Corrosion acid theory

Carbon dioxide was once thought essential for the rusting of ferrous metals (viz. the carbonic acid theory of rusting) but is now considered of relatively minor importance . However, basic zinc carbonate is frequently found in the corrosion products of zinc and small amounts of siderite (FeCOs) are found in ferrous rusts. 

The most important outcome of this theory is that the rate of dissolution should be potentially greater as the pH increases, which is in conflict with simple concepts of corrosion kinetics. However, the theory has been proved to be applicable to many systems, and BonhoeflFer and Heusler found that iron in sulphuric acid corroded at a greater rate with increase in pH, whilst Kabanov etal. found that it corroded faster in alkaline solution than in acid solution for the same electrode potential. 

The evidence at present available concerning the corrosion of high-silicon irons by sulphurous acid is insufficient to allow the formation of any theory about the mechanism by which the silica film barrier is broken down in the presence of this acid. As far as is known, this acid is corrosive to all types of high-silicon iron. 

The second chapter is by Aogaki and includes a review of nonequilibrium fluctuations in corrosion processes. Aogaki begins by stating that metal corrosion is not a single electrode reaction, but a complex reaction composed of the oxidation of metal atoms and the reduction of oxidants. He provides an example in the dissolution of iron in an acidic solution. He follows this with a discussion of electrochemical theories on corrosion and the different techniques involved in these theories. He proceeds to discuss nonequilibrium fluctuations and concludes that we can again point out that the reactivity in corrosion is determined, not by its distance from the reaction equilibrium but by the growth processes of the nonequilibrium fluctuations. ... 

It next became interesting to investigate some chemical processes in which hydrogen overvoltage is intimately involved in order to see whether changes in these processes, produced by variations in the external pressure, are also in the direction predicted by the theory. Three such processes are (1) the corrosion of metals in acid solutions,... 

ACIDS AND BASES. The conventional definition of an acid is that it is an electrolyte that furnishes protons, i.e.. hydrogen ions. H+. An acid is sour to the taste and usually quite corrosive. A base is an electrolyte that furnishes hydroxyl ions, OH . A base is bitter to die taste and also usually quite corrosive. These definitions were formulated in terms of water solutions and. consequently, do not embrace situations where some ionizing medium other than water may be involved. In the definition of Lowry and Brnsted, an acid is a proton donor and a base is a proton acceptor. Acid-base theory is described later. 

ACTIVITY SERIES- Also referred to as the electromotive series or the displacement series, this is an arrangement of the metals (other elements can be included) in the order of their tendency to react with water and acids, so that each metal displaces from solution those below itiu the series and is displaced by those above it. See Table 1. Since the electrode potential of a metal in equilibrium with a solution of its ions cannot be measured directly, the values in the activity series are, in each case, the difference between the electrode potential of the given metal tor element) in equilibrium with a solution of its ions, and that of hydrogen in equilibrium with a solution of its ions. Thus in the table, it will be noted that hydrogen lias a value of 0.000. In experimental procedure, the hydrogen electrode is used as the standard with which the electrode potentials of other substances are compared. The theory of displacement plays a major role in electrochemistry and corrosion engineering. See also Corrosion and Electrochemistry. 

Civilization depends on the protection of metals, for most of them are unstable in normal environments unless they are protected by some kind of oxide film. The basic idea about the theory of corrosion is that the metal gets involved in a kind of local fuel cell in which it consumes itself. The partner to most of this self-dissolution is the deposition of hydrogen (favored in acid solutions) or the reduction of oxygen (favored in alkaline). Corrosion is measured in many ways, but the quick way in the laboratory is to move the potential a little bit away ( 5 mV) from the corrosion potential in both anodic and cathodic directions and measure the corresponding current. A simple equation takes the data from this type of measurement and produces the corrosion rate. 

Since these experiments were not carried out under ideally defined flow conditions the dependence of corrosion rate on flow rate will be discussed only in a qualitative manner. Under laminar flow conditions and mass transfer control one would have expected the corrosion rate to increase with the square root of the velocity while under turbulent conditions proportionality would prevail. However, in Fig.15 one finds that the corrosion rate varies approximately with the 0.2 to 0.3 power of the flow rate. It appears therefore that the observed dependence on the flow rate does not obey conventional mass transfer theory. A flow effect might be expected in uninhibited hydrochloric acid because hydrogen bubbles, formed on the surface of the metal, are faster and more easily removed at higher flow rates. While this argument could be applied in discussing Fig.15, we find in Fig.16 that the flow effect at similar corrosion rates is much less pronounced under deaerated conditions. We therefore have to conclude that the observed flow effect is not mechanical and cannot be related to pure mass transfer control either. In Fig.17, the flow dependence of the corrosion rate is shown for 2-butyne-l,4-diol in deaerated UN hydrochloric acid. Note that the corrosion rate appears to be noticeably affected only at the higher flow rates. Finally, in Fig.18, we observe that increased flow rate can either increase or decrease the corrosion rate in the presence of an inhibitor. This effect was observed reproducibly only in 6N hydrochloric acid with 2-butyne-l,U-diol under deaerated conditions for 0.2% and 0.1% inhibitor concentration. This behavior indicates that the corrosion rate is controlled by the superposition of two partial reaction rates each of which is mass transfer dependent to a certain extent. In terms of the model delineated in Table 6, it is suggested that the three-dimensional polymeric layer made up by inhibitor molecules is in fact a three-dimensional chelate made up of iron ions and inhibitor molecules. The corrosion rate is then... 

Applying mixed potential theory, the corrosion current and corrosion potential when 1 cm" Sn is coupled to 1 cm of Pt in an acidic solution are calculated by solving Eqs. (E6.1) and (E6.2). 

HX-f H20=F H30 -fX-The ion HaO is the oxonium ion (or hy-droxonium ion or hydronium ion). This definition of acids comes from the Arrhenius theory. Such acids tend to be corrosive substances with a sharp taste, which turn litmus red and give colour changes with other indicators. They are referred to as protonic acids and are classified into strong acids, which are almost completely dissociated in water (e.g. sulphuric acid and hydrochloric acid), and weak acids, which are only partially dissociated (e.g. ethanoic acid and hydrogen sulphide). The strength of an acid depends on the extent to which it dissociates, and is measured by its dissociation constant. See also base. 

Wherever particles are involved, the problem of fouling, or deposition on surfaces arises. In the case of heat exchangers for combustion gases, thermal transfer efficiency may be drastically reduced by the deposition of the relatively highly insulating soot and flyash particles. In other contexts sulfuric acid and other corrosive vapor droplets diffuse or impact upon conduit surfaces thereby shortening their useful lifetime [1.22,25]. In all of these cases, numerous questions of kinetic theory arise including all the phoretic forces. In addition, particle interaction forces are ultimately responsible for delivery to the surface in question. 

This evidence of the uptake of inhibitive anions into oxide films forms the basis of the chemical or pore plugging theory of inhibition, associated originally with Evans etal. In this theory the rdle of the inhibitive anion is to promote the repair of weak points or pores in the oxide film, where corrosion has started, by reacting with dissolving iron cations to form insoluble products of separate phase, which plug the gaps. These insoluble products may contain the inhibitive anion either as a salt, e.g. phosphate, or a basic salt, e.g. azeiate , or as an insoluble oxide, e.g. CrjOj from chromate Precipitation of such solid products is favoured if the pH in the region of the pores does not become acid. Thus, on the basis of this theory, inhibition by anions such as phosphate, borate, silicate and carbonate, is enhanced by their buffer properties which serve to prevent a fall in pH in the anodic areas. Since ferric salts are usually more insoluble than... 

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