Patent Application
20120058005
The present application is directed to a machinable silicon brass alloy, and more particularly to a copper (Cu) corrosion resistant, machinable silicon brass alloy with low amounts of lead (Pb).
In the continuing effort to reduce the health hazards due to heavy metals such as lead (Pb), Assembly Bill 1953 (“AB 1953”) by the State of California has mandated a substantial reduction of lead (Pb) in plumbing hardware. As a result, the use of low or no lead (Pb) materials (especially in brass) is now essential for continued sale of products starting in year 2010. It is noted that statutory and legal references to “no lead (Pb)” requirements, in fact do not require zero lead (Pb) content, but permit small amounts of lead (Pb), such as amounts less than 0.25% by weight, while still allowing the alloy to be commercialized as a “no lead (Pb)” product. As set forth herein, lead (Pb) content references shall specify the lead (Pb) alloy percentages present as “no” or “low,” which when used in this context shall mean zero to low percentage amounts, as well as by using specific percentages. It should be understood that the governmentally permitted use of small amounts of lead (Pb), as opposed to a lead (Pb)-free mandate, more readily enables the use of recycled materials, which are typically very difficult to ensure are entirely free of all lead (Pb). Moreover, the use of somewhat higher levels of lead (Pb), such as up to 1% by weight, may continue to be acceptable in non-potable water conduit components.
The reduction or elimination of lead (Pb) has also brought new challenges to manufacturing. That is, the reduction or removal of lead (Pb) greatly reduces the machinability, and this adds significant cost burden to the manufacturing operations due to the increased cycle times. Furthermore, because the amount of lead (Pb) removed is typically replaced with corresponding amounts of zinc (Zn) at fixed (Cu) contents in the brasses, the additional zinc (Zn) increases the amount of zinc (Zn)-rich beta′ (β′) phase and hence the susceptibility of the brass to a dezincification failure.
It may be easily noted in the copper-zinc (Cu—Zn) phase diagram, a one % point variation in zinc (Zn) content within the two phase (α+β′) phase field affects the amount of β′ by as much as 15% points as shown in the binary phase diagram of copper (Cu) and zinc (Zn). This is because the two phase (α+β′) field is only about 7% point wide. As it is the β′-phase that is more susceptible to dezincification, it is a widely held belief that keeping the zinc (Zn) content low is the first easy step to mitigate the potential for dezincification. The zinc (Zn)-rich β′-phase is in the trade commonly referred to as β-phase and as such, β-brass contains β′-phase as well as a matrix phase.
In the brass and plumbing industry, in order to address such potential dezincification effects, a low zinc (Zn), high copper (Cu) brass is often used. The zinc (Zn) concentration is kept at 20% by weight or lower, compared to 35-40% zinc (Zn) of more commonly used brasses. However, low zinc (Zn) brasses have extremely poor machinability. This has emerged as a major barrier to process transferability from highly machinable conventional leaded brass to a variety of no or low lead (Pb) brass options. In the recent years, a new class of brass alloys has become available as an answer to the machinablity and dezincification concerns. The alloys contain 2.5-3.5% silicon (Si) by weight. The addition of silicon (Si) is believed to improve machinability.
Laboratory studies, however, have found that such silicon (Si) brasses have serious weaknesses in other types of corrosion. Naturally, due to the low zinc (Zn) level in the brass, the tendency of dezincification has greatly reduced; but the tendency for corrosion of copper (Cu) has increased so much to the point that the silicon (Si) brass may be inappropriate or unfit for certain applications where corrosion of copper (Cu) may, under some circumstances, over time, compromise the functionality of the component made of such brass.
The prior art silicon (Si) brass alloys generally contain a small amount (less than 0.4%) of tin (Sn) for certain types of non-copper corrosion resistance, but this is not considered enough to suppress the corrosion of copper (Cu). The use of such silicon (Si) brass alloys, or lead (Pb)-free products, for example, such as commercially available ECO BRASS® from Chase Brass & Copper Company, Inc. and Ingot Metal Company Limited of Toronto, Ontario, which was developed by Sambo Copper/Mitsubishi Shindoh Ltd., as set forth in numerous U.S. patents, including, for example, U.S. Pat. Nos. 6,413,330 and 7,056,396, focus on the environmental advantages and governmental requirements of lead (Pb)free alloys, but have yet to address the potential risks associated with structural weakness due to copper (Cu) corrosion. It is noted that such prior art materials focus on zinc (Zn) corrosion or de-zincification and erosion corrosion, which is the acceleration or increase in rate of deterioration or attack on a metal because of relative movement between a corrosive fluid and the metal surface. Copper (Cu) corrosion is not addressed. In erosion corrosion, metal is removed from the surface as dissolved ions, or it forms solid corrosion products that are mechanically swept from the metal surface. Such alloys certainly have resistance to zinc (Zn) corrosion or dezincification, and in many circumstances of water chemistry have some resistance to erosion corrosion, due to the creation of a skin or layer of oxidation at the internal diameter surface of the components. Studies have not been conducted to determine the time line for potential risks of component failure due to copper (Cu) corrosion, and particularly in plumbing products where a thin walled, low lead (Pb) or no lead (Pb) product is under pressure and is required to be fit for passing potable water. Commonly conducted dezincification tests fail to address the corrosion of copper (Cu) because the tests are conducted in a highly concentrated aqueous solution of copper chloride (CuCl2), and as such the high concentration of copper (Cu) ions in the test solution prohibits or suppresses the corrosion of copper (Cu) in the brass test articles.
This application provides an alloy for specifically improving copper (Cu) corrosion resistance, with a particular interest in dezincification resistance and resistance to copper (Cu) corrosion, of low lead (Pb) brass materials; and at the same time, enhancing the machinability of the low lead (Pb) brasses. The application provides an alloy which reduces or replaces lead (Pb) in brass with metals with physical, mechanical, chemical and electrochemical properties that will improve the corrosion resistance and machinability of low lead (Pb) brass alloys. Alloying element candidates are bismuth (Bi), antimony (Sb), tellurium (Te), phosphorous (P), silicon (Si), sulfur (S) for machinability improvement; and tin (Sn) for corrosion resistance improvement. These elements along with other elements of brass alloy become metallurgically integral such that some parts of the added metals form solid solution with brass to impart corrosion resistance, and some stay as an isolated metallurgical phase(s) to facilitate good machinability.
It is further understood that the use of the following alloy composition, along with the specific additions mentioned to reduce copper (Cu) corrosion, is preferred. The alloy having an excellent machinability and exhibiting a high degree of copper (Cu) corrosion resistance, which is composed of: 69 to 79%, by weight, of copper (Cu); 2 to 4%, by weight, of silicon (Si); 1 to 3%, by weight, of tin (Sn); 0.01 to 1%, by weight, of lead (Pb); and the remaining %, by weight, of zinc (Zn). This copper (Cu) alloy will be called the “first alloy.” It is understood to those of skill in the art, and is likewise to be understood herein, that the stated amounts of the referenced components include a common margin of error resulting from the weight measurement calculation.
Tin (Sn) is effective in improving not only the machinability but also the copper (Cu) corrosion resistance properties of the alloy. The first alloy is thus improved in copper (Cu) corrosion resistance by such properties of tin (Sn). To raise the copper (Cu) corrosion resistance, tin (Sn) would have to be added in an amount of at least 1% by weight, and preferably about 2% by weight. If the addition of tin (Sn) is higher than 2% by weight, for example, about 3% by weight or higher, additional alloy corrosion resistance and hardness may be obtained, but if it is added to between about 3 and 6% by weight, the corrosion resistance and machinability will continue to improve, but the additional amounts of tin (Sn) may render the alloy uneconomical, depending on the application.
Additional improvements to the first alloy may be had by addition of the various metals indicated. Bismuth (Bi) has a rhombohedral (R-3m) crystal structure that is structurally incompatible with copper (Cu, Fm-3m) or zinc (Zn, P63/mmc), and therefore has very low solid solubility into the brass (Fm-3m, Im-3m, Pm-3m). Furthermore, bismuth (Bi) does not even form any stoichiometric intermetallic compounds with either copper (Cu) or zinc (Zn). Consequently, the added bismuth (Bi) will solidify as independent grains, which embrittle the chips during machining and hence an improvement of the machining speed and productivity. The amount of the metal addition may be considered small (preferably 2% or less by weight, but may be as much as 5%), but the net effect on the machinability is large enough to consider the alloys comparable to the leaded brass alloys.
A further improvement in machinability may be achieved by metallurgically forming or by directly adding particles of metal oxides, silicides, sulfides or phosphides (for example, cerium oxide, chromium oxide, manganese oxide, molybdenum silicide, molybdenum sulfide, copper phosphide, iron phosphide) in the melting and casting process of brass making The optimum level of phosphorous or sulfur from the addition of phosphides or sulfides for the desired machinability may be varied and selected for each different applications based on the optimum combination of mechanical, thermal, electrical, electrochemical, chemical and physical properties of the resulting brass. The phosphides and sulfide phases that forms within brass embrittles brass and as a result the brass becomes easily machinable. The phosphorus or sulfur levels and the resulting degree of brittleness will be determined based on the applications of the brass. For oxide additions, maintaining the median particle size of the metal oxide particle size below 2 microns, a substantial improvement in the strength is also achieved.
Similar to copper (Cu), zinc (Zn) in brass also may react with phosphorus (P) and sulfur (S) to form brittle compounds (zinc phosfides or zinc sulfides) and improve the machinability of brass. Further similar effects of improving machinability may be attempted by using selenium (Se) or arsenic (As), but the toxicity concerns make them unsuitable for potable water services even if they are not controlled under AB 1953.
An additional second formulation of the first alloy keeps the zinc (Zn) content to below 20% by weight such that the fraction of beta′ (β′) phase is at 5% or lower or approaches zero % by volume to minimize the potential risks of dezincification failure during service in the widely varying ranges of water chemistry.
It is further understood that the use of the following third alloy composition, along with the specific additions mentioned above to reduce copper (Cu) corrosion, is preferred. The alloy having an excellent machinability and exhibiting a high degree of corrosion resistance, which is composed of: 69 to 79%, by weight, of copper (Cu); 0.01 to 1%, by weight of lead (Pb); 2 to 4%, by weight, of silicon (Si); 2 to 3%, by weight, of tin (Sn); and the remaining %, by weight, of zinc (Zn), but less than 20%. This third alloy will be called the “third alloy.”
It is further understood that the use of the following fourth alloy composition, along with the specific additions mentioned above to reduce copper corrosion, is preferred. The alloy having an excellent machinability and exhibiting a high degree of corrosion resistance, which is composed of: 69 to 79%, by weight, of copper (Cu); 0.01 to 1%, by weight of lead (Pb); 2 to 4%, by weight, of silicon (Si); 3 to 6%, by weight, of tin (Sn); and, and the remaining %, by weight, of zinc (Zn), but less than 20%. This alloy will be called the “fourth alloy.”
Desired improvement in copper (Cu) corrosion resistance is achieved by the addition of tin (Sn) into the brass alloy. Tin (Sn) in particular, as can be noticed in the Potential-pH Diagram (
The present application provides an improved copper (Cu) corrosion resistant, machinable brass alloy with at least low amounts of lead (Pb), such as 0.01 to 1%, by weight, and between 1 and 3%, by weight, of tin (Sn), to resist copper (Cu) corrosion during use, and particularly during use in high pressure, thin walled plumbing fixtures. To confirm the ideal desired combination of copper (Cu)corrosion resistance, machinability and hardness, a test series of sample alloys was used having the compositions set forth in Tables 1, 2 and 3 below, in the form of cylindrical stock having a 1-1.025 inch diameter.
A sample of prior art brass alloys were tested to obtain bench marks regarding corrosion resistance. During the testing, an electrode area of 0.25 cm2, was mounted in epoxy. An electrolyte of 80 mL of simulated tap water was used (Schock's 4 Water, US EPA) having a pH of 8, alkalinity of 475-525 ppm, free chlorine (Cl2) of 2 ppm, and a temperature of 22° C. The reference electrode used was a Saturated Calomel Electrode separated by a cracked stopcock bridge and Luggin capillary tip. A counter electrode was platinum separated by glass frit. The test was open to the air with no active aeration. A 1 cm magnetic stir bar provided agitation at about 60 rpm.
Still further chronoamperometric corrosion tests were also conducted. The results are presented in Ampere versus time, i.e., the variation of corrosion current with respect to time. In the chart, the corrosion current decreases rapidly with time in materials with higher corrosion resistance; whereas the curve will persist to stay at a higher current if the material is susceptible and stays susceptible to corrosion. The results of the commercial alloy silicon brass, C69300, shown by the solid line, and a common brass, C27450, indicated by the dashed line, as shown in
Chronoamperometric corrosion tests were also conducted on the applicant's improved copper (Cu) corrosion resistant, machinable brass alloys containing tin (Sn) samples in 3 test series. Test series No.1 comprised the test alloys listed in Table 1 below, where 0% by weight of tin (Sn) was included in all samples, and the weight % of lead (Pb) was varied as shown from 0 to 1 over the 5 samples. In test series No. 2, shown in Table 2 below, the weight % of tin (Sn) was 3% in all samples, and the weight % of lead (Pb) was varied as shown from 0 to 1 over the 5 samples. In test series No. 3, shown in Table 3 below, the weight % of tin (Sn) was 6% in all samples, and the weight % of lead (Pb) was varied as shown from 0 to 1 over 5 samples. Measurements were done in simulated tap water (Schock's 4, EPA); the electrode potentials in volt versus saturated calomel electrode (SCE); the electrode potential held at +250 mV versus SCE, with the further testing as described above.
The test results shown in
In addition to copper (Cu) corrosion testing, hardness testing was also conducted by measuring the Rockwell B-scale hardness (HRB) of each test series sample at mid-radius of the test sample cylinder. The results, shown in
Cutting tests were also conducted to review the machinability of applicant's alloys in comparison with the conventional alloys. In the cutting tests, evaluations were made on the basis of chip color, size and shape. The tests were conducted by mounting the cylindrical test samples on a lathe, where a tool cut the samples at a cutting speed of 250 feet per minute, and a feed of 0.01 inches per revolution. The chips from the cutting work were examined and are shown for example, for the conventional brass alloys, C36000 (a free-cutting brass having the components previously described), C36500 (a brass composed of 60% Copper (Cu), 0.6% Lead (Pb), and 39.4% Zinc (Zn)) and C69300 (a lead (Pb)-free silicon brass having the components previously described), in
Additionally a further qualitative tests for hardness and machinability was conducted. The test measured the time to cut the test series samples of bar stocks with a hack saw blade. The saw blade was kept at a constant load. As lead (Pb) content increases the machinability tended to increase, but only up to a certain point, beyond which the saw blade becomes less effective due to lead (Pb) stuck in the teeth of the blade. The results of the time to cut a sample with a hack saw test is shown in
It should be understood that applicant's current corrosion resistant brass or copper (Cu) alloy generally contains a balance of components to achieve improved copper (Cu) corrosion resistance, and machinability as well as lower cost conduit applications and plumbing fixtures of the type shown in
While the present brass alloy has been described with reference to certain preferred embodiments, one of ordinary skill in the art will recognize that additions, deletions, substitutions, modifications and improvements can be made while remaining within the spirit and scope of the present invention as defined by the appended claims.
The present application claims priority from U.S. Patent Application Nos. 61/265,208 and 61/265,215, both of which having a filing date of Nov. 30, 2009, and the entire disclosures of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61265208 | Nov 2009 | US | |
| 61265215 | Nov 2009 | US |
