Aluminium Corrosion Treatment

In the process of doing some some work on my 2003 110 SW I temporarily removed the 60/40 middle row seats. There are three mounting points and the aluminium body work around both the outside ones has corroded quite badly. I think it is caused by galvanic corrosion as the aluminium body work was sandwiched between a steel seat hinge plate and a steel bracket attached to the cross member.

I've searched for information on the best way forward and I am proposing to:
- brush down the corrosion on the inside with a nylon brush to remove as much of the white corrosion dust as possible - steel wool could lodge bits of steel in the aluminium and cause further problems
- paint the area with some Hammerite Special Metals Primer
- paint with a suitable top coat, probably some regular Hammerite because I have some in the garage
- make new aluminium patch panels to go over the top of the affected area
- prime and paint the patch panels
- further insulate the patch panels from the steel on both sides as best I can (ideas welcome!)
- rivet and silkaflex the patch panels in place
- treat the chassis side with a very generous coating of waxoyl

Does this sound reasonable?

Also, in terms of bolts, although the zinc coating on galvanised bolts has a lower galvanic difference to aluminium, it does get consumed and then leaves the aluminium exposed to the steel. Lots of people use stainless steel which does have a higher galvanic difference to the aluminium but I using stainless bolts in aluminium is a widespread practice in the world beyond landies. This site is a bit dry but suggests that stainless bolts in aluminium is low risk: http://www.bssa.org.uk/topics.php?article=89.

NickMcKee said:

In the process of doing some some work on my 2003 110 SW I temporarily removed the 60/40 middle row seats. There are three mounting points and the aluminium body work around both the outside ones has corroded quite badly. I think it is caused by galvanic corrosion as the aluminium body work was sandwiched between a steel seat hinge plate and a steel bracket attached to the cross member.

I've searched for information on the best way forward and I am proposing to:
- brush down the corrosion on the inside with a nylon brush to remove as much of the white corrosion dust as possible - steel wool could lodge bits of steel in the aluminium and cause further problems
- paint the area with some Hammerite Special Metals Primer
- paint with a suitable top coat, probably some regular Hammerite because I have some in the garage
- make new aluminium patch panels to go over the top of the affected area
- prime and paint the patch panels
- further insulate the patch panels from the steel on both sides as best I can (ideas welcome!)
- rivet and silkaflex the patch panels in place
- treat the chassis side with a very generous coating of waxoyl

Does this sound reasonable?

Also, in terms of bolts, although the zinc coating on galvanised bolts has a lower galvanic difference to aluminium, it does get consumed and then leaves the aluminium exposed to the steel. Lots of people use stainless steel which does have a higher galvanic difference to the aluminium but I using stainless bolts in aluminium is a widespread practice in the world beyond landies. This site is a bit dry but suggests that stainless bolts in aluminium is low risk: http://www.bssa.org.uk/topics.php?article=89.

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Basically isolation of all the metals is the best idea and using the the fewest different types helps, stainless steel fasteners are considered (as you said) safe in aluminium but not aluminium fasteners in SS.

If you could use lots of paint during assembly and then paint the whole lot to try and seal it that will make a huge difference but it is not always easy to get 100% seal unless on the underbody areas you can blast lots of gravitex or similar (not waxoyle) to encapsulate the lot.

I would not mess about with hammerite special metals primer, just use a proper etch primer. I have used special metals and really was not impressed with it, it's adhesion is no where as good as an etch, fine for painting an aluminium garage door or something but that's about it.

UPOXY sell a litre can and litre of hardener - so 2 litres mixed for about £20 which can be brushed and sprayed and you can mix up as little or as much as you want, or just get some cans of etch primer.

I would grind out aluminium oxide with a silica carbide or similar grinding disc and remove the aluminium oxide in it's entirety, get back to clean bare aluminium which will then take a coat of etch with excellent adhesion. When I fixed my tub where there were oxide patches or spots right through the panel I drilled it right out to ensure it was gone - I was lucky as most of these holes were only 1/2mm.

One the whole lot is painted and assembled get a coast of underbody seal on it, gravitex or indasa stonechip are good. It looks much better than waxoyle and does a better job, you can waxoyle over it after it as set if you want but it is not needed, that is the stuff car manufacturers use from the factory.

Mine looked like that. After in started cleaning it up it all fell to bits and needed repair sections fitted.

You need to cut the rot out, not paint over it.

Thanks everyone - much appreciated.

I've got rid of most of the aluminium oxide and will keep going until it is nice and clean. I've also had three patch panels (175x110mm) cut and shaped at a local engineering firm. They made them in 3mm alu as that's what they had in stock. They also made me a new steel spreader plate as I seem to have lost the one from the middle row - I was hacked off at the time because the last bolt sheared as I was taking it out. £10 for all four which I though was a good deal. Grinding, etching, painting, sealing here we come.

just to pick up on a point stainless steel IS NOT safe on aluminium, they are miles apart (nearly opposite ends) of the galvanic table so will react very quickly and aggressively

DarrenHS3 said:

just to pick up on a point stainless steel IS NOT safe on aluminium, they are miles apart (nearly opposite ends) of the galvanic table so will react very quickly and aggressively

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Thanks for this Darren - is this your experience? I ask because I have no experience either way but the official recommendation seems to be that stainless fasteners in alu panels are ok. The article I linked to highlights this combination in use in major civil engineering construction.

Nick, how old is your vehicle? 2003? So that makes it what 14 years old? The original lasted 14 years, don't look too deeply into making a fix that will outlast the rest of the vehicles life span,

nobber said:

Nick, how old is your vehicle? 2003? So that makes it what 14 years old? The original lasted 14 years, don't look too deeply into making a fix that will outlast the rest of the vehicles life span,

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Too true!

DarrenHS3 said:

just to pick up on a point stainless steel IS NOT safe on aluminium, they are miles apart (nearly opposite ends) of the galvanic table so will react very quickly and aggressively

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No, you have misunderstood how it works - you don't just look at the table and go, "Oh they are far apart they won't work together".

You need to take into consideration surface area of the metals. Bear in mind aluminium is anodic to stainless.

SS fasteners in aluminium are one of the safer combinations you will get.

NickMcKee said:

Thanks for this Darren - is this your experience? I ask because I have no experience either way but the official recommendation seems to be that stainless fasteners in alu panels are ok. The article I linked to highlights this combination in use in major civil engineering construction.

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The chemistry behind this and the practical use of SS fasteners confirms that aluminium fastened with SS fasteners is a good choice.

To further complicate things, SS fasteners are often not a good idea in... stainless steel!

If it offers you any comfort I have used SS fasteners all over my LR including direct contact to aluminium - been there for about 5 years now... no issues with corrosion.

When it comes to metals the best combination would be to use the same metal for everything, not just the same type of metal, but the same exact material specification - which you will never get as sheet steel and bolt steel for example are different grades!

discomania said:

No, you have misunderstood how it works.

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ok? then enlighten us.

the difference in nobility determines the rate of corrosion. mild steel and aluminium are only 2 steps apart on the galvanic series. (and look how they got on) aluminium and 304 stainless (random example based on A2 bolts) are 26 steps apart.

i agree though that its circumstantial

DarrenHS3 said:

OK. Then enlighten us.

The difference in nobility determines the rate of corrosion. Mild steel and aluminium are only 2 steps apart on the galvanic series (and look how they get on). Aluminium and 304 stainless (random example based on A2 bolts) are 26 steps apart.

I agree, though that it's circumstantial.

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What you are saying is on the face of it quite correct, however there is a lot of chemistry behind it and this can explain why we can use SS fasteners in Aluminium.

Right the first thing you need to consider is that the galvanic series is not set in stone this is due to metal specifications always changing and being varied, so what ever table you are looking at will probably differ from the next. From a metallurgists galvanic series table I have here, the likely grade of aluminium you are looking at in your table is actually a very hard aluminium with relatively high chrome content, and at that, it is about 5 steps away from a 1010 steel which your table will probably be using as "steel". The chrome content in that aluminium actually reduces the reactivity of that type of aluminium as chrome is more noble.

The bad news is that aluminium that LR's are made from (originally Birmabright which was a trade name for a 5251 aluminium) and then later just a 5251/52?? range aluminium had a fair bit of magnesium in it, that magnesium (about one of the least noble common metals) makes LR aluminium even more reactive, to a point it is about 23 steps away from "steel".

So the basic chemistry behind galvanic corrosion it is that two metals with different potentials in presence of an electrolyte, e.g. water or worse salt water will set up a cell with the metals forming the cathode and anode, just like a battery they will react and the current flows from the anode (electronegative) to the cathode (electropositive), the further apart on the table they are the more reactive they will be, the bigger the potential difference therefore the bigger the current and the worse the corrosion will be. The corrosion is caused as the electrons rearrange themselves, i.e. leave the anode and flow to the cathode, so the anode must release these, this all happens by means of a chemical reaction.

So take the Defender door, the aluminium is against the steel, water or salty water gets into the door and the reaction starts, a current will flow from the anode (aluminium) to the cathode (steel), as it does this, electrons leave the aluminium, the chemical reaction which causes this converts the aluminium into aluminium oxide and this cycle continues depleting the aluminium. All aluminium has a layer of aluminium oxide on it, if you remove this it will react even in air and form a new layer, if this is left on it's own with no contributing factors the aluminium will stay like this, but if you keep removing the oxide you will eventually remove all the aluminium until there is a big hole or pitting.

Now, if you then take stainless bolts which will be the cathode, the more noble, the electropositive, when combined with aluminium the area of aluminium is huge in comparison, therefore as the aluminium passes electrons to the SS by means of the chemical reaction as there is a lot of aluminium and not a lot of SS the reaction will not be aggressive.

If you used an aluminium rivet in SS it would corrode quick-time as the tiny little bit of aluminium passes all it can to the large area of SS.

Where the aluminium and steel are in contact on the Defender the issue is similar, the rear tub mount for example is hard up against the steel chassis, once the paint protection is breached and they get wet you now have a huge battery!

If the chassis was aluminium and the body was light steel the issue would all but disappear.

In galvanic corrosion this is refereed to as the area ratio and is a very important factor when designing bimetallic structures or joints - so yes, the difference in potential will determinate the aggressiveness of the reaction but other factors will give different results as the chemical reaction rate will differ.

Another thing to bear in mind is that there are quite a few factors that will reduce the reaction aggressiveness overtime and that is the buildup of the corrosion itself, as this layer builds the ability for the aluminium for example, to release the oxygen reduces and the reaction can almost stop.

To further complicate things, the galvanic series changes depending on what the electrolyte is!

excellent write up, thanks @discomania

to answer the personal experience, its with SS bolts holding the sidelight and indicator (to the vehicle) and vapourising the wings. i've read about people using nylon nuts and bolts instead, which is excellent idea.

for other areas i have made essentially gaskets out of damp roof course (the stuff that comes on a roll from builders merchant used for the lower course of a brick wall, as opposed to the large sheets for laying a slab of concrete) in the case of nuts and bolts cutting out penny washers and piercing a hole through the middle, in case of body mountings large rectangle pads to put between dissimilar metals with holes pierced for bolts.

there's also a commercial paste called duralac which achieves the same thing, from electrolyte/reactivity angle

DarrenHS3 said:

excellent write up, thanks @discomania

to answer the personal experience, its with SS bolts holding the sidelight and indicator (to the vehicle) and vapourising the wings. i've read about people using nylon nuts and bolts instead, which is excellent idea.

for other areas i have made essentially gaskets out of damp roof course (the stuff that comes on a roll from builders merchant used for the lower course of a brick wall, as opposed to the large sheets for laying a slab of concrete) in the case of nuts and bolts cutting out penny washers and piercing a hole through the middle, in case of body mountings large rectangle pads to put between dissimilar metals with holes pierced for bolts.

there's also a commercial paste called duralac which achieves the same thing, from electrolyte/reactivity angle

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Darren, thanks.

You have made a very valid point, your SS screws going into the body from the sidelight and indicator, does one of the tabs have copper as well? and it acts as a sort of earth.

This just helps to set up an even better reaction to be honest, another thing to remember is that any bare aluminium i.e. around the thread of a fixing will oxidise, then salt water and alkali road spray will get in, help to rinse off the oxide and it starts all over.

Your DPM solution is a good one. I used 3mm neoprene to isolate the tub from the chassis all over, even the seat-belt mounts have a layer between them. I also like to think it will help the cushion the body a little...

discomania said:

Darren, thanks.

You have made a very valid point, your SS screws going into the body from the sidelight and indicator, does one of the tabs have copper as well? and it acts as a sort of earth.

This just helps to set up an even better reaction to be honest, another thing to remember is that any bare aluminium i.e. around the thread of a fixing will oxidise, then salt water and alkali road spray will get in, help to rinse off the oxide and it starts all over.

Your DPM solution is a good one. I used 3mm neoprene to isolate the tub from the chassis all over, even the seat-belt mounts have a layer between them. I also like to think it will help the cushion the body a little...

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And I thought I was being cautious! Love the attention to detail. On my little repair job I'm going to great lengths to isolate steel from stainless from alu. Im feeling like a part timer compared to complete chassis isolation

How did you clean up the panels from behind where they are attached to the body crossmember? Mine are in direct contact with the crossmember with no space between the two surfaces. So the only way I see to get access is to remove the crossmember. Or is there a better way? If you just treat the fronts corrosion will come right back from behind won't it?

Slightly off-topic: Is it necessary to put the bolts back in if I don't put the seats back in? Or can I just seal the holes?

Kitai said:

How did you clean up the panels from behind where they are attached to the body crossmember? Mine are in direct contact with the crossmember with no space between the two surfaces. So the only way I see to get access is to remove the crossmember. Or is there a better way? If you just treat the fronts corrosion will come right back from behind won't it?

Slightly off-topic: Is it necessary to put the bolts back in if I don't put the seats back in? Or can I just seal the holes?

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I had the tub off when I did mine. If the back is corroded and the front is only treated the treated bit will start to blister a year or so later so it is not a long term repair. You could just clean up the back the best you can and blast the area with underseal to try and stop moisture getting into the space to stop the corrosion.

If the seats are not going back in then you don't need the bolts no - unless there is anything the bolts also fasten down but they should just go into the little captive nut plates. A little grommet or something would work.

discomania said:

I had the tub off when I did mine.

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By tub you mean the crossmember? Did you just unbolt it and move it back a bit? Or did you also have the side panels out?

Kitai said:

By tub you mean the crossmember? Did you just unbolt it and move it back a bit? Or did you also have the side panels out?

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No I had most of the body off the chassis. The rear tub and seatbox was off the chassis sitting on the floor so I could work on it.