Copper alloys crevice corrosion

Most cases of crevice corrosion take place in near-neutral solutions in which dissolved oxygen is the cathode reactant, but in the case of copper and copper alloys crevice corrosion can occur owing to differences in the concentration of Cu ions however, in the latter the mechanism appears to be different, since attack takes place at the exposed surface close to the crevice and not within the crevice in fact, the inside of the crevice may actually be cathodic and copper deposition is sometimes observed, particularly in the Cu-Ni alloys. Similar considerations apply in acid solutions in which the hydrogen ion is the cathode reactant, and again attack occurs at the exposed surface close to the crevice. 

A summary of the results of the long-term exposures is shown in Table 7.6. After the exposure to the seawater fog, crevice corrosion had initiated under one site on the 8090 alloy. Some minor corrosion was also observed along rolling marks, which became visible after the exposure. Deep pits were observed on the surface of the 2024 alloy, which also suffered crevice corrosion under 6 of the 24 initiation sites, with 3 of these sites showing substantial penetration. Deep pits were also observed on the surface of the 2090 alloy, but crevice corrosion had not been initiated. Pitting had occurred on the surface of the 7075 alloy as well, but the depth of attack was not as severe as that which occurred on either the 2090 or 2024 alloy. This observation was consistent with the higher copper concentration present in the 2000-series alloys. Crevice corrosion was observed under one initiation site on the 7075 alloy. ... 

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. ... 

Crevice corrosion of copper alloys is similar in principle to that of stainless steels, but a differential metal ion concentration cell (Figure 53.4(b)) is set up in place of the differential oxygen concentration cell. The copper in the crevice is corroded, forming Cu ions. These diffuse out of the crevice, to maintain overall electrical neutrality, and are oxidized to Cu ions. These are strongly oxidizing and constitute the cathodic agent, being reduced to Cu ions at the cathodic site outside the crevice. Acidification of the crevice solution does not occur in this system. 

Copper has excellent resistance to some corrosive environments, including fresh waters and fluoride-containing atmospheres. Alloying is necessary to achieve good strength, but copper limiting with steel for strength is an alternative (BS 5624). Copper and some of its alloys are susceptible to crevice corrosion, but the mechanism is different from that which affects stainless steels. 

Metals with protective oxide layers are not immune from attack, especially stainless steels and copper alloys, which may suffer aggressively from crevice corrosion. 

Austenitic SS s are also used in freshwater. However, because of cost their use is limited mainly to applications in which copper-zinc alloys are unsuitable, as in tubes in which the process side is incompatible with copper base alloys. To avoid pitting, type 304 (UNS S30400) SS is normally limited to services in which the chloride ion concentration is at a maximum of 100 ppm, and type 316 SS is limited to services in which the chloride ion is a maximum of 500 ppm. The relative pitting and crevice corrosion resistance of SS alloys can be approximated by the following equation ... 

Nickel-copper and nickel-chromium-molybdenum alloys are the nickel-base alloys that are t5fpically used in seawater. The nickel-copper alloys have good corrosion resistance in high velocity seawater, but do exhibit localized corrosion in quiescent seawater [79]. Alloy 625, a nickel-chromium-molybdenum alloy, is susceptible to crevice corrosion in both quiescent and flow conditions [97-700]. Other nickel-chromium-molybdenum alloys, such as Alloys C-276, C-22, 59 and 686 have increased seawater crevice corrosion resistance as compared to Alloy 625 [97,98],... 

ASTM G 78 is employed specifically for crevice corrosion testing of iron and nickel-base type alloys. If susceptible, these materials exhibit attack within the crevice area. Other nonstainless-type alloys such as copper-nickels often tend to suffer localized corrosion just outside the crevice rather than within it. Traditional crevice corrosion test are performed routinely on Cu-Ni alloys. 

An acceptable life for undergound pipelines can sometimes be realized through the use of corrosion-resistant piping. Copper, aluminum, and stainless steel piping are sometimes used for this purpose. All three alloys can have greater corrosion resistance than carbon steel or cast iron. However, they are not immune to corrosion and are often more susceptible to localized corrosion such as pitting, crevice corrosion, and SCC than carbon steel or cast iron. Further, corrosion protection in the form of coatings and cathodic protection are frequently used. 

The high nickel and molybdenmn contents provide improved resistance to chloride SCC. Copper has been kept to a residual level for improved performance in seawater. The high alloy composition resists crevice corrosion and pitting in oxidizing chloride solutions. 

For copper alloys, increase in temperature accelerates film formation. While it takes about 1 day to form a protective film at 15°C, it may take a week or more at 2°C. It is important to continue initial circulation of clean seawater long enough for initial film formation for all copper alloys. For stainless steels and other alloys that are prone to pitting and crevice corrosion, an increase in temperature tends to facilitate initiation of these types of attack. However, data on propagation rate suggest... 

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