Brass galvanic corrosion

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). 

When possible, avoid coupling materials having widely dissimilar galvanic potentials. If this cannot he avoided, make use of favorable area ratios by giving the active metal a large exposed area relative to the noble metal. For example, copper or copper-based alloy tubes may be joined to a steel tube sheet. Because of the favorable area ratio in this case, a relatively inexpensive steel tube sheet may be intentionally substituted for a bronze or a brass tube sheet if thickness specifications allow for a small amount of galvanic corrosion of the steel. 

These galvanic corrosion processes take place when one or more elemental constituents of an alloy is leached, often leaving a weak, porous structure, although the component dimensions often are unchanged. Dealloying particularly affects equipment constructed of cupronickels, bronzes, brasses, and gunmetal, such as FW heaters, strainers, valves, and pump impellers. 

Blade castings can also be produced from brass and stainless steel, but their use is generally limited because of their relatively high cost and weight. If the blade shanks and hub are of dissimilar metals, they should be insulated from each other to prevent galvanic corrosion from occurring. 

X X Corrosion-resistant alloys brass galvanizing steel iron products rubber treatment of zinc deficiency in humans pesticides 111,23,3512, 3522,355,372, 38, 932... 

Fig. 1 Galvanic corrosion of a steel blank flange around a threaded brass coupling from a liquid fertilizer storage tank. (View this art in color at www.dekker.com.)...

Two dissimilar metals in contact in an aqueous environment does not necessarily give rise to galvanic corrosion. Turner [1990] cites an example of a shell and tube heat exchanger (steam condenser), containing aluminium brass tubes expanded into carbon steel tube plates. It could be anticipated that with the relatively large area of the noble aluminium brass, severe galvanic corrosion would occur on the smaller area of exposed steel resulting in a short service life. In fact the condenser had been in service for 26 years when the examination took place ... 

E6.1. Predict the possibihty of galvanic corrosion in sea water for the following coupled pairs of alloys and metals (i) aluminum alloys and aluminum brass, (ii) cadmium and manganese bronze, (iii) zinc and tin, (iv) low alloy steel and stainless steel 410, (v) low alloy steel and stainless steel 430, (vi) nickel 200 and Ni-Cr-Mo-Cu-Si... 

E6.2. Predict whether or not galvanic corrosion will cause the following alloys to be subjected to leaching (i) carbon and carbon steel alloys in an oxidizing atmosphere, (ii) steel rivets in aluminum drain gutters, (iii) copper-nickel alloy in refinery condenser tubes, (iii) graphite fiber-reinforced aluminum composites, (iv) brass in water, (v) iron-chromium alloys, and (vi) carbon steel pipe in contact with the weld to stainless steel pipe. 

Forty and Humble [51] su ested the presence of vacancy-saturated defect structure modes. According to this model, the vacancies are injected into the crack during anodic dissolution of the alloy caused by galvanic corrosion. These vacancies create conditions for the formation of a crack structure that may initiate SCC of brass. Revie and UMig [62] found that vacancies were causing increased creep by the galvanic anodic reactions. 

E6.1. Predict the possibility of galvanic corrosion in sea water for the following coupled pairs of alloys and metals (i) aluminum alloys and aluminum brass,... 

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. 

Galvanic corrosion is an accelerated corrosion of a metal due to formation of a corrosion cell with a metal or non-metallic conductor that exhibits a higher corrosion potential. For example, if a water pipe made of zinc-coated steel (galvanized steel) is connected to a brass fixture and caution is not taken to electrically isolate the two metals, a corrosion cell is established (Figure 7.5). To simplify the situation, we have replaced, in Figure 7.5, the zinc-coated steel by pure zinc and the brass by copper. The cathodic reaction is the reduction of dissolved oxygen, which takes place on both metals. The corrosion cell formed between the zinc and the copper leads to an accelerated corrosion of zinc near the joint. 

Even in the absence of an electrical contact between two parts of a structure made of different metals, galvanic corrosion phenomena can still occur locally, when displacement reactions take place. A well know example are domestic heating systems consisting of a water heater made of copper or brass combined with galvanized steel piping (Figure 7.12). Intrinsic corrosion of the water heater liberates copper ions that... 

FIG. 8—Galvanic corrosion caused by contact of dissimilar metals. Steel is the anode, which suffers corrosion, and brass Is the cathode. Electrons leave the steel and flow to brass. 

Corrosion caused by the connection of two or more different metals also occurs underground. This electrochemical corrosion cell is commonly referred to as bimetallic or galvanic corrosion. Typical examples include brass or bronze valves connected to steel or cast iron pipes and stainless steel fasteners coimected to steel or cast iron. These couplings of dissimilar metals will locally affect the corrosion rate. Aluminum can be severely corroded if directly connected to most other engineering alloys, such as steel, iron, copper, or stainless steel—dielectric isolation must be used. 

Structural materials that are direcdy exposed, such as in cabinets, housings, and heat sinks, are fabricated fix>m materials such as steels, stainless steels, brass, zinc, aluminum, and other metals and alloys with appropriate prof>erties. The types of corrosion encountered in these structures depend on the environment and material as would be exp>ected. Both uniform and localized corrosion can be important when cosmetic corrosion is of concern. In structural applications, crevice corrosion, corrosion fatigue, stress corrosion cracking (SCC), galvanic corrosion, and intergranular corrosion causing reduction in mechanical properties are important. 

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. 

Although copper and brass typically have good corrosion resistance in aqneous solntions, they may be subject to corrosion in plant environments, depending upon the process stream. The presence of sulfides and ammonia componnds in wastewater can lead to dissolntion of cuprous compounds. Further, if copper is coupled to a less noble metal like steel or alnminnm, galvanic corrosion of the less noble metal may resnlt. Because copper is a fairly soft material, it is also subject to erosion. This type of corrosion is accelerated by high fluid velocities, high temperatures, and abrasive particulate matter. 

The surface area of the electrode in the corrosive medium is known as the relative area, which influences the rate of galvanic corrosion. For instance, the experimental data on brass/steel couple in 20%NaCl at room temperature reported by Jones [19] can be used to determine that increasing the surface area ratio increases the the galvanic corrosion potential. Therefore, the corrosion potential of galvanic couplings is strongly dependent on the cathode-to-anode surface area ratio. 

The galvanic series indicates that alloys can be coupled without being corroded. For instance, alloys close to each other in the series can be safely coupled. As shown in the table, monel can be coupled to copper, or bronze, without any risk of galvanic corrosion. However, brass cannot be coupled... 

Galvanic corrosion of steel pipe with brass fittings. 

Galvanic corrosion of the body of the ship in contact with brass or bronze propellers. 

It is assumed that the only sources of lead in drinking water are lead service lines and any internal lead pipes within premises and that lead contamination is due solely to the process of dissolution. The model does not simulate lead releases from brass or galvanic corrosion, nor particulate lead releases however, correction factors can be apphed if necessary. 

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. 

While contact between aluminium alloys and copper and cuprous alloys (bronze, brass) causes no appreciable galvanic corrosion of aluminium under atmospheric conditions, it is nevertheless advisable to provide insulation between the two metals to localise surface corrosion of the aluminium. 

Except in rare cases, there is no homogeneity of materials, metals or alloys in buildings or in mechanically joined structures. For example, metallic fittings in aluminium are always joined with screws in stainless steel, while accessories such as hinges, filters, lift-off hinges are in stainless steel or chrome-plated steel, or even in brass. Under appropriate conditions, there is a risk of galvanic corrosion of aluminium. 

Galvanic corrosion occurs when two metals or alloys having different compositions are electrically coupled while exposed to an electrolyte. This is the type of corrosion or dissolntion that was described in Section 17.2. The less noble or more reactive metal in the particular environment experiences corrosion the more inert metal, the cathode, is protected from corrosion. As examples, steel screws corrode when in contact with brass in a marine environment, and if copper and steel tubing are joined in a domestic water heater, the steel corrodes in the vicinity of the junction. Depending on the natiue of the solntion, one or more of the reduction reactions. Equations 17.3 through 17.7, occius at the surface of the cathode material. Figure 17.14 shows galvanic corrosion. 

Galvanic corrosion may be a problem when dissimilar metals are placed in contact with each other and are located in corrosive environment. This problem should be carefully considered during design and material selection. The severity of this problem depends on the environment and the connecting metals. For example, brass connecting to steel may cause rapid corrosion of the steel, whereas aluminum connected to steel may cause rapid corrosion of the aluminum. In contrast, connecting steel to a corrosion-resistant alloy may not cause significant corrosion but should be evaluated. 

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