Red SLR Posted September 18, 2002 Share Posted September 18, 2002 Need a technical response here.... peter..... See X777CAT here Link to comment Share on other sites More sharing options...
stevefoster Posted September 18, 2002 Share Posted September 18, 2002 Yes it does. Every time you polish it with a mildly abrasive polish you remove microns of oxide only to leave a fresh surface to oxidise all over again. The oxide is the protective layer and as it doesn't flake like iron oxide it does some good at protecting the metal beneath. I await Peter for a chemical, electron related explanation. /Steve My racing pics, 7 DIY, race prep. Updated often here Hants (North) and Berkshire area club site here Link to comment Share on other sites More sharing options...
Red SLR Posted September 18, 2002 Author Share Posted September 18, 2002 Thanks, PC.... See X777CAT here Link to comment Share on other sites More sharing options...
Andy Clayson Posted September 19, 2002 Share Posted September 19, 2002 As Steve suggests, you very rarely see pure Aluminium. It oxides to Aluminium Oxide on contact with air. So the act of polishing removes the oxide layer, exposes Aluminium which then instantly oxides again, this time looking slight prettier since there should be no dirt and the surface should be smooth. I recall my old Chemistry teacher telling me that if a next door neighbour really irritated you and they had an Aluminium greenhouse, you should paint the metal with Sliver Nitrate solution. This removes the aluminium oxide (it used in industry when they need to get at pure aluminium rather than aluminium oxide) so effectively that it generally causes the greenhouse to collapse since they only use thin bits of aluminium... Andy Link to comment Share on other sites More sharing options...
Graham Perry Posted September 19, 2002 Share Posted September 19, 2002 Is a Caterham actually skinned in Ali, or do they use that Birmabight (Spelling ) alloy stuff that Land-rovers are made of ? Link to comment Share on other sites More sharing options...
Nigel Riches Posted September 20, 2002 Share Posted September 20, 2002 I think Caterhams are skinned with "alclad" which is a surface skin of aluminium over a base, (sandwich) of some form of aluminium alloy, this allows greater strength and better corrosion resistance with out a weight penalty, similar material is used for aircraft skins. Link to comment Share on other sites More sharing options...
Pierre Gillet Posted September 20, 2002 Share Posted September 20, 2002 How do you explain then that Caterham are subject to corrosion, and not Land Rovers? My sons just bought a 1979 Lighweight SIII (or Half Ton, also known in Europe as "Commando") which does not show any corrosion of the ali body! Link to comment Share on other sites More sharing options...
John E Posted September 20, 2002 Share Posted September 20, 2002 Landrovers do corrode. I have a 1996 Disco that has recently had some corrosion sorted on the rear door. When I bought the car it was 2 years old and there was some corrosion on the drivers door just under the widow sealing rubber. This was sorted under warranty. Link to comment Share on other sites More sharing options...
Bricol Posted September 20, 2002 Share Posted September 20, 2002 But aren't Discoverys that nasty steel stuff? And as for REAL Land Rovers - I bet on the rear of the panels where they touch steel, they are nice powdery white. Plus most of them by now are protected by several layers of multi-hues paint on top of the oxide layer on the outside. Bri Link to comment Share on other sites More sharing options...
Nick Woods Posted September 20, 2002 Share Posted September 20, 2002 Landrovers do corrode where the alloy and steel meet, just like a caterham does. If bare metal touches bare metal then an electrical circuit forms and the steel effectively eats away at the aluminium. The other cause of aluminium corrosion can be impact damage if a panel vibrates against something hard. The surface layer of oxide gets scraped off and reforms, over time this removes a lot of metal. My neighbours Discovery has exactly the same problem on his doorskins and I'm sure he said they where alloy although we could both be wrong. Nick P8MRA - The green one with red wings. Which is now bent ☹️ 🙆🏻 ☹️ 🙆🏻 ☹️ 🙆🏻 ☹️ 🙆🏻 ☹️ Link to comment Share on other sites More sharing options...
stevefoster Posted September 20, 2002 Share Posted September 20, 2002 The ali being more reactive than the steel gives it's electrons to the steel and thus corrodes. The missing electrons allow the metal to form other allianses with oxygen in this case. Loose old element oxygen (lucky for us eh!). This could explain why when you lose your powercoat you get little more than surface rust on the chassis. The ali is saving it. Steel hulled ships have zinc blocks bolted on every few metres for just this electron donation corrosion protection. Valencies and periods tables... ah some of it comes back every now and then... /Steve My racing pics, 7 DIY, race prep. Updated often here Hants (North) and Berkshire area club site here Edited by - stevefoster on 20 Sep 2002 14:43:35 Link to comment Share on other sites More sharing options...
John Hughes Posted September 20, 2002 Share Posted September 20, 2002 Most of the Disco's panels are ali, however unlike a "classic" Range Rover the roof is steel - helpful when using a mag mount ariel. The corrosion on the door is due to galvanic action between the steel inner frames and the ali outer skin John Link to comment Share on other sites More sharing options...
Joe 90 Posted September 20, 2002 Share Posted September 20, 2002 The term aluminium is commonly used for aluminium alloy. Pure aluminium is very soft, so an alloy is used for panels, skins, greenhouses etc. The most widely used alloys have a silicon content of about 4% to 6%. Just like steel, these different alloys have different corrosion resistance properties, different strengths and different work hardening properties (and cost of course). The old SII and SIII landrovers were made of some appalling stuff which was all that was available after the war, and yes they do rust! Corrosion looks like it is going to be the death of my 7 in the near future, so tips on protecting the next one are appreciated. P.S. The bits covered by the oil leaking out of XFlows don't corrode! 99,000 miles so far Link to comment Share on other sites More sharing options...
Pierre Gillet Posted September 20, 2002 Share Posted September 20, 2002 The ali panels of my 1979 Land Rover "Light Weight" SIII, ex Belgian army (I think) do not rust. In fact they are in very good condition. Pierre Link to comment Share on other sites More sharing options...
Alias Posted September 20, 2002 Share Posted September 20, 2002 If I remember my chemistry GCSE correctly, Aluminium is highly reactive. It is covered by a thin layer of aluminium oxide which is transparent. If you stripped off the aluminium oxide layer with silver nitrate and held a match nearby, you could set light to it, and watch the whole bar oxidise very quickly - in a very similary way to magnesium would - i.e. with a very bright and hot flame. Link to comment Share on other sites More sharing options...
R2D2 Posted September 20, 2002 Share Posted September 20, 2002 Birmabright - is the name of a range of alloys that used to be produced by Birmetals of Quinton near Birmingham, unfortuantely no longer with us. There are sevearl similar specification alloys produced by companies such as Alcoa. Birmabright is an Aluminium/Magnesium alloy that has very good work hardening properties. The ability of a metal to work harden is quite important when making pressings. It gains its basic properties from "solid solution strengthening" rather than the cold rolling of "quarter and half hard" sheet products. It is relatively corrosion resistant but is not any better at resisting bi-metallic corrosion than cheaper materials. It was used mostly for early Land Rover bodies but ist hasn't been around as Birmabright for quite a long time. I don't think that the traditional Seven is skinned in this type of material. It is likely to be either a soft 1000 or 2000 series alloy for the body with a half hard material for the floor, to give better wear resistance. the new caterhams are quite likely to be something much more modern and I am sure Arch would have the information. As everyone has said Aluminium has a very stable oxide. The Elligham Diagram, printed in most basic metallurgy books such as Rollason, shows that the thermodynamics for the formation of an oxide are very favourable. When a freshly exposed aluminium surface is exposed to air it will oxidise rapidly. Aluminium oxide is virtually transparent, has very good mechanical strength, relatively high surface hardness (Sapphire) and most impotantly has virtually no volume change compared to the base material. The reason that steels corrode so badly is that Iron Oxides are not very strong, are quite brittle and have a significant volume expansion compared to iron. They do protect for a very short time but because of the poor strength and expansion they break free from the surface and expose more fresh metal. Aluminium oxide tends to "seal" the surface and provide a protective layer. The reason that pin-holing can occur in simple sheet materials is that the oxide layer tend to vary slightly in thickness and can have a few holes. Obviously anodising is a means of significantly improving the corrosion resistance of aluminium and this technique simply increases the thickness of the oxide layer by making the component the anode in an electrochemical cell. Interestingly aluminium oxide has good thermal conductivity but is a very good electrical insulator. If a component is anodised after holes are punched and the surface isn't damaged by screw or rivet heads bi-metallic corrosion should be quite limited. The natural oxide film that forms on a polished surface just isn't thick enough to give good electrical insulation. Edited by - chris flavell on 20 Sep 2002 19:01:33 Link to comment Share on other sites More sharing options...
Graham Perry Posted September 21, 2002 Share Posted September 21, 2002 Interesting Stuff Chris, thanks Link to comment Share on other sites More sharing options...
Rob Morris Posted September 21, 2002 Share Posted September 21, 2002 Problem with steel is that when it oxides (red rust) the oxide surface is pourus therefore failing to for a barrier against further corrosion. Aluminum on the other had suffers the same initial oxide (white rust) problem but the resulting surface is not pourus so providing the surface is left alone no further rusting will form. Problem with many Alloy vehicles is that of steel meeting alloy with them being at different ends of the element table they in effect form a crude battery. This results in them eating away at each other just like the old batteries we had as children that would leak if not removed when flat. these "old" batteries used the outer case as one element of the power source. Modern "leak free" batteries use different elements in there construction and no longer eat the case! Hope this explains why in simple terms. Rob M. 40th Anniversary Vauxhall Man www.slipstream-trackdays.co.uk Link to comment Share on other sites More sharing options...
Red SLR Posted September 21, 2002 Author Share Posted September 21, 2002 So it does oxidize then! Thanks Simon. See X777CAT here Link to comment Share on other sites More sharing options...
Morls Posted September 21, 2002 Share Posted September 21, 2002 I've filled my garage with brine and hung a zinc block from my steering rack mount to avoid corrosion. Can anyone reccomend a good antifoul that won't harm the cats? Mark Link to comment Share on other sites More sharing options...
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