Silicon Bronze vs Stainless Steel Fasteners
What is silicon bronze?
Silicon bronze is a copper based alloy which contains a significant addition of silicon. This low lead alloy exhibits superior corrosion resistant properties and hardness due to that addition of silicon. Don't make the mistake of thinking that all bronze alloys are alike. There are in fact many: tobin bronze, manganese bronze, statutory bronze, and others are out there but are not suitable for fasteners such as wood screws and bolts due to their different alloy makeups. In fact, many unscrupulous manufacturers will advertise and sell a fastener as being silicon bronze when they are the much less expensive and much less corrosion resistant tobin bronze for example. Here at Fair Wind Fasteners we've had our silicon bronze fasteners laboratory tested to be certain they are the alloy we say they are. Many suppliers are supplying "silicon bronze" but don't realize that an inferior bronze is being supplied by their overseas manufacturer in an unscrupulous effort to save a few bucks on the alloy. You can read all about the practice on another one of our articles here on the site.
Silicon bronze is readily weldable, joinable as well as soldered, and cast. Moreover, bolts produced from this alloy not only have good resistance to corrosion, but they exhibit high tensile strength as well. All of our bolts at Fair Wind Fasteners are made of 655 silicon bronze, of which you can find the tech specs here. Silicon bronze has a slightly higher density as compared to stainless steels in general. Many woodworkers prefer using silicon bronze bolts and silicon bronze screws as they are often regarded as one of the better looking copper based alloys. More importantly, and especially when dealing with boat building, silicon bronze is highly resistant to corrosion both above and below the waterline.
Its superior anti-corrosion properties in conjunction with its aesthetics, it's conductive properties, and hardness permit silicon bronze fasteners to be used in many applications involving decorative and architectural uses, boat building, electrical work, and work in any highly corrosive environment such as water treatment plants or offshore oil rigs. Its attractive reddish-gold coloration makes silicon bronze fasteners favored amongst various craftsmen.
Silicon bronze fasteners tend to resist deformation due to the alloy's high tolerance of cycle loading as well as fatigue induced stress. However, in the event of silicon bronze fasteners breaking, the alloy first gets deformed by indicating certain visual confirmatory signs such as elongated holes, twists and bends. Thus rendering the user to understand that these fasteners need to be replaced.
What is stainless steel?
Stainless steel is a low carbon, non-magnetic alloy that exhibits a greatly improved corrosion resistance over other grades of steel. This chemistry of stainless steel is what prevents the fasteners from getting rusty. While stainless steel in general envelopes many stainless steel grades, austenitic alloys 304 (18-8) and 316 are amongst the most popular, especially when dealing with an environment where corrosion resistance is of the utmost importance. Be careful of lower stainless grades such as 304 (18-8) in a corrosive environment such as seawater or around chlorines. 316 stainless steel has a much higher strength and durability than 304. Know what grade you are purchasing, most hardware stores only carry 304 stainless. Like bronze, not all stainless is the same!
While 316 stainless steel is highly corrosion resistant, it's corrosion resistance (and the corrosion resistance of other stainless alloys) relies on an invisible protective passive layer that forms when the stainless steel is exposed to oxygen. If stainless is not exposed to oxygen crevice corrosion can occur.
Stainless Steel Crevice Corrosion?
Crevice corrosion and pitting occur on stainless steels when oxygen cannot freely circulate such as in tight joints, under fastener heads, and when enveloped in wood! Salts, chlorides, and pollutants from the natural environment will slowly seep in to those tight joints and crevices, causing a minor amount of rust that would otherwise just look like surface rust that is easily polished. However in that crevice the rusting process removes oxygen from the environment which breaks down the invisible protective layer that normally would form on stainless steels. A more scholarly and scientific explanation can be found here if you're interested in reading on.
This issue is particularly applicable to wooden boat building. Boat builders on a budget might be tempted to go for the much cheaper stainless steel wood screws to fasten planks below the waterline for example, only to find that within a couple of short years that their fasteners have broken down and the boat is literally coming apart at the seams. That's why silicon bronze wood screws are recommended for that type of application.
Silicon bronze doesn't require a protective passive layer to retain it's anti-corrosion properties, and while it is a much more expensive alloy it can be enveloped in wood and be expected to last for decades to come.
Silicon Bronze vs Stainless Steel Fasteners
Other than of course being a beautiful decorative alloy, silicon bronze fasteners are used in applications involving the marine industry as well as in electrical components, and chemical processing equipment. Their enhanced resistance to corrosion in both freshwater and saltwater is what makes these bolts highly useful for marine applications. Silicon bronze fasteners are also resistant to alkalis, acids, fog/mist, and organic chemicals. When it comes to corrosion resistance, silicon bronze is at the top of the list of alloys. It's only downside is the relatively high price when compared to stainless steels as well as the difficulty in sourcing them.
We're talking a lot about corrosion resistance because when comparing silicon bronze and stainless steels, their properties of corrosion resistance are why they were created.
Stainless steel fasteners come in a much greater range of head types, drives, shapes, and sizes. Need something non-standard? You can call us and have it made for you out of silicon bronze, but it is most likely already done in stainless steel. While absolutely NOT recommended for use in corrosive environments such as under the waterline on a vessel where the fastener won't be exposed to oxygen, 316 stainless fasteners are completely okay on the deck of a vessel as long as they get the odd bit of polish here and there. They are cheaper, more available, and will be up to the task on many different applications. But as I said before, 304 and 316 stainless steels are not the same so know what grade you are buying!