Electronic – Preventing corrosion of copper PCB tracks

Because that board has been gold plated. Ironically, that saves money; copper tarnishes, and would need expensive cleaning immediately before soldering. And gold on the contact fingers makes for reliable connections. Other platings are possible : tin or silver, and formerly lead-based solder. Scrape some green solder-resist off the earth track and you would see a more reddish metal, though it would take a few months to tarnish to the colour of those pipes.

EDIT : the gold plating on the contact fingers may be a different process (thicker plating, to resist wear) - plating on the board itself will be VERY thin.

None that I know of. 15+ years ago I also worked for a small engineering firm that did what you described above. Today, in my basement, I can layout PC boards with free or low cost software, generate a Gerber file, email those files to a circuit board house and have boards made for less than $3/sq in with better quality that I can dream of doing. Mask and silkscreen are typically included for the same price. Turnaround is between 5-15 working days depending on how much you want to pay.

It's no longer reasonable to make PC boards yourself unless you need turnaround time measured in minutes. It's so cheap that if you place a reasonable value on your time, you lose money doing it onsite.