Dissimilar metal corrision

There has been quite a lot of activity in the Samba Vanagon forum on the subject of stainless steel cooling lines causing dissimilar metal corrosion in the aluminum alloy engine. About a week ago I decided to do a little experiment to see if I could detect any visible corrosion on a bit of aluminum connected to stainless steel, both in shared electrolyte of engine coolant.

The stainless is 304, the aluminum is 6061. Both samples are 0.125″ X 0.750″ and immersed in the coolant to approx 2″.

The coolant in a 50/50 mix of OAT based coolant and distilled water.

I did a casual buffing of both samples with tripoli, and then degreased. The buffing was to allow any corrosion to be more visible. In the picture, the aluminum is on the right.

The samples are connected by tinned copper wire.

Ambient temp in the lab, I mean the barn, ranges from 7 C to 10 C.

Initial measurements between the unconnected samples in the coolant was, 150 microAmps and 540 milliVolts.

Experiment started at 6:30 pm, pacific time, October 26 2010.

I’ll leave it undisturbed for a week or so, then take pics of the metals.

It will be interesting to see if anything happens

Nov. 2 1220 hrs – I pulled the metals out of the jar and rinsed off coolant. The aluminum looks pretty good. I don’t know if the pic shows it but there is a very slight haze on the surface.

I replaced the coolant with tap water (well water, pretty hard), and set the expt. up again, same conditions as before. Initial readings are:
326 mV and 195 microAmps between the 2 samples.
(I don’t know why wordpress is sticking the “I” up there to the right of the image… I have tried to get it back down, the html looks right to me, but…)

Update, Nov. 4, 2010.

There are a few spots on the aluminium (none on the stainless) that look like a white “fluffy” deposit.

  1. #1 by David R. on November 2, 2010 - 10:30 pm

    Thanks Alistiar for doing theses tests! Really appreciate your incentive, time and effort.

    David R
    Oakland CA.
    Orbital-D

  2. #2 by David R. on November 4, 2010 - 2:07 pm

    And the white fluff is after replacing the antifreeze with tap water? And after only two days? Interesting!

    • #3 by albell on November 4, 2010 - 2:18 pm

      Exactly. But remember our water comes from a well, and its pretty hard. I have seen this exact same kind of deposit on water tunnels I have made. Have a look here on this modest website for what I mean by water tunnels. There are no deposits on the stainless sample.

      I suppose you could call this the worst case scenario for cooling systems šŸ™‚

  3. #4 by David R. on December 15, 2010 - 5:10 pm

    Any update on the “Dissimilar metal corrosion” project?

    • #5 by albell on December 15, 2010 - 7:53 pm

      coming soon…. it was put on back bench and actually froze solid in recent cold snap. I’ll take pics of results and set up a battery of other tests during the holidays.

      alistair

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