Galvanic corrosion surface effects

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 life of equipment subjected to corrosive environments can be increased by proper attention to design details. Equipment should be designed to drain freely and completely. The internal surfaces should be smooth and free from crevasses where corrosion products and other solids can accumulate. Butt joints should be used in preference to lap joints. The use of dissimilar metals in contact should be avoided, or care taken to ensure that they are effectively insulated to avoid galvanic corrosion. Fluid velocities and turbulence should be high enough to avoid the deposition of solids, but not so high as to cause erosion-corrosion. 

The basic objective of the conversion coating process is to provide a corrosion-resistant film that is integrally bonded chemically and physically to the base metal and that provides a smooth and chemically inert surface for subsequent application of a variety of paint films. The conversion coating processes effectively render the surface of the basis material electrically neutral and immune to galvanic corrosion. Conversion coating on basis material coils does not involve the use of applied electric current to coat the basis material. The coating mechanisms are chemical reactions that occur between solution and basis material.1-4... 

The type of conversion process will depend on the substrate, the nature of the oxide layer on its surface, and the type of adhesive or sealant used. The formation of a noncon-ductive coating on a metal surface will also minimize the effect of galvanic corrosion. 

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... 

Fig. 8 Effect of the area of the cathodic reaction (the fully exposed surface) relative to the area of the anodic reaction (the occluded region). An increase in the dissolution rate of the crevice will occur (just as in galvanic corrosion) owing to need to satisfy the summation of the anodic reaction rates equaling the sum of the cathodic reaction rates.

The effect of the phosphate layer as a cathodic inhibitor under atmospheric conditions is illustrated in Fig. 12. In this experiment, the Volta potential of a galvanized steel surface is measured as a function of time during a transition from air to Ar atmosphere, as indicated [51]. The measurement is performed with a Kelvin probe, and the Volta potential of the corroding surface is directly proportional to the corrosion potential with appropriate calibration [58]. The potential jump induced by the presence of air is a measure of the sensitivity of the surface to the oxygen reduction reaction. Here, we see that the galvanized steel surface shows a very large potential jump, on the order of 200 mV. However, the phosphated surface shows only... 

Case Study 6.1—Effect of the Ratio of the Surface Area of the Cathode to the Surface Area of the Sacrificial Anode on Galvanic Corrosion of the Tin-Platinum Galvanic Couple... 

Fig. 6.6 Effect of the cathode-sacrificial anode surface area on galvanic corrosion of a tin-platinum galvanic couple.

E6.6. Sn and Pt are immersed in an acidic solution with unit hydrogen ion activity. Using the electrochemical parameters listed below, construct the Evans diagram and evaluate the effect of the cathode-sacrificial anode electrode surface area ratio on galvanic corrosion of a tin-platinum galvanic couple (see Case Study 6.1). 

The LSP mechanism proposes that SCC results from the effect of the structure ahead of the crack tip [61]. This mechanism assumes that a galvanic corrosion between active sites (weakened passive site) and surroimding passive surfaces produces large anodic currents at the rupture site. Repassivation of the active sites is prevented by the presence of weakened passive films on the surface. It has been su ested that the weakened passive film... 

Ferrous ions from the anodic reaction Fe Fe + 2e react with from the cathodic depolarization reaction and with OH from the water dissociation reaction and form ferrous sulphide, FeS, and hydroxide, Fe(OH)2. FeS can play an important role. Where the sulphide forms a continuous film on the surface it acts as protection and as an effective site for the cathodic reaction. If the film is injured or there is a lack of continuity in the film for other reasons, local galvanic corrosion will occur. Experiments and experience indicate that also the anodic reaction (Fe —> Fe +2e ) is depolarized as a result of the SRB environment. This is of interest in connection... 

When metals are used as coatings the possibility of occurrence of galvanic corrosion exists, since cracks or pores in the coating enable the corrosive medium to contact the substrate. Mainly for this reason metallic coatings have not been used in internal implants. However, surface treatments with inert materials have been widely applied and are now in clinical practice. The effect of these and other surface treatments will be addressed in this section. 

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