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That corrosion is not normal, at least not in my Volvos. If you have used the Volvo antifreeze with distilled water, the coolant isn't likely the problem, except in the past owner's time. It is possible that your engine has an electrolytic corrosion problem. By this I mean that an electrical current is passing through the coolant and corroding the metal electrically.
There have been some discussions about this in the forums I read. Information on this can be found with the search function. The other forum I watch is a Mercedes forum:
http://www.mercedesshop.com/shopforum/
What I remember is this:
1. Take a digital voltmeter and connect the negative probe to the battery negative terminal. Open the coolant tank (when cool) and dip the positive probe into the coolant. Don't touch the sides with the probe, only the coolant.
2. The voltage you read should be very low, only a few millivolts. If more, the antifreeze may need replacement or you may have electrolytic corrosion from poor ground connections.
3. Now turn on the things that use battery power, one at a time, and see if the voltage reading goes up. If it does, try cleaning and tightening the ground for whatever is using power. Examples would be the headlights, the radio, the heater fan.
4. While reading the voltage, have someone start the engine. If there is a big increase in voltage when the starter motor is running, there may be a ground problem in the starter circuit. This has been known to cause much corrosion because the current to the starter is very high.
As you can see, much of the cause of this type of corrosion can be traced to old antifreeze or to poor grounds. If it were my car, I'd clean every ground I could find, lightly coat each ground with grease, silicone grease is best, and reconnect tightly. You live in a humid climate and moisture will affect these grounds.
If you do a Google search using 'electrolytic corrosion', you will get more information. This will be mostly about corrosion in boats but it will help you to understand this problem.
Good luck in finding the cause of the corrosion. You need to correct the problem since the corrosion could lead to head gasket failure. If you can't find the problem, at least change the antifreeze often, say yearly.
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