Aluminum Frames and Stainless Steel Screws in Hawaii and Puerto Rico

You are convinced that30 years is the number? I'm not sure you will get 20 years useful life out of photovoltaics, maybe not even 15...

I'm thinking the aluminum would be attacked by the screw. I think that you might be surprised to find taht stainless steel screws will cause corrosion faster than carbon steel, based on the potential difference in the galvanic series.

While the aluminum may be anodized, THEDRILLED HOLES ARE UNLIKELY TO BE.

I'd epoxy the holesand surrounding surfaces.

Heres a mil spec for epoxy:

MIL-PRF-23377J-A1 PRIMER COATINGS -- EPOXY, HIGH-SOLIDS

http://assist.daps.dla.mil/quicksearch/basic_profile.cfm?ident_number=16259

This will give you a good tutorial courtesy of uncle sam:

MIL-STD-808A FINISH, MATERIALS AND PROCESSES FOR CORROSION PREVENTION AND CONTROL IN SUPPORT EQUIPMENT

http://www.everyspec.com/MIL-STD/MIL-STD+(0800+-+0899)/download.php?spec=MIL_STD_808A.966.pdf.

cheers

milo

I agree with Milo. The better way is to "paint" tha mating surfaces with an electric isolating primer. The use of nylon washers could also help. Well, even if you're considering that long operational time for the assembly, I'm affraid the aluminum frame itself could fall with corrosion pitting in such environment. So, I suggest to paint all the frame with primer.

Griggs paints make good ones that comply with the mentioned specs. I know them by the Boeing spec BMS 10-11 type I. Really good ones, used to protect airframes and high strengh steel parts.

Powder coating is an expensive solution and while I love the process and use it often, experience has shown that powder coating can prevent early detection of serious structural flaws. In some cases, moisture trapped between the powder coating and the metal accelerated corrosion.

The engine mount for a smaller of the Lycoming piston engines (an O-235) failed in flight causing an off-airport landing. No one one was hurt and the aircraft received only minor damage.

On close inspection it was discovered that failure of a weld had occurred and that the subsequent vibrations stressed other clusters to where the engine mount failed catastrophically.

That mount was examined visually every time the aircraft got it's FAA mandated annual inspection and the flawed welds were not discovered. The consensus of opinion was that the use of a powder coating on the mount prevented early detection.

I suspect that shouldered plastic bushings will prove to be the most cost effective solution.

L. J.

I also would suggest the combination of nylon washers and epoxy paint. I have trucks that are aluminum, painted with epoxy. They are less than 1 mile from the Atlantic. As long as the Paint does not crack, or chip the combination stays well. I usually use cadmium plated cap screws. When I do use stainless steel I try to use Never-seize on the bolts.

Another way to buy some time, is powder coating. If the finish is not damaged, you should be good for your 30 year mark. Again, I would suggest using broad nylon flat washers. If you are forced to drill holes in your prepared aluminum, you might consider filling the drilled hole with marine grade silicone before inserting the mounting screws.

This might not be workable for your needs, but street signs are commonly mounted with aluminum bolts to prevent oxidation.Good luck.

i think that the best thing to do is to use isolating washers. This will eliminate the possibility of galvanic corrosion.

"In contrast, in a marine environment, severe localised pitting corrosion to the aluminium treads has been observed where un-insulated stainless steel bolts were used to secure the treads in place.
On the same ladder however, bolts with sound insulating washers did not show any pitting on the surrounding aluminium."

http://www.bssa.org.uk/topics.php?article=89

S.

corrosion protection

I suggest you consider the use of aluminium pop-rivets insted of stainless steel screws. They are faster, less costly, and eliminate the electolytic corrosion potential.

Seems obvious...build frame out of non metallic/non conductive high grade plastic AND MOUNT ACCORDINGLY.

MR.GUY

What High grade plastic is warranted for 30 years in High UV exposure?

I see expensive (not necessarily high grade mind you) plastic fade in a a single week out by my pool in the summer. Some is embrittled by end of season, Some fails in three years or less...

milo

Off the shelf composite parts from fiberglass and properly cured epoxy can easily provide the resistance to harsh environmental conditions, especially when painted white using an epoxy paint with UV inhibitors. Marine paints excel at this.

In fact, a structure optimized through the use of composites can be lighter in non load bearing structures, stronger where loads are concentrated and less costly too. You can't do that with aluminum which is one reason why the Aerospace industry has moved away from aluminum.

Not one General Aviation design I know of that's been certified by the FAA in recent years, is made using decades old monocoque construction methods. They have all switched to cored fiberglass for the most part and to a lessor degree carbon fiber.

They'll last a lot longer than 30 years!

L.J.

Fiberglass might be OK in Hawaii, but in the tropics I would be dubious about it's life.

In Mt Isa (dry tropics about 17.5S), fiberglass grid flooring lasted less than 18 months where it was exposed to direct sunlight. Around cairns (16.5S) the only thing which gives a fiberglass boat a decent life is the sacrifice of the gel coat, which chalks after a few years.

If you use Al, use anodised 5083 or 5086. The normal 6063 etc types used for window frames etc don't survive salt exposure very well.

Use insulating washers between your Al and everything else, including SS bolts.

If it is feasible you could use Al pop rivets, with an Al mandrel. The normal ones with the steel mandrel will corrode.

The combination of high ambient temperature, salt and sulphur make this a tough environment.

Good luck

...build frame out of non metallic/non conductive high grade plastic AND MOUNT ACCORDINGLY"

I don't know of any plastic that will withstand indefinite exposure in the tropics.

Even worse for plastic in the dry tropics because of the very high UV.

With salt spray exposure as specified by the original post, isolate with plastic washers (the fastener shields from the sun) or use Al pop rivets.

Don't like your chances of 30+ year life with Al in this environment. especially if you use the normal 6061 or 6063 alloys.

5083 will probably give the best life.

Another thought is WHICH aluminum alloy are you using? Different alloys have different response to corrosion.

Bill

Hello im4pvru,

If you are talking the dry side of Hawaii you may get 20 years every where else less than 5.

Aluminum pop rivets as suggested then coat with a UV resistant vinyl. (none can resist me for ever)

Better yet TIG weld it. Isolate your clamps by vinyl coating them.

Another way to handle it is sacrificial anodes.

There are ship systems that use voltage in the opposite direction of the galvanic corrosion, but unless this is a large array it may be cost prohibitive.

A few ideas

Brad

Just another two cents.

If you look at the manufactures cut sheet, not the pretty glossy one at the dealers, the one with warranty and specs. Almost every one of them have exclusions regarding being located within 500 meters of salt water, and Mitsubishi panels also have a volcano exclusion.

Revision:

Technical May 1, 2008

Bulletin PV Module Grounding Approved:

D. Buttz

Mitsubishi Electric USA, Inc.

5665 Plaza Drive, Cypress CA 90630 www.mitsubishielectricsolar.com pv@meus.mea.com

© 2008 MEUS PVDMEUS003A

This document describes the manufacturer approved PV module grounding methods for installation in the USA. The

four methods given in this document include clarifications of the two methods described in the installation manual.

WARNING

The grounding method should satisfy both the local code and the National Electrical Code consistent with the

specifications of the installed PV system. The module should be grounded properly as appropriate for the

conditions at the installation site. Be careful not to damage module backfilm when using drills and sharp objects.

Method # 1

• To the grounding hole, secure a self-tapping stainless steel screw

of at least 32 threads per inch (e.g. 10-32).

• Stainless steel washers (e.g. flat #10) should be inserted between

copper wire and the frame to avoid galvanic corrosion.

• As shown in the figure, wrap copper wire around the screw

between the washers and tighten the screw.

Method # 2a

• If the mounting holes are not used to fix the module, one of them

can be used for grounding.

• To the mounting hole, secure a grounding lug (e.g. tin-plated

copper) with a stainless steel bolt and nut.

• Insert a stainless steel washer (e.g. flat, locking) between the

grounding lug and frame.

• A stainless steel toothed washer can be inserted between the nut

and the frame to break the anodized layer of frame.

• If needed, sand the anodized layer of frame near the hole and

add anti-oxidant compound.

• As shown in the figure, tighten the grounding lug set screw to the

copper wire.

Revision:

Technical May 1, 2008

Bulletin PV Module Grounding Approved:

D. Buttz

Mitsubishi Electric USA, Inc.

5665 Plaza Drive, Cypress CA 90630 www.mitsubishielectricsolar.com pv@meus.mea.com

© 2008 MEUS PVDMEUS003A

Method # 2b

• To the grounding hole, secure a grounding lug (e.g. tin-plated

copper) using a self-tapping stainless steel screw of at least 32

threads per inch (e.g. 10-32).

• Insert a stainless steel washer (e.g. flat, locking, toothed)

between the grounding lug and frame. A stainless steel toothed

washer can aid in breaking the anodized layer of the frame.

• If needed, sand the anodized layer of frame near the hole and

add anti-oxidant compound.

• As shown in the figure, tighten the grounding lug set screw to the

copper wire.

Method # 3

• If there is not convenient access to

locate the grounding device at the

grounding hole or mounting hole, a

hole may be drilled into the side or

bottom of the module frame within

the designated drilling zones.

• Make an appropriately-sized hole

(less than 5/16" diameter) on the

frame suitable for the grounding

device.

• Secure grounding device via

Method 1 or Method 2.

Method # 4

• Grounding devices listed to UL467, such as barbed washers, that are designed to be used to bond photovoltaic

modules to the mounting structure may be used for grounding as described in National Electric Code sections

250.134 and 250.136.

• Follow instructions of UL467 listed grounding device manufacturer.

Thanks to all who have replied. Much appreciated.

I haven't seen any solar companies go to pop rivets - intriguing. All of them want to avoid repetitive maintenance, such as re-coating very few years. Thousands of solar modules have been installed in Hawaii, and there's zero information that I've found about how they are doing after 10, 15 or 20 years.

The solar cells should last for a long time (decades - most have a 25 year performance warranty) and, based on utility rates across Hawaii (and the Caribbean), solar should be very cost effective.

Best to all...