Pourbaix steel corrosion

Effect of the potential of the external surfaces on the pH and potential inside an artificial C-steel crevice. Same numbers refer to the corresponding conditions on the free surface and inside the crevice. (From Pourbaix, M, Corrosion, 26,431,1970.)... 

Pourbaix, A., Characteristics of Localised Corrosion of Steel in Chloride Solutions , Corrosion, 27, 449 (1971)... 

Pourbaix, M., Klimezak-Mathieu, Martens, C. and Meunier, J., Potentiokinetic and Cor-rosimetric Investigations of the Corrosion Behaviour of Alloy Steels , Corros. Sci., 3, 239 (1963)... 

The principle of the cathodic protection may be elucidated for the case of carbon steel. The Pourbaix diagram for iron in water consisting of the plot E (potential) vs pH is shown in Figure 1.68. The regions of passivity, immunity and corrosion are seen in the figure. 

Next we consider the Pourbaix diagram for iron, which is, of course, of paramount importance for the understanding of corrosion of ferrous alloys such as the many types of steel and stainless steel. This is a rather complex diagram, since two oxidation states of iron exist both in the liquid and the solid state and the metal is amphoteric to some extent. Figure 16M is a simplified version of the diagrams shown in the original work of Pourbaix. The two soluble species in acid solutions are Fe and Fe. The relevant equilibria are... 

Parallel use of the Pourbaix diagram and overvoltage curves is sometimes useful when evaluating how typical and stable the uniform corrosion is under different conditions. Let us consider a case with iron or unalloyed steel in two different environments ... 

Pourbaix diagram [3] maps out possible stable equilibrium phases of an aqueous electrochemical system and indicates that piue iron is passive at pH values from 9 to 12.5 to form iron hydroxide. Considering the interplay of atmospheric factors this diagram was used as guide to the steel dissolution process to form passivity on WS in laboratory. The passive films formed on pure iron are not so stable and consequently the passivation state of iron is not maintained for prolonged time periods [147, 148]. The mst layers of steels play a role as a barrier against corrosion, and their growth rate is decreased to a rate similar to that of the passive films, when suitable elements are added to the steel [149, 150]. 

Pourbaix, M. (1970). Corrosion de tubes en acier galvanisd utilises pour la distribution d eau (Corrosion of galvanized steel pipes used in water distribution systems). CEBELCOR Rapp. Tech., 7/4(180) (in French), 10 pp. 

Second, the critical potential may be a repassivation potential. It has been shown in artificial active crevices that lowering the potential of the free surfaces causes the local environment to become less aggressive (see, for example, Pourbaix [36]). Thus, at some point, the environment becomes not aggressive enough for active dissolution to be sustained and the metal surface in the crevice becomes passive. In this case, a subsequent increase of the corrosion potential does not produce immediate reactivation inside the crevice. Starr et al. [82] observed this situation on 12% Cr stainless steels in near-neutral environments Dunn and Sridhar [83] observed the same behavior on alloy 825. However, the repassivation may be attributed to different environment changes an increase of local pH in the crevice [82,84], a destabilization of the salt film that controls the... 

Traditionally, metallic coatings serve only one or two functions. For example, zinc has excellent corrosion resistance and functions as a sacrificial anode (Tsura, 2005). Zinc galvanizing provides sacrificial cathodic protection and acts as a barrier (Jones, 1996) but does not nsnally supply inhibitor ions. The release of zinc ions during the sacrificial protection of galvanized steels (Tsuru, 2005 Pourbaix, 1974) only provides a small additional benefit compared with galvanic protection provided by the potential driving force. Metallic coatings used to protect Al alloys (Reddy et al., 2000 Walton et al, 1953) consist of a thin layer of nearly pure Al mechanically bonded to standard precipitation age... 

As already pointed out, atmospheric corrosion penetration usually is not linear with time. The buildup of corrosion products often tends to reduce the corrosion rate over time. Pourbaix utilized the so-called linear bilogarithmic law for atmospheric corrosion, to describe atmospheric corrosion damage as a function of time on a mathematical basis. This law was shown to be applicable to different types of atmospheres (rural, marine, industrial) and for a variety of alloys, such as carbon steels, weathering steels, galvanized steels, and aluminized steels. This mathematical model has also been apphed more recently... 

A further important feature of the hydration reactions of cement with water is that the resulting pore solution in concrete is highly alkaline [refer to Eq. (2.28) above]. In addition to calcium hydroxide, sodium and potassium hydroxide species are also formed, resulting in a pH of the aqueous phase in concrete that is typically between 12.5 and 13.6. Under such alkaline conditions, reinforcing steel tends to display completely passive behavior, as fundamentally predicted by the Pourbaix diagram for iron. In the absence of corrosive species penetrating into the concrete, ordinary carbon steel reinforcing thus displays excellent corrosion resistance. 

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