Dura-Con® Advanced Fastening Systems in Solar Racking
About the Author: Paul Mudge is the founder of Mudge Fasteners, Inc., creator of the solar mounting component website solarfastenerexpert.com and originator of Dura-Con® Advanced Fastening System used in solar racking systems. He has nearly 40 years of experience in fastener distribution.
In most situations fasteners are a last-thought item when designing a solar PV, solar thermal or CSP system. Although the system won’t go together without fasteners the fact that fasteners are relatively low-cost and usually readily available leaves them at the bottom of the priority list.
For that reason little time is spent on selecting the fastening system to be used to assemble a racking system or support structure for a solar-power system.
Designers have historically chosen stainless steel bolts, nuts, washers and screws as the fasteners of choice for these applications. The main concern has been to assure that the fasteners last as long as the other components of the racking system. Stainless steel seems an obvious solution. It is readily available, has superior corrosion resistance and when marketing the system, it is seen as a positive feature when you say your system uses “stainless steel hardware”.
Drawbacks of using stainless steel fasteners include thread galling, galvanic corrosion and high prices.
Unfortunately, stainless steel fasteners also have certain drawbacks. Among them are: • Galling(1) • Galvanic Corrosion(2) when used in combination with dissimilar metals (i.e. aluminum) • Price volatility
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Solutions to Thread Galling of stainless steel fasteners have their drawbacks.
If you have ever experienced galling when assembling a stainless steel bolt and nut you know how frustrating it can be when they “seize up” and have to be cut apart and replaced. Preventing galling is usually attempted in one of a few different ways. But there are drawbacks with each of these “solutions.” • • • •
• •
SOLUTION: Reducing the RPM of the impact wrench or power driver will reduce the likelihood of thread galling. DRAWBACK: Slower fastening extends production time and increases cost. SOLUTION: Applying lubricant. DRAWBACK: Lubricants must be applied in advance in a batch process which is an added cost or at point of assembly using an “Anti-seize” type liquid which is time consuming and messy. SOLUTION: Using different grades of stainless for the bolts and mating nuts. DRAWBACK: Using 18-8 grade bolts, which is the most common grade used for commercial fasteners in combination with 316 grade nuts adds costs and creates problems with availability. 316 grade is less in demand and therefore not always available.
(1)
Thread galling seems to be the most prevalent with fasteners made of stainless steel, aluminum, titanium, and other alloys which self-generate an oxide surface film for corrosion protection. During fastener tightening, as pressure builds between the contacting and sliding thread surfaces, protective oxides are broken, possibly wiped off, and interface metal high points shear or lock together. This cumulative clogging-shearing-locking action causes increasing adhesion. In the extreme, galling leads to seizing - the actual freezing together of the threads. If tightening is continued, the fastener can be twisted off or its threads ripped out. Source: Industrial Fastener Institute's 6th Edition Standards Book (page B-28),
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1
The Galvanic Series determines the susceptibility of metals to Galvanic Corrosion
Galvanic corrosion occurs when two dissimilar metals are in direct contact in the presence of an electrolyte. In a galvanic ‘cell’, the greater the distance between the two metals on the Galvanic Series, the more electrolytic potential exists and the Anodic metal will sacrifice to the Cathode. In typical solar racking systems the potential for galvanic corrosion exists where the stainless steel fasteners are in contact with an aluminum structural piece if there is moisture present. Moisture can be in the form of rain, dew, standing water, fog or irrigation water. The affect of galvanic corrosion of the fastenings on a solar racking system is to cause the aluminum components to sacrifice to the stainless steel bolts. The loss of aluminum in the joint will cause the joint to loose its tension pre-load or clamp force and the bolts will loosen.
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Source: Specialty Steel Industry North America, Stainless Steel Information Center 1
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Aluminum is on the Anode end of the scale and will corrode when in contact with stainless steel.
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Source: Specialty Steel Industry North America, Stainless Steel Information Center 1 1
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The speculative trading in commodities like nickel can affect the cost of stainless steel fasteners. 1 1
The price volatility seen in stainless steel is the result of the varying cost of nickel on the commodities market. The past few years have seen wide fluctuation in the price of this metal. No longer a function of supply and demand but affected by speculative trading, the price of nickel is hard to predict. We have seen nickel prices range from under $5.00 per pound to nearly $25.00 per pound during this timeframe. (See Fig.1) Nickel constitutes 8% to 10% of the 300 series stainless steel alloys, so steep fluctuations in the price of nickel can greatly affect the cost of raw material used in producing stainless steel fasteners.
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THE BETTER SOLUTION
The Stainless Steel Alternative that Saves You Money
Dura-Con® Fasteners resist corrosion as well as stainless steel
Dura-Con® Fasteners have a galvanic barrier coating that prevents Galvanic Corrosion
Now there is a proven way to overcome the drawbacks of using stainless steel fasteners in Solar applications while at the same time realizing substantial savings in purchase cost. Dura-Con® Advanced Fastening System is the only system that has been tested specifically for use in solar racking materials. In a side-by-side corrosion resistance test, DURA-CON™ fasteners and 18-8 stainless steel fasteners were installed in 6063 and 6105 aluminum material and exposed to 1500 hours of Salt Spray Testing in accordance with ASTM B-117. At the end of the Salt Spray Testing, both the DURA-CON™ fasteners and the stainless steel fasteners showed equal resistance to Environmental Corrosion. When the fasteners were removed from the aluminum, there was no sign of Galvanic Corrosion in the DURA-CON™ assembly but the stainless steel assembly showed signs of Galvanic Corrosion as the aluminum was beginning to corrode. (Fig.2)
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Fig. 2
Thread galling is eliminated with the Dura-Con®
Advanced Fastening System
Dura-Con® Advanced Fastening System is stronger and goes together more smoothly than stainless steel fasteners.
The SAVINGS can be as high as 40%
Not only is it now possible to have corrosion resistance equal to that of stainless steel fasteners while eliminating the threat of galvanic corrosion; the occurrence of galling is eliminated entirely using the Dura-Con® Advanced Fastening System . Predictable torque-tension values and higher tensile strength are added benefits of the Dura-Con® Advanced Fastening System. Dura-Con® Fasteners are made from carbon alloy steel that is stronger than 300 series stainless steel. They are then treated in a three-step coating process that provides corrosion resistance, a galvanic barrier and added lubricity. The result is a predictable co-efficient of friction that achieves desired clamp load within a much narrower range of torque values than other types of fasteners. And finally, the Dura-Con® Advanced Fastening System SAVES YOU MONEY! Savings typically are 25% - 30% and can be as high as 40% when compared to the purchase cost of stainless steel fasteners. Considering the savings in time not lost to galling problems and the savings in the future costs of repairing the affects of galvanic corrosion the actual savings are much, much greater. The choice is clear. It is time to consider switching the fasteners used in solar PV, solar thermal and CSP systems away from stainless steel to the Dura-Con® Advanced Fastening System. Contact information: Mudge Fasteners, Inc. 3050 Palisades Dr. Corona, CA 92880 1-800-634-0406 sales@mudgefasteners.com www.duraconfastening.com
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