Mercruiser sterndrive Zinc Chromate

Discussion in 'Propulsion' started by Ike, Apr 12, 2016.

  1. Ike
    Joined: Apr 2006
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    Location: Washington

    Ike Senior Member

    I have been cleaning up my sterndrive, preparing to do some touch up (it needs a whole re-paint but it can wait) and I think some time in the past long before I bought it some one put zinc chromate on this thing. There is yellow primer peeking out from under the paint.

    I have always been told that you should not use zinc chromate on an aluminum sterndrive. It's the zinc that is the problem. After four to five months in the water the unit is dotted with little deposits of a white powder that looks a lot like sodium chlorate (or aluminum oxide?) or something similar. You can see some in the photos. The local Mercury repair place told me it's from the fertilizer chemical runoff in the lake. Big freshwater lake. Good fishing lake, but occasional algae blooms in the summer. Lots of big houses with large lawns around the lake. Anyway, now I am wondering if I am also seeing a reaction between the zinc and aluminum.

    Any thoughts.

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    In any case after this season I am going to have the sterndrive rebuilt including a completely paint job. So I am just curious about what's going on here
     
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Mercury uses zinc dichromate on their drives and recommends its use when re-painting. I have used it for decades with no problems. Never had customers complain.
     
  3. Ike
    Joined: Apr 2006
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    Location: Washington

    Ike Senior Member

    Thanks Gonzo. I have always used their light grey primer which is recommended by Mercury for sterndrives. I have no idea what's in it. It seems to work fine. But I have done this routine every year for the last three years and never noticed the yellow coat before, of course I haven't taken it down to bare metal before. So perhaps you're right. However, since this boat is 44 years old and has the original sterndrive I wonder how many times it's been repainted. No way to tell. Frankly I am amazed that it still works well.
     
  4. sdowney717
    Joined: Nov 2010
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    sdowney717 Senior Member

  5. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Parts of the drive leg that are immersed at speed need etch primers, otherwise the paint will have doubtful adhesion.
     

  6. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    You barely need to dust the aluminum with the zinc chromate. It bonds chemically and etches the surface.
     
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