Nickel-copper alloys pitting corrosion

Nickel is usually alloyed with elements including copper, chromium, molybdenum and then for strengthening and to improve corrosion resistance for specific applications. Nickel-copper alloys (and copper-nickel alloys see Section 53.5.4) are widely used for handling water. Pumps and valve bodies for fresh water, seawater and mildly acidic alkaline conditions are made from cast Ni-30% Cu type alloys. The wrought material is used for shafts and stems. In seawater contaminated with sulfide, these alloys are subject to pitting and corrosion fatigue. Ammonia contamination creates corrosion problems as for commercially pure nickel. 

Effect of velocity. The combination of low general corrosion rates and high resistance to pitting and crevice corrosion ensures that the copper-nickel alloys will perform well in quiet, clean, and aerated seawater. As the flow rate of seawater increases, the corrosion rate remains... 

Water environments can also have a variety of compositions and corrosion characteristics. Freshwater normally contains dissolved oxygen as well as minerals, several of which account for hardness. Seawater contains approximately 3.5% salt (predominantly sodium chloride), as well as some minerals and organic matter. Seawater is generally more corrosive than freshwater, frequently producing pitting and crevice corrosion. Cast iron, steel, aluminum, copper, brass, and some stainless steels are generally suitable for freshwater use, whereas titanium, brass, some bronzes, copper-nickel alloys, and nickel-chromium-molybdenum alloys are highly corrosion resistant in seawater. 

Metals which owe their good corrosion resistance to the presence of thin, passive or protective surface films may be susceptible to pitting attack when the surface film breaks down locally and does not reform. Thus stainless steels, mild steels, aluminium alloys, and nickel and copper-base alloys (as well as many other less common alloys) may all be susceptible to pitting attack under certain environmental conditions, and pitting corrosion provides an excellent example of the way in which crystal defects of various kinds can affect the integrity of surface films and hence corrosion behaviour. 

Alloys such as 304 and 316 stainless steel or nickel-chromium cdloys exhibit deep pitting in low flow conditions, yet at high seawater velocities their corrosion rate decreases to less than 25 pm per year. Contrary to this, iron and copper show significantly lower corrosion rates at low flow velocities than rmder high seawater flow conditions [37]. 

The nickel-copper alloys show higher resistance to cavitation than the copper-nickel materials or the austenitic nickel cast alloys and are therefore frequently used in pump parts and in heat exchangers, whereby however cathodic protection is recommended due to the risk of pitting corrosion [130,199]. 

Almost all types of engineering materials have been reported to experience MIC by SRB copper, nickel, zinc, aluminium, titanium and their alloys [96, 97, 98], mild steel [72, 99, 100], and stainless steels [68, 80, 101, 28] are just some examples. Among duplex stainless steels, SAF 2205 has been reported for its vulnerability to MIC [44, 102, 103]. According to these studies, SAF 2205 can corrode and have pitting initiated due to the presence of SRB after immersion in seawater for more than one year (18 months) [102]. Corrosion rates of lOmm/year [5] in oil treatment plants and 0.7 mm/year to 7.4mm ear due to the action of SRB and/or acid-producing bacteria in soil environments [8] have been reported. 

Figure 1. Light optical micrographs of (a) plan view of a corrosion pit in a nickel alloy tube, with concentric ring illumination, at a magnification of lOX, and (b) cross section of a corrosion pit in copper metal with the same illumination and 25X magnification.

Nickel, containing 0.6 rf-electron vacancy per atom (as measured magnetically), when alloyed with copper, a nontransition metal containing no rf-electron vacancies, confers passivity on the alloy above approximately 30-40 at.% Ni. Initiation of passivity beginning at this composition is indicated by corrosion rates in sodium chloride solution (Figs. 6.12 and 6.13), by corrosion pitting behavior in seawater (Fig. 6.13), and, more quantitatively, by measured values of /critical and passive (Fig. 6.14), [41-43] or by decay (Flade) potentials (Fig. 6.15) [44] in IN H2SO4. 

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