Corrosion anodic inhibition

Batteries. Many batteries intended for household use contain mercury or mercury compounds. In the form of red mercuric oxide [21908-53-2] mercury is the cathode material in the mercury—cadmium, mercury—indium—bismuth, and mercury—zinc batteries. In all other mercury batteries, the mercury is amalgamated with the zinc [7440-66-6] anode to deter corrosion and inhibit hydrogen build-up that can cause cell mpture and fire. Discarded batteries represent a primary source of mercury for release into the environment. This industry has been under intense pressure to reduce the amounts of mercury in batteries. Although battery sales have increased greatly, the battery industry has aimounced that reduction in mercury content of batteries has been made and further reductions are expected (3). In fact, by 1992, the battery industry had lowered the mercury content of batteries to 0.025 wt % (3). Use of mercury in film pack batteries for instant cameras was reportedly discontinued in 1988 (3). 

Copper Corrosion Inhibitors. The most effective corrosion inhibitors for copper and its alloys are the aromatic triazoles, such as benzotriazole (BZT) and tolyltriazole (TTA). These compounds bond direcdy with cuprous oxide (CU2O) at the metal surface, forming a "chemisorbed" film. The plane of the triazole Hes parallel to the metal surface, thus each molecule covers a relatively large surface area. The exact mechanism of inhibition is unknown. Various studies indicate anodic inhibition, cathodic inhibition, or a combination of the two. Other studies indicate the formation of an insulating layer between the water surface and the metal surface. A recent study supports the idea of an electronic stabilization mechanism. The protective cuprous oxide layer is prevented from oxidizing to the nonprotective cupric oxide. This is an anodic mechanism. However, the triazole film exhibits some cathodic properties as well. 

One of the methods used in corrosion control is anodic inhibition. The method applies in particular to iron and its steels. The electrode is moved in the anodic direction (at first stimulating the corrosion rate), but soon an oxide film forms and reduces the dissolution current. There are certain types of oxide film, passive films, that are particularly protective. Indeed, such films are involved in the way metals preserve themselves in nature. There is much to be found out about these films (why they are so protective) and some of the material that allows us to understand them and their eventual breakdown by aggressive ions such as chloride, has been given in this chapter. 

Overall, these results indicate that chromates inhibit corrosion by elevating the pitting potential on aluminum with respect to the corrosion potential, which decreases the probability for the formation of stable pits. In general a chromate chloride concentration ratio in excess of 0.1 is necessary to observe significant anodic inhibition. 

A Catalytic Mechanism of Anodic Inhibition in Metallic Corrosion... 

B. H. Cartledge Oak Ridge National Lab.) While I appreciate Huddle and Anderson s (Lecture 41) sympathetic reference to any work on inhibition by the pertechnetate ion, I must record any objection the interpretation of any views which they present. The processes of corrosion and inhibition are usually too complex to be related uniquely to single anodic and cathodic reactions. Thus, in aerated aqueous systems containing one of the inhibitors of the X04 type, there are at least three possible cathodic processes namely, reduction of oxygen, of hydrogen ions, or of the inhibitor anion, if it is reducible. The kinetics, as well as the thermodynamics, of the several reactions, determines which of them will predominate in any particular case. 

Anodic inhibition of stress corrosion cracking was first provided on an industrial scale in a large plant for the production of hydrogen by electrolysis of potassium hydroxide solution. After preliminary trials on a laboratory scale, the chemical industry s first anodically protected large-scale plant, a sodium hydroxide solution evaporator with a capacity of 142 t, was put into operation in 1968. Since then, plants have been equipped in the same way (Grafen 1971). 

Inhibition is a method of corrosion protection by molecules that are adsorbed on the metal surface and reduce the rate of either metal dissolution (anodic inhibition) or the rate of the... 

Corrosion in neutral environments is most often due to the reaction of dissolved oxygen with the metal. The risk of attack is therefore considerably reduced if one limits the access of oxygen to the metal surface (cathodic inhibition). Alternatively, one can diminish the rate of corrosion by bringing about passivation of the metal (anodic inhibition). Corrosion can also be reduced by inhibitors that adsorb onto the surface and block the reactive sites. 

The corrosion potential of carbon steel is shifted in the anodic direction by the addition of HEDP due to the anodic inhibition effect of the phosphonate. In the presence of Zn " or Ca ", the corrosion potential tends to shift in the cathodic direction. By combined application of HEDP and or... 

Molybdate pigments are calcium or zinc salts precipitated onto an inert core such as calcium carbonate [47,96-98], They prevent corrosion by inhibiting the anodic corrosion reaction [47]. The protective layer of ferric molybdate, which these pigments form on the surface of the steel, is insoluble in neutral and basic solutions. 

Aluminum Batteries. Aluminum is another attractive anode material with a high theoretical energy density, but problems such as polarization and parasitic corrosion have inhibited the development of a commercial product. It, too, is being considered for a number of applications, with the best promise as a reserve or mechanically rechargeable battery (see Chaps. 9 and 38). 

Brunet and Turluer made an extensive evaluation of corrosion and inhibition in EDTA solutions containing citric acid and/or hydrazine. Inhibitors tested include benzotriazole (BZT), benzimidazole (BZI), BZI with gelatin, and a commercial inhibitor. All of these inhibitors reduce the corrosion of several types of steels. When the steel is coupled to magnetite, the potential is shifted in the anodic direction. With a large enough ratio between magnetite and bare steel, most of the inhibitors desorb. Only the commercial inhibitor was effective at a pH of 9 in the presence of magnetite. 

Figure 2.2 Principle of anodic inhibition. When no anodic inhibitor has been added, the corrosion current (rate) is h. When an anodic inhibitor is added, the anodic potential is modified and rotates up so that the corrosion rate is reduced (I2)...

In many early studies iron dissolution was investigated as part of corrosion and inhibition at open-circuit potential imder an Rj-saturated atmosphere, no accoimt being taken of the interference of the Hj/H reaction with the Tafel slopes [63,64], Even under an inert atmosphere, the anodic parameters may be affected by slow hysteresis phenomena resulting in steeper Tafel lines [65-69],... 

Anodic inhibition shows an increase in the polarization (a large potential change results from a small current flow) of the anode in the presence of an anodic inhibitor. Addition of the inhibitor causes the corrosion potential to shift in a cathodic direction. 

Given some basic information about the corrosion inhibition mechanisms of chromates, many studies have been conducted for chromate replacements. For effective replacement of hexavalent Cr, however, an inhibitor has to inhibit the oxygen reduction reaction as well as anodic dissolution/pitting, and several studies indicate that hybrid formulations seem to be the best way to do just that. Typically, in these hybrid formulations an organic oxygen reduction reaction inhibitor is included with environmentally benign anodic inhibiting anions. 

Methods that have evolved over the years for combating corrosion include inhibition, material selection, cathodic or anodic protection coating and control of the environment. 

This is essentially a corrosion reaction involving anodic metal dissolution where the conjugate reaction is the hydrogen (qv) evolution process. Hence, the rate depends on temperature, concentration of acid, inhibiting agents, nature of the surface oxide film, etc. Unless the metal chloride is insoluble in aqueous solution eg, Ag or Hg ", the reaction products are removed from the metal or alloy surface by dissolution. The extent of removal is controUed by the local hydrodynamic conditions. 

Calcium carbonate (calcite) scale formation in hard water can be prevented by the addition of a small amount of soluble polyphosphate in a process known as threshold treatment. The polyphosphate sorbs to the face of the calcite nuclei and further growth is blocked. Polyphosphates can also inhibit the corrosion of metals by the sorption of the phosphate onto a thin calcite film that deposits onto the metal surface. When the polyphosphate is present, a protective anodic polarization results. 

Alternatively, there are also iuhibitors that decrease the rate of hydrogen generation and thus decrease corrosion. Mercury, effective at inhibiting 2iac corrosion, has long been used as an additive to 2iac anodes. More recentiy, however, because of iacreased iaterest ia environmental issues, the amount of mercury ia alkaline cells has been reduced. 

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