Was I lucky...

Big lumps of metals and spanners.

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dwayne
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Re: Was I lucky...

Post by dwayne »

R0B wrote:.Its the metal on metal that will cause corrosion.

I got an advisory on my MOT a couple of months ago , all my exhaust clamps had virtually disintegrated , tester said it was because they were steel clamps on a stainless exhaust , I didn;t even know it was stainless :oops: thought it looked cleaner than most I had seen . He said he sees it all the time and can't understand why fitters don't use stainless clamps . Needless to say all new stainless clamps fitted :ok
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Oldiebut goodie
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Re: Was I lucky...

Post by Oldiebut goodie »

I have experienced severe corrosion in marine grade stainless in a wet exhaust system in a boat - admittedly there will be corrosive gases in addition to heated sea water passing through it - so I will not take it that stainless is the perfect solution to the problem with the vans system.
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Re: Was I lucky...

Post by footstuck »

Hi Rob
Could you ask your man if sectioning and welding in pieces of stainless
on internal chassis repairs, will have any detrimental long term corrosive effect.
It seemed logical at the time, but in light of your post . . . .?
Regards FS

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Re: Was I lucky...

Post by Oldiebut goodie »

I personally wouldn't use it due to the risk of it fracturing.
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dwayne
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Re: Was I lucky...

Post by dwayne »

As above , I think it will corrode faster than you think and the weld would fracture. I remember changing 2 of my exhaust clamps 2 years ago and they had totally disintegrated where as other clamps changed at the same time are good as new .
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Re: Was I lucky...

Post by AdrianC »

R0B wrote:He said the water flowing through the pipes should cause very little.But hey hes only a metallurgist.

Since the _inside_ of the rotten pipe on mine was immaculate, apart from where the outside was bursting in, I think we can say that practice has proven the theory.
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Rich Tea
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Re: Was I lucky...

Post by Rich Tea »

I just came across this post and thought I might add my thoughts as I know a little about Metallurgy (Classics Monthly even claimed I was a " boffin" a couple of months back :rofl ).

Stainless is more noble than mild steel, so if you put the two together in the presence of an electrolyte (water) you will get a galvanic reaction (think battery) and it's the mild steel that will go all crusty. Aluminium - Stainless is also not a good idea.

Welding Stainless is a specialist job. The main problem is that when welded the Chromium in the steel forms Chromium Carbides at the grain boundaries instead of being nicely alloyed and preventing the steel going rusty. So the Chromium is now not doing it's intended job, on top of that it left nice brittle chromium carbides on the now susceptible grain boundaries. Corrosion can then attack these chromium depleted areas aggressively. Internal stresses resultant from the welding can also result in stress corrosion cracking. If you know what you're doing these effects can be somewhat be addressed by factors such as alloy selection, filler metal, post weld heat treatment etc (but now we're getting really technical)

One way of reducing the issue is to use low carbon Stainless alloys (less carbon = less carbides) e.g. 304L or 316L modifications to the popular 304 and 316 alloys.

As for welding Stainless to mild steel I'd say you need to keep a close eye on that, especially if it's structural.

So stainless is not as ideal as we might hope. Let's not forget, the best way to avoid corrosion is to isolate the parts, a robust paint scheme not only keeps out the environment but electrically isolates one metal from another.

I hope that's of some use to anyone that's interested.

Rich
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Oldiebut goodie
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Re: Was I lucky...

Post by Oldiebut goodie »

Thanks for that - it's amazing what I only half remember - I remember the not to do's but not the reasons why! :)
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Re: Was I lucky...

Post by Cruz »

So what are we saying about front to back coolant pipes then? Surely adding coolant to water makes a difference?

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Oldiebut goodie
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Re: Was I lucky...

Post by Oldiebut goodie »

It will make it more conductive I would have thought. Introducing stainless into the equation would be outside the design parameters for the coolant/antifreeze I suspect.
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Re: Was I lucky...

Post by toomanytoys »

Those steel pipes are shockingly thin and under painted.. a small rust bubble can be a hole with a light sand blast...

Having good quality coolant in there in the right concentration is the most important bit.. the electrolytic reactions wont be an issue.. not in the lifespan of the vehicle.. :wink:

direct stainless to alloy should be avoided in any weather beaten situation, but on our engines I wouldnt worry.. there is usually enough oil about protecting things and a smear of copperslip goes a long way on assembly..

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Re: Was I lucky...

Post by Ian Hulley »

toomanytoys wrote: a smear of copperslip goes a long way on assembly..

That was what I thought, there's no contact with the thermostat housing (plastic anyway in 2wd) and the pump end there's an O-ring (and in my case Hylomar) so the only contact is the 2 socket head capscrews which have been stainless on mine since I fitted the engine anyway.

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Rich Tea
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Re: Was I lucky...

Post by Rich Tea »

Cruz wrote:So what are we saying about front to back coolant pipes then? Surely adding coolant to water makes a difference?

I'm not a Chemist so don't feel confident commenting on how the inhibitors in coolant work. But you do need them, and they do need to be at the correct concentration and to the correct spec for the engine and replaced at the recommended intervals. If not then you could get all kinds of problems.

I wouldn't worry too much about the Stainless front to back pipes, you need direct contact between the two dissimilar metals as well as an electrolyte path (think back to those experiments at school when you dip dissimilar metals in a beaker of acid, you need to connect the anode and cathode together via the volt metre to get current to flow). The coolant pipes terminate on rubber hoses so are isolated from mild steel parts. However if you use mild steel P-clips or the like to bolt the pipes to the van there would be an issue. Due to the relatively large surface area of the stainless and the vastly smaller surface area of the clip corrosion could be driven by anode cathode ratio.

I don't want to get too deep into this stuff but it goes like this.

If you put a small steel bolt in a large stainless steel sheet, the bolt would corrode quickly, if you put a small stainless bolt into a large steel sheet the corrosion of the steel sheet would much be less significant, and you'd probably be ok.

I'm spouting a bit of theory here, but the reality is that in practice we very often get away some of these combinations due to other factors, see Ian's example that his stainless pipe is potentially isolated from the water pump by the o-ring and hylomar = probably fine. But sometimes we don't, see dwaynes previous example of the corroded exhaust clamps.
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Re: Was I lucky...

Post by ghost123uk »

California Dreamin wrote:I think I caught this just about right lol good job I wasn't on my hols somewhere in Europe isn't it..

Image

Could be a timely reminder for others to have a close look at their metal coolant pipes (mines a 2.1 wasserboxer)

Martin

Mine went 1/2 way a long it's main length, I sawed it in half, removing the corroded bit, cleaned and painted it (black Hammerite) and joined the 2 ends with a bit of normal Rad hose, working ok for now.

JK do them at = http://www.justkampers.com/shop/water-p ... 10912.html" onclick="window.open(this.href);return false; = £56 :twisted: :twisted: :twisted:

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Re: Was I lucky...

Post by Red Westie »

I think we all get carried away with stainless products and forget that the original mild steel pipes have lasted 21 years plus..Now honestly I'm not going to worry about fitting mild steel for this application as another 20 years is fine by me.

I guess if I'm going to worry about anything it would be the quality of the aftermarket replacements that are available and that is why I opted for a near mint second hand GENUINE VW part as a replacement.
And
As I said earlier...there was no evidence of internal corrosion just rust from the outside...so my used pipe was stripped back to almost bare, primed and given 5 - 6 spray coats of paint which is more than can be said for the original finish, which looks like is has barely any paint on it at all and I suspect the REAL reason these pipes corrode through.

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