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steel and aluminium

steel and aluminium

steel and aluminium

(OP)
Hi,

Should bolting a steel plate to an aluminium surface cause corrosion problems?

I recently had an ABS problem on a car and traced the fault to the gap between the sensor and the wheel rotor. The gap had increased due to corrosion build up.

The sensor is mounted onto a steel plate at the rear of the wheel hub. The wheel hub is made from aluminium and the corrcsion had built up between the two materials to such a degree, that it pushed the steel plate away from the hub; thus increasing ABS sensor gap.

Thanks

RE: steel and aluminium

Without a protective finish, yes.

RE: steel and aluminium

(OP)
Thanks EWH

I dont believe either surface had any kind of a finish on it.

The corrosion - is it due to differences in electrcical properties of the two dis-similar materials you think?

RE: steel and aluminium

Yep, it's called galvanic corrosion.  Try googling it for more info.

RE: steel and aluminium

(OP)
Will try a google search for it.

I'm convinced "galvanic corrosion" is what i'm dealing with here. But it strikes me odd that a car manufacturer has taken no preventitive measures ?!?!

What finish would you suggest using?

RE: steel and aluminium

Nick93,
A very common problem in the boating world.
Perhaps a check with a boat place or an electronics place may  provide a suitable product.

HTH, Cheers , Pete.

RE: steel and aluminium

In the past I've used an organic coating between a stainless stud pressed into HSLA steel to discourage galvanic corrosion.
I don't recall the name of the coating though, the studs came pre-coated.

I've seen where a washer with heavy zinc coating is used between joints of dis-similar metals. The zinc becomes a sacrificial anode and is eroded while the other materials are protected.

regards,

Hydroformer

RE: steel and aluminium

Electrically isolate the two pieces. Use a flat stock btw the two. then use plastic washers and sleeves to isolate the bolts.

Nick
I love materials science!

RE: steel and aluminium

Standard practice in the auto industry is to coat the steel piece, usually with cathodic electrocoating (e.g. epoxy at 25 µm.  Then, use a multi-layer zinc organic coating for the fasteners.  Leave the aluminium part bare.

Regards,

Cory

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RE: steel and aluminium

Right-o Cory,

I should have mentioned, in my stainless stud application, the
steel stamping was e-coated prior to stud insertion. Further more, a teflon (stretched mesh) bushing was used which had to maintain electrical continuity through the joint.

At our tech-center we had one of a very few all aluminum Sables. Under the hood were various labels warning mechanics to use ONLY organically coated fasteners.

regards,

Hydroformer

RE: steel and aluminium

(OP)
Hi all,

thanks very much for the input. I've got a lot to work with.

regards

nick93

RE: steel and aluminium

Hmm, Toyota seem to have no problem bolting an aluminium thermostat housing to a cast iron block. Now try and find one in a scrapyard. Mine lasted 21 years.

Cheers

Greg Locock

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RE: steel and aluminium

Greg,

Salt spray doesn't reach the thermostat/block area as readily as it does the suspension area, which reduces corrosion risk.

As far as internal corrosion due to coolant as the electrolyte, coolants are produced with additives to prevent corrosion of the Al parts (especially the radiators) along the coolant path.

Regards,

Cory

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RE: steel and aluminium

(OP)
Hi guys,

i'm left thinking that two dissimilar metals joined would coorode on their own without an electrolyte, and that having an electrolyte (water)would accelerate the corrosion.

RE: steel and aluminium

Nick93- I think you still need some sort of electrolyte, otherwise all those steel bolts that thread into aluminium on my car would be forever frozen.

RE: steel and aluminium

(OP)
I see your point. in my case, driving water spray was the electrolyte then?

RE: steel and aluminium

Galvanic corrosion is a simple cell. As I remember from High School (early 1960's) a cell needs dissimilar metals and an electrolyte.

The cell works by transfer of electrons. The transfer of electrons requires conductivity. The force driving the transfer of electrons is the difference in affinity for electrons of the two substances connected by the electrolyte. I am digging deep into the memory cells on this one, so I might be way off base, but it seems logical.

Regards

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RE: steel and aluminium

(OP)
in my early days, ive always been told not drill into an aluminium chassis and insert a steel screw - and in places where water cant reach. Maybe an electrolyte merely accelerates the corrosion.

RE: steel and aluminium

An electrolyte MUST be present for galvanic corrosion to occur.  You can perform a simple Internet search for supporting information.

Regards,

Cory

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RE: steel and aluminium

Water with the slightest salt content, or acid from dissolved gasses is an electrolyte

Regards

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RE: steel and aluminium

(OP)
Hi guys again,

Going back to my original problem layed out at the start, would GROUNDING the "wheel hub"/"steel backing plate" assembly to chassis potential disipate all electrical potential held within the different metals?

RE: steel and aluminium

No, that won't fix your problem.  As long as the dissimilar metals contact each other, and are connected with an electrolyte, then adding an electrical path to another part won't help.  My message from 11 May 05 9:21 is really your best solution based on price and performance.

Regards,

Cory

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RE: steel and aluminium

(OP)
Thanks for the input Cory. Its been an interesting subject to research.

What profession are you in?

RE: steel and aluminium

I work as a Materials Engineer for an automobile manufacturer.  You can learn more about any user on this site by clicking on his/her handle.

Regards,

Cory

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RE: steel and aluminium

patprimmer wrote,
"Water with the slightest salt content, or acid from dissolved gasses is an electrolyte"

Rain, for example.

Just a few years back,  Ford ran a little science experiment by putting aluminum decklids on Lincolns.  It was a short-lived experiment, as the steel hinges contacting the aluminum skin prompted the aluminum to corrode right through,  usually in a year's time or sligthly more.

They've since done quite a bit of study of galvanic corrosion...

RE: steel and aluminium

(OP)
Hi everyone,

Once again thaks for all the input. I'l echo a point i made earlier on in that this has been an interesting topic research.

If anyone is interested, see my final case study WWW.HIBTECH.COM then FERRARI 456 ABS part 2.

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