Greg,

Nothing in Nature is ever an "always" situation. In the case where a fuse is sized to protect some light duty components, say an 8 amp fuse, and you have corrosion on the connections of some part of that circuit, the circuit will draw more amps and can possibly blow the light duty fuse. You may have seen that in the past, and you will likely see it again.

The corrosion on the contacts looks exactly like some kind of load to the electrical system (greater resistance), and the wasted energy in the circuit produces heat. So, a line intended to draw under 8 amps can start drawing more than 8 amps and pop a fuse.

The power in a Diesel car battery (really any car battery) can easily pop any of the fuses in the lines (without damage to the battery or the system), and any extra load on any of the lines due to corroded connections will bring the line load closer to the fused limit by using up the engineered margin. In severe cases it is feasible the battery and alternator will droop in output Voltage from the 13.8 nominal no load condition but maintain around 12 Volts across the higher resistance connection, which can be dangerous. If it is dangerous it should drive the power demand of the line over the amperage limit of its fuse, causing it to pop before it does any other damage.
In a case where the corrosion is so bad that it is better simulated by an open circuit, next to no current will flow.

A case we have all experienced where corrosion drives the battery performance down is on the battery posts themselves, when they get so corroded the battery uses too much of its stored energy just pushing electrons across that layer of scuz between the terminal and the clamp to start the motor (Diesel engine turns too slow to start). When that is the problem, try touching the battery terminals after you have been cranking the starter motor for a while. I have done this on an outboard motor in a boat and changed the shape of my finger for a week or two.

Hope this helps. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)