Prevention of Galvanic Corrosion

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

The concentration of inhibitor used for prevention of galvanic corrosion in pipe systems should be increased, as compared with protection of single metal. 

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. 

The cost of CP systems varies depending on the type of system used. In this regard, Virginia DOT has published a report entitled, Evaluation of Anodes for Galvanic Cathodic Prevention of Steel Corrosion in the Prestressed Concrete Piles in Marine Environments in Virginia (30). The data in this report and the data in the literature published by Virmani (2) suggest that the sprayed Al-Zn-In alloy or the zinc-hydrogel systems with a life of 10-20 years cost 108-129/m. ... 

Galvanic corrosion can usually be prevented by reasonable means (see below). Nevertheless, more or less serious cases of galvanic corrosion occur now and then. A drastic example concerns a safety valve on a foam tank with compressed air. Normally, the valve should open at a pressure somewhat above six bars. But because the valve ball material was low-alloy steel, and the valve housing was made of brass and other parts of martensitic stainless steel, the ball rusted and stuck to the seat. Together with other occasional malfimctions, this caused the tank to explode. Cases mentioned earlier (pp. 99 and 100) also represent examples of failure experienced in real life. 

The most frequently used materials for buried metal structures are the carbon steels. For prevention of their corrosion the most recommended, economical, and effective method is cathodic protection (CP). The use of CP is now standard procedm-e for long-term corrosion protection of imderground pipelines, oil and gasoline tanks, and other structures. With a shift of the metal potential to more of a negative value of -0.85 V versus a C11/CUSO4 reference electrode, it is possible to make the metal surface a cathode, which ensures an immune (no corrosion) state of the carbon steel. Cathodic polarization is achieved by direct current, which can be supplied either by sacrificial anodes in galvanic contact with the steel structure, or by impressed current from a rectifier. 

One practical way of preventing possible galvanic corrosion is to insulate the two metals in contact from one another as carefully as possible [17]. This is achieved by placing a high ohmic resistance, i.e. an insulating material (Figure B.3.3) such as neoprene or any other suitable polymer, between them. 

These tests as well as experience shows that any contact between aluminium and steel (such as reinforcements) should be avoided in concrete. Neither anodising nor painting prevents the risk of galvanic corrosion. 

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. 

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. 

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