Corrosion on hot water heater pipes

Is this hot water heater pipe corrosion normal?
(1) Needs to be connected by a nipple, only brass available http://img4.imagetitan.com/img.php?image _image6071.jpg
(2) Will the new brass nipple corrode just like the old did? http://img4.imagetitan.com/img.php?image _image6072.jpg
(3) Hot water outlet nipple seems to be steel? http://img4.imagetitan.com/img.php?image _image6074.jpg
(4) New & old nipples are both brass so why corrode? http://img4.imagetitan.com/img.php?image _image6075.jpg
(5) Cold water inlet has less corrosion it seems but why? http://img4.imagetitan.com/img.php?image _image6076.jpg
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