Copper alloys impingement attack

This example of aluminium illustrates the importance of the protective him, and hlms that are hard, dense and adherent will provide better protection than those that are loosely adherent or that are brittle and therefore crack and spall when the metal is subjected to stress. The ability of the metal to reform a protective him is highly important and metals like titanium and tantalum that are readily passivated are more resistant to erosion-corrosion than copper, brass, lead and some of the stainless steels. There is some evidence that the hardness of a metal is a signihcant factor in resistance to erosion-corrosion, but since alloying to increase hardness will also affect the chemical properties of the alloy it is difficult to separate these two factors. Thus althou copper is highly susceptible to impingement attack its resistance increases with increase in zinc content, with a corresponding increase in hardness. However, the increase in resistance to attack is due to the formation of a more protective him rather than to an increase in hardness. 

This form of attack, especially as affecting copper alloys in sea water, has been widely studied since the pioneer work of Bengough and May . Impingement attack of sea water pipe and heat exchanger systems is considered in Sections 1.6 and 4.2. In such engineering systems the water flow is invariably turbulent and the thickness of the laminar boundary layer is an important factor in controlling localised corrosion. 

The addition of 1 to 2 mass% Fe was shown to improve the corrosion resistance impingement attack, deposit attack, pitting, of copper alloys containing 10 mass% Ni [1951Bai]. Copper precipitation has been... 

Figure 20.3. Trends of dezincification, stress-corrosion cracking, and impingement attack with increasing zinc content in copper-zinc alloys (brasses).

Aluminum brass resists high-velocity waters (impingement attack) better than does admiralty metal. Cupro-nickel alloys are especially resistant to high-velocity seawater when they contain small amounts of iron and sometimes manganese as well. For the 10% Ni cupro-nickel alloy, the optimum iron content is about 1.0-1.75%, with 0.75% Mn maximum for the analogous 30% Ni composition, the amount of alloyed iron is usually less (e.g., 0.40-0.70% Fe accompanied by 1.0% Mn maximum) [46]. It is found that supplementary protective films are formed on condenser tube surfaces when iron is contained in water as a result of corrosion products upstream or when added intentionally as ferrous salts. Accordingly, the beneficial effect of iron alloyed with copper-nickel alloys is considered to result from similar availability of iron in the formation of protective films. 

Copper alloys suffer from impingement attack under turbulent flow conditions, when corrosion is accelerated by solids and gas bubbles in the liquid stream. This behavior is often seen at the inlet end of a heat exchanger and is called inlet attack. 

Copper and some copper alloys do exhibit corrosion under advanced velocity conditions. Their corrosion resistance is dependent on the growth and maintenance of the protective layers formed on the metal surface. The maximum velocity recommended for copper in seawater is 0.9 m/sec. Alloying with nickel or aluminum increases the resistance of copper alloys to impingement attack [72,79]. 

Additions of tin and phosphorus to copper produce alloys having good resistance to flowing seawater and to most nonoxidizing acids except hydrochloric. Alloys containing 8 to 10 % tin have high resistance to impingement attack. Phosphor bronzes are much less susceptible to SCC than brasses and are similar to copper in their resistance to sulfur attack. 

The next major group of the copper alloys are the bronzes that, from a corrosion standpoint, are very similar to the brasses. Copper-aluminum (aluminum bronze), copper-silicon (silicon bronze), and copper-tin (tin bronze) are the main cast bronze alloys. The addition of aluminum to the bronzes improves resistance to high-temperature oxidation, increases the tensile properties, and provides excellent resistance to impingement corrosion. They are resistant to many nonoxidizing acids. Oxidizing acids and metallic salts will cause attack. Alloys having more than 8% aluminum should be heat treated because it improves corrosion resistance and toughness. Aluminum bronzes are susceptible to SCC in moist ammonia. 

Copper-based alloys. The copper-based alloys are velocity-limited, as impingement attack occurs when the hydrodynamic effect caused by seawater flow across the surface of such alloys exceeds the value at which protective films are removed and erosion-corrosion occurs. Thus, if these alloys are to exhibit high corrosion resistance, they must be used at design velocities below this limiting value. A more detailed coverage of the marine usage of two important copper-nickel alloys is presented in the section on copper alloys. 

The two main wrought copper-nickel alloys chosen for seawater service contain 10 and 30% percent nickel, respectively. When comparing international specifications, the compositional ranges of the two alloys vary slightly between specifications, as can be seen in Tables 8.15 and 8.16 for 90-10 and 70-30 copper-nickel alloys. In practice, these variations have little influence on the overall service performance of the alloys. Iron is essential for both alloys because it provides added resistance to corrosion caused by velocity effects called impingement attack. An optimum level is between 1.5 and 2.5% iron, probably as a result of solid solubility. The corrosion resistance improves with increasing iron so long as it remains in solid solution. The specification limits for alloys were set by this observation. 

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. 

High velocities of aqueous solutions impinging on copper and brass tubes produce impingement on the metal or alloy. The aluminum brass and cupronickel alloys have great resistance to flow-induced attack up to a well-defined maximum for the flow rate, beyond which the film on the metal surface will be disrupted. Admiralty brass and aluminum brass have lower values for maximum velocity of flow than cupronickels. Admiralty brass and aluminum brass are preferred to cupronickels for use in media containing sulfide species. Coatings have been developed for cupronickel and aluminum brass condenser tubes for land-based and marine systems. 

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