FYORRG Posted March 12, 2014 Share Posted March 12, 2014 Having read about galvanic action in Peter Parkinson's Architectural Forgework, has caused me to think closely about how I will be doing an awning for a client. The awning is basically a copper sheet riveted to mild steel bars. I'm wondering if the nobler "copper" will concentrate the corrosion to the mild steel rivets causing them to rust out in no time. So now... I'm thinking copper rivets to fasten the copper to the steel. Now, I theorize that the galvanic action will spread out over the bars in a similar fashion to a copper rivet spreading the rust over the area of sheet steel. Now another question comes to mind, will the combined mass of the copper rivets and sheet make for a harsher life for the steel brackets? So now I'm thinking it needs galvanizing to insulate it from the copper, and then i can rivet it cold, being careful not to tarnish the zinc coating. This takes me to another point. The client wants the copper awning to patina naturally. Will the zinc affect this? Is there any preparation necessary to facilitate a natural greenish patina? Or will it potentially be quite different than what he expects? Any thoughts would meet great welcome from here at Pike Lake Forge :) Quote Link to comment Share on other sites More sharing options...
doc Posted March 12, 2014 Share Posted March 12, 2014 Using two dissimilar metals together will generally cause some galvanic action. Combining the copper with a low iron alloy stainless would be your best bet but I'm sure for you it's probably too late for that now. I don't think what you're doing requires alot of over thought. If the job will be near salt water ( think coastline location) I believe you'd have more to worry about. In an average environment your rivets should last quite a few years without a problem. If you''re still concerned a good urethane based paint or powder coat on your iron should suffice if you don't chip the powder coating.You can also paint the shanks of your rivets during assembly. I'm not certain but I believe you would probably compound your worries if you introduce zinc into the mix, as i believe it would be more reactive with the copper that the iron would be. Quote Link to comment Share on other sites More sharing options...
HWooldridge Posted March 12, 2014 Share Posted March 12, 2014 Your reasoning is sound but mass and the potential for electrolysis will also have an effect. Canada likely has a higher corrosion rate than Arizona simply based on frequency of moisture so you probably have to design for worst case. Rather than zinc, you could consider insulating with an inert material (nonmetallic). I think the best natural patina would be initiated by chemically or mechanically cleaning the copper so it is evenly exposed to the elements in the immediate area. You would be amazed at what patterns can develop from tarnish. Leaves and other detritus can affect it - water drainage onto the awning will also be a variable. The clients might change their mind about natural patina after a few years of exposure. Quote Link to comment Share on other sites More sharing options...
ianinsa Posted March 12, 2014 Share Posted March 12, 2014 Copper + Zink = battery... your riveting should be either copper or s/s. Get some 12mm soft/malleable plastic strips that you rivet to your m/s frame and ....and countersink those rivets. Then rivet the copper to the plastic. This will prevent the problem. When the job is complete wash the roof with laundry detergent ie. Tide and then wipe down with white vinegar diluted 2:1 with water this should give you "next day" patina. What may be worth considering is to slightly "offset" the plastic strip in order to achieve a toggle action when copper expands and contracts thermally . Dependent on the sheet ie hard drawn or soft drawn it will either elongate the holes or shear the rivets. Good luck Ian Quote Link to comment Share on other sites More sharing options...
SimonW Posted March 12, 2014 Share Posted March 12, 2014 The understanding I have is that galvanic action between steel (the anode) & copper (cathode) will erode the steel. The easy fix I'd think is to switch to copper rivets. So long as the copper awning protects any connection between dissimilar metals from water, you greatly reduce the effect of galvanic action. With copper rivets the connection between the two metals would be under the awning where the rivet pierces the steel bracket, and should be protected from rainfall. This would also give you a much larger anode to erode over time as your new anode would now be the bracket & not the much smaller steel rivet. As mentioned above, in a very humid or salty environment any reactions are magnified & accelerated, so if this is the case you may need to find a different solution. Any way to avoid galvanization is most likely best, its bloody expensive to hot dip steel! Simon Quote Link to comment Share on other sites More sharing options...
ianinsa Posted March 13, 2014 Share Posted March 13, 2014 Just to clarify , for those that don't quite get it! When you hot dip steel it gets coated with wait for it ..... Zinc!When you put copper and zinc together it makes a basic battery! You merely need a conductive liquid like salt water/acidic rain/dirty or soapy water etc.Ian Quote Link to comment Share on other sites More sharing options...
ianinsa Posted March 13, 2014 Share Posted March 13, 2014 Sorry for the late addition, a good( think cheap here) source of suitable plastic is cheap breadboards from the like of dollarstore . Quote Link to comment Share on other sites More sharing options...
L Smith Posted March 13, 2014 Share Posted March 13, 2014 The idea of powder coating is not bad. In aviation the rivets are coated with good primer before setting and that works also. You need to break the galvanic bond. Quote Link to comment Share on other sites More sharing options...
FYORRG Posted March 13, 2014 Author Share Posted March 13, 2014 Thank you all for your time! Yes. Of course the budget at hand prohibits the hot-dip. The zinc complicates the equation quite a bit so I reckon simpler is better. Powder coating seems a sound way to go. Copper + Zink = battery... your riveting should be either copper or s/s. Get some 12mm soft/malleable plastic strips that you rivet to your m/s frame and ....and countersink those rivets. Then rivet the copper to the plastic. This will prevent the problem. What may be worth considering is to slightly "offset" the plastic strip in order to achieve a toggle action when copper expands and contracts thermally . Dependent on the sheet ie hard drawn or soft drawn it will either elongate the holes or shear the rivets. Good luck Ian I can see this working a whole lot. But the copper rivet in the mild steel hole is still in contact... Perhaps some powder coating of the rivets will help to some degree? I'm wondering how well that will hold up when setting the rivet. Thanks for all your input. Very helpful to a guy who's only done interior work. Quote Link to comment Share on other sites More sharing options...
ThomasPowers Posted March 13, 2014 Share Posted March 13, 2014 I know that a traditional method of getting your copper roof off to a good patination was to buy the roofing crew a barrel of beer at the end of the job and have them apply the results of drining it to the roof in the natural way----probably not a good idea these days...l Quote Link to comment Share on other sites More sharing options...
ianinsa Posted March 13, 2014 Share Posted March 13, 2014 Using the plastic strip = copper(sheet & rivet) to plastic & aluminum rivet(countersunk)plastic to mild steel frame, thus no direct copper to steel....Ian Quote Link to comment Share on other sites More sharing options...
ianinsa Posted March 13, 2014 Share Posted March 13, 2014 I know that a traditional method of getting your copper roof off to a good patination was to buy the roofing crew a barrel of beer at the end of the job and have them apply the results of drining it to the roof in the natural way----probably not a good idea these days...lNot that the royal bodyguard(Gurkas) to Sheik Hamdan bin Zayed(UAE) have no sense of humor, but if they were to catch anyone(especially a non believer) taking a leak(After consuming alcohol) on a palace it may just be the start of a long and unpleasant ordeal. At the very least a sure fire way to ruin an otherwise perfect day....Ian Quote Link to comment Share on other sites More sharing options...
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