Tin bronze (C90300–C91700 family) is the default Cu-Sn workhorse for sleeve bearings, worm gears, valve trim and seawater fittings because it combines a 300–650 MPa tensile band with self-lubricating Cu-Sn phases that protect against galling under boundary lubrication [S1].
The same tin that hardens the matrix also drives cost volatility and pulls conductivity down to 15–20% IACS — so spec'ing tin bronze for bus bars or weight-sensitive aerospace parts is a classic misuse that still shows up on RFQs [S2].
Composition, Mechanical Bands and Hardening Mechanism
Wrought and cast tin bronzes run 3–20% tin, with phosphorus (0.01–0.35% P) added as a deoxidizer that also forms the Cu₃P hard phase which raises the as-cast tensile ceiling above 450 MPa in C90300/C90500 [S1]. Higher-tin grades (C91700, 11–12% Sn) shift toward 500–650 MPa tensile with 150–220 HB hardness, the range engineers reach for when gear-tooth contact stress climbs past 250 MPa [S1]. Tin dissolved in copper produces solid-solution strengthening, while the δ-phase (Cu₃₁Sn₈) only forms above ~15% Sn and is best avoided in ductile applications because it embrittles the matrix — a frequent failure mode in overheated sand castings [S2].
Corrosion, Seawater and Cavitation Behavior
Tin bronze resists neutral chloride attack because the tin-rich surface film stays adherent, withstanding flow velocities up to 4.5 m/s in seawater before the corrosion rate spikes past 0.5 mm/yr, per classic naval-architecture data [S1]. Compared to aluminum bronze, tin bronze trades some resistance to high-velocity cavitation for better anti-friction behavior — a clear decision criterion: pick aluminum bronze (C95400) for pump impellers >30 m/s, but stay on tin bronze (C90500) for pump liners and line shafts [S2]. Dezincification is not a tin-bronze failure mode (tin bronzes have ≤0.05% Zn in marine grades), but partial substitution of zinc-bearing leaded bronzes can re-introduce it, so marine spec'ers should pin the Cu+Sn+Ni ≥ 99.5% rule on the drawing [S1].
Wear, Bearing Load and the Self-Lubrication Myth
Tin bronze sleeve bearings tolerate PV values of 60–100 MPa·m/s at low sliding speeds because the hard δ-phase carries load while the softer Cu-Sn matrix wears preferentially and creates micro-channels that retain oil [S1]. The self-lubrication story is partial: under starved lubrication, leaded tin bronzes (C93200, 6–8% Pb) drop friction further because lead smears over the contact zone, raising the seizure load by roughly 30% versus unleaded C90500 [S2]. Engineers comparing bearing alloys against the same four criteria — max PV, fatigue life, embeddability, thermal conductivity — get a clean ranking: aluminum bronze wins PV, leaded tin bronze wins embeddability, manganese bronze wins shock load, babbitt wins conformability, and steel-backed PTFE composite wins dry running [S1].
Castability, Machinability and Cost Volatility
Tin bronze has a long freezing range (~150 °C between liquidus and solidus in 10% Sn alloys), which feeds interdendritic shrinkage porosity in sand castings above 25 mm section thickness and pushes foundries toward investment casting or chill casting for hydraulic valve bodies [S1]. Machinability sits at 20–30% of free-cutting brass (C36000 = 100% rating) because the work-hardening Cu-Sn matrix resists chip flow, so leaded tin bronze (C93200) at 70–80% rating is the spec'd grade for finished bushings and threaded components [S2].
Electrical, Thermal and Weight Penalties
Electrical conductivity drops from 100% IACS for pure copper to 15–20% IACS at 10% Sn and 11–14% IACS at 15% Sn because tin scatters conduction electrons — so tin bronze is the wrong alloy for slip rings, commutator segments and grounding straps [S1]. Thermal conductivity follows the same curve, falling from 390 W/m·K for copper to ~75 W/m·K at 10% Sn, which is fine for heat-radiating bearings but disqualifies the alloy for heat-exchanger plates that copper-nickel (C70600) handles at 45 W/m·K with better fouling resistance [S2]. Density is roughly 8.8 g/cm³ across the family, which is heavier than aluminum bronze (7.6 g/cm³) and aluminum-silicon casting alloys (2.7 g/cm³) — a structural dead weight that limits tin bronze in aerospace bearing housings and high-speed rotating equipment [S1].
Typical Service Ceilings and Failure Modes
Continuous service temperature for tin bronze is generally capped at 260 °C before creep and oxidation accelerate, with steam service limited to about 200 °C because of hot-water dezincification in the zinc-bearing variants [S1]. Common field failures trace back to three root causes: (1) hot shortness above 600 °C in high-tin grades from liquation at grain boundaries, (2) stress-corrosion cracking in ammonia or moist SO₂ environments where tin bronze falls behind aluminum bronze, and (3) galvanic corrosion when tin bronze couples with stainless steel in seawater with a cathode-to-anode area ratio above 5:1 [S2]. Spec'ing a small area-ratio insulator or stepping up to aluminum bronze (C95500) for ammonia service are the two reliable mitigations, while material selection rubrics for industrial valve trim still default to C90500 for water/steam and C95400 for sour hydrocarbon service [S1].
Selection Rules and Standards Discipline
Pick tin bronze when the dominant criterion is corrosion + wear in a low-to-medium speed, lubricated bearing or gear application, and pick something else — aluminum bronze, manganese bronze, leaded steel-backed PTFE, or forged steel — when the dominant criterion is cost, weight, conductivity, or shock loading [S1]. For more on how these material choices interact with adjacent components like the flow meter housings and the pressure transmitter manifolds that share the same process fluids, see the linked reference pages. A broader review of material trade-offs in industrial rubber and adjacent elastomer selections makes the same cost-vs-corrosion vs-temperature ranking method easier to read across an RFQ package.
Two trackable signals worth watching: LME tin settlement and ASTM B584/B505 revision ballots — both directly shift the C90300/C90500 line-card pricing and the permissible impurity ceiling for marine grades within 1–2 quarters.