Lead-Free Copper-Base Alloys
Copper-based alloys are used widely in the metalcasting industry because of their ability to economically deliver a wide range of properties. One of copper’s more appealing characteristics is its good corrosion resistance, which can result in increased flow capacity. Because of this, plumbing components in North America have traditionally been made from leaded red and semi-red brasses (alloys C83600 and C84400) via green sand molding.
For copper-based alloys to maintain their desirable characteristics and remain a viable metalcasting option, certain elements are added to them. Included in these is lead, which is added to improve machinability and ensure pressure tightness.
The problem with lead is it has been linked to a variety of adverse health concerns. Swallowing or breathing in lead paint chips and dust causes the greatest exposure to lead. But lead used in plumbing fixtures can find its way into drinking water and become hazardous as well.
With the passage of the 1996 Amendment to the Safe Drinking Water Act, which limits the amount of lead in drinking water, the need for a lead replacement in brass castings used for potable water systems emerged with a new sense of urgency.
Why Lead is Being Controlled
The Safe Drinking Water Act, which governs drinking water quality in municipalities and rural water districts, was established in 1974 to minimize chemical and bacterial contaminants in drinking water. The 1996 Amendment includes the regulation of leaded plumbing fittings and fixtures and provides for the establishment of voluntary standards for lead leaching. All faucets, drinking fountains, water coolers and other drinking water conveyance devices must pass National Sanitation Foundation (NSF) Standard 61.
The standard requires certification of devices, not alloys, because devices constructed from the same alloy may release different amounts of lead depending on differences in surface area exposed to water. In addition, the 11 parts per billion (ppb) limit for lead under NSF 61 is less than the 15 ppb limit in the EPA’s Lead and Copper Rule, which is the level at which public water utilities are required to take action to control water chemistry to reduce corrosion. The reason for the discrepancy is that EPA assumes that up to 4 ppb of lead could be picked up from other sources.
Role of Lead in Brass
Lead is immiscible in copper and its alloys, and it forms discrete globules that are retained when the metal solidifies. These globules are uniformly dispersed in the alloy matrix and act as a lubricant during machining, especially at the high speeds typical in high-volume manufacturing. In addition, lead fills the intra-dendritic spaces that form when castings solidify. This results in pressure tightness, thus preventing the seeping of water though a series of voids that may interconnect to form a continuous, but tortuous, path from the inner to the outer surface of plumbing castings.
The application of non-leaded brass castings (which also may be described as low-lead alloys because no lead is intentionally added to them) is an important approach being taken to reduce the lead intake in drinking water. A variety of other approaches to meet the lead-release provisions of NSF 61 also have been attempted or are being utilized, including organic and inorganic coating, chemical removal of interior surface lead and reduction of internal surface areas by design changes.
Lead-Free Copper Alloys
Most lead-free alloys contain bismuth as the major alloying element. Bismuth replaces lead in the copper alloys and contributes to the machinability and pressure tightness characteristics. Similar to lead, bismuth almost is completely insoluble in copper and has a low melting point. Further, it is not known to be toxic to humans.
Some lead-free alloys also contain selenium as the alloying element due to its effect on machinability. Initial research at the Asarco Technical Center, Salt Lake City, revealed that selenium, in combination with bismuth in brass casting applications, considerably increases the machinability. This also reduces the total amount of bismuth in the alloy. Selenium, similar to copper, is an essential nutrient for humans.
In response to this need to reduce the amount of lead in drinking water applications, the Copper Development Assn. (CDA), New Yor, entered into an alloy development program with the American Foundry Society (AFS), Schaumburg, Ill., the Brass and Bronze Ingot Manufacturers Inc. (now the Brass and Bronze Ingot Industries Inc.) and the Canadian Materials Technology Laboratory (CANMET), Ottawa, Ontario, Canada. The goal was to find a substitute for lead in brass plumbing castings. Specifically, the objective was to develop non-leaded replacements for the three most common leaded brass casting alloys.
The most common red brass, UNS Alloy C83600, is called 85 metal, or 85-5-5-5. It contains 85% copper and 5% each of tin, lead and zinc. The next is UNS Alloy C84400, a semi-red brass known as 81 metal, or 81-3-7-9. It contains 81% copper, 3% tin, 7% lead and 9% zinc. These two alloys are traditionally sand cast. UNS Alloy C85800 is a standard yellow brass for permanent mold casting, containing 31-41% zinc and 1.5% each of lead and tin, with the remainder being copper.
The lead replacement choices were narrowed to bismuth and selenium. The result was a family of non-leaded bismuth- and selenium-containing brass casting alloys called EnviroBrass alloys (earlier known as SeBiLOYs).
EnviroBrass I and II (C89510 and C89520) were developed to replace C83600 leaded red brass and C84400 semi-red brasses. EnviroBrass III (C89550) is a substitute for C85800 leaded yellow brass. The compositions of these alloys are presented in Table 1.
Table 1. Compositions of EnviroBrass Alloys
|Nickel (including Cobalt)||1%||1%||1%|
EnviroBrass I and II have received ASTM accreditation and are listed in ASTM B584. EnviroBrass III currently is under evaluation for an ASTM listing.
In addition to the EnviroBrasses, there are several other patented copper-based low-lead or lead-free alloys available. All of them invariably contain bismuth. Other conventional copper alloys, such as silicon-brasses and bronzes, contain little to no lead and could be considered lead-free alloys. These alloys offer excellent casting characteristics in sand and permanent molding operations and are potential candidates for drinking water applications.
The EnviroBrasses were developed to have characteristics (melting and casting) as well as properties (physical and mechanical) similar to the existing leaded alloys (Table 2). In other words, there are no differences in the metalcasting practices currently being used for leaded alloys. Further, existing cast metal components do not need design modifications to satisfy property requirements of lead-free alloys. However, the data should be interpreted with caution as it is from laboratory or pilot-scale trials. In large-scale production, some problems may lead to lower production rates and increased costs.
Studies on gating and risering of these alloys have revealed that conventional systems employed for leaded alloys could successfully be used. In addition, bismuth-containing alloys perform better in respect to hot tearing, which could be used to improve the design of many existing components.
Table 2. Casting Characteristics of EnviroBrass Alloys and Common Leaded Alloys
|Alloy||Liquidus||Freezing Range||Fluidity (cm at 100F superheat)||Relative Hot Tearing Resistance||Drossing||Soldering||Brazing||Effect of Section Size on Mech. Properties|
Even though EnviroBrasses possess similar physical and mechanical properties as leaded alloys, they suffer from low ductility (Table 3). As a result, care should be exercised while machining these alloys.
Table 3. Mechanical Properties of EnviroBrass Alloys & Common Lead Alloys
|Alloy||Ultimate Tensile Strength||0.5% Yield Strength||% Elongation||Hardness [(500 kg) BHN]|
|C83600 (Typical)||37 ksi||17 ksi||30||60|
|C84400 (Typical)||35 ksi||14 ksi||28||55|
|C89510 (Typical)*||30 ksi||20 ksi||12||71|
|C89510 (Minimum)*||27 ksi||17 ksi||8||66|
|C89520 (Typical)*||31 ksi||21 ksi||10||73|
|C89520 (Minimum)*||21 ksi||18 ksi||6||68|
|C85800 (Typical)||48 ksi||28 ksi||9||60|
|C89550 (Typical)*||48 ksi||29 ksi||8||63|
|C89550 (Minimum)*||35 ksi||21 ksi||5||60|
These new alloys have machining characteristics similar to the leaded brasses but require fluids for optimum machining performance (Table 4).
Reports of the alloys cracking during their machining operations may be due to excessive loads applied during un-lubricated machining. But moderate loading and some cooling could mitigate this problem. Another aspect in which the lead-free alloys differ from their leaded counterparts is marginally lower impact strength, which could be an effect of lower ductility.
Table 4. Machinability Ratings of EnviroBrass Alloys & Common Leaded Alloys
|Alloy||Machinability Rating (multiple operations)|
|C36000 (Free-Cutting Brass)||100|
|C89550 (with 0.7% Bi)*||75|
|C89550 (with 1.1% Bi)*||97|
Physical & Chemical Treatments
Alternate techniques for controlling lead leaching include metal treatments and coatings. Metal treatments that remove the surface lead in contact with potable water while maintaining the existing base metal characteristics are commercially available now.
Coatings also are being applied to the water contact surfaces to prevent lead from leaching into the drinking water. Many of these alternate treatment and coating methods already have obtained proper NSF product approvals.
Life Without Lead
Copper-based castings are at their best when the end use calls for a component that can hold pressure tightness while delivering good corrosion-resistant characteristics.
The use of lead-free copper-based casting alloys has gained acceptance recently due in part to EPA and NSF standards. Combined, the lead-free alternative alloys have displaced about 8% of the market traditionally dominated by leaded red brass.
As the trend continues to move away from the use of leaded metals, these bismuth- and selenium-containing copper-based alloys have proven they can move into an already established waterworks marketplace without compromising casting quality.