Brass erosion-corrosion

The heat-transfer quaUties of titanium are characterized by the coefficient of thermal conductivity. Even though the coefficient is low, heat transfer in service approaches that of admiralty brass (thermal conductivity seven times greater) because titanium s greater strength permits thinner-walled equipment, relative absence of corrosion scale, erosion—corrosion resistance that allows higher operating velocities, and the inherently passive film. 

Figure 11.5 Erosion-corrosion at elbow of a brass tube. Note also the borseshoe-shaped depressions and comet tails aligned with flow direction in the straight section.

Figure 11.9 Erosion-corrosion damage on the surface of a brass tube facing the steam inlet nozzle.

Because alterations to equipment design can be cumbersome and expensive, a more economical approach may be to change the metallurgy of affected components. Metals used in typical cooling water environments vary in their resistance to erosion-corrosion. Listed in approximate order of increasing resistance to erosion-corrosion, these are copper, brass, aluminum brass, cupronickel, steel, low-chromium steel, stainless steel, and titanium. 

The resistance of a metal to erosion-corrosion is based principally on the tenacity of the coating of corrosion products it forms in the environment to which it is exposed. Zinc (brasses), aluminum (aluminum brass), and nickel (cupronickel) alloyed with copper increase the coating s tenacity. An addition of V2 to 1)4% iron to cupronickel can greatly increase its erosion-corrosion resistance for the same reason. Similarly, chromium added to iron-base alloys and molybdenum added to austenitic stainless steels will increase resistance to erosion-corrosion. 

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. 

In addition to ductile iron and PVC, copper and lead are used in pipes, and brass in fixtures and connections. Lead is released because of uniform corrosion. Copper is also released because of uniform corrosion, localized-attack cold water pitting, hot water pitting, MIC, corrosion fatigue, and erosion-corrosion. Lead pipes and lead-tin solder exhibit uniform corrosion. Brass corrosion includes erosion-corrosion, impingement corrosion, dezincification, and SCC. The direct health impacts are because of increased copper, lead, and zinc concentrations in the drinking water. Mechanical problems because of corrosion include leaks from perforated pipes, rupture of pipes, and the loss of water pressure because of blockage of pipes by corrosion products. 

The original saltwater condenser tube made of admiralty brass was found to be susceptible to erosion-corrosion at tube ends. Aluminum brass containing 2% aluminum was more resistant to erosion in saltwater. Inhibition with arsenic is necessary to prevent dezincification as in the case of admiralty brass. The stronger naval brass is selected as the tube material when admiralty brass mbes are used in condensers. Cast brass or bronze alloys for valves and fittings are usually Cu-Sn-Zn compositions, plus lead for machinability. Aluminum bronzes are often used as tube sheet and channel material for exchangers with admiralty brass or titanium tubes exposed to cooling water. 

Bronzes are copper-tin alloys which upon prolonged contact with the atmosphere form a dark patina that is much appreciated in the art world. In presence of certain pollutants such as chloride the dark patinas eventually turn to green. Aluminum-containing bronze forms surface films containing AI2O3 which improves the resistance to erosion corrosion compared to copper or brass. 

The repeated impact of liquid against a solid surface using an intermittent jet at sufficiently high speeds leads to a form of erosion. At impact, a liquid drop produces a very high compressive stress in the vicinity of the area of contact, and this is followed by outward, radial flow of liquid at very high speed which shears and erodes the surface. Erosion-corrosion in alpha-brass comprises two parts. The initial progressive plastic indentation is followed by the formation of an annulus where pits gradually form. 

Figure 6.40 Erosion-corrosion of a brass tube carrying out seawater. (Courtesy of Defence R D Canada-Atiantic)...

Admiralty brass is not particularly resistant to erosion-corrosion. It should not be used in water flowing at velocities exceeding 2 m/s. They give good service as condenser tube materials in ships condensers. 

Aluminum brass is resistant to erosion-corrosion and dezincification. It is used as condenser heat exchanger tube material for marine applications. A good example is the use of Al-Brass (76-79 Cu, 1.8-2.5 Al, 0.005 As, 0.007 Pb) as in marine service. 

An example of this type of attack is erosion-corrosion of brass tubes in boiler reheaters. The attack takes place where the direction of flow changes. Overheating and local boiling takes place with a disruptive effect on protective films, particularly at the exit and the entry, where turbulence is the greatest. 

Figure 13.15 The tricolor internal surface of a leaded-brass bushing. Greenish-blue corrosion product overlies the red of the generally dezincified surface. To the extreme right, only bare metal is visible due to erosion.

Dents in tubing can induce erosion failures, especially in soft metals such as copper and brass. Welding and improper heat treatment of stainless steel can lead to localized corrosion or cracking through a change in the microstructure, such as sensitization. Another form of defect is the inadvertent substitution of an improper material. 

Due to using traditional materials-brass nozzle in 131419 working face, this type of spray nozzle are likely to cause destruction of the surface material owing to high velocity flow erosion and corrosion in the process of using, which make nozzle increase the amount of water but decrease the degree of spray. 

Early rotors were often manufactured of steel or brass, but now, they are more commonly constructed of aluminum and titanium (Table 2). Newer composite materials have made tremendous gains in popularity, with plastics for small-scale applications, and stainless steel for industrial-scale units in common use. Though somewhat more expensive, titanium is particularly suitable as it has both a higher strength-density ratio and a high resistance to corrosion and erosion. 

Of interest in this connection is the fact that atmospheric corrosion mainly occurs in the form of a more or less uniformly distributed scar-shaped erosion. However, other forms of corrosions are also encountered. Thus, for instance, the impact of a humid atmosphere on zinc alloys may produce intercrystalline corrosion, and the impact of a humid atmosphere containing NH3 may lead to stress cracks in brass components. 

A corrosion mechanism similar to brass dezincification, known as dealuminification, can occur to the beta phase or eutectoid structure depending on environmental conditions. Proper quench and temper treatments produce a tempered beta structure with reprecipitated acicular alpha crystals, a combination often superior in corrosion resistance to the normal annealed structure. The nickel component in the more complex nickel aluminum bronzes alters the corrosion characteristics of the beta phase because of the nickel additive and gives greater resistance to deaUoying and cavitation-erosion in most liquids. 

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