Corrosion reactions, suppression

Based on the polarization curves of figure C2.8.4 tliere are several possibilities for reducing or suppressing tire corrosion reaction. The main idea behind every case is to shift tire corroding anode potential away from E. This can be done in tire following ways. 

Chromates are excellent inhibitors of oxygen reduction in near neutral and alkaline solutions. In these environments, they can stifle corrosion by suppressing this cathodic partial reaction. The inhibition mechanism appears to involve reduction of Cr(VI) to Cr(III) at a metal surface and formation of Cr(III)-0-substrate metal bonds (38). This surface complex is likely to be substitutionally inert and a good blocker of oxygen reduction sites, as suggested by the exceedingly small water exchange rate constant for the first coordination sphere of Cr3+ (39). 

Evolution of hydrogen can be suppressed by addition of appropriate inhibitors to the negative active material. Then, the amount of water lost will be equal to the amount of oxygen involved in the corrosion reaction. Since the corrosion current is too low, VRLA batteries will need no maintenance for years of service. 

Anodic inhibitors - suppressing the anodic corrosion reaction. 

The reinforcement network then becomes cathodic and corrosion is suppressed. However, another cathodic reaction can occur if the potential gets too negative ... 

The basic principle of all CP techniques is that the unwanted anodic corrosion reactions are suppressed by the application of an opposing current forcing the local anodes to be polarized to the potential of the local cathodes therefore stifling corrosion cells. If less than this amount of cathodic current is supplied some corrosion would still occur, but the level of corrosion would be less without any CP. From a thermodynamics point of view, the application of a CP current basically reduces the corrosion rate of a metallic structure by reducing its corrosion potential toward its immune state (see Figs. 4.14 and 4.17). The two main methods of achieving this goal are by either ... 

Natural waters. The corrosivity of natural waters depends on their constituents, such as dissolved solids, gases, and sometimes colloidal or suspended matter. The effects may either stimulate or suppress the corrosion reaction. Constituents or impurities in water include dissolved gases such as oxygen, COj, SOj, NHj, HjS, some of which are the result of bacterial activity. Dissolved mineral salts are mostly calcium, magnesium sodium, bicarbonate, sulfate, chloride, and nitrate. The effect of each of these ions on corrosion rate is different, but the chlorides have received the most study in this regard. Organic contaminants of water can directly affect the corrosion rate of metals and alloys. Bacteria, under optimum conditions can double their number in 10-60 minutes. This characteristic is typical of the widespread biodeterioration caused by microbes in aU indnstries, of which corrosion is a special case. With a few exceptions such as synthetic polymers, all materials can be attacked by bacteria. 

The most favorable conditions for equation 9 are temperature from 60—75°C and pH 5.8—7.0. The optimum pH depends on temperature. This reaction is quite slow and takes place in the bulk electrolyte rather than at or near the anode surface (44—46). Usually 2—5 g/L of sodium dichromate is added to the electrolysis solution. The dichromate forms a protective Cr202 film or diaphragm on the cathode surface, creating an adverse potential gradient that prevents the reduction of OCU to CU ion (44). Dichromate also serves as a buffering agent, which tends to stabilize the pH of the solution (45,46). Chromate also suppresses corrosion of steel cathodes and inhibits O2 evolution at the anode (47—51). 

Metals immersed or partly immersed in water tend to corrode because of their thermodynamic instability. Natural waters contain dissolved solids and gases and sometimes colloidal or suspended matter all these may affect the corrosive projjerties of the water in relation to the metals with which it is in contact. The effect may be either one of stimulation or one of suppression, and it may affect either the cathodic or the anodic reaction more rarely there may be a general blanketing effect. Some metals form a natural protective film in water and the corrosiveness of the water to these metals depends on whether or not the dissolved materials it contains assist in the maintenance of a self-healing film. 

Fig. 12.4 Corrosion diagram for a zinc diecasting in a nickel plating bath, pH 2-2. There are two possible cathodic reactions, hydrogen evolution (H) and nickel ion reduction (AO. The corrosion current is the sum of the partial cathode currents. Even with live entry the potential is still too high to suppress corrosion, though the rate is reduced to...

In order to inhibit corrosion, it is necessary to stop the flow of current. This can be achieved by suppressing either the cathodic or the anodic reaction, or by inserting a high resistance in the electrolytic path of the corrosion current. These three methods of suppression are called cathodic, anodic znA resistance inhibition respectively (Section 1.4). 

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. 

Both batteries and fuei cells utilize controlled chemical reactions in which the desired process occurs electrochemically and all other reactions including corrosion are hopefully absent or severely kinetically suppressed. This desired selectivity demands careful selection of the chemical components including their morphology and structure. Nanosize is not necessarily good, and in present commercial lithium batteries, particle sizes are intentionally large. All batteries and fuel cells contain an electropositive electrode (the anode or fuel) and an electronegative electrode (the cathode or oxidant) between which resides the electrolyte. To ensure that the anode and cathode do not contact each other and short out the cell, a separator is placed between the two electrodes. Most of these critical components are discussed in this thematic issue. 

Corrosion can be controlled by Isolation of the metal from the corrosive environment by suppression of the anodic dissolution of metal and by suppression of the corresponding cathodic reaction. Isolation of corrosion prone metals from corrosive environments is probably the most general mechanism of the corrosion protection afforded by paint films, sealers, and similar polymer-based materials. Effective isolation requires that polymeric materials have good barrier properties and remain adherent in the presence of water and the products of metallic corrosion. Barrier properties and adhesion aspects of corrosion control are discussed in detail in subsequent sections. 

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