162 9.Degradation of Materials (Corrosion) TABLE 9.2.Galvanic series of some commercial metals and alloys exposed to sea watera Platinum Cathodic Gold Graphite Titanium Stainless steel(passive) Copper-nickel alloys Bronze Copper Tin Sn-50%Pb solder Stainless steel (active) Cast iron Iron and steel Aluminum alloys Aluminum Galvanized steel Zinc Magnesium Anodic "Note that each metal or alloy is anodic with respect to those above it. marine environment.Examples are steel rivets that have been (improperly)used to join aluminum sheets,or steel ball bearings of sliding doors that sit on an aluminum track.The list of "gal- vanic corrosion sins"observed in daily life is almost endless and, of course,is not restricted to iron and steel. Galvanic corrosion is also encountered due to concentration dif- ferences of ions or dissolved gases in an electrolyte.The respec- tive mechanism involved has been termed a concentration cell.The deterioration often occurs at the site at which the solution is most diluted.For example,the oxygen concentration is lower immedi- ately under a drop of water sitting on an iron plate.In contrast, the oxygen concentration at the periphery of the water drop is much higher due to its increased exposure to the atmosphere.As a consequence,an iron plate corrodes under the center of the wa- ter drop and forms what is called a corrosion pit;see Figure 9.5(a). Similar observations are made immediately below the waterline Anode Cathode (corrosion) FIGURE 9.4.Galvanic cor- rosion of two dissimilar H2O metals (water pipes)(see Table 9.1).marine environment. Examples are steel rivets that have been (improperly) used to join aluminum sheets, or steel ball bearings of sliding doors that sit on an aluminum track. The list of “gal￾vanic corrosion sins” observed in daily life is almost endless and, of course, is not restricted to iron and steel. Galvanic corrosion is also encountered due to concentration dif￾ferences of ions or dissolved gases in an electrolyte. The respec￾tive mechanism involved has been termed a concentration cell. The deterioration often occurs at the site at which the solution is most diluted. For example, the oxygen concentration is lower immedi￾ately under a drop of water sitting on an iron plate. In contrast, the oxygen concentration at the periphery of the water drop is much higher due to its increased exposure to the atmosphere. As a consequence, an iron plate corrodes under the center of the wa￾ter drop and forms what is called a corrosion pit; see Figure 9.5(a). Similar observations are made immediately below the waterline 162 9 • Degradation of Materials (Corrosion) TABLE 9.2. Galvanic series of some commercial metals and alloys exposed to sea watera Platinum Cathodic Gold Graphite Titanium Stainless steel (passive) Copper–nickel alloys Bronze Copper Tin Sn–50% Pb solder Stainless steel (active) Cast iron Iron and steel Aluminum alloys Aluminum Galvanized steel Zinc Magnesium Anodic aNote that each metal or alloy is anodic with respect to those above it. Anode (corrosion) Cathode H Fe 2O e– e– Cu FIGURE 9.4. Galvanic cor￾rosion of two dissimilar metals (water pipes) (see Table 9.1)