 | Home > Maintenance & Modifications > Anodic corrosion help please? |
 
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| ZeDefender Member Since: 15 Sep 2011 Location: Munich Posts: 4731   
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Good morning!
For various reasons, I have to use aluminium rivnuts and countersunk screws to fix my chequerplate 
I would like the screws to be black for optics so normal black steel ones would achieve this - but steel and ally aren't usually very happy together  I could also use stainless ones that would need painting... but from experience the paint usually wears off 
With possibilities like wet assembly with Sikaflex or ACF, can anyone give me some advice (preferably from experience)? If I use steel, will the screws or rivnuts corrode first? (there are so many steel and ally types in published anodic tables...) Cheers Matt Tell someone you love them today because life is short. But shout it at them in German because life is also terrifying and confusing... |
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28th Feb 2012 7:00am |
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| ZeDefender Member Since: 15 Sep 2011 Location: Munich Posts: 4731
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Thanks Brendan.
I was previously looking at anodised ally screws - 2 quid a pop  and I need 24
Hadn't thought about anodised stainless
Cheers, Matt p.s. I still owe you money  Tell someone you love them today because life is short.
But shout it at them in German because life is also terrifying and confusing... |
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28th Feb 2012 7:57am |
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| Green Machine Member Since: 19 Nov 2010 Location: North Yorkshire Posts: 1226  
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Matt,
Duralac jointing compound may be another option - it effectively isolates the fixing to prevent galvanic corrosion (primarily used in the boating world I believe). I used it when I replaced all of my door bolts with stainless, you just coat the threads, let it go tacky and then assemble. It seems to be good stuff - recommended by a few on here. Tris. 2005 Td5 | 90 Station Wagon | Tonga Green |
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28th Feb 2012 8:24am |
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| Zagato Site Supporter Member Since: 08 Jan 2011 Location: Billingshurst West Sussex Posts: 5009
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Anodising kits here
http://www.frost.co.uk/automotive-electrop...-kits.html Cheaper if you are buying a lot of stuff. Some of the false anodised windscreen hinges are the same price as the kit alone. It also wears away eventually, especially the dye so to have your own kit for subsequent dippings is a good idea if you are going down this route  |
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28th Feb 2012 8:29am |
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| leeds Member Since: 28 Dec 2009 Location: West Yorkshire Posts: 8580
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As a matter of interest what happens to the electrolytic corrosion between steel or stainless and Ali if the steel is anodised?
Or put question the other way do you need anti corrosion paste if using anodised bolts? Brendan |
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28th Feb 2012 8:37am |
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| Honker Member Since: 26 Jun 2011 Location: St Albans Posts: 353  
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As Brendan suggests check out http://www.pro-bolt.com/ it where we've bough anodised fastners for years. Good quality and quick.
Stu |
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28th Feb 2012 8:48am |
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| Caterham Member Since: 06 Nov 2008 Location: Birmingham Posts: 6295  
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sorry going off subject a little hear but has anyone tried to install cathodic protection for there vehicle???
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28th Feb 2012 9:16am |
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| davew Member Since: 02 Jan 2012 Location: North Yorkshire Posts: 888 
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yes and no... Land Rover galvanic corrosion is generally localised and is a result of differing metals and water/damp soil acting as an electrolyte. On a boat, for example, adding a sacrificial anode is relatively simple because you are dealing with what is effectively a single structure fully immersed in an electrolyte. Having said that, having a galvanised (zinc coated) chassis will potentially offer some protection to the bodywork as the zinc is present in many areas but it won't help if you're bolting something to a panel. For a sacrificial anode to be effective on a Land Rover you would need the node to be independently electrically connected to both the aluminium part and the steel part. This isn't really practical on a Land Rover as it's not fully immersed in water and there are so many potential paths between dissimilar metals. The best solutions are fairly simple really. Electrically insulate the dissimilar metals from each other using one of the coatings mentioned above. Use a high zinc content primer between the two metals. Cover the joint with Dinitrol or another quality coating that will dispel moisture once it's assembled. Where a steel panel meets an aluminium panel either fit an insulating gasket between them or add a "sacrificial" aluminium shim. Using BZP bolts is also a very good idea as the zinc in the coating provides a small amount of protection, using stainless is a bad idea as stainless is further away from aluminium then mild steel so will encourage galvanic corrosion. |
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28th Feb 2012 11:11am |
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| Laurie Member Since: 22 Feb 2008 Location: Sussex, England Posts: 2897 
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Back in the days when tow sockets were made of zinc alloy instead of plastic, we all had built in sacrificial anodes. Had to replace them annually.
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28th Feb 2012 11:29am |
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| ZeDefender Member Since: 15 Sep 2011 Location: Munich Posts: 4731
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[quote="davew"]
After some more research, this is my provisional conclusion. In order of increasing destruction/reactivity when in electrical contact 1) Stainless steel 2) Steel (e.g. mild) 3) Aluminium 4) Zinc (e.g. galvanised steel) Since I am bolting ally plate into ally rivnuts that are set into ally wings... ideally I should use ally bolts - but that would cost me 50+ quid
Stainless bolts don't corrode themselves but whatever they're bolted into often does (unless it's also stainless). Galvanised bolts are great on their own and being close to aluminium in reactivity shouldn't harm the ally. However the zinc may corrode over time (not sure what happens then as zinc and steel straddle ally in reactivity?) Plain old steel bolts might rust on their own anyway but since they're not far from ally in reactivity, they should only damage it slowly. Think I may just use the steel bolts I already had and make sure the threads are all dry and full of Sikaflex. If I get corrosion problems I will see it pretty quick from the plate and switch to another material Tell someone you love them today because life is short.
But shout it at them in German because life is also terrifying and confusing... |
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28th Feb 2012 1:10pm |
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| ZeDefender Member Since: 15 Sep 2011 Location: Munich Posts: 4731
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I used to "blue" my rifle barrels (had a touch-up pen too) - is this the same chemical? Tell someone you love them today because life is short. But shout it at them in German because life is also terrifying and confusing... |
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28th Feb 2012 1:13pm |
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| VeeTee Member Since: 06 Mar 2011 Location: Somewhere Posts: 1512 
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You 'd better use Duralac jointing compound instead of Sikaflex if you use SS bolts on ally. As posted before, it really works (I have 20+ years of experience in marine applications). Cheers, Vincent
1959 Polynorm 1/4 Ton Trailer, Olive Drab Green (sold) 1970 M416 Military Trailer (Camping Trailer Conversion), Epsom Green (sold) 1975 Series III 88 V6, Light Green (sadly sold) 1996 Defender 110 CSW 300 Tdi, Epsom Green (sold) 2000 Freelander 1 TD4 3-drs, Silver (sold) 2006 Freelander 1 TD4 5-drs Facelift Automatic, Tonga Green (sold) MySite |
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28th Feb 2012 5:00pm |
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| ZeDefender Member Since: 15 Sep 2011 Location: Munich Posts: 4731
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I suppose I better had
Means having to order it from UK and wait though 
... ordered... now wait... Cheers (and yes, you told me before  )
Matt Tell someone you love them today because life is short. But shout it at them in German because life is also terrifying and confusing... |
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28th Feb 2012 7:04pm |
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| SteveS Member Since: 05 Oct 2010 Location: Devon & Berkshire Posts: 388 
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A reputable post market specialist dealer tried to sell me a cathodic protection system (electrical trickle) for my truck....I wrote to them basically with the same arguments that davew put forward above. Just think how would the charge get to the two sets of dissimilar metals throughout the truck - doors for example?
I politely declined their kind offer |
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29th Feb 2012 7:54pm |
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