Mystery corrosion. 2007 F150

[vroom ZOOM]

I found a free motor at the scrapyard (2007 F150). The lower unit looked good, without any corrosion at all, so it was a no brainer to load it up. It was partially disassembled, but with all the parts there. The oil was all milky. I pulled the head and found this:

Funny that the anodes aren't even touched...
Here is a closeup of the extreme corrosion. The head gasket obviously has no surface to seal against.


Here is the kicker: This is a FRESHWATER motor. The engine holder, for example, doesn't show any corrosion AT ALL. The head is good too.



I am planning on finding a bare engine block and throwing the pistons, rods and crank into it. The question is, what can cause this corrosion, and how do I prevent the nex block from being eaten up like this?

 

[racerone]

I have a 50 HP Yamaha 4 stroke with the same corrosion.-----Fresh water motor.----Scrapped by a boat shop.------I have another with a mechanical knock.----Fresh water motors.-----Need to get busy and make a good runner I think.----There must be hundreds of them like that.----Poor quality material.------Have never seen this issue with Johnson / Evinrude motors !!