Pourbaix diagram

Added: 2026-05-12

[00:00:02]It's my pleasure to welcome you all to today's lecture on mobile. I am Dr. Babaj Bazar, associate professor in the department of chemistry at SRM Institute of Science and Technology, Katangur. Uh today I'm delighted to present to you one of the most uh fundamental and powerful tools in corrosion science, the 4 by diagram. In this session, uh we will explore how forward diagrams map the thermodynamic stability of metals as a function of electro potential and pH and how they help us uh predict whether a metal will corrode uh passivate or remain immune under a given set of electrochemical conditions.

[00:00:43]What are four by diagrams and what information uh do they provide? uh phob diagrams are electrochemical tools that plot the thermodynamic stability of different redux states of an element as a function of uh pH. But what exactly do they map?

[00:01:01]Essentially functioning as phase diagrams for aqua systems. Uh they map the regions of electro potential and pH under which different redux species remain stable in solution. What parameters are taken into account? Uh the two key parameters considered are voltage and solution pH which together define the prevailing electrochemical environment. And what can these diagrams ultimately tell us? By considering these parameters, PB diagrams allow us to identify and predict the possible electrochemical and corrosion reactions uh that may occur for a given metal making them broadly applicable to a wide range of metallic system including iron, aluminium, cadmium and zinc and so on so forth.

[00:01:52]As mentioned in the previous slide, uh for diagrams serve as an exceptional valuable tool in corion science. At their core, these diagrams are plots of electrochemical potential versus pH uh defining the precise conditions under which your given metal is susceptible to corrosion. To understand the foundation of these diagrams, we first examine the electrochemical stability of water using nonst equation. The stability of water is governed by two pH uh independent electrochemical reactions uh namely the reduction of protons and the reduction of oxygen. The pH dependence of these reactions can be expressed graphically by plotting the obsal potential against pH yielding two boundary lines. As you can see here the line A it's a lower stability line hydrogen evolution wherein the two protons and two electrons they combine and produce the hydrogen gas. And uh when it comes to upper stability line line B where you can see it's indicated in blue oxygen evolution where water is getting oxidized to produce oxygen plus 4 H+ and four electron and also uh this region within the boundary where you can see the region between these two parallel lines separated by a certain volt and each carrying a slope of molt per ph corresponds to thermodynamic stability zone water.

[00:03:42]However, pro diagram extends well beyond simply mapping water stability. Uh it is a comprehensive uh plot of potential versus pH that is uh simultaneously illustrates the stable phases of a given element under varying electrochemical uh conditions. Uh taking 4 by diagram of ion as an example. The distinct region of the uh diagram represents the thermodynamically stable states in which ion can exist including uh dissolved ionic species such as iron 2+ and iron 3+ in solution and metallic ion in its pure form. the solid oxide phases uh such as iron 3 oxide F23 each occupying a well- definfined zone governed by the prevailing potential and pH conditions. Therefore we can think that B diagram as a phase diagram in which uh instead of temperature and pressure the axis are potential versus pH. The axis are potential and pH. But how are the phase boundary lines of your phob diagram derived? The line indicated by letter A where you can see that here line A it's a horizontal line right so A represents the equilibrium between in 0 and in2 plus ions. Whereas the line indicated by the letter B where you can see that represents equilibrium between iron 2 plus and iron 3 plus ions. Both these equilibrium reactions are pure redux in nature where you can see that it's if you alter these horizontal line uh certainly the potential uh difference is there. Hence you can say that these horizontal lines are potential dependent and pure redux in nature. The potential of these lions can be expressed from the nonst equation. In both cases, uh we find that the potential is independent of pH as no protons are involved in these uh reactions only they uh you know depend on the only potential. So we can use this uh nun equation in order to uh find out the potential dependence.

[00:06:05]Right? So now therefore the lines A and B are horizontal. The line indicated by letter C where you can see that C represents the equilibrium between 3+ and iron 3 plus oxide where this is the corrosive region whereas this is a passive region. uh yet both the ion species are in the same oxidation state that is nothing but uh 3+ the oxidation number of ion does not change and this reaction is an acid base dependent where the uh vertical line clearly indicates it's pH dependent by altering the pH you can in fact move from corrosion to passivation or passivation to corrosion corrosion for a given system it's uh here what we discuss is ion therefore it is represented by a line that uh does not depend on the potential and is vertical all other lines in the for diagrams have a certain slope uh apart from the horizontal as well as vertical a bc what we have seen uh now also we see that slope the slope uh this region that is d line represented by d it's clear that uh they refer to reactions in which both electrons and protons are transferred and the corresponding potentials uh are function of pH by applying n equation the pH dependence of these potentials can be obtained by using the given equation and uh however uh we also discussed different regions and line that's equilibrium etc the next slide Okay, having discussed about the brief about the different species present in the phob diagram. Uh the phob diagram of iron and as well as uh that of other metals can be simplified by indicating only three regions uh such as immunity, corrosion and passivation. As you can see here clearly this protection region is nothing but uh immunity region where iron remains in intact. This region is the corrosive region where Fe2 plus and Fe3+ are the two solid species present in it. And the other one is a passivation region where iron oxide present.

[00:08:34]And uh in the immunity region uh the stable phases is the metal that is F0.

[00:08:41]uh in our uh case metallic iron under these conditions it cannot corrode due to a negative potential where you can clearly see that this environment is the electronrich despite the uh environment is quite acidic it's quite uh prone for the corrosion however due to the negative potential in this area zone uh ion is protected therefore you can see the cathodic protection is taking place and the next one is a corrosion region marks the thermodynamic uh conditions that favor the oxidation because the potential is moving slightly to the positive side and uh that soluble products would tend to form for iron.

[00:09:23]This means that stable phases that form under these conditions are dissolved ionic species uh especially Fe2 plus or F3 plus ions in solution. And when it comes to the move to the uh passivation region, the thermodynamically stable phases take the form of insoluble metal oxide. Uh under these condition, the oxidation products can deposit as a thin film over a metal surface. Uh it's a tiny invisible one. Uh potentially acting as a barrier against a further corrosion. However, uh one should notice that the degree of protection uh this film offers is strongly influenced by two factors. How proh porous the oxide layer is. We have already seen uh in our earlier PPT uh that's filling pen ratio based on that one can effectively say whether the corrosion is porous or non-porous. Therefore here you can say that how effectively it bonds to the underlying metal surface. Uh if the volume of the oxide film is lesser than the consumed part then that could be uh porous in nature. Whereas the volume of oxide form is greater than the metal consumed then it could be non-porous uh in nature. A closer examination of the phobi diagram for iron reveals that within the region of water stability iron exist if you look at the two parallel lines that is nothing but uh region of water stability. Uh iron iron exist either as dissolved ionic species or as an oxide like Fe2 plus or F3+ or iron oxide. never in its pure metallic form where you can see that in a pure metallic form there you don't see the uh water stability zone. This indicates that iron is inherently unstable when in contact with water. Nevertheless, the specific outcome varies with the prevailing conditions. Iron may either undergo active corrosion or uh transition into a passivated state depending on the electrochemical environment that we can see that while Popa diagram are valuable tools for mapping the thermodynamic stability of various phases they offer no insight into the actual rates at which corrosion reaction may proceed. In other words, thermodynamics favorability does not necessarily translate into observable reaction kinetics. A distinction that is critical to understanding real world corrosion behavior. So overall if you look into that uh if you carefully notice there are three different regions in the given by diagram. One is the protection region where iron remains intact. And uh when you move to the corrosion region where negative potential in moving into positive one in2 plus in the slightly positive potential and the more positive potential it becomes in3 plus and also in the passivation region where thin film of metal oxide formation takes place. Uh that's what we call it as passivation and the thin metal oxide layer formation on the surface of the metal that either it can uh slow down the corrosion or depends on the uh kind of nature of the oxide film formed on the metal surface.

[00:13:11]It could be a porous or non-porous uh we can clearly understand from the filling bedwars rule. However, there could be even other points one needs to remember how to protect uh or how to uh ensure uh which species would be present that is uh redux species. If iron we when we want to protect it then we need to certainly uh maintain negative potential despite there is a acidic or basic environment and also when we that that clearly indicated by the uh horizontal line when it comes to the vertical line that's clearly uh pH dependent and slope is indep uh dependent on both protons as well as the electrons.

[00:14:03]In summary, uh if you carefully look at the phob diagram of ion in aqua system, the horizontal line between in3+ and in2 plus regions represents the reaction uh that is the F3+ will get reduced to F2 plus or vice versa which has a standard potential of plus.77 volt. Uh while we could use standard potential for all these lines in practice phob diagrams are usually plotted for lower ion concentrations that is often 1 mill that are more relevant to corrosion and electrochemical experiments. uh areas in the phobia diagram uh as you have seen earlier marks uh the regions where a single species either F2+ or F3+ present in the corrosive region or in the passivation region like iron oxide etc is stable most stable species tend to occupy larger areas the lines uh between two uh faces mark the two species exist in equilibrium pure redux reactions are horizontal lines whereas pure acid base reactions are vertical lines and the reactions that are both acid base and the redux have the slope.

[00:15:24]Thank you.