous solutions because of a passive film that forms over its surface. The metal acts like an oxygen electrode instead of like chromium hence, when coupled with iron, chromium becomes the cathode and current flow accelerates the corrosion of iron. In the active state (e.g., in hydrochloric acid), the reverse polarity occurs that is, chromium becomes anodic to iron. Many metals, especially the transition metals of the periodic table, commonly exhibit passivity in aerated aqueous solutions. 

Mesnage, A., G. Deniau, and S. Palacin. Grafting polyphenyl-like films on metallic surfaces using galvanic anodes. RSC Advances 3, 2013 13901-13906. 

This chapter presents electrochemical reactions and corrosion processes of Mg and its alloys. First, an analysis of the thermodynamics of magnesium and possible electrochemical reactions associated with Mg are presented. After that an illustration of the nature of surface films formed on Mg and its alloys follows. To comprehensively understand the corrosion of Mg and its alloys, the anodic and cathodic processes are analyzed separately. Having understood the electrochemistry of Mg and its alloys, the corrosion characteristics and behavior of Mg and its alloys are discussed, including self-corrosion reaction, hydrogen evolution, the alkalization effect, corrosion potential, macro-galvanic corrosion, the micro-galvanic effect, impurity tolerance, influence of the chemical composition of the matrix phase, role of the secondary and other phases, localized corrosion and overall corrosivity of alloys. 

Even single metals, however, are subject to aqueous corrosion by essentially the same electrochemical process as for bimetallic corrosion. The metal surface is virtually never completely uniform even if there is no preexisting oxide film, there will be lattice defects (Chapter 5), local concentrations of impurities, and, often, stress-induced imperfections or cracks, any of which could create a local region of abnormally high (or low) free energy that could serve as an anodic (or cathodic) spot. This electrochemical differentiation of the surface means that local galvanic corrosion cells will develop when the metal is immersed in water, especially aerated water. 

The LSP mechanism proposes that SCC results from the effect of the structure ahead of the crack tip [61]. This mechanism assumes that a galvanic corrosion between active sites (weakened passive site) and surroimding passive surfaces produces large anodic currents at the rupture site. Repassivation of the active sites is prevented by the presence of weakened passive films on the surface. It has been su ested that the weakened passive film... 

Ferrous ions from the anodic reaction Fe Fe + 2e react with from the cathodic depolarization reaction and with OH from the water dissociation reaction and form ferrous sulphide, FeS, and hydroxide, Fe(OH)2. FeS can play an important role. Where the sulphide forms a continuous film on the surface it acts as protection and as an effective site for the cathodic reaction. If the film is injured or there is a lack of continuity in the film for other reasons, local galvanic corrosion will occur. Experiments and experience indicate that also the anodic reaction (Fe —> Fe +2e ) is depolarized as a result of the SRB environment. This is of interest in connection... 

Both resistance of the electrolyte and polarization of the electrodes limit the magnitude of current produced by a galvanic cell. For local-action cells on the surface of a metal, electrodes are in close proximity to each other consequently, resistance of the electrolyte is usually a secondary factor compared to the more important factor of polarization. When polarization occurs mostly at the anodes, the corrosion reaction is said to be anodically controlled (see Fig. 5.7). Under anodic control, the corrosion potential is close to the thermodynamic potential of the cathode. A practical example is impure lead immersed in sulfuric add, where a lead sulfate film covers the anodic areas and exposes cathodic impurities, such as copper. Other examples are magnesium exposed to natural waters and iron immersed in a chromate solution. 

The principle of this mechanism is based on the fact that conductive (p-doped) coatings could provide anodic galvanic protection to the metal substrate and act as an oxidizer to maintain the metal in the passive domain. This mechanism can lead to passivation of the exposed metal surface at small defects in the passive layer. A considerable number of reports have been made claiming the formation of passive films at the exposed site, with subsequent inhibition of further metal dissolution [129]. 

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