Corrosion zone diagrams

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... 

In contrast with the Pb-HjO system, it can be seen in Figure 4.13 that in the presence of SO " the corrosion zone in the region of low pH no longer exists, owing to the thermodynamic stability of PbSO. The Pb-HjO-COj system has been expressed in a similar pH/potential diagram in which account has been taken of insoluble carbonates and basic carbonates of lead. 

The Zn-HjO (Fig. 1.17) diagramshows that extensive corrosion zones exist at both low and hi pH values (compare the very restricted corrosion zone in the Fe-H20 diagram at high pHs) similar zones in the region of low and high pH are obtained with other amphoteric metals such as aluminium, lead and tin. The diagram for Zn-H20 predicts with some accuracy the behaviour of the metal in practice, where it has been established that zinc corrodes rapidly outside the range pH 6-12 5 but is passive within... 

At high pH values and low potentials, Fe, Fe, Fe , Fe(OH)2 and Fe(OH)j, etc. will be thermodynamically unstable with respect to FeO H" and a further limited zone of corrosion will appear on the right-hand side of the diagram. 

Although the zones of corrosion, immunity and passivity are clearly of fundamental importance in corrosion science it must be emphasised again that they have serious limitations in the solution of practical problems, and can lead to unfortunate misconceptions unless they are interpreted with caution. Nevertheless, Pourbaix and his co-workers, and others, have shown that these diagrams used in conjunction with E-i curves for the systems under consideration can provide diagrams that are of direct practical use to the corrosion engineer. It is therefore relevant to consider the advantages and limitations of the equilibrium potential-pH diagrams. 

The Af-HjO diagrams present the equilibria at various pHs and potentials between the metal, metal ions and solid oxides and hydroxides for systems in which the only reactants are metal, water, and hydrogen and hydroxyl ions a situation that is extremely unlikely to prevail in real solutions that usually contain a variety of electrolytes and non-electrolytes. Thus a solution of pH 1 may be prepared from either hydrochloric, sulphuric, nitric or perchloric acids, and in each case a different anion will be introduced into the solution with the consequent possibility of the formation of species other than those predicted in the Af-HjO system. In general, anions that form soluble complexes will tend to extend the zones of corrosion, whereas anions that form insoluble compounds will tend to extend the zone of passivity. However, provided the relevant thermodynamic data are aveiil-able, the effect of these anions can be incorporated into the diagram, and diagrams of the type Af-HjO-A" are available in Cebelcor reports and in the published literature. 

The region of immunity [Fig. 1.15 (bottom)] illustrates how corrosion may be controlled by lowering the potential of the metal, and this zone provides the thermodynamic explanation of the important practical method of cathodic protection (Section 11.1). In the case of iron in near-neutral solutions the potential E = —0-62 V for immunity corresponds approximately with the practical criterion adopted for cathodically protecting the metal in most environments, i.e. —0-52 to —0-62V (vs. S.H.E.). It should be observed, however, that the diagram provides no information on the rate of charge transfer (the current) required to depress the potential into the region of immunity, which is the same (< —0-62 V) at all values of pH below 9-8. Consideration of curve//for the Hj/HjO equilibrium shows that as the pH... 

Fig. 1.38(Equilibrium potential-pH diagram for the Cr-H20 system and (< ) potential-pH diagram showing zones of corrosion, passivity and immunity (after Pourbaix )... 

As may be seen from the potential-pH diagram " (Fig. 6.3) platinum is immune from attack at almost all pH levels. Only in very concentrated acid solutions at high redox potentials (i.e. under oxidising conditions) is there a zone of corrosion. This accounts for the solubility of platinum in aqua regia. Platinum is also prone to complex-ion formation, and this can lead... 

Fig. 9.29 Corrosion sites in stainless steel welds. The typical peak temperatures attained during welding (°C) are given at the foot of the diagram. Note that knifeline attack has the appearance of a sharply defined line adjacent to the fusion zone...

Essentially, the pH is controlled to suppress the hydrogen evolution cathodic reaction- The Pourbaix Diagram for iron indicates that high pH values as well as low values may lead to corrosion. The construction of these diagrams for higher than ambient temperatures - shows how the area of the alkaline zones increases considerably under boiler conditions, so that the risk of corrosion is correspondingly higher. Many feed systems contain copper alloys. 

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. 

Dissolution of the chlorides from the corrosion products is an essential part of the conservation process. It is essential that the artefact is immersed in an electrolyte that will not corrode the metal any further, while this dissolution is taking place. Corrosion scientists have developed redox potential - pH diagrams from thermodynamics in order to predict the most stable form of the metal. These diagrams are divided into three zones. Where metal ions are the most stable phase, this is classed as a zone of corrosion. If the metal itself is the most stable species, this is said to be the zone of immunity. The third zone is where solid metal compounds such as oxides, hydroxides, etc, are the most stable and may form a protective layer over the metal surface. This zone is termed passivity and the metal will not corrode as long as this film forms a protective barrier. The thickness of this passive layer may only be approximately 10 nm thick but as long as it covers the entire metal surface, it will prevent further corrosion. 

Fig. 3 Electrochemical framework for intergranular corrosion described by an Evans diagram depicting the anodic half-cell reaction kinetics for the grain boundary zone and the grain matrix. In this case, enhanced active dissolution occurs in both the grain boundary region and in the matrix. At a fixed potential, given by Egpp, the anodic dissolution rate is accelerated along the grain boundary compared to the matrix.

One frequently asked question concerning cathodic protection systems is what happens at the anode edge Is there a risk of accelerated corrosion This is a valid question and the risk is supported by the Pourbaix diagram which shows areas of imperfect passivity, pitting and corrosion around the immune and passive regions (Figure 7.2). However, the author knows of no atmospherically exposed reinforced concrete structure that is totally protected by cathodic protection. Most have anode zones that end before the reinforced concrete does. No cases of accelerated corrosion have been reported between zones or at the end of zones. 

Equations (2.17)-(2.19) represented by the lines 1, 2, and 3 are stable electrochemical reactions. Equations (2.20) and (2.21) represent chemical reactions. Zinc will dissolve at the suitable potential to form soluble zinc ions, i.e., Zn when the solution pH is less than 7. When the solution pH is in between 8 and 11, anodic reactions produce zinc hydroxide Zn(OH)2 which is less soluble in water and may deposit on the metal surface as a protective film. This zone is designated as passivity region. When the pH value is more than 11 corrosion of zinc is possible by forming complex zincate ions Zn02. The bottom region of the diagram represents thermodynamically stable solid zinc. Line 6 and... 

In MS, there were regions of high O2 and Fe concentration indicating formation of lumpy oxides at certain regions and regions of Fe rich zones on the surface. Based on the above results, higher corrosion rate of MS can be expected and same was found from polarisation diagram. 

The well-known Pourbaix diagrams (showing specific zones in which no corrosion and passivation occur), in which pH is plotted against electrode potential, are valid only in pure water at 25°C. A comparison of the behavior of zinc and other metals offers a deeper insight into corrosion by water, however. 

Pourbaix Diagrams. Plots of equilibrium pH vs electrochemical potential E describe the effects of aqueous corrosion on borosilicate and silicate glasses. They are applicable to weathering studies and to ground water attack on nuclear waste glasses. The diagrams display any immune zone between active corrosion... 

Fig. l.lS(top) Equilibrium potemial-pH diagram for the Fe-HjO system showing the zones of stability of cations, anions and solid hydroxides (after Deltombe and Pourbaix ) and (bottom) simplified version showing zones of corrosion, immunity and passivity (curve / is the HjO/Hj equilibrium at Phj= 1 and cur s the Oj/HjO equilibrium at Poj = )... 

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