Leaded Copper and Brass
Lead is added to copper, brass, and bronze alloys to increase their machinability. The role of lead in copper alloys is two-fold, it acts as a lubricant and, in the free machining grades, the lead assists in chip break up. Lead is added to many copper alloys, making all types of free machining alloys. Lead does not affect the structure and properties of copper because it is practically insoluble in solid copper. The pure copper solidifies first, leaving the lead to solidify last as almost pure lead globules. The size and concentration of lead particles depends upon the concentration of lead in the alloy. Leaded coppers are categorized as low lead alloys, or free machining alloys and high lead alloys. In the free machining alloys, the lead acts as chip breaker and lubricant making these alloys easier to machine than their non-leaded counter parts. The high leaded copper alloys are used in bearing applications. In the bearing materials, the lead acts as a solid lubricant and the copper is the load bearing support.
The microstructure of the leaded copper alloys is similar to the structure of the unalloyed copper materials with the addition of almost pure lead particles in the grain boundaries. The size and amount of lead particles in the structures depends on the concentration of lead in the alloy. The higher the lead content of the alloy the more lead globules present in the structure. In the wrought structures, the lead is present as discrete particles between the alpha copper grains.
Lead has been a lubricant for brass, bronze, and copper processing since ancient times. Brass can contain several hundred ppm to 2% residual surface lead. Lead is hard to remove because the alloy metals in brass (primary Zinc) and bronze (primarily Tin) can also be plucked out with lead by conventional means and ruin the alloy. Many strategies for removing lead require harsh conditions, produce low yields and have waste problems.
The November 2015 issue of Production Machining, pages 38-41, highlights the many problems of integrating the more expensive No-Lead Brass production with traditional “leaded” brass production. Metal finishing shops face new challenges in fabrication and segregation of a unique product. Since No-Lead Brass wears differently than traditional brass, changes in cycle times, coolants, and cutting tools are required. Stone Chemical Company CU400L will clean, brighten, and remove leachable surface lead from brass, copper, and bronze parts and allow them to be used in the same applications as
No-Lead applications without the headaches.
The Stone Chemical Company passivation process uses a proprietary product CU400L for the removal of leachable lead from wrought brass, bronze, and copper parts down to near 1 ppb. The obstacles to removal of leachable lead from the surface of the brass, bronze, and copper are:
1. The coating of processing oils on the surface that physically prevent water containing the lead removal solution from contacting lead.
2. The presence of some lead metal in crystal “holes” rather than sitting on the surface of the metal in globules.
3. The presence of lead metal just below the surface of the part that might “pop up” later treatment.
The removal of lead occurs in two stages:
1. The surface conditions on the metal parts are adjusted by using CU400L to:
A) First remove surface oils and other debris blocking access to surface lead
B) Then “prime” the exposed lead for removal by making it more reactive.
2. CU400L then removes lead smoothly.
A) The surface lead is dissolved out of the holes by the mildly acidic CU400L, which alters the oxidation state of the lead while minimizing the loss of Copper and Zinc from Brass; and Copper and Tin from Bronze.
B) The mildly acidic CU400L gently rearranges the metal crystal structures to “fill in” the empty lead “hole”. This will have a brightening effect on the brass.
C) The phosphate component of CU400L passivates the surface by combining with some of the Brass (primarily Zinc) or Bronze (primary Tin). Lead just below the surface of the brass is now sealed in.
Stone Chemical Company has obtained two United States Patents (US Patents Nos. 7,771,542 and 8,618,038) for the invention to safely remove surface lead from metals including brass and copper which are used in potable water systems. The removal of lead from the environment is currently an EPA goal for health and wellness as reflected in the 1991 National Primary Drinking Water Regulations and the 1996 Safe Water Drinking Act amendments.
For mildly oily parts, CU400L has adequate cleaning ability. However, for heavily soiled parts, the parts can be pre-cleaned by either SW100L (a neutral dispersing cleaner) or SW150L (an alkaline non-dispersing cleaner).
CU400L Copper, Brass & Bronze Cleaner
CU400L is a unique product that cleans, brightens and removes other metal contaminants, including lead, from the surface of copper and brass while providing a passivated surface which resists tarnishing. CU400L meets EPA mandated NSF61 requirements for the removal of lead from copper and brass for drinking water systems.
Heavy Duty Copper, Brass and Bronze Cleaners To Pre-Treat Parts Before CU400L
SW100L Neutral Spray Cleaner
SW100L Spray Cleaner is a neutral, dispersing, spray wash cleaner for removing oil and grease after machining and fabrication. SW100L cleans and brightens copper, brass and bronze and prevents tarnishing. For use in industrial parts washers.
SW150L Alkaline Spray Cleaner
SW150L alkaline spray cleaner is a non-dispersing, spray wash cleaner for removing oil and grease after machining and fabrication. The hydrocarbon soils float to the surface for easy removal via overflow or skimming. For use in industrial parts washers.