Exhaust Manifold/Riser question

[jmr]

It is widely recommended that exhaust manifolds be replaced every 5-8 years. What I do not understand is, the block and cylinder heads are made out of the same material, are circulated with the same raw water, and have similar water passages as the exhaust manifolds. Why then, are the exhaust manifolds more sucseptible to corrosion than the other components?

 

[bondo]

It is widely recommended that exhaust manifolds be replaced every 5-8 years. What I do not understand is, the block and cylinder heads are made out of the same material, are circulated with the same raw water, and have similar water passages as the exhaust manifolds. Why then, are the exhaust manifolds more sucseptible to corrosion than the other components?

Ayuh,...... If yer running in saltwater, the manifolds, especially at the elbow junction, is where the rust/ erosion is the worst,.... the elbows usually go 1st, as they see more air/ water, 'n rust more,...
I believe it's the wet/ dry, hot/ cold, that promotes excessive rust,....

If running in sweetwater, they usually last, pretty much forever,....

 

[makomark]

And add the corrosive nature of the exhaust gas - once the process starts, its very difficult to stop....

 

[sandkicker]

.. and the high temp