Choosing the Right Lag Bolt - Silicon Bronze or 316 Stainless?
At Fair Wind Fasteners we specialize in high quality, corrosion resistant fasteners, and have tens of thousands of lag bolts sitting on the shelves ready to ship out. Half of them are made of silicon bronze, and the other half are 316 stainless steel - both highly corrosion resistant alloys that do really well in a corrosive environment. So when would you choose a fastener made of 316 stainless steel vs one made of silicon bronze? We can help!
Lag bolts, more properly known as lag screws, are a type of fastener that is commonly used to secure heavy objects or materials to wood. They are called lag bolts because they were originally developed and used to secure the staves of a barrel, which are also known as lags.
We offer lag bolts made of either silicon bronze or 316 stainless steel, and each type has its own unique properties and characteristics. In this article, we will compare silicon bronze lag bolts and 316 stainless steel lag bolts and discuss their common uses and when you might choose to use one over the other.
First, let's take a look at the properties of silicon bronze and compare them with the properties of 316 stainless steel, the corrosion resistant alloys we're addressing here:
Properties of Silicon Bronze Lag Bolts
Silicon bronze is a type of copper alloy that contains small amounts of silicon and a couple of other trace elements. It is known for its high corrosion resistance, strength, and durability. You'll often find it being used to fasten timbers on wooden boats and in the marine industry due to it's high resistance to corrosion. Other industries will also use silicon bronze fasteners for that same reason, as well as it's high electrical conductivity due to the fact that the alloy is 97% copper (a highly conductive element). Here are some of the key properties of silicon bronze lag bolts:
- High corrosion resistance: Silicon bronze is resistant to corrosion from saltwater, acids, and other chemicals, making it a good choice for use in marine environments or other corrosive environments. Silicon bronze will never rust!
- Strength: Silicon bronze has a high tensile strength. While not quite as strong as 316 Stainless Steel, it's still a very strong alloy that will be much stronger than any wood a lag bolt could possibly be driven in to.
- Durability: Silicon bronze is a tough and durable material that can withstand wear and tear very well. Bronze is a common material in bearings due to it's ability to withstand wear so well.
Properties of 316 Stainless Steel Lag Bolts
316 stainless steel is a type of stainless steel that contains molybdenum, which gives it an added level of corrosion resistance over many other stainless steels such as the more common 304 stainless steel. Like bronze, it is known for it's excellent corrosion resistance, strength, and durability. Here are some of the key properties of 316 stainless steel lag bolts:
- Excellent corrosion resistance: 316 stainless steel is resistant to corrosion from saltwater, acids, and other chemicals, much like silicon bronze is, making it a good choice for use in marine environments or other corrosive environments. As corrosion resistant as it is, 316 stainless steel will still rust given enough time in a corrosive environment. "Stainless" does not mean "Stain-Proof," it simply stains LESS.
- Strength: 316 stainless steel is even stronger that silicon bronze. In a lag bolt, both are (much) stronger than the holding power of the threads of the bolt, but if the absolute highest strength possible is completely necessary, then stainless takes the win.
- Crevice Corrosion: As corrosion resistant as stainless steel is, it's unfortunately susceptible to one type of corrosion that doesn't have an effect on silicon bronze, and that's crevice corrosion. That's a big topic in and of itself and why stainless isn't recommended for use on wooden boats. You can read more about crevice corrosion here.
Comparison Table
| Property | 316 Stainless Steel | C65500 Silicon Bronze |
|---|---|---|
| Chemical Composition | Fe: 70%, Cr: 16-18%, Ni: 10-14%, Mo: 2-3% | Cu: 95-97%, Si: 2-4%, Sn: 1-2% |
| Tensile strength (psi) | 75,000 - 85,000 | 40,000 - 65,000 |
| Yield strength (psi) | 30,000 - 40,000 | 25,000 - 35,000 |
| Elongation at break (%) | 40 - 50 | 15 - 25 |
| Hardness (BHN) | 170 - 220 | 110 - 140 |
| Modulus of elasticity (E) (psi x 10^6) | 28 - 30 | 10 - 11 |
| Specific mass (g/cm^3) | 7.93 | 8.9 |
| Cost | Moderate | Moderate/High |
Common Uses for Lag Bolts
Now that we've looked at the properties of silicon bronze and 316 stainless steel lag bolts, let's discuss some common uses for lag bolts.
Lag bolts are commonly used in a variety of construction and DIY projects to secure heavy objects or materials to wood. They are used when a regular wood screw simply isn't long, wide, or generally strong enough for the task at hand. Some examples of common uses for lag bolts include:
- Attaching stems, floor timbers, and even large frames on wooden boats.
- Attaching ledger boards to a house
- Securing a railing to a deck or porch
- Securing framing on docks and pilings
- Attaching large wooden beams
- Hanging a heavy chandelier, or other heavy overhead equipment
Notice what everything above has in common? It's all BIG, HEAVY stuff, and requires a big, heavy fastener. That's where lag bolts come in to their own.
When to Use a Bronze Lag Bolt
Now that we've discussed some common uses for lag bolts, let's talk about when you might choose to use a bronze lag bolt over a stainless steel one.
One key factor to consider when choosing a lag bolt is the environment in which it will be used. As we mentioned earlier, silicon bronze lag bolts have high corrosion resistance, making them a good choice for use in marine environments or other corrosive environments. They are also not susceptible to that nasty crevice corrosion and will never rust. If you're working near saltwater for example and never want to deal with rust or worry that your fastener is corroding and becoming compromised then bronze is the way to go. It's especially important to used bronze in applications that will be submerged in saltwater, since they'll stay corrosion free despite constant submersion.
Another factor to consider is the appearance of the lag bolt. Bronze lag bolts have a distinct golden color that can add a decorative touch to your project. If you are working on a project where aesthetics are important, a bronze lag bolt might be the better choice. We sell a lot of bronze lag bolts to architectural firms that are building custom decking and railings. They love the polished or patinaed look of bronze.
In a lag bolt application we can forget about any strength differences between silicon bronze and stainless steel. Both are very strong alloys and as we mentioned previously, both are much stronger than any timber they might be fastened in to.
When to Use a Stainless Steel Lag Bolt
So when might you choose to use a stainless steel lag bolt over a bronze one?
You're working in a corrosive environment, near a lake or the sea, or even in a chemical plant and you don't want to spend the extra cash? 316 stainless is the way to go. While it IS subject to crevice corrosion which could compromise the fastener, that only really happens to 316 stainless steel when it is semi-regularly submerged. Also, while it will still rust to some degree, it will generally just be a small amount of surface rust as opposed to completely disintegrating in to a brown flaky mess.
But let's not forget the appearance aspect either, because plenty of people make their choice based on appearance. Stainless has that lovely, shiny, white metal appearance which is a huge contrast to the copper, golden, or brownish hues of bronze.
Conclusion
In conclusion, both silicon bronze and 316 stainless steel lag bolts are strong, durable, and corrosion resistant options for use in a variety of construction and DIY projects. The short version is that stainless steel is cheaper, but is subject to more corrosion, and silicon bronze is more expensive but will stave off corrosion much better. Both are excellent choices, and we'll be happy to answer any of your questions about either one, just give us a shout!