Bimetalic corrosion

Messed up fitting a ventura rack - drilled too far into the undertray/rear frame causing the captive nuts or threaded whatever it is to disappear leaving nothing to screw holding bolts into.:banghead

Solution - insert a couple of rivnuts:thumbup

I would prefer to use stainless steel rivnuts and bolts. - I know that I could use aluminium rivnuts & bolts but I already have the stainless ones

Question - what is the potential for corrosion of the rear frame which is made of aluminiium. I believe there is a possibility of the aluminium becoming a sacrficial anode causing the hole for the rivnut to become oversize?:dunno

So long as the area is water-proofed then corrosion should not be an issue?:dunno

Has anyone out there used stainless fixings into aluminium or seen this type of corrosion?

MW whit bolts are yer going to fix the rack wae?

Yer stainless to ally might cause the ally tae oxidise a bit but I widnae bother aboot it to much if yer gonae waterproof the area.

Aldo said:

MW whit bolts are yer going to fix the rack wae?

Yer stainless to ally might cause the ally tae oxidise a bit but I widnae bother aboot it to much if yer gonae waterproof the area.

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Thanks Aldo

I am using M6 rivnuts & bolts all in 304 stainless

Thought I might use silicone sealant to keep the water out.

know of but not used Duralac...

http://www.eliseparts.com/products/show/60/408/duralac/

Hi i use duralac at work ocassionally and would suggest if you use rivnuts to coat them in duralac prior to inserting them into the frame,also smear some over the hole in the frame to be doubly sure...HTH

felix & milky

Thanks for the information on Duralac:thumbup

For the sake of £7 it's worth the belt & braces approach so I think I will get some:yes

any zinc chromate based jointing compound will do the job just as well.

i use it all around the bike.

bi-metallic corrosion is something ah encounter in ma line o work wi subsea lifting gear. However it taks a fair time fur it tae hae any real effect, other than cosmetic, even when accelarated wi seawater immerssion etc. Theres other factors effect how quick as weel such as any electrical current activity, oxygen levels, water salinity an ph etc etc .

Ah reckon if ye coat everything well wi some o the gubbins the lads suggest ye will be fine fur a good few years.

Wankopedia welly says

"Dissimilar metals and alloys have different electrode potentials and when two or more come into contact in an electrolyte a galvanic couple is set up. A galvanic couple can also be set up on a single metal or alloy due to the metal surface not being homogeneous or if the electrolyte varies in composition, forming a concentration cell.

The electrolyte provides a means for ion migration whereby metallic ions can move from the anode to the cathode. This leads to the anodic metal corroding more quickly than it otherwise would; the corrosion of the cathodic metal is ******** even to the point of stopping. The presence of electrolyte and a conducting path between the metals may cause corrosion where otherwise neither metal alone would have corroded".

Look at Landrover Defenders

Great vehicle but the silly twats still haven't sussed out that the dissimilarity between steel and aluminium causes a galvanic reaction which makes the paint blister at all the rivet points

*** I hope that's right or some rag head Indian will sue my arse!

welly, a jist spent 30mins on a reply and a wis sneezin aw oor the place.

A'll get this boy a know to really give us the heads up on SS tae Ally or any other dissimilar metal connections.

Any hoo whit dae yer think of they Blade Levers ya fud.

rico said:

bi-metallic corrosion is something ah encounter in ma line o work wi subsea lifting gear. However it taks a fair time fur it tae hae any real effect, other than cosmetic, even when accelarated wi seawater immerssion etc. Theres other factors effect how quick as weel such as any electrical current activity, oxygen levels, water salinity an ph etc etc .

Ah reckon if ye coat everything well wi some o the gubbins the lads suggest ye will be fine fur a good few years.

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I wis thinking the same thing Rmin. Saying that I was wondering if it wid hay a greater effect on the fulcrum point due ti the leverage o the ventura rack. Mair o shear stress thin a tensile een.

slomogadgie said:

I wis thinking the same thing Rmin. Saying that I was wondering if it wid hay a greater effect on the fulcrum point due ti the leverage o the ventura rack. Mair o shear stress thin a tensile een.

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Where the rack bolts on at the exhaust hanger the bolts will be in shear. Where the rack bolts to the ally seat frame the bolts will be in tension.

I think the key point that has been raised is the need for water / electrolyte - so as long as I coat the rivnut with a suitable product and water proof as best I can there is not a real problem

Great information many thanks to everyone