Galvanic prediction

The galvanic series in Table 16.1 is generally useful for predicting the tendency toward galvanic corrosion between coupled metals. The arrangement of this series is, however, based on data generated under controlled laboratory conditions on clean, bare metals. 

Zinc is often used as a protective coating over iron to form galvanized iron. In industrial settings exposed to SOj and humidity, this zinc coating is subject to sufficient corrosion to destroy its protective capacity. Haynie and Upham (5) used their results from a zinc corrosion study to predict the useful life of a zinc-coated galvanized sheet in different environmental settings. Table 9-2 shows the predicted useful life as a function of SO concentration. 

Predicted Useful Life of Galvanized Sheet Steel with a 53-/iRi Coating at an Average Relative Humidity of... 

Astley, D. J., Prediction of Galvanic Corrosion in Marine Heat-exchangers , Institute of Metals Conference, Bristol (1986)... 

There are in addition several other factors that accelerate corrosion and must betaken into account these include crevices, galvanic coupling, tensile stress, aeration, presence of impurities, surface finish, etc. If these were also taken into consideration then several million experiments would have to be performed to compile such data. There are many instances where two or more chemicals exert a marked synergistic action such that low dissolution rates obtained in either environment become much greater in the presence of both. Further, the corrosiveness of a chemical will be affected by the presence of certain impurities, which may act as either accelerators or inhibitors. To take all these factors into account would add to an already impossible task and as Evans has remarked, There are not enough trained investigators in the world to obtain the empirical information to cover all combinations of conditions likely to arise . Unfortunately corrosion science has not yet reached the stage where prediction, based on a few well established laws, allows selection of materials to be made without recourse to a vast amount of data. 

Because of the many variables which can influence the corrosion reaction, the use of the e.m.f. series of metals to predict the behaviour of galvanic couples in a given service environment can be hazardous and misleading. Numerous examples of coatings expected to act cathodically which have, in fact, been anodic have been reported in the literature and specialised lists of galvanic couples in different environments have been compiled. ... 

Guide for development and use of a galvanic series for predicting galvanic corrosion performance... 

Predict the standard emf of each of the following galvanic cells ... 

Predict the standard cell emf and calculate the standard reaction Gibbs free energy for galvanic cells having the following cell reactions ... 

Predict whether the cell potentials of galvanic cells depend on the electrodes and electrolytes in the half-cells. Give reasons for your prediction. 

Predict any other factors that you think might affect the voltage of a galvanic cell. Describe an investigation you could complete to test your prediction. 

You know that galvanic cells have positive standard cell potentials, and that these cells use spontaneous chemical reactions to produce electricity. You also know that electrolytic cells have negative standard cell potentials, and that these cells use electricity to perform non-spontaneous chemical reactions. Thus, you can use the sign of the standard cell potential to predict whether a reaction is spontaneous or not under standard conditions. 

Predict the standard cell potential and calculate the standard free energy for the following galvanic cells (the standard potentials of these cells were obtained in Exercise 12.19). 

Many other parameters tend to influence the corrosion of metals immersed in sea water. When two metals of different potentials are galvanically coupled, the acceleration of the attack on the less noble metal of the two is observed frequently. A small area of an anodic metal coupled to a large area of a second metal that is cathodic can be particularly dangerous. A useful guide to help predict unfavorable combinations is the galvanic series of metals in sea water (0). The reverse situation—namely, a small cathode coupled to an anode that is large in area—often proves satisfactory in service. 

Electrochemistry electrolytic and galvanic cells Faraday s laws standard halfcell potentials Nernst equation prediction of the direction of redox reactions... 

E. Polarization Curve Measurements for Galvanic Corrosion Prediction... 

One example of the application of polarization curves in a predictive manner involves their use in galvanic corrosion. Galvanic corrosion occurs when two dissimilar metals are in electrical and ionic contact as is schematically shown in Fig. 29. Galvanic corrosion is used to advantage in sacrificial anodes of zinc in seawater and magnesium in home water heaters. It slows corrosion of millions of tons of structural materials. The darker side of galvanic corrosion is that it also causes major failures by the accelerated dissolution of materials that are accidentally linked electrically to more noble materials. 

Figure 34 The steps involved in determining the depth of container wall penetration under Canadian nuclear waste disposal conditions using data obtained in an electrochemical galvanic coupling experiment. (A) Crevice propagation rate (R cc Ic) as a function of temperature (T) (B) RCc as a function of 02 concentration [02] (C) calculated evolution of container surface temperatures and vault 02 concentrations with time in the vault (D) flux of 02 (Jo2) to the container surface as a function of time (E) predicted evolution of Rcc up to the time of repassivation (i.e., at [02]p) (F) total extent of crevice corrosion damage expressed as the total amount of 02 consumed (Q) up to the time of repassivation (G) experimentally determined maximum depth of wall penetration (Pw) as a function of 02 consumed (Q) (H) predicted maximum value of Pw up to the time of repassivation (fP)-...

ASTM G82-83, Standard Guide for Development and use of a Galvanic Series for Predicting Galvanic Corrosion Performance, Philadelphia, PA, 1983. 

Avoid contact between metals with different potentials. Complete isolation of one from the other should be made and inspected periodically. To predict or avoid galvanic corrosion, the potential of every metal or conductor should be evaluated as a function of time in the intended medium. The variation of the properties at the interface should be carefully evaluated as a function of time. 

Prediction of the anodic and cathodic components of the galvanic cell and inversion of polarity of the cell should be considered. 

REDOX REACTIONS IN GALVANIC CELLS When discussing oxidation-reduction reactions we have not mentioned ways in which the directions of such reactions can be predicted. In other words, discussions in the previous chapters were aimed at understanding how oxidation-reduction reactions proceed, but there was no mention of why they take place. In this and the next few sections the problem will be dealt with in some detail. 

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