Nickel-aluminum bronze, corrosion

J.A. Duma, Heat Treatments for Optimizing Mechanical and Corrosion-Resisting Properties of Nickel-Aluminum Bronzes, Nov. Eng. J., Vol 87, 1975, p 45-64... 

Figure 15.4 Mapping electrochemical material loss against mechanical erosion rates for a nonpassivating surface carbon steel (AISI1020) along with two potentially passivating surfaces of nickel aluminum bronze (NAB) one that has been thermally sprayed by high-velocity oxy-fuel deposition as a coating on carbon steel ( j and another which has been cast (A.). These results were obtained from jet impingement erosion-corrosion tests. Reprinted from Ref. [7]. Copyright (2007) with permission from Elsevier.

Figure 15.5 Sliding wear-corrosion of nickel aluminum bronze in 3.5% NaCl solution. Stroke 21.5 mm, 6 mm-diameter alumina ball reference electrode Ag/AgCI counterelectrode graphite Static 60 min wear-corrosion 60 min and 10 min post wear-corrosion.

Figure 15.6 Sliding wear-corrosion of nickel aluminum bronze in 3.5% NaCI solution at 35 N and room temperature.

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. 

Some metals depend on formation of a protective film for corrosion resistance in sea water. A fresh supply of oxygen brought to the surface of the metal tends to promote the corrosion reaction in some cases, and in others it helps form desired protective films. If a critical velocity of flowing sea water is exceeded, the film may be eroded away. The velocity for useful corrosion resistance is low for copper, higher for aluminum, cupro-nickels, and aluminum bronzes, and highest for stainless steels, Hastelloy C, and titanium. 

Although the degree of atmospheric corrosion of copper and its alloys depends upon the corrosive agents present, the corrosion rate has been found to generally decrease with time. The copper and its alloys such as silicon bronze, tin bronze usually corrode at moderate rates, while brass, aluminum bronze, nickel silver, and copper-nickel corrode at a slower rate.51 The most commonly used copper alloys are Cl 1000, C22000, C38500 and C75200. 

Dealuminification occurs in aluminum bronzes containing the y-2 phase microstructure and the process is more severe when the y-2 phase forms a continuous grain boundary network. Dealuminification can be averted by rapid cooling from >600°C, or by addition of 1-2% iron or more than 4.5% nickel. Microstmctural changes can still occur during welding and lead to corrosion in the heat-affected zone. 

Hydrochloric acid as a hydrolyzer and as a product of hydrolysis in the absence of alkali has been the source of much trouble as it is one of the most corrosive chemicals known. However, even at slightly elevated temperatures, completely dry hydrogen chloride gas has very little corrosive action and is easily handled in iron equipment. Nickel and Monel metal are fairly resistant to low hydrochloric acid concentrations. With dilute acid,. several of the copper-base alloys, si ch as phosphor bronze, aluminum bronze, manganese bronze, ahd Everdur metal, have fairly good... 

The brasses also perform well in unpolluted freshwaters but may experience dezincification in stagnant or slowly moving brackish or slightly acidic waters. The copper-nickels, silicon and aluminum bronzes display excellent resistance to corrosion. 

The superior performance of the inhibited admiralties, aluminum brass, aluminum bronzes, and copper-nickels over copper results from the combination of their corrosion product insolubility in seawater, erosion, and biofouling resistance. 

Aluminum bronzes containing 7% Al, 2% Ni, show an outstanding resistance to de-alloying and cavitation corrosion in most fluids and seawater, because of nickel addition which is highly resistant to corrosion. Aluminum bronze, such as 76 Cu-22 Zn-2 Al, are used for marine heat exchangers and condenser because of its excellent corrosion resistance. Aluminum is responsible for increased corrosion resistance. But the velocity must not exceed a safe threshold to avoid erosion-corrosion. 

IV. 12% Cr 400 series steels, pH corrosion-resistant steels, 18% Cr 400 series steels, chromium, brass, bronze, copper, beryllium copper, aluminum, bronze alloys, 300 series stainless steels. Monel, Inconel, nickel alloys, titanium alloys... 

The outstanding properties of copper-base materials are high electrical and thermal conductivity, good durabihty in mildly corrosive chemical environments and excellent ductility for forming complex shapes. As a relatively weak material, copper is often alloyed with zinc (brasses), tin (bronzes), aluminum and nickel to improve its mechanical properties and corrosion resistance. 

The metals recommended for use with ethanol include carbon steel, stainless steel, and bronze [3.10]. Like methanol, metals such as magnesium, zinc castings, brass, and copper are not recommended. Aluminum can be used if the ethanol is very pure, otherwise it should be nickel-plated or suitably protected from corrosion by another means. The metals compatible with ethanol represent a much wider range than those for methanol and represent most of the metals currently used in fuel systems, so few changes would be anticipated when using ethanol. 

Low resistance to heat flow was obtained in the wall of the tube by using oxygen-free copper for the tube. This material possesses high conductivity and excellent corrosion resistance in most saline water environments. For special applications where copper is not suitable, bronze or aluminum brass could be used. Low conductivity material such as Monel and copper-nickel is not suitable. 

These tests illustrate graphically the unsuitability of copper, brass, bronze, galvanized iron, and aluminum as materials to be used with lithium at any temperature above its melting point. There is appreciable attack on most of the common corrosion-resistant alloys containing nickel at temperatures below 500°C. (12). 

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