Bronze corrosion rates

Copper-alloy corrosion behavior depends on the alloying elements added. Alloying copper with zinc increases corrosion rates in caustic solutions whereas nickel additions decrease corrosion rates. Silicon bronzes containing between 95% and 98% copper have corrosion rates as low as 2 mil/y (0.051 mm/y) at 140°F (60°C) in 30% caustic solutions. Figure 8.2 shows the corrosion rate in a 50% caustic soda evaporator as a function of nickel content. As is obvious, the corrosion rate falls to even lower values as nickel concentration increases. Caustic solutions attack zinc brasses at rates of 2 to 20 mil/y (0.051 to 0.51 mm/y). 

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. 

Bronze, Zinc, Aluminum, and Galvanized Steel Corrosion Rates as a Function of Space and Time over the United States... 

Bronze data is sparse. An alloy of 92% Cu exposed from 1931 1951 showed no corrosion in the desert environment and very little at the rural sites. Coincident zinc data indicates that over the 20 year period the average corrosion rates were higher for bronze at marine sites but much lower for bronze than for zinc at rural sites (Figure 2). 

At the rural State College, PA, site the corrosion rate for two different bronze alloys during the 1958-1978 period was nearly twice that of the earlier bronze exposure, while the industrial Newark area corrosion rates appeared to have declined. 

A bronze with 99% Cu in alloy with 1.25% Sn and P was exposed during the 1958-1978 period at the four standard ASTM sites. The corrosion rates at the rural site and the western marine site were less than half those at the eastern marine and industrial sites. 

A bronze alloy more representative of statuary bronzes (5% Sn and P) was also exposed at the four standard sites. The rural and Western marine sites again show corrosion rates less than half those of the eastern marine and the industrial site. The corrosion rates at the industrial and the marine site increase with exposure time for this alloy, in marked contrast to the usual pattern of declining corrosion rate with time. It is also curious that the measured loss of strength at the rural PA site is disproportionately large for its corrosion rate (1.4% vs 2.0% at the industrial site). 

The bronze alloys were exposed at the same time and place as the zinc during the period 1958-1978, with measurements at 2, 7 and 20 year exposures. Examination of the corrosion rates revealed two separate patterns a marine trend of high initial corrosion rate with sharp reduction in rate after the first few years, and an industrial-rural trend correspond to different chemical mechanisms at work, with only the industrial-rural corrosion being related to sulfur compounds. 

The corrosion rates of mild steel, zinc, aluminum, and copper are based on the results of 5 years of exposure. The rates for bronze are based on 4 years of exposure at the same sites, except for Cape Town, where the bronze metal was exposed at Ysterplaat, near Cape Town, not at the docks. 

Cathodic protection can be successful only under immersed conditions and, therefore, only the parts of a ship below the waterline (and holds filled with seawater ballast) can be protected by this method. Ideally, the ship is painted in dry dock to the required standard, and zinc anodes are fitted below the waterline to protect the steel and paint. Since, in general, a zinc anode in seawater will protect all the steel within a radius of about 3 m, anodes are spaced at distances of about 6 m. Because of the turbulence around the stem and because bronze propellers often are present, corrosion rates are higher in this area and more anodes are placed there. 

Studies on samples exposed underground have shown that tough pitch coppers, deoxidized coppers, silicon bronzes, and low-zinc brasses behave essentially alike. Soils containing cinders with high concentrations of sulfides, chlorides, or hydrogen ions corrode these materials. In this type of contaminated soil, alloys containing more than 22 % zinc experience dezincification. In soils that contain only sulfides, corrosion rates of the brasses decrease with increasing zinc content and no dezincification occurs. 

Corrosion rates of uncoupled brass and bronze were determined both by potentiodynamic polarization and by weight loss and dezincification weight loss depth measurements. 

Corrosion caused by the connection of two or more different metals also occurs underground. This electrochemical corrosion cell is commonly referred to as bimetallic or galvanic corrosion. Typical examples include brass or bronze valves connected to steel or cast iron pipes and stainless steel fasteners coimected to steel or cast iron. These couplings of dissimilar metals will locally affect the corrosion rate. Aluminum can be severely corroded if directly connected to most other engineering alloys, such as steel, iron, copper, or stainless steel—dielectric isolation must be used. 

Analysis of the results after 1 and 2 years exposure indicates that the influence of SO2 on the corrosion rate of carbon steel, weathering steel, zinc, bronze, sandstone, limestone, and nickel is significant. Conversely, no influence of NO2 has so far been observed in any of the materials studied. This discrepancy between the laboratory exposures and the field tests must be investigated. Perhaps as a result of catalysts or oxidizers in field exposures, which aid in promoting the oxidation of S(IV) to S(VI), the effect of NO2 was hidden. 

Silicon bronze has approximately the same corrosion resistance as copper, but better mechanical properties and superior weldability. The corrosion rates are affected less by oxygen and carbon dioxide contents than with other copper alloys. Silicon bronzes can handle cold dilute hydrochloric acid, cold and hot dilute sulfuric acid, and cold concentrated sulfuric acid. They have better resistance to SCC than the common brasses. In the presence of high-pressure steam, silicon bronze is susceptible to embrittlement. 

Bronze containing 2.4% tin showed a corrosion rate of. 05 mm/year at a velocity of 0.6 m s The corrosion rate of important brasses, bronzes and copper in seawater are shown in Table 9.35. 

Copper-tin alloys (76 Cu-22 Zn-2 Al), also known as tin bronze, show increased corrosion rates with increased temperature compared to Cu-Al alloys. The corrosion resistance of copper-aluminum alloys increases with increasing aluminum content. Alpha alloys (or-aluminum bronze) consists of single (alpha) phase up to 7% aluminum and two phase (or -t- or a -f y) above 7%. Aluminum bronzes exhibit an outstanding resistance to corrosion in seawater and offer a good resistance to impingement corrosion. 

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