Data and lead notes. CDA register composition, density and specifications for C66100 only — there is no published machinability rating, so although the 0.20–0.8% lead is added expressly to improve machining, we are not quoting a figure for it. Separately, the lead addition brings two practical consequences: the alloy will be hot short in a weld heat-affected zone, so an unleaded grade is the safer choice for welded assemblies; and lead is restricted in drinking-water-contact components, at a weighted average of 0.25% of wetted surfaces under the US Safe Drinking Water Act, with equivalent European controls and RoHS limits on electrical equipment. At 0.20–0.8% this grade can exceed that limit — confirm the regulatory position before specifying it for potable water.
Silicon bronze is copper alloyed with roughly 1–4% silicon, and it occupies a position no other copper alloy quite matches: high strength, excellent corrosion resistance, and — unusually — outstanding behaviour in every fabrication process at once. Most copper alloys force a trade. A
leaded brass machines superbly and welds badly. An
aluminium bronze resists seawater superbly but fights the alumina film at every joint. A
phosphor bronze makes a fine spring but hot works Poorly and machines at a rating of 20. Silicon bronze declines most of those trades.
CDA rate both documented grades here Excellent for cold working and Excellent for hot forming — a combination almost nothing else on this site manages. They are rated
Excellent for gas shielded arc welding, spot welding, butt welding and brazing, Good for oxyacetylene welding, and
C65100 Excellent for soldering too. There is no refractory oxide film to remove before joining, which is precisely what makes the aluminium bronzes difficult. Machinability at
30 is modest but still half again better than a phosphor bronze.
Silicon does the strengthening by solid solution, and the price is conductivity. C65100 at 0.8–2.0% silicon carries 12% IACS;
C65500 at 2.8–3.8% carries only
7% IACS and 36 W/m·K. Silicon in solution scatters conduction electrons hard. These are not electrical alloys — for that see
C50500 at 48% IACS — they are structural and fastening alloys that happen to be copper-based.
Silicon bronze has one of the best all-round corrosion records of any copper alloy, and two specific advantages worth naming. The first is that
dezincification does not apply — zinc is an incidental element here, capped at 1.5%, so the characteristic failure mode of the
brasses is off the table. The second is that there is no gamma-2 phase and therefore no
dealuminification risk either, which is the corresponding weakness of the
aluminium bronzes. Silicon bronze simply does not have a selective-leaching failure mode.
In service it resists
seawater and brackish water, fresh water, neutral salt solutions, non-oxidising acids, alkalis, organic acids and industrial and marine atmospheres. It weathers to a stable brown-bronze patina rather than the green of a high-copper alloy, which is why it is specified for exposed architectural metalwork, and it carries the copper matrix's natural biofouling resistance. It is
non-sparking and essentially non-magnetic, and it retains strength and toughness well at low temperature.
The limits are the normal copper-alloy ones. Oxidising acids, notably nitric, attack it. Ammonia, ammonium compounds and amines will cause stress-corrosion cracking in material carrying residual tensile stress, so
stress relieve heavily cold-formed and cold-headed parts before service. In a galvanic couple it is cathodic to steel, aluminium and zinc — which is exactly why silicon bronze fasteners are specified
with bronze and copper hardware and
against steel only with deliberate isolation.
How three grades divide — and silicon content is only half the story.C65100, CDA's
Low-Silicon Bronze B, carries 0.8–2.0% silicon. Less silicon means more conductivity (
12% IACS against C65500's 7%) and better cold formability, and it is the grade CDA rate Excellent for soldering. As ASTM F468
Cu 651 it is specified at up to
690 MPa in small diameters but at only 8% minimum elongation — the strong, less forgiving fastener choice. It reaches 724 MPa in extra hard wire.
C65500,
High-Silicon Bronze A, carries 2.8–3.8% silicon plus a deliberate 0.50–1.3% manganese addition. It is
the most widely used silicon bronze in the world and the one most people mean by the term: sixteen listed specifications,
1000 MPa in spring wire, and as ASTM F468
Cu 655 a fastener requirement of 345–552 MPa at
20% minimum elongation — two and a half times Cu 651's. That ductility is why it dominates marine and architectural fastening.
C66100 is the
free-machining grade, at 2.8–3.5% silicon with a deliberate
0.20–0.8% lead addition where the other two hold lead to 0.05% as an impurity. As ASTM F468
Cu 661 it is specified at 483–690 MPa with 15% minimum elongation — Cu 651's strength with nearly twice its ductility.
CDA register composition, a density figure and the specification list for it and nothing else, so its mechanical table here comes from ASTM F468 rather than from typical-property data.
