steel and aluminium

[Nick93]

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

 

[ewh]

Without a protective finish, yes.

 

[Nick93]

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?

 

[ewh]

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

 

[Nick93]

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?

 

[Peter7307]

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.

 

[Hydroformer]

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

 

[NickE]

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!

 

[CoryPad]

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

Regards,

Cory

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[Hydroformer]

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

 

[Nick93]

Hi all,

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

regards

nick93

 

[GregLocock]

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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[CoryPad]

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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[Nick93]

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.

 

[NickE]

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.

 

[Nick93]

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

 

[patprimmer]

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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[Nick93]

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.

 

[CoryPad]

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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[patprimmer]

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

Regards

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