| Author | Message | ||
     Neville Davies Experienced User Username: nev_davies Post Number: 29 Registered: 9-2006 |
I've been looking at some of the later Bentley's i.e.1993 to 1998 vintage.In nearly all cases there has been evidence of corrosion of the alumnium panels usually near badges or window trim but in some cases just odd patches away from other metals.The question is would zinc anodes as fitted to boats etc overcome this problem? If so where would you fit them? Nev | ||
| David Gore Moderator Username: david_gore Post Number: 1042 Registered: 4-2003 |
The answer is basically no as Aluminium and Zinc are close in the Electrochemical Series which determines how effective galvanic protection between dissimilar metals will be. In fact, "galvanised" iron often comprises a coating of an alloy of Aluminium and Zinc instead of the traditional pure Zinc [Zincalume is the brand name]. Unfortunately, I was a ferrous rather than non-ferrous Metallurgist and cannot advise on possible causes/solutions for this problem with Aluminium panels. | ||
| Laurie Fox Frequent User Username: laurie_fox Post Number: 91 Registered: 6-2004 |
David Has zinc ever been used as a sacrifical anode to reduce corrosion in the cooling system for a car with a cast iron cylinder block? Laurie | ||
| David Gore Moderator Username: david_gore Post Number: 1043 Registered: 4-2003 |
Laurie, I cannot give a definitive answer as I have no experience with this application however it would be very similar to a conventional vitrified glass hot water heater cylinder which uses these anodes for corrosion reduction. On this basis, I think they would work but positioning them appropriately in the cooling system would be the main determinant of their effectiveness. I suggest you contact the UK manufacturers of sacrificial anodes listed in the following link and they should be able to answer your question in detail: http://www.buysacrificialanodes.com/sacrificial_anodes_uk_suppliers.html Hope this helps and best regards David P.S. Just had an after-thought, anodes may have been used for this purpose in marine engines which use straight-through salt water cooling so this may be another source of information. | ||
| Jeff Young Frequent User Username: jeyjey Post Number: 57 Registered: 10-2010 |
Aluminium alloys occupy quite a wide range on the galvanic scale. It's possible some of them might get some protection from Zinc, but the other end of them nearly overlaps with Zinc. Magnesium gives you some distance, but I'm not sure if there would be other issues with using it. But what's causing the galvanic action away from other metals? (Perhaps a rub-down with steel wool prior to painting?) Jeff. | ||
| David Gore Moderator Username: david_gore Post Number: 1044 Registered: 4-2003 |
Jeff, the emf range between Zinc and Aluminium plus their alloys is not sufficient for reliable galvanic protection especially where large surface areas are involved. Aluminium [like stainless steel] relies on the formation of a protective oxide coating on the metal surface for its corrosion resistance. This coating occurs naturally on exposure to air but is often enhanced by specific processes. The anodisng process used to colour Aluminium products relies on electrically enhancing the oxidation process and the use of dyes to toughen this surface coating. If the oxide film is damaged by abrasion, chemical attack etc, the underlying metal can suffer localised corrosion regardless of the proximity of dissimilar materials which are either anodic or cathodic thus giving the problems you mention. This is not general galvanic corrosion but localised corrosion due to specific circumstances such as differential aeration to name one. | ||
| Jeff Young Frequent User Username: jeyjey Post Number: 58 Registered: 10-2010 |
David, Interesting; thanks for taking the time to explain. Jeff. | ||
| Neville Davies Experienced User Username: nev_davies Post Number: 30 Registered: 9-2006 |
David. Jeff. Thanks for the input,it seems a shame that Rolls Royce could not protect against this problem at manufacture.When I served my apprenticeship I made my tool boxes out of aluminium after welding I sprayed them with what I believe was zinc chromate (it was a long time ago)a yellow paint before using a top coat.At present I have not found anybody in the UK that can supply this product that's why anodes came to mind. Nev | ||
| David Gore Moderator Username: david_gore Post Number: 1045 Registered: 4-2003 |
Neville, Zinc chromate is a barrier coating usually applied as a paint on Aluminium to improve its corrosion resistance. It is used extensively in the aircraft industry for this reason. It may provide some galvanic protection however I cannot comment specifically on this due to my lack of experience in this field. I remember reading somewhere that the use of zinc chromate was a contributing factor to the loss of the Challenger space shuttle. The paint should still be available from aviation and marine industry suppliers however there may be OH&S restrictions due to the Chromium content. A quick check revealed the following supplier: http://www.supermarinepaint.com/primer-zinc-chromate.asp | ||
| Neville Davies Experienced User Username: nev_davies Post Number: 31 Registered: 9-2006 |
David. Aquired a Bentley today newer than my 8 but with evidence of minor corrosion would like to think when the paint is repaired it will be permenant.If zinc chromate will provide a barrier then I will try it,unfortunately as with your site the supplies are in the U.S.but I will make some inquires with the aviation sites. Nev | ||
| Dave Puttock Experienced User Username: ariel Post Number: 23 Registered: 5-2010 |
You can get it in aerosol cans in the UK for about GBP12 http://shop.airworlduk.com/aviation-paints---zinc-chromate-primer-300616-1040-p.asp | ||
| Neville Davies Experienced User Username: nev_davies Post Number: 32 Registered: 9-2006 |
Dave Thanks for the contact will give them a try Nev |
