dougs968

Both covers are magnesium. The two spring pins that align the covers are steel as are the three zinc electroplated 6mm bolts.

MB968

And, one of the spring pins that align the covers is in the right boss, and the other is in the top hole of the castings.

tamathumper

Where different metals touch, the "less noble" of the two becomes the sacrificial anode, which is why we put zinc discs on our aluminum and stainless outboard motors. Magnesium, zinc, and aluminum metals are all less noble than steel or copper.

But I'm no engineer or chemist, I'm just thinking this rhymes with my maritime experience that when metals touch one of them is going to get eaten.

MB968

Guess I should go back to my books and read up again on galvanic action. As I reacall, the difference in location on the periodic table has a lot to do with the tendency for this to happen. We use steel screws in our aluminum castings all the time without this issue. And, as noted above, there is little difference between the right boss on the bottom and the top mating between the two castings. They both have the steel spring locating pins, and steel screws. So, why always the bottom right boss that corrodes and not the other.

dougs968

I have a theory as to why the lower right boss fails. Simply, the manufacturing process. After the parts are finished being machined, they need to be treated and then painted. They have to be hung somehow to do this so that they look pretty afterword. By whatever method it appears to me that the hole in the right hand boss was used. Therefore, an unprotected spot remained for the elements to start eating away at it from the inside out.

At my work we go through a fair amount of aluminum extrusion for heat sinks. After all the mounting holes for all the various components are added, off they go to the anodizers. When they come back there is always one hole that didn't get anodized. That is the hole they screwed a bolt in so they could hang it in the solution.

Anyway, that's how it happens in my view.

Doug

Eric_Oz_S2

Is the inside of these covers actually protected though (ie What is the protection that is not locally applied)? Perhaps the reason they fail is the roll pin creates a concentration of galvanic current at the post? It seems like galvanic corrosion as mentioned earlier is a likely culprit. Steel bolts and magnesium is a bad mix given there isn't that much magnesium relative to the bolts - especially considering the shape of the posts which would result in a concentration of current. Usually aluminium is used to isolate the two different metals (eg aluminium washers), but in the case of the cover, it is not easily to isolate the steel bolt from the threaded hole in the casting.

Normally galvanic corossion also needs a conductor such as moisture to accelerate the corrosion, so perhaps corrosion is worse in damp or humid climates.

Woo hoo 1000 posts...