zinc passivate

[miked]

I was considering what to do with some new rear A frames and some very clean front wish bones.

I was considering the above but am aware that it makes metal brittle. I recall a plater saying about hydrogen brittle-isation (spelling). I know that the yellow type gives pretty good corrosion resistance. I was thinking of doing this and then a laquer.

Is this a silly idea and will it have an effect or am I being paranoid. Shall I just get them powder coated.

I like the idea of being able to see the welds and braze. It is for a Hill climb car.

Mike

[bigvalvehead]

Hi
TTR has all his 26R wishbones passivated.
Regards
Dave
Elan 26R

[cliveyboy]

You are correct about passivating making steel brittle but it is mainly on high tensile steels (bolts etc) that you get the problems.
You can debrittle the steel by heat treating it (a bit like tempering or annealing).

Clive

[bcmc33]

Mike,

My rear suspension is in pieces and stripped down ready for refurbishing.

Next week I shall take the bones and diff tie rods to a local company to be shot blasted, zinc plated and epoxy coated prior to reassembly.

There will be no problem if you prefer zinc + passivation, as Cliveboy says, embrittlement is only an issue with high tensile and heat treated steels. I would still paint over the passivation, though.

[miked]

Thanks Guys.

I will do them. The A frames are TTR but I notice a lot of his kit in bare form now. Must be to let you decide. I just picked up a roll bar from him for my mate and had that done.

I was told about the heating business. It was in relation to some bolts for brake calipers. I see that they are always just blacked for this reason.


Mike

[handi_andi]

Mike
The risk with zinc passivation, as far as I know and stand to be corrected, depends on the acid chosen to clean the items before plating in the main and to a lesser extent on the cathodic voltage applied during the plating process. I would agree that hydrogen embrittlement has always traditionally been associated with high tensile steels and this is where the majority of problems are seen. Recent research has shown though that it can occur in low tensile steel in sea water at none cathodic protected levels. Hence whether the increase in risk due to zinc passivation alone is probably open to debate, and other fatigue mechanisms are far more likely to be a problem in suspension units.

To put the hydrogen embrittlement question into perspective I have just seen a technical paper that suggests shot blasting can increase the risk of hydrogen embrittlement!

Or to put it another way, the hydrogen embirttlement mechanism isn't actually that well understood.

Hence, I would suggest you go with whatever you are happiest with, as lets face it if you didn't paint them and they started rusting then we could start a discussion on corrosion induced fatigue cracks.

Cheers

Andy

[GrUmPyBoDgEr]

Hi Mike,

I would go ahead & get your bits Zinc plated if that's the way you want to go.
Both the tubular & pressed steel wishbones will be made of mild steel or something only a little bit better. Certainly not heat treatable steel.
I've had some first hand experience with Hydrogen embrittlement.
I had some "Banjo" bolts on an external oil feed (4 bar) start to "let go".
The "cacky" really hits the fan when something like that happens
Hydrogen embrittlement occurs on heat treated components mostly (even an 8.8 quality bolt is heat treated).
Hydrochloric acid is used to clean & descale after heat treatment (pickling) This prepares the surface for Zinc plating.
If the component is not completely cleaned free of the HCL, this can remain trapped betwen the component & the Zinc layer. This usualy occurs in small irregularities in the surface e.g. roots of threads, sharp corners in shoulders etc.
So that is the part of the process that needs that bit of extra care.
The effect of this is to further weaken already highly stressed areas of the component.
None of this has any connection with the passivation process, which is an additional Chromating process that provides a bit more corrosion protection to the Zinc. The most common form is yellow Chromating which gives the part the often seen Gold colour.

Hope to have been of help.
John

[miked]

Thanks guys for all the input, didn't get on last night.

As all things it sounds like it can get pretty technical. I dropped them off today. I will keep a good eye on the welds on the A frames and the braze on the bones. I figured I would have more chance of capturing a crack without paint or powder coat.

I am note sure about his voltage and acid strength but do know that this guy and his late father did all sorts with data and spec sheet for lots of businesses. He told me about the process. I note he does lots for automotive applications, so must be familir with all their declared spec's (I hope).


Mike

[bcmc33]

Mike,

A couple of things.
Did I understand correctly that you had an ARB zinc plated? That is one of the items I would paint only as it is made from heat treated high carbon steel.
Secondly, I would not have any zinc plated items open to the environment under the car - it will corrode very quickly. Even with a chromate seal it won't last very long.

Unless of course like me, you don't take your car out in the wet.

[miked]

Brian,

Yes, you did hear right about the ARB. My mate asked me to pick it up for him from TTR and than have it plated. When I was at TTR's Stuart told me that the reason that they dont do them themselves anymore is that they could not get the plater to move the (non bushed) drop links about while the plating was being carried out. As consquence they ended up with missed bits in that region. So it was not really the right thing to do! It is the heavy duty one!

Re: my car, this is for sprint/hill climbs and should not get that much weather abuse. I was hoping that with quality laquer I could keep them unpainted for examination purposes. Dont you not think so?


Mike

[bcmc33]

Mike,

I?m still taking my parts to be zinc plated and powder coated tomorrow. However, I think the handbrake rods will be zinc plated and chromated.

This whole subject has got to me a little, and as naturally argumentative person, I thought I?d throw in the following:

I was well versed in the hydrogen embrittlement subject for some years with high tensile bolts, and the baking standards and controls that were very strict in order to dissipate the hydrogen atoms following various electro plating processes.

However, this thread has highlighted the fact that although I was reasonably knowledgeable about how embrittlement occurs and how to treat it, I had absolutely no insight as to the actual failure modes caused by it. As I?ve been out of the loop for some years on this subject, I spoke to an old contact that was considered a leading expert on the subject ? even though he says the subject is not as well understood as it should be to always make the right decisions. Theories seem to increase exponentially, but practices remain stable. He suggested I read a couple of published papers on embrittlement failures of fasteners on oil rigs and welded pipelines ? from which I came to the following (paraphrased) conclusions:
1. Failed high tensile parts like nuts and bolts showed that the embrittlement was critical when coming into contact with external corrosion.
2. Embrittlement in welded seams is due to poor housekeeping during the welding process. When the weld area is clean and dry there is little chance of introduction of hydrogen atoms into the steel. And failures occur when external corrosion meets the embrittlement area(s).
3. Failures occur after long periods in service.
4. Such potentially critical items/areas are now protected to prevent external corrosion taking place.
5. If/when embrittlement should be present in mild steel parts like the front and rear wishbones, and it is affected by external corrosion, nothing will happen as the parts are not permanently highly stressed (i.e. in a permanent state of plastic deformation), and it is extremely unlikely that any transient stresses involved in fast track driving with have a detrimental affect. (Although having said this, maybe a 24 hour race may need some different thoughts).

[miked]

Brian,

Very interesting reading. You have eased my mind. I have used the bare metal high tempertaure laquer from the Frost book. I have used it before and it does seem to wok well.

PS what is chromated?


Thanks Mike

[bcmc33]

Mike,

Sorry - chromating is the usual trade word for yellow passivation. The active ingredient of yellow passivation is chrome 6 or chrome 3. There is also clear passivation which does not have the same level of corrosion resistance as yellow passivation.

I don't know what is the latest situation is regarding yellow passivation as I've been retired three years, but before I retired there was a European automotive ban on chrome 6 & chrome 3 (dangerous heavy metals). But the last I heard was that chrome 3 was reinstated pending development of alternative "eco friendly" processes.

Yellow passivation with chrome 6 is still in use with some industries, but I'm sure it will be phased out eventually.

[GrUmPyBoDgEr]

You're righr Brian,
in the Automobile industry Chromating has been outlawed for new designs for a few years now.
Presently all components have to be "Chrome free" & "Lead free" which does make life difficult.
The available alternatives that are affordable all have their own problems
I'm still "in the loop" but looking forward escaping
John