Galvanic corrosion, prevention

Corrosion prevention—material selection, corrosion inhibition, cathodic protection, galvanic corrosion prevention, and extensive nondestructive and destructive inspection protocols. ... 

It is obviously not appropriate to rely on possible anodic or cathodic polarisation phenomena in order to limit the damage caused by galvanic corrosion. Prevention is necessary at the assembly design stage. 

Because the corrosion resistance of lead and lead alloys is associated with the formation of the protective corrosion film, removal of the film in any way causes rapid attack. Thus the velocity of a solution passing over a surface can lead to significantly increased attack, particularly if the solution contains suspended particulate material. Lead is also attacked rapidly in the presence of high velocity deionised water. The lack of dissolved minerals in such water prevents the formation of an insoluble protective film. In most solutions, lead and lead alloys are resistant to galvanic corrosion because of the formation of a nonconductive corrosion film. In contact with more noble metals, however, lead can undergo galvanic attack which is accelerated by stray electrical currents. 

D. L. Hawke, J. E. HiUis, and W. Unsworth, Preventive Practice for Controlling the Galvanic Corrosion of Magnesium Alloys, International Magnesium Association, McLean, Va., 1988. 

The most serious form of galvanic corrosion occurs in cooling systems that contain both copper and steel alloys. It results when dissolved copper plates onto a steel surface and induces rapid galvanic attack of the steel. The amount of dissolved copper required to produce this effect is small and the increased corrosion is difficult to inhibit once it occurs. A copper corrosion inhibitor is needed to prevent copper dissolution. 

The nature of galvanic corrosion is such that successful avoidance generally requires implementing preventive rather than corrective techniques. Therefore, consideration of galvanic corrosion problems must be integrated into the design of equipment. Corrective techniques applied to existing equipment can be expensive and less than satisfactory. 

Consideration of the basic elements characteristic of the galvanic corrosion process, as discussed above, points to the principles of sound preventive techniques. Since a galvanic potential difference is the driving force for corrosion reducing the magnitude of this difference can reduce or prevent galvanic corrosion. 

Aluminum components are sensitive to ions of heavy metals, especially copper. To avoid localized galvanic corrosion of the aluminum by metallic copper reduced from copper ions, care must be exercised to prevent heavy metal ions from entering aluminum components. Note the recommendations under Elimination. ... 

The basic mechanisms involved in graphitic corrosion are familiar and easily understood. Hence, remedial and preventive measures are relatively simple to implement. Although commonly categorized as a form of dealloying, graphitic corrosion has much in common with galvanic corrosion. 

To prevent or reduce galvanic corrosion we can employ several techniques. Any one of these techniques may be used either by itself or in combination of two or more of the techniques. These techniques are as follows ... 

Bimetallic corrosion in atmospheres is confined to the area of the less noble metal in the vicinity of the bimetallic joint, owing to the high electrolytic resistance of the condensed electrolyte film. Electrolytic resistance considerations limit the effective anodic and cathodic areas to approximately equal size and therefore prevent alleviation of atmospheric galvanic corrosion through strict application of the catchment area principle. 

The coating is, in addition, useful in preventing galvanic corrosion . Plated on steel which is to be used in contact with aluminium alloys, it protects the steel and does not stimulate the corrosion of the light alloy and is itself not consumed as rapidly as a 100% zinc coating. 

The most common overcoats, however, are sputtered carbons. Their role in disk corrosion has been described in contradicting ways. Whereas Garrison [141] clearly observed that carbon, like Rh, can enhance galvanic corrosion, Smallen et al. [131] believe that carbon decreases corrosion by preventing lateral growth of corrosion products. Results of similar tests are sometimes contradictory Nagao et al. [145] have shown an improvement of the corrosion resistance of carbon-coated CoCr alloys on T/H test (with either SOz gas or NaCl mist), whereas Black [146] finds that pyrolitic carbon over a CoCrMo alloy results in elevated corrosion rates. 

Blade castings can also be produced from brass and stainless steel, but their use is generally limited because of their relatively high cost and weight. If the blade shanks and hub are of dissimilar metals, they should be insulated from each other to prevent galvanic corrosion from occurring. 

Minimizes or prevents galvanic corrosion between dissimilar metals. 

Figure 1.42 Insulating material between dissimilar metals, such as copper and aluminum prevents galvanic corrosion...

The usual preventive measures79 in the galvanic corrosion are (i) use of indoor and shelters for outdoor exposure (ii) seal faying surfaces (iii) proper design for joining... 

Prevention of Galvanic Corrosion. Galvanic corrosion can be prevented or reduced by ... 

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