[RE-wrenches] iron & aluminum

Fri May 29 13:23:54 PDT 2015

Hi:

One thing which has come up a lot since the solar industry really took off 
in the last 10 years is that of corrosion and degradation of exposed metal 
on racks and modules.

Corrosion due to dissimilar metals, aka galvanic corrosion, is due to the 
natural characteristics of the metals. 

To get a relative sense of the magnitude of the problem, all the metals 
can be stacked up in table where at one end are the metals which don't 
corrode and the other, the ones which do.

With this layout, the farther apart they are in table, the faster the 
corrodible metal will corrode.  (This shows that zinc is a good 
sacrificial metal for every other metal except Magnesium).

There is another effect which is at least as, or in some cases even more 
significant.  That is the dissimilar coefficients of thermal expansion. 
The table below lists a bunch of common metals.  The units for the numbers 
are, micro-meters per meter per degree Celsius.

Most Expansive

Zinc
30
Cadmium

30

Lead

28

Magnesium

25

Tin

23

Aluminium 

22

Silver

20

Brass

19

Copper

16

Stainless Steel

16

Gold

14

Steel/Iron

11

Titanium

8

Graphite

8

Least Expansive

As in the previous table, the farther apart the metals are the worse is 
the situation as the metals are cycled through mid winter lows to high 
noon summer temperatures.  You can also see that steel/iron has an issue 
with almost every other metal.

One way to deal with the galvanic corrosion is by having a zinc coating or 
perhaps zinc anodes up in the framework in such a place so that the zinc 
residue will run down the posts as the zinc oxidizes.

With the thermal expansion, the use of rubber/plastic/nylon bushings will 
allow for thermal cycling without allowing metal fatigue and/or damage. 

In the case of iron, the galvanic potential between itself and aluminium 
is virtually non-existent.  It's the thermal expansion which messes things 
up.  Using bushings gives flexibility at the joint to allow for 
non-detrimental thermal expansion cycling.

One other related thing to note is the the thermal coefficient of 
expansion for masonry is in the range of 3-4.  That's a lot different than 
aluminium, so the "rubbery" joints help a lot in this case.

JARMO

_____________________________________________________________________________________ 

Jarmo Venalainen  |   Schneider Electric   |  Xantrex Brand  |   CANADA  | 
  Sales Application Engineer 
Phone: +604-422-2528  |   Tech Support: 800-670-0707  |   Mobile: 
+604-505-0291 
Email: jarmo.venalainen at schneider-electric.com  |   Site: www.Xantrex.com 
|   Address: 3700 Gilmore Way, Burnaby, BC V5G4M1 

*** Please consider the environment before printing this e-mail

From:
RM You <solareagle at solareagle.com>
To:
RE-wrenches <re-wrenches at lists.re-wrenches.org>, 
Date:
05/28/2015 07:21 PM
Subject:
[RE-wrenches] iron & aluminum
Sent by:
"RE-wrenches" <re-wrenches-bounces at lists.re-wrenches.org>

Hi Wrenches, I have had a couple of customers want to fabricate their own 
PV pole mounts and racks out of iron. Aside from the galvanic corrosion 
and weight issues is there anything else I should caution these guys 
about? Any stories about failures etc? My druthers are to stay with the 
engineered aluminum pole mounts but when you’re dealing with someone with 
fabrication experience and the cost for pre-fab is high then the obvious 
outcome occurs. If the unit is properly treated/painted I imagine it could 
last many years. Is there a way to attach an anode to such a rack or would 
simple non conductive insulation between panels and rack be the best 
answer?

Ron Young
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