Exterior Fastening Systems
Understanding the Use of Stainless Steel Fasteners
Before looking into the benefits of using stainless steel products in construction applications let’s first touch on what stainless steel actually is and its characteristics. The broad definition of stainless steel is a blend of iron base alloys containing at least 11.5% of the element chromium. It is a relatively strong metal alloy. In the fact the lateral load strength for stainless steel fasteners is 20% higher than common steel fasteners. The chromium found in stainless steel contributes to corrosion resistance. It quickly forms a thin, tough invisible film of chrome oxide over the surface when exposed to oxygen. This film acts to insulate the metal alloy from caustic attack.
The most common grades of stainless steel fasteners found in the construction industry are 304,305 and 316. These numbers are referred to as Austenitic grades in that they contain nickel as well as chromium and are inherently non-magnetic and will not harden by heat treatment. Grades 304 and 305 provide adequate protection from corrosion in most inland applications but are not suitable salt water exposure. Type 316 contains slightly more nickel than 304 and 2-3% molybdenum giving it better corrosion resistance properties especially when exposed to the pitting effects of salt environments. Type 316 is the grade of choice for sea water exposure and even resists corrosion when exposed to sulfuric acid compounds.
Why consider using stainless steel fasteners?
- The labor is the same whether you use common, galvanized or stainless steel
- You insure a project will not fail from fastener deterioration.
- Staining or fastener bleeding will not ruin a finished surface
- By expanding the lifetime of a project you can consider stainless a "green" choice
Where is it specifically important to use 316 grade stainless steel:
- Anyplace where salt water may contact the project.
- Areas that in close proximity to salt water especially areas of:
- High humidity
- Significant rain
- Salt moisture carried by wind
- Higher temperatures. Galvanic corrosion rates can double for every 20 degree increase in temperature.
A few simple precautions should be taken for storing and installing stainless fasteners. Storage should always be in a clean dry area. The presence of impacted dirty or other contaminants may cause pitting by denying oxygen to access the metal surface. This can interrupt the formation of clear protective oxide. Painting over stainless fasteners especially with latex coating can be problematic if the fasteners have encapsulated dirt or in seaside areas where salt residue is embedded on the fasteners. Salt particles can react with paint and give rise to reddish stains which will bleed through the surface coating. Fastener heads should be painted as soon after installation as possible or cleaned before coating are applied. Always remember when working near salt water to use 316 grade fasteners.
When Pressure Treated Wood Requires Stainless Steel
In 2003 the pressure treating industry started to remove potentially toxic arsenic (CCA treatment) from their treatment formulas to newer alternative formulas. Most of these new formulas were copper based and referred to as ACQ. Shortly after the formula change traditional galvanized fasteners started to exhibit corrosion problems not seen with CCA treatments. In some cases the deterioration was up to five times faster. In moist salt air environments the results were even worse. Scientist went to work to determine why this was happening.
They found that a phenomenon called "Galvanic corrosion" was causing the different reactions with ACQ treated lumber versus the traditional CCA treated lumber. In the case of CCA treated lumber the zinc coated galvanized steel fasteners are protected from corrosion. With CCA treated lumber the zinc coating will start to corrode until a layer of zinc oxide forms on the outer surface. The zinc oxide scale protects the underlying zinc from oxygen and moisture while the steel that sits underneath the zinc remains protected. The better the zinc coating the longer the steel will remain protected. The scientists discovered the copper in the ACQ lumber made the traditional galvanized protective process unreliable. In the presence of moisture, the copper leaches from the ACQ wood and attacks the zinc until the steel core of the fastener is exposed. With out that protective coating the steel quickly rusts and eventually fails. Researchers also discovered some of the new formulas which contained other ingredients such as ammonia or chloride that actually sped up the copper attack on the zinc.
The lesson is you have to be very careful using galvanized fasteners on treated lumber especially in high moisture and salt environments. It is especially important to know how the lumber you are using is treated. Some of the newer treatments such as sodium borate (SBX) seem to be less caustic to zinc than the older CCA treatments but using stainless steel fasteners might be a prudent investment.
In deciding whether to use galvanized versus stainless steel fasteners with pressure treated wood, the following should be kept in mind.
- Examine the extra price of stainless steel versus galvanized steel. Is the extra investment worth it to prolong the life of the project?
- If pressure treated wood is kept dry and not exposed to rain or ground moisture and maintain a 12% moisture content of less, galvanized fasteners should work satisfactorily.
- Where salt air corrosion is likely to occur always play it safe and use 316 grade stainless steel fasteners in all applications.