Galvanic corrosion

Galvanic corrosion occurs when a metal or alloy is electrically coupled to another, usually dissimilar, metal in the same electrolyte. Severe localized attack is often seen when aluminium alloys are coupled with more noble metals. As this 

Pits can exist on some fuel when it enters the storage basin. Not all spent fuel cladding is initially in pristine condition. A more severe environment exists in the pit beneath the nodule or crusted oxide corrosion product. Corrosion will likely continue beneath the nodular oxide. However, high quality basin water could minimize or eliminate any new corrosion. In addition, the high quality water could slow pit growth by dilution of the severe environment within the pit. 

Galvanic corrosion is either a chemical or an electrochanical corrosion. The latter is due to a potential difference between two different metals connected through a circuit for current flow to occur from more active metal (more negative potential) to the more noble metal (more positive potential). 

In addition, galvanic corrosion can be predicted by using the electromotive force (emf) or standard potential series for metal reduction listed in Table 2.1. These reactions are reversible. The standard metal potential is measured against the standard hydrogen electrode (SHE), which is a reference electrode having an arbitrary standard potential equals to zero. Details on types of reference electrodes are included in chapter 2. 

Lithium Ambient-Temperature Batteries (LAMBS) These are high energy density devices, in which the JA anode is passivated. The solid cathode can be made of LiCuO, LiMnO, IAV2O5, or IABi2Ph20. Some LABS rrse liquid cathodes, such as IASO2 [7]. For negative cathodes made of carbon (graphite or coke) in lithium-ion batteries, the reaction on this electrode is 

The reduction reaction is for charging and the oxidation is for discharging processes of the Uthium-ion cells. Detail analysis of these cells, including side reactions can be found elsewhere [13-15]. 

Lead-Acid Battery The basic operation of a lead-add (Pb — H SO ) battery is based on groups of positive and negative plates immersed in an electrolyte that consists of diluted sulfuric(fl2 S 04) acid and water. Hence, the mechanism of this t5T)e of battery is based on the electron-balanced anodic (-) and cathodic (+) reactions. Hence, the ideal electrode reactions are reversed 

Galvanic corrosion (also called dissimilar metal corrosion or wrongly electrolysis ) refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. In a bimetallic couple, the less noble material becomes the anode and tends to corrode at an accelerated rate, compared with the uncoupled condition and the more noble material will act as the cathode in the corrosion cell. 

Values of potential can change from one solution to another or in any solution when influenced by such factors as temperature, aeration, and velocity of movement. Consequently, there is no way, other than by direct potential measurements in the exact environment of interest, to predict the potentials of the metals and the consequent direction of a galvanic effect in that environment. As an example, zinc is normally very negative or anodic to steel at ambient temperatures, as indicated in the galvanic series shown in Fig. 6.31. However, the potential difference decreases with an increase in temperature until the potential difference may be zero or actually be reversed at 60 C [18 19]. 

Another important factor in galvanic corrosion is the area effect or the ratio of cathodic to anodic area. The larger the cathode compared 

18-8 Mo Stainless Steel (Passive) 18-8 Stainless Steel (Passive) Chromium Steel 11% Cr (Passive) Inconel (Passive) 

Silver Solder Monel Bronzes Copper Brasses Chlorimet 2 Hastelloy B Inconel (Active) 

Galvanic corrosion has appeared ever since two different metals were put together in a liquid medium, that is to say since the Iron Age, when iron was put in contact with brass and copper. Marine archaeology shows such cases of galvanic corrosion in wrecks of ancient ships. 

The first technical report on galvanic corrosion was addressed to the British Admiralty in 1763. The hull of the English frigate Alarm had been lined with copper plate in order to prevent the devastating effect of shipworms on the wood of the ship and prevent barnacles from attaching themselves to the hull, resulting in a reduction in the vessels speed. The plates had been fixed with iron nails. 

After 2 years of navigation in the Caribbean Sea, the ship was careened in the dry dock, and it was discovered that many copperplates had been lost during navigation over the sea because the nail heads were corroded. A detailed inspection showed that those nail heads that had resisted corrosion were insulated from the copper plate by pieces of brown wrapping paper that had been used as a packaging material for the plates [1]. 

From this misadventure, the engineers of the time drew the conclusion that any direct contact between iron and copper in seawater should be avoided. 

The coupling of titanium with dissimilar metals usually does not accelerate the corrosion of titanium. The exception is in reducing environments where titanium does not passivate. Under these conditions, titanium has a potential similar to aluminum and will undergo accelerated corrosion when coupled 

For most environments, titanium will be the cathodic member of any galvanic couple. It may accelerate the corrosion of the other member of the couple, but in most cases the titanium will be unaffected. As a result of this, hydrogen will be evolved on the surface of the titanium, proportional to the galvanic current flow. This may result in the formation of surface hydride films that are generally stable and cause no problems. However, if the temperature exceeds 170T (77 C) hydriding can cause embrittlement. 

The surest way to avoid problems with galvanic corrosion is to construct equipment of a single metal. If this is not practical, select two metals that are close in the galvanic series. If contact of dissimilar metals with titanium is necessary, the critical parts should be constructed of titanium because it is not usually attacked. 

Galvanic corrosion occurs when two metals with different electrochemical potentials are in contact in the same solution (Figs. 1.5 and 1.6). In both cases the corrosion of iron/steel is exothermic, and the cathodic reaction controls the rate of corrosion. The more noble metal, copper, increases the corrosion rate through the cathodic reaction of hydrogen ion reduction and hydrogen evolution. A passive oxide film on stainless steel can accelerate hydrogen reduction reaction. 

In general, the galvanic cell is influenced by (i) the difference in potential between two metals/materials, (ii) the nature of the medium or environment, (iii) polarization of the metals, and (iv) the geometry of the cathodic and anodic sites such as shape, relative surface areas, distance. 

For example, it is not desirable to have a small anode connected to a large cathode as this favors accelerated localized anodic dissolution. Rivets of copper on a steel plate and steel rivets on a copper plate on immersion in seawater for a period of 15 months resulted in the steel plate covered with corrosion products while the steel rivets were corroded completely and disappeared. As copper is more noble than iron, it accelerated the hydrogen reduction reaction for the oxidation of the steel plate. In the case of the copper plate with steel rivets, the steel rivets corroded because of the relatively important cathodic surface of copper. The same reasoning applies to the corrosion of noncoated auto parts in contact with a large stainless steel surface (Fig. 1.6). 

Galvanic corrosion occurs because of (i) nonmetalUc conductors and corrosion products, (ii) metallic coatings and sacrificial anodes, (iii) polarity inversion, (iv) deposition corrosion, (v) hydrogen cracking or damage, (vi) high temperature. 

The series of standard reduction potentials of various metals can be used to explain the risk of galvanic corrosion however, these potentials express thermodynamic properties, which do 

If dissimilar metals are placed in contact, in an electrolyte, the corrosion rate of the anodic metal will be increased, as the metal lower in the electrochemical series will readily act as a cathode. The galvanic series in sea water for some of the more commonly used metals is shown in Table 7.4. Some metals under certain conditions form a natural protective film for example, stainless steel in oxidising environments. This state is denoted by passive in the series shown in Table 7.4 active indicates the absence of the protective film. Minor 

Admiralty brass (Cu 71 per cent, Zn 28 per cent, Sn 1 per cent) Nickel (active) 

Cast iron Mild steel Aluminium Galvanised steel Zinc 

Pitting is the term given to very localised corrosion that forms pits in the metal surface. If a material is liable to pitting penetration can occur prematurely and corrosion rate data are not a reliable guide to the equipment fife. 

Aluminum alloys 2117, 2017,2024, in this order Low-carbon steel Wrought iron Cast iron 

Copper alloys C46400, C46500, C46600, C46700 (naval brass) 

Copper alloys C44300, C44400, C44500 (admiralty brass) 

Copper albys C60800, C61400 (aluminum bronze) 

Copper alloys C65100, C65500 (silicon bronze) 

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The vessel, as weU as the wick, must be compatible with the working fluid. Where possible, the wick and vessel are made of the same material to avoid the formation of galvanic corrosion ceUs in which the working fluid can serve as the electrolyte. In addition to its role within the heat pipe, the vessel also serves as the interface with the heat source and the heat sink. 

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. 

G lv nic Corrosion. Galvanic corrosion is an electrochemical process with four fundamental requirements (/) an anode (magnesium), 2) a cathode (steel, brass, or graphite component), (J) direct anode to cathode electrical contact, and (4) an electrolyte bridge at the anode and cathode interface, eg, salt water bridging the adjacent surfaces of steel and magnesium components. If any one of these is lacking, the process does not occur (133,134). 

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. 

Vanadium is resistant to attack by hydrochloric or dilute sulfuric acid and to alkali solutions. It is also quite resistant to corrosion by seawater but is reactive toward nitric, hydrofluoric, or concentrated sulfuric acids. Galvanic corrosion tests mn in simulated seawater indicate that vanadium is anodic with respect to stainless steel and copper but cathodic to aluminum and magnesium. Vanadium exhibits corrosion resistance to Hquid metals, eg, bismuth and low oxygen sodium. 

Localized corrosion, which occurs when the anodic sites remain stationary, is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching (eg, dezincification), galvanic corrosion, crevice or underdeposit corrosion, intergranular corrosion, stress corrosion cracking, and microbiologicaHy influenced corrosion. Another form of corrosion, which caimot be accurately categorized as either uniform or localized, is erosion corrosion. 

Galvanic Corrosion. Galvanic corrosion occurs when two dissimilar metals are in contact in a solution. The contact must be good enough to conduct electricity, and both metals must be exposed to the solution. The driving force for galvanic corrosion is the electric potential difference that develops between two metals. This difference increases as the distance between the metals in the galvanic series increases. 

Galvanic corrosion can be controlled by the use of sacrificial anodes. This is a common method of controlling corrosion in heat exchangers with Admiralty tube bundles and carbon steel tube sheets and channel heads. The anodes are bolted direcdy to the steel and protect a limited area around the anode. Proper placement of sacrificial anodes is a precise science. 

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. 

Aluminum is not embrittled by low temperatures and is not subject to external corrosion when exposed to normal atmospheres. At 200°C (400°F) its strength is less than half that at room temperature. It is attacked by alkahes, by traces of copper, nickel, mercuiy, and other heaw-metal ions, and by prolonged contact with wet insiilation. It suffers from galvanic corrosion when coupled to copper, nickel, or lead-... 

Plastic Pipe In contrast to other piping materials, plastic pipe is free from internal and external corrosion, is easily cut and joined, and does not cause galvanic corrosion when coupled to other materials. Allowable stresses and upper temperature limits are low. Normal operation is in the creep range. Fluids for which a plastic is not suited penetrate and soften it rather than dissolve surface layers. Coefficients... 

Galvanic Corrosion Galvanic corrosion is the corrosion rate above normal that is associated with the flow of current to a less active metal (cathode) in contact with a more active metal (anode) in the same environment. Tables 28-1 7 and 28-li show the galvanic series of various metals. It should be used with caution, since exceptions to... 

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. ... 

See Chap. 3, Tuberculation Chap. 7, Acid Corrosion and Chap. 16, Galvanic Corrosion. ... 

Corrosion tests of metals under static conditions reveal nothing relating to erosion-corrosion susceptibilities. It is entirely possible that a metal tested under static conditions will fail in service when sufficient fluid velocity produces erosion-corrosion. Similarly, it has been observed that galvanic corrosion between coupled, dissimilar metals may be accelerated or even initiated under flow conditions when little or no galvanic corrosion is observed under static conditions (see Chap. 16, Galvanic Corrosion ). 

Changing the pump metallurgy to a more corrosion- and cavitation-resistant material, such as stainless steel, is a potential solution to this type of problem. Note, however, that all other cast iron pump components that have sustained graphitic corrosion should be replaced to avoid the possibility of galvanic corrosion (see Chap. 16) between retained graphitically corroded cast iron components and new components. 

Generai description. Galvanic corrosion refers to the preferential corrosion of the more reactive member of a two-metal pair when the metals are in electrical contact in the presence of a conductive fluid (see Chap. 16, Galvanic Corrosion ). The corrosion potential difference, the magnitude of which depends on the metal-pair combination and the nature of the fluid, drives a corrosion reaction that simultaneously causes the less-noble pair member to corrode and the more-noble pair member to become even more noble. The galvanic series for various metals in sea water is shown in Chap. 16, Table 16.1. Galvanic potentials may vary with temperature, time, flow velocity, and composition of the fluid. 

Locations. Galvanic corrosion of any type is most severe in immediate proximity to the junction of the coupled metals. Galvanic corrosion of weld metals is frequently microstructurally localized. The less-noble weld material will corrode away, leaving behind the skeletal remnants of the more-noble metal (Figs. 15.1 and 15.2). 

Critical factors. Two factors are critical in galvanic corrosion of weld metal. The first is the existence of sufficient compositional differences... 

Identification. The primary identif3dng feature is confinement of metal loss to the weld bead (Fig. 15.3), although in advanced stages base metal immediately adjacent to the weld bead may also be affected. Note that this feature seems to distinguish galvanic corrosion of welds from other weld-related corrosion, such as weld decay, which preferentially attacks the immediately adjacent base metal (Fig. 15.4). 

Elimination. Recall that the critical factors governing galvanic corrosion of welds are the presence of substantial compositional differences within the weld metal and the exposure of such a weld to a sufficiently aggressive environment. If the aggressiveness of the environment cannot be sufficiently reduced, significant compositional differences within the weld metal must be avoided. This requires following proper... 

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