Does anyone know what has caused this corrosion on hot water heater?
The corrosion is being caused be a poor solder job. The joints are slowly leaking. Next time get a real plumber to do the job and don't call this company back. By the way, the discharge pipe for the T&amp;P valve is to be run in a full pipe size not a reduced pipe or tubing as it looks like yours was installed. That in itself shows that the "butcher" that installed this water heater needs to find a different line of work. If by chance the installing company is a good and reputable company, call them back and show them work done by their employee and maybe they will re-do the installation for free or at a reduced rate.