Thats not corrosion around the welds.
The grills are resistance spot welded together, stainless steel when heated and exposed to "air" will oxidise slightly and change colour. Depending on how hot and how much air determines the colour. Generally from straw yellow at the colder end to purple/blue at the hot end. If the weld is a dull grey then it means all the chromium has precipitated out of the weld. Not only is this a weak weld but but will corrode faster.
This was a Ginetta G27 Fuel tank I made a couple of years ago, as it was to be a Christmas Present, I made the welds Christmas coloured...
Simon @ Mulfab
