Galvanic corrosion effects

Massive electrochemical attack known as galvanic corrosion [58,59] is the most severe form of copper corrosion. It can completely remove the copper from the structures (Figs. 17.25 and 17.26). It can occur when the wafers are exposed to a corrosive electrolyte for an extended period. It can also occur if the slurry does not contain enough or effective corrosion inhibitor. The source of such a galvanic potential on the patterned copper surface may be due to the fact that some copper structures connected to transistors have a different electrical potential than the rest of the wafer surface. Another possible cause of this type of galvanic potential is related to the barrier material induced metal metal battery effect. Most copper CMP slurries have been developed for Cu structures with Ta or TaN as a barrier material. In some cases, other metals may also be used in addition to the barrier metal. For example, a metal hard mask could contribute to the galvanic corrosion effects. It is also possible that some types of copper are more susceptible to corrosion that others. The grain... 

A word of warning is necessary in the use of these tables. The choice of materials by this method is a tentative one and should by no means be considered as the final selection. The presence of impurities or galvanic corrosion effects can give entirely different resulte than predicted by the tables. It is for this reason that the outlined plan shown on page 85 stresses laboratory testing of possible materials of construction in contact with process materials. A procedure for testing can be found in Perry s Chemical Engineers Handbook," 3d ed., p. 1458. 

It is occasionally desirable to evaluate galvanic corrosion effects from atmospheric exposures (see ASTM G 116, Conducting Wire-on-Bolt Test for Atmospheric Galvanic Corrosion and ASTM G 149, Conducting the Washer Test for Atmospheric Galvanic Corrosion). Several different types of sj)ecimens have been used for this purpose. One approach is... 

The in-service cable plant is tested to establish the presence or absence of stray direct currents, the galvanic corrosion effect of "foreign plants, the corrosivity of local environmental conditions, and the corrosive effect of long cells formed by the changes in the environment. The corrosion of the in-place plant is detected through potential surve5fs, current tests, soil resistivity tests, redox potential tests, and pH tests. 

Ivey D, Luo J, Ingrey S, Moore R, Woods I (1998) Galvanic corrosion effects in InP-based laser ridge structures. J Electron Mater 27 89-95... 

The plumbosolvency of a water supply is determined by the quality of the source water(s) and by water temperature. There are exceptions, such as lead leaching from brass and galvanic corrosion effects (see Chapter 1), but case studies indicate that generally the worst lead in drinking water problems relate to the presence of lead pipes. We can therefore focus on the interaction of water with lead pipes. 

It is important to realize that galvanic corrosion effects can be manifested not only on the macroscopic level but also within the microstructure of a material. Certain phases or precipitates will undergo anodic dissolution under microgalvanic effects. Because the principle of galvanic corrosion is widely known, it is remarkable that it still features prominently in numerous corrosion failures. Figure 7.17 illustrates the main factors affecting the formation of a galvanic cell [14]. 

Boyd J., Chang G., Webb W., Speak S., Gerth D., Reck B., Galvanic Corrosion Effects on Carbon Fiber Composites, 36th Intematiorud SAMPE Symposium, 1, vol. 36, San Diego, 1991, p. 1217-1231. 

The natural differences in metal potentials prodnce galvanic differences, such as the galvanic series in sea water. If electrical contact is made between any two of these materials in the presence of an electrolyte, current must flow between them. The farther apart the metals are in the galvanic series, the greater the galvanic corrosion effect or rate wfll be. Metals or alloys at the upper end are noble while those at the lower end are active. The more active metal is the anode or the one that will corrode. 

Control of galvanic corrosion is achieved by nsing metals closer to each other in the galvanic series or by electrically isolating metals from each other. Cathodic protection can also be used to control galvanic corrosion effects. 

Galvanic corrosion effects have also been observed and have caused unexpected failure of piping tankage and pressure vessels where the welds are anodic to the base metal. The following examples illustrate the point... 

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. 

Area effects in galvanic corrosion are very important. An unfavorable area ratio is a large cathode and a small anode. Corrosion of the anode may be 100 to 1,000 times greater than if the two areas were the same. This is the reason why stainless steels are susceptible to rapid pitting in some environments. Steel rivets in a copper plate will corrode much more severely than a steel plate with copper rivets. 

Copper alloys often show only weak crevice corrosion. This is especially the case if the copper alloy is coupled to a less noble alloy such as steel. The corrosion of the steel is stimulated by the galvanic effect caused by the coupling of dissimilar metals. Hence, the sacrificial corrosion of the steel protects the copper alloy (Fig. 2.9). See Chap. 16, Galvanic Corrosion. ... 

The magnitude of this resistance phenomenon can vary from metal to metal. Hence, the rate and extent of galvanic corrosion for any particular couple may be less than anticipated when these effects are operating. 

The possible effects of fluid velocity on galvanic corrosion are sometimes overlooked. Fluid velocity can affect the apparent potential of metals in a given environment. Depending on the environment, a metal under the influence of relatively rapid flow may assume either a more noble or a more active character than that indicated by the galvanic series. Occasionally, this shift in potential may result in galvanic corrosion that would not occur under stagnant or low-flow conditions. 

Copper, aluminium, steel and galvanized iron are the most widely used metals for the purpose of grounding. Choice of any of them will depend upon availability and economics in addition to the climatic conditions (corrosion effect) at the site of installation. In Table 22.3 we provide a brief comparison of these metals for the most appropriate choice of the metal for the required application. 

Note that Reference" draws attention to the possibility of an increase of anodic polarisation of the more negative member of a couple leading to a decrease in galvanic corrosion rate. There can also be a risk of increased corrosion of the more positive member of a couple. Both these features can arise as a result of active/passive transition effects on certain metals in certain environments. 

Two dissimilar metals, such as iron and aluminium, may cause aggravated corrosion effects even if they are not in electrical contact. This subject is, however, outside the scope of this section, and has been treated in detail elsewhere. Heavy metal ions, such as copper ions, are particularly liable to produce galvanic effects by redeposition on a less noble metal the phenomenon is discussed in Sections 4.1, 4.2 and 9.3. 

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. 

Contact of brass, bronze, copper or the more resistant stainless steels with the 13% Cr steels in sea-water can lead to accelerated corrosion of the latter. Galvanic contact effects on metals coupled to the austenitic types are only slight with brass, bronze and copper, but with cadmium, zinc, aluminium and magnesium alloys, insulation or protective measures are necessary to avoid serious attack on the non-ferrous material. Mild steel and the 13% chromium types are also liable to accelerated attack from contact with the chromium-nickel grades. The austenitic materials do not themselves suffer anodic attack in sea-water from contact with any of the usual materials of construction. 

There is an accelerating trend away from the use of lead-containing solders in contact with potable water. The effects of galvanic corrosion of one of the substitute alloys (Sn3%Ag) in contact with a number of other metals including copper have therefore been studied . The corrosion of tin/Iead alloys in different electrolytes including nitrates, nitric and acetic acids, and citric acid over the pH range 2-6 were reported. The specific alloy Pb/15%Sn was studied in contact with aqueous solutions in the pH range... 

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