The Transition To Lead-free Products - Bismuth vs. Silicon

By "Apollo" Flow Controls | August 15, 2017

Abstract

Those buying and installing lead-free plumbing hardware have the choice of valves and fittings made from two different brass alloys: bismuth or silicon. Both meet the requirements of the Reduction of Lead in Drinking Water Act, but there are differences in their properties and behavior. The significance of these will depend upon the specific application. Copper-silicon alloys can be more difficult to solder or braze than the copper-bismuth equivalents. Possibly of less concern to the plumber or installer, alloys incorporating silicon also pose manufacturing challenges which may be reflected in the price.

Introduction

It's long been known that lead is a health hazard. Indeed, it's sometimes suggested that citizens of the Roman Empire suffered lead poisoning from drinking water distributed through lead pipes. (1) (There may have been other causes.) In retrospect, it seems surprising that it took until the late 20th Century to start taking lead out of plumbing fixtures.

The problem is that lead is a very useful metal. It's very ductile and easy to work, solders or brazes well, and also aids machinability of copper alloys. Taking it out of the alloys forced the adoption of other constituent metals that aided machinability. Those are primarily bismuth and silicon. The merits of the two are hotly debated among material specialists and those engaged in the manufacturing of flow control devices. This White Paper provides some background on the move to lead-free products and assesses the advantages and disadvantages of the two alternatives. Individual sections address:

The Reason for Including Lead in Brass 

Plumbing products – valve bodies, elbows, tees and so on – have long been cast from brass, an alloy of copper and zinc. (Bronze, also used in some plumbing applications, is an alloy of copper and tin.) Adding a small percentage of lead provides two benefits: it fills open pores in the alloy matrix, so preventing leaks, and it improves machinability.

The reason for this behavior stems from the insolubility of lead in brass. Instead of actually dissolving when processed correctly, it becomes dispersed evenly throughout the alloy. As the alloy has a higher melting point, on cooling it freezes first, allowing the lead to fill any gaps.

When brass castings are machined this dispersed lead breaks up the metal chips which otherwise tend to be long and stringy. It also acts as a lubricant that allows faster cutting, which increases output and lowers costs. In short then, brass plumbing products incorporated lead because it produced a superior product and aided manufacturing.

The Move to Lead-free Products

Moving away from lead required three things:

  1. An understanding of the hazards of lead.
  2. Legislative action.
  3. New, lead-free brass alloys.

Lead and Health

The second half of the 20th Century saw growing recognition of how lead in the environment negatively affects human health. According to the World Health Organization, lead, “... causes long-term harm in adults, including increased risk of high blood pressure and kidney damage.” It is particularly damaging to children where it affects, “the development of the brain and nervous system” and to pregnant women where it, “.. can cause miscarriage, stillbirth, premature birth and low birth weight, as well as minor malformations.” (2)

For these reasons lead was phased-out of gasoline in the early 1970's, and with the arrival of the Safe Drinking Water Act (SDWA) in 1974, the process of removing it from the water supply began.

Legislation

Initially, the SDWA required only that plumbing products contained less lead than previously. A 1996 amendment set the maximum lead content of fluxes and solders at 0.2%, and that of products themselves at 8% lead.

In 2011 the Reduction of Lead in Drinking Water Act further lowered these limits. Effective January 4th, 2014, this reduced the maximum lead content of wetted surfaces to 0.25% and today is often referred to as the “lead-free act.” (3)

For manufacturers of lead free products, this posed a problem. How to economically produce brass components at prices comparable to what customers were comfortable paying while eliminating lead?

New Brass Alloys

The solution adopted by most companies was to switch to copper-bismuth brass alloys. The first practical versions of these alloys appeared around 1990, having been developed by AT&T and IMI of the UK. (4)

Like lead, bismuth doesn't dissolve in the copper-zinc alloy, so it has no impact on metal properties. However, it does provide similar benefits to lead in terms of machinability without the health hazards. From an installation and use perspective, these alloys act in very similar ways to the older copper-lead materials, meaning plumbers see little practical difference.

A few manufacturers instead switched to copper silicon alloys. One of the first of these goes by the trade name ECO Brass, which only received a US patent in 2002. (5) In some regards these offer superior metallurgical properties, but they also behave somewhat differently to the earlier copper-lead alloys.

(Note that lead-free plumbing components are required to be certified to the NSF Standard 61 and labeled accordingly. Details of certification markings are available from http://www.nsf.org/newsroom_pdf/Lead_free_certification_marks.pdf )

The Differences Between Bismuth and Silicon Copper Alloy Materials

Copper alloys are designated by a five digit Unified Numbering System (UNS) managed jointly by the ASTM and the SAE. This system numbers cast alloys from C80000 to C99999. (6) The copper-bismuth alloys most often used for plumbing products are C89833 and C89836. (The latter being used by Apollo Valves, among others.)

Copper-silicon alloys, sometimes sold under brand names such as ECO Brass TM and Performance BronzeTM, use designations including C87600, C87850 and C69300. (This last number being a wrought alloy.)

Precise specifications for all copper alloys are available from the Copper Development Association, through their website, www.copper.org. For comparison, abbreviated compositions of one bismuth and one silicon alloy are set out below.

  CU Pb Zn Bi Si
 C89836 (Bismuth)  87-91%  0.25% (max)  2.0-4.0%  1.5-3.5%  
 C87600 (Silicon)  88% (min)  0.09% (max)  4.0-7.0%    3.5%-5.5%


Differences in composition result in differences in properties and behavior. Those that are significant are set out below.

 

Tensile Strength
(ksi)

Brinell
Hardness

Thermal Conductivity
(Btu / sq ft / fthr / °F
)

Machinability
Index
C89836(Bismuth)   37.7  65  41*  85
C87600  (Silicon)  66  110  16.4  40

* This number is for the very similar C89833 alloy.

Health Concerns

With the experience of lead as well as that of asbestos, whenever a new material is substituted for an old one there is always concern about the potential for adverse effects. What follows is an overview of current knowledge regarding both bismuth and silicon.

Bismuth

A metallic element very similar to tin and lead, bismuth is found in the same periodic table group as antimony and arsenic. This inevitably prompts concern about safety. Based on information available on the OSHA, NCBI and CDC websites as well as in MSDS documents, these concerns are unfounded. Bismuth is sufficiently safe for use in cosmetics and pharmaceutical products such as those used to treat diarrhea. (7), (8) The most well-known use of bismuth may well be in Pepto-Bismol!

Silicon

Related to carbon and germanium in the periodic table, silicon is a crystalline element found almost everywhere around the world, most commonly as sand. Today it has many uses such as in electronics and construction materials. It is considered non-toxic, although the dust can be hazardous if inhaled. (9), (10), (11), (12)

Manufacturing Issues

Bismuth typically costs under $5/lb while silicon, depending on purity, can be $50/lb. Consequently, copper-silicon alloys are typically more expensive than those incorporating bismuth.

While both copper-bismuth and copper-silicon alloys cast readily, machinability is a different matter. Bismuth aids metal cutting in much the same way as lead: it provides lubrication and acts as a chip-breaker. Silicon however dissolves in the copper-zinc alloy and increases hardness. The result is higher cutting forces and increased tool wear. (13) In combination, these increase the cost of machining products cast from copper-silicon alloys.

Practical Differences for the Plumbing Professional

By the eye it's hard to distinguish between bismuth and silicon copper alloys. However, there are some differences the buyer or plumber should consider. These are:

Differences in Strength

Adding silicon to the copper alloy undoubtedly increases strength and hardness, particularly at high temperatures, (above 350°F – far higher than the boiling point of water.) However, in the overwhelming majority of applications this is of no benefit: flow control devices such as valves, tees and elbows are rarely required to maintain their integrity under very high loads and temperatures.

Heat Transfer Behavior

In practical terms a bigger issue arises with thermal conductivity. As shown in the table above, alloys such as C87600 have much lower conductivity than the copper-bismuth C89833 and the closely-related C89836. This has significant implications for the contractor or plumber making soldered or brazed joints. (Note that the thermal conductivity of C89836 is very similar to that of the earlier lead-containing brasses, so it requires no changes to soldering or brazing practice.)

When thermal conductivity is higher heat flows away faster from where the torch is acting. The whole fitting warms up, encouraging better flow of filler metal. In the case of low conductivity copper-silicon alloy products, the installer must take greater care to ensure the whole joint is hot enough for good filler metal flow. Failing to do so may leave cold spots that will subsequently leak. It's also important to remember that the joint will be slower to cool down. (14) Plumbers soldering or brazing copper-silicon castings should allow extra time for each joint.

For further guidance on soldering of lead-free copper alloys, the reader is encouraged to consult “Recommended Practice for Soldering of No-Lead Copper Alloys”, published by the Copper Development Association as document A4108. (15)

Products Affected

The SWDA, as amended by the “lead-free” act, has a broad scope. It addresses all pipes, valves, plumbing fittings and fixtures, as well as solders and fluxes used in public water systems or any plumbing that delivers water for human consumption. (There are exemptions for systems where water will not be drunk, such as in industrial processes.) (3)

A partial list of plumbing products that must be lead-free includes ball valves, mixing valves, gates, backflow preventers, pressure reducing valves, strainers, couplings, adapters and elbows. Reputable manufacturers such as "Apollo" Valves provide catalogs detailing their lead-free products along with guidance on certification and marking.

Technological Maturity

Plumbers and contractors weighing bismuth versus silicon may also look at length of time on the market as an indicator of product maturity. In this case it's worth noting that the copper-bismuth alloys have been around considerably longer, allowing the development of greater application experience.

Summary (Buyer Beware!)

Since 2014 all new plumbing systems for potable water are required to be lead-free. In practical terms this means surfaces in contact with water must have a lead content lower than 0.25%. Achieving this required component manufacturers to remove lead from the brass materials used. As lead significantly improves machinability, to avoid incurring substantially increased manufacturing costs it was necessary to find a substitute material.

For many manufacturers that substitute was bismuth, incorporated into copper-bismuth alloys such as C89833 and C89836. Others chose to adopt silicon as found in C87600, C87850 and C69300. Silicon imparts higher strength, (a benefit of questionable value in most plumbing applications,) but at the expense of greater manufacturing and installation challenges. Installers of lead-free plumbing components would be well-advised to educate themselves on the merits of these two types of copper alloys. If in doubt about the type of alloy used in a specific fitting, seek advice from the distributor or manufacturer.

Sources:

  1. http://penelope.uchicago.edu/~grout/encyclopaedia_romana/wine/leadpoisoning.html
  2. http://www.who.int/mediacentre/factsheets/fs379/en/
  3. https://www.awwa.org/portals/0/files/legreg/documents/reducutionofleadindrinkingwateract.pdf
  4. http://www.nytimes.com/1991/05/15/business/business-technology-a-new-form-of-brass-to-cut-lead-in-drinking-water.html
  5. http://www.ecobrass.com/html_e/topix-e.html#4
  6. https://www.copper.org/resources/standards/uns-standard-designations.html
  7. http://www.espimetals.com/index.php/msds/51-Bismuth
  8. https://www.ncbi.nlm.nih.gov/pubmed/2682129
  9. https://www.cdc.gov/niosh/ipcsneng/neng1508.html
  10. http://www.lenntech.com/periodic/elements/si.htm
  11. https://www.cdc.gov/niosh/npg/npgd0554.html
  12. https://www.cdc.gov/niosh/pel88/7440-21.html
  13. https://www.researchgate.net/publication/257736977_Machinability_characteristics_of_lead_free-silicon_brass_alloys_as_correlated_with_microstructure_and_mechanical_properties
  14. https://www.constructionspecifier.com/properly-soldering-no-lead-copper-alloys/2/
  15. https://www.copper.org/publications/pub_list/soldering.html