Current galvanic

Dust-tight electrical systems should be used in areas where activated carbon is present, particularly powdered products (66). When partially wet activated carbon comes into contact with unprotected metal, galvanic currents can be set up these result in metal corrosion (67). 

Hard plating is noted for its excellent hardness, wear resistance, and low coefficient of friction. Decorative plating retains its brilliance because air exposure immediately forms a thin, invisible protective oxide film. The chromium is not appHed directiy to the surface of the base metal but rather over a nickel (see Nickel and nickel alloys) plate, which in turn is laid over a copper (qv) plate. Because the chromium plate is not free of cracks, pores, and similar imperfections, the intermediate nickel layer must provide the basic protection. Indeed, optimum performance is obtained when a controlled but high density (40—80 microcrack intersections per linear millimeter) of microcracks is achieved in the chromium lea ding to reduced local galvanic current density at the imperfections and increased cathode polarization. A duplex nickel layer containing small amounts of sulfur is generally used. In addition to... 

Separated Anode/Cathode Realizing, as noted in the preceding, that locahzed corrosion is usually active to the surrounding metal surface, a stress specimen with a limited area exposed to the test solution (the anode) is elec trically connec ted to an unstressed specimen (the cathode). A potentiostat, used as a zero-resistance ammeter, is placed between the specimens for monitoring the galvanic current. It is possible to approximately correlate the galvanic current 7g and potential to crack initiation and propagation, and, eventually, catastrophic fail-... 

Sufficient temperature differences between sites on the same component can cause a galvanic current flow. In these cases, the site with the higher temperature is usually the corrosion site (see Case History 16.3). Galvanic corrosion of this form can potentially affect heat exchangers and refrigeration equipment. 

The attraction of rubbed amber and some other effects of electricity were known in ancient times. We know from finding nails in an old wreck that the Romans knew about contact corrosion combined with electric current flow. A skin of lead as a protection against boring worms covered the wooden planks of the ship and was nailed down with copper nails. Galvanic coupIe.s formed between the lead and the copper nails and the less noble lead sheets around the nails corroded in the seawater and fell off. The shipbuilders discovered a simple solution and covered the heads of the copper nails with lead as well. Galvanic current flow between the two metals was eliminated and corrosion was prevented (26). 

Stray currents from foreign sources are to be regarded in the same way as galvanic currents. The explanations for Eq. (4-11) are relevant. Protective measures afed cribed in Chapters 9 and 15. 

There are several plastic coatings available commercially, some being more effective than others. However, the selected plastic coating must possess flexibility, resistance against impact, chemical attack, flow of galvanic currents and... 

The corrosion potentials of the two metals in the environment under consideration will determine the direction of the transfer of electrons, but will provide no information on the rate of electron transfer, i.e. the magnitude of the galvanic current. Thus if E an.. is more positive than corr..B thc transfer of electrons will be from to with a consequent increase in the corrosion potential (more positive) of and a decrease in that of A/ the corrosion rate of will consequently increase and the corrosion rate of A/ will decrease compared with the rates when the metals... 

Fig. 1.62 Potential/current curves for a metal polarised (a) cathodically and (b) anodically. The horizontal intercepts xy, x y, x"y" with AB and CB represent the local cell currents respectively, and yz, y z, y z" the externally applied currents (cathodic and anodic). In bimetallic corrosion yz, y z, etc. will be the galvanic current /gjjv, flowing from to (see...

Accordingly, a bare copper surface is soon presented to the electrolyte. Aluminium, on the other hand, maintains an oxide-covered surface under these conditions, and it is evident from Fig. 1.64, which is constructed from the work of Pryor and Keir , that the reduction of dissolved oxygen is highly polarised and severely limits the galvanic current flow. Aluminium is... 

The existence of oxide films of varying resistance on the surface of a cathode results in a corresponding variation of galvanic current flow. This... 

It is significant that most of the data from which a remarkable uniformity of attack is deduced are derived from small isolated panels. This is the most convenient form of specimen for measurements of corrosion rates by loss of weight but it eliminates the important effect of galvanic currents passing between remote parts of a large structure. It is believed that the experience of civil engineers and other users would not support the conclusion suggested by panel tests that corrosion is no faster in tropical than in temperate waters. 

Galvanic effects If niobium is cathodic in a galvanic couple the results can prove disastrous because of hydrogen embrittlement. If niobium is the anode in such a couple it anodises so readily that no damage occurs and the galvanic current drops to a very low value due to the formation of an anodic oxide film. 

To protect buried metals from premature breakdown it must suffice to say that protective coatings and other methods must be applied against factors such as the effects of galvanic currents, composition of the moisture in the ground, humus acids, bacteria, etc. (See Section 14.8.)... 

In many cases it will suffice to include in the circuit a shunt of appropriately low resistance over which 7/ -drop potential measurements can be made for ready calculation of the magnitude of the current flow. This technique permits measurements to be made as required without opening the circuit even momentarily for the introduction of current-measuring devices. It is also possible to arrange instruments in a circuit so that no measuring resistance is introduced in the galvanic current circuit . 

Galvanic current Measurement of the galvanic current between two different metals can be easily measured using a zero resistant ammeter ". This method can have specific application, e.g. to provide a signal indicating failure of a protective coating in a process vessel. Commercial probes are available for industrial monitoring. 

With their rest potential much higher than that of the steel medium listed in Table 1.6, sulphide minerals act as a cathode while the steel medium acts as an anode. During grinding, minerals and grinding media (steel) come in repeated contact with each other, and galvanic current flows between the two surfaces of the sulphide minerals may be altered. 

Galvani, Luigi (1737—1798). An Italian physician and anatomist who discovered the galvanic current in the course of working with frog muscles... 

If a galvanic current is conducted through acid or neutral aqueous solutions of aniline, there is formed at the positive... 

---