Guide
Silicon Bronze or 316 Stainless: Which Should You Machine Marine Hardware From?
Silicon bronze is a copper alloy, mostly copper with a few percent silicon and some manganese, that is machined from bar as C65500 for marine fittings, fasteners and bushings, while 316 is the molybdenum-bearing stainless steel that most hardware catalogs default to. The choice is made by where the part sits (above or below the waterline, stagnant or flowing), what the hull is made of, and whether the finish is bright metal or a patina, not by which material is “better”.
By Saola Manufacturing engineering team · Published · Updated
This guide is for marine hardware brands, boatbuilder engineers and sourcing agents who get quotes for both materials and need a reason to pick one. It uses a naval architect’s published comparison, a bar producer’s data sheet and a stainless association’s seawater guidance. None of the three gives a single numerical corrosion rate, so the guide stays with selection logic and the drawing wording, and does not rank alloys by a number.
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Should marine hardware be silicon bronze or 316 stainless?
Choose silicon bronze for parts that sit below the waterline or in crevices and stagnant water, and for traditional deck hardware on wood or fiberglass hulls. Choose 316 when the hull is aluminum or steel, when a bright polished finish must stay bright, or when the design is a thin section that needs stiffness.
Dave Gerr, a naval architect, puts the case for bronze strongly in his 2009 article on marine metals: he says good-quality marine bronze does not suffer from pitting, crevice corrosion or weld corrosion cracking, and that stainless is in its worst environment underwater in stagnant water, which is why he argues against stainless keel bolts (Gerr, High Strength with High Corrosion Resistance, 2009). That is an opinion piece from a practitioner, and it is stronger than most data sheets would be, but the direction matches the stainless side’s own guidance.
The British Stainless Steel Association says 316 is susceptible to crevice and pitting corrosion in seawater, that stagnant conditions must be avoided, and that any form of crevice must be avoided; it also says slow-moving water can encourage biofouling that shields the metal and starts crevice attack (BSSA, seawater selection note, 2026). A fitting with a gasket, a washer face or a threaded joint in a wet cavity is a crevice by construction. So the same location that makes bronze attractive is the location where 316 needs the most design care.
The reverse is also true. Bronze is not a universal answer, because it is a poor galvanic partner for aluminum and steel hulls and has its own finish trade-off. The table below sets the two materials side by side by situation.
| Situation | Silicon bronze (C65500 bar) | 316 stainless (bar) |
|---|---|---|
| Above waterline, exposed deck hardware | Good; polish it or let it develop a green patina, which Gerr calls harmless | Good; keep crevices out of the design |
| Below waterline or stagnant water | Preferred by Gerr; Wieland cites excellent corrosion properties in seawater | Risk: BSSA says stagnant seawater must be avoided and crevices cause attack |
| Fiberglass or wood hull | Gerr’s first choice for fittings | Acceptable with design care for crevices |
| Aluminum or steel hull | Avoid direct contact: Gerr says bronze causes corrosion in aluminum or steel it is fastened to | Preferred by Gerr to limit galvanic corrosion |
| Bright polished finish that must stay bright | Needs polishing or chrome plate, which Gerr says must be done properly | Name the polish grade and any passivation step on the drawing |
| Threaded fasteners and bushings | Common; Wieland lists nuts, bolts and bushings among the uses | Common |
| Cast instead of machined from bar | True silicon bronze is harder to cast cleanly and to machine after casting (Gerr) | Cast CF8M is a separate grade; see the CF8M guide |
What does silicon bronze bar offer a machined part?
A wrought copper alloy of typical composition 95% copper, 4% silicon and 1% manganese, strong in the hard-drawn temper, resists seawater and is sold as round bar. Properties fall as bar diameter grows, so the size and temper on the drawing matter.
Wieland’s C65500 data sheet gives hard-drawn (H04) rod and bar tensile strength of 90 to 100 ksi (615 to 689 MPa) up to a quarter inch, falling to 70 to 80 ksi (485 to 552 MPa) between 1.5 and 3 inches, with elongation rising as strength falls. It describes excellent corrosion properties in seawater and lists marine hardware, pump and valve parts, fasteners and bushings among the uses.
Gerr quotes a similar tensile strength of 90,000 psi in hard temper and 55,000 psi in soft temper for the best-known silicon bronze, which his article calls his all-around favorite marine metal. The two sources agree on the hard-temper figure, which is why it is reasonable to write the temper on the drawing.
The comparison with 316 is not a simple “stronger or weaker”. Gerr says bronze has nearly the same tensile strength as stainless and is more ductile, so it absorbs more energy before failure. ASTM A276/A276M-25 covers stainless bar in several conditions, from annealed to strain hardened, and the minimums depend on condition and size, as they do for bronze. Compare minimums for the exact temper, condition and diameter you will buy, and do not assume one material is stronger or stiffer without checking your own data sheets.
What does the hull or mating metal change?
Everything about the galvanic pair. Bronze is a good choice on wood and fiberglass hulls and a bad one in direct contact with aluminum or steel, where stainless or a compatible insulated design is the safer call.
Gerr’s position is that on aluminum or steel boats the fittings should be proper marine stainless to minimize galvanic corrosion, because a bronze fitting fastened directly to steel or aluminum will cause corrosion in the aluminum or steel. That is the same logic as the galvanic corrosion guide: the hull is the anode and the larger the cathode, the faster the loss.
Two practical drawing points follow. First, the drawing should state the hull material and the isolating hardware (bushings, sleeves, sealants) if the customer has already decided them, because the machine shop cannot see the boat. Second, if the choice is not settled, quote both materials from the same drawing with the material as a variant, so the comparison is on price and delivery to the same print, not on two different drawings.
Can silicon bronze be cast instead of machined from bar?
It can, but Gerr notes that true silicon bronze is harder to cast cleanly with defined edges and harder to machine after casting. Casters add zinc to improve castability, and past a point the alloy is a brass, not a bronze.
Gerr’s rule of thumb (2009 article) is that copper alloys with less than 15% zinc are unlikely to suffer dezincification, and that alloys with less than about 6% zinc can be considered bronzes and immune. His example is C83600 (Gerr, page 2), cast leaded red brass at 85% copper, 5% tin, 5% lead and 5% zinc, which he calls effectively a bronze and a good alloy for casting many marine fittings. HG Houston gives the same threshold for practical immunity: brasses with 15% or less zinc are practically immune (HG Houston, dezincification).
The warning is for the high-zinc alloys sold under bronze names. Gerr lists manganese bronze, Tobin bronze, commercial bronze and naval brass as brasses, says they give adequate service on deck and excellent service in the interior, and warns that they are potential problems below the waterline. A casting RFQ that says “bronze” without a UNS number can come back as one of them. See the dezincification guide for how that failure looks and how it is tested.
Casting is placed with a partner foundry that Saola manages and does not name; the bores, faces and threads are then machined at the lead plant. The CNC milling page describes brass and bronze valve bodies handled that way. Whether cast or bar is right is a process question covered in the casting vs machining drawing guide.
What should the drawing and RFQ say?
State the alloy by UNS number (C65500 for silicon bronze, or the 316 designation), the product form, the temper or condition, the certificate type and the finish. Trade names such as Everdur are not specifications.
A workable bronze note reads: “Material: UNS C65500 silicon bronze, round bar, temper H04 or as noted, mill certificate with chemistry.” A workable stainless note names the grade, the bar standard and the condition, as in the 316 vs 316L guide. In both cases write what is not allowed: no free-machining substitutes, no leaded substitutes for a part that will be in drinking-water or food contact, and no brass in place of bronze.
Finish deserves its own line. If the part is to be polished, say to what finish; if it is to be left to patina, say so, so nobody oils it for shipping. Gerr notes that chrome plating on bronze must be properly prepared over nickel base coats to avoid peeling, which is a partner-plant process and not something to leave to a generic “plate” note. For stainless, name the passivation or electropolish step and the standard. See the grade guide and the passivation guide.
Saola’s CNC turning page lists silicon bronze among the bar materials turned at the lead plant. For 316 or any other alloy, send the drawing and the quote will state the material source and the production site. The marketing term “marine grade” has no industry-recognized definition according to Wikipedia’s note on marine grade stainless, so it should not stand in for an alloy name on your print.
Frequently asked
Is silicon bronze stronger than 316?
Not by a simple rule. Gerr says bronze has nearly the same tensile strength as stainless; Wieland shows hard-drawn C65500 bar falling in strength as diameter grows. Compare minimums at the exact temper and size.
Does silicon bronze corrode in seawater?
Wieland describes excellent corrosion properties in seawater. Gerr says it does not pit or crevice-corrode, but it still causes galvanic corrosion in aluminum or steel it touches.
Why is 316 a risk below the waterline?
BSSA says stagnant seawater must be avoided and any form of crevice must be avoided, and those are common below the waterline and in gasketed joints.
Can I use a high-zinc brass with a bronze name?
Not below the waterline. Gerr lists manganese bronze, Tobin bronze, commercial bronze and naval brass as brasses that are limited to deck and interior use.
Is silicon bronze cheaper to machine than 316?
This guide does not give prices. Ask for both materials quoted from the same drawing and compare.
Sources
- High Strength with High Corrosion Resistance (marine metals, part 2), Dave Gerr, Gerr Marine, 2009
- Bronze CDA 65500 data sheet, Wieland Diversified, accessed October 2026
- Selection of 316, 304 and 303 types of stainless steels for seawater applications, British Stainless Steel Association, accessed October 2026 (page copyright 2026)
- ASTM A276/A276M-25, Stainless Steel Bars and Shapes, ASTM International store page, 2025 edition
- Dezincification, material property data, H&G Houston (undated; accessed October 2026)
- Marine grade stainless, Wikipedia, accessed October 2026