[TheMidnightNarwhal]

Okay so I jacked my car this week-end and notice some surface rust, especially the diff as the main part of it is in iron or steel.

Now this thought came to mind... when rust starts it usually spreads onto the piece and will rot through right, for example a door panel, it will rot through and make some holes possibly.

Now why is it surface rust doesn't go through the whole diff if it's all made of iron? If we go by the logic of rust spreads and gets worse, technically the diff should rust completely inside out after some point?

Is it because the metal is very thick or?

[whyzee125]

Originally Posted by TheMidnightNarwhal

Okay so I jacked my car this week-end and notice some surface rust, especially the diff as the main part of it is in iron or steel.

Now this thought came to mind... when rust starts it usually spreads onto the piece and will rot through right, for example a door panel, it will rot through and make some holes possibly.

Now why is it surface rust doesn't go through the whole diff if it's all made of iron? If we go by the logic of rust spreads and gets worse, technically the diff should rust completely inside out after some point?

Is it because the metal is very thick or?

It would eventually. It happens on 4runner's up north all the time. It would take a LONG time in severe salt with a major lack of cleaning for it to happen but it's possible. The coating is porus so bits of salt, etc. can get into the top layer and form surface rust. Left untreated it will continue to burrow deeper into the metal but chances are you've got nothing to worry about.

[ryan stewart]

Baiscally the answer is yes. Because its a thick metal. Its also usually a denser alloy.

And rust requires oxygen, so moving deeper into metal is slow. Where a thin door panel is all surface area, so its easy for rust to just keep going.