Susceptibility copper-based alloys

Essentially all industrial metals are susceptible to SCC in some specific environment. Of the metals commonly used in cooling water systems, copper-based alloys and stainless steels are most frequently affected. Common specific corrodents causing SCC in these and other heat exchanger metals are listed in Table 9.1. 

Most metals are subject to erosion-corrosion in some specific environment. Soft metals, such as copper and some copper-base alloys, are especially susceptible. Erosion-corrosion is accelerated by, and frequently involves, a dilute dispersion of hard particles or gas bubbles entrained in the fluid. 

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. 

Titanium. Unlike other metals, titanium normally does not pit, is not susceptible to stress corrosion, is free from local corrosion under fouling organisms, is free from impingement and cavitation attack at velocities which attack copper-base alloys, and is not susceptible to sulfide attack in contaminated sea water. Experiments with water velocities at 20 to 50 feet per second show no attack on titanium. 

A number of variables contributing to sulfide-influenced corrosion include duration of sulfide exposure, system operating velocity, and degree of turbulence must be assessed in conjunction with the sulfide concentration in order to accurately predict the sulfide susceptibility of copper-nickel alloys. Other research [72] on copper-base alloys has found that 3-ppm hydrogen sulfide significantly increases the corrosion rates of several copper alloys as indicated in Table 5. 

BRUSH ALLOY 25, a heat-treatable beryllium copper product contains 1.80 to 2.00% beryllium. BRUSH ALLOY 25 is resistant to hydrogen embrittlement, and not susceptible to either sulfide stress cracking or chloride stress cracking. Moreover, in marine and certain industrial environments this alloy outperforms stainless steel, titanium, and most copper based alloys. Beryllium copper is available in a wide range of forms, including strip, tube, rod, bar, extrusions, casting and master alloy, and forging billet. 

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],... 

The AlSi-alloy types are optimally castable because of the solidified eutectic at a low temperature of 570 °C (at 12.5% Si in the Al-base compound). They are particularly suitable for castings with large wall thickness variations and thin ribs. The alloy is very soft and tends to smear when machining. The corrosion resistance is described as good, if free of copper. The addition of Mg or Cu to the Al-Si alloys causes harden-ability and thus an increase in strength. The three-component alloys are also easy to cast, can also be better machined because of their higher hardness, and can be well protected by anodic oxidation. The copper-containing alloys if unprotected are more susceptible to corrosion than copper-free versions. 

A number of cold-rolled alloys based on aluminium, copper and zinc are susceptible in varying degrees to recrystallisation on exposure to heat. This can have a detrimental effect on the adhesion of paint films. While there may, at first, be no sign of trouble, the defect will become obvious by brittleness of the film after some storage time has elapsed. 

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. 

Cupronickels (90/10 that contains 10% nickel or 70/30 with 30% nickel or Monel 400) have been used for many years in applications where sea water has been involved, for their good corrosion resistance. This fitness for purpose is specifically because of the cupronickels passive cuprous oxide (CU2O) film, which retards both the anodic dissolution of the alloy and the rate of oxygen reduction [7]. Based on studies by Gouda et al. and reported by Lee et al. [8], alloy 400 (= Monel 400 containing 66.5% nickel, 31.5% copper, and 1.25% iron) is much more susceptible to SRB-induced MIC compared to 70/30 cupronickel or brass. 

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