Inside of kettle markings?

Last night I added a blichman brew mometer to my 10G brew kettle. I did a test run to check for leaks and calibration. After I was done I left the water in the kettle with the lid in overnight and today when I went to start a brew day, there is this weird scum, or I don't know what really, with just plain tap water left over night in it... Here's a pic. Any ideas? All safe still? I scrubbed it with soap and water, and it really didn't come off. Is it just cosmetic?

Thanks!

Looks like an aluminum kettle. Oxidation from the water.

yes, aluminum kettle... guess i forgot that detail. all good? just cosmetic at this point then?
thanks!

Brewing with a raw aluminum kettle on the first brew will almost certainly draw aluminum into the wort. That kettle does not appear to have been conditioned to a state suitable for dealing with acidic fluids such as wort.

Typically the process is clean the kettle bright - might need to use a green scotch brite and some metal cleaning powder. Then fill it with water to a level at least a few inches above your brewing capacity needs, and get it boiling for a half hour or so. What you're going to want to see is a distinctly dark coating, which is the aluminum oxide you want to create that will resist the hot acidic wort and block aluminum from dissolving into your young beer. The black spots you're seeing now is essentially what you want the entire usable interior to look like...

Cheers!

Brewing with a raw aluminum kettle on the first brew will almost certainly draw aluminum into the wort. That kettle does not appear to have been conditioned to a state suitable for dealing with acidic fluids such as wort.

Typically the process is clean the kettle bright - might need to use a green scotch brite and some metal cleaning powder. Then fill it with water to a level at least a few inches above your brewing capacity needs, and get it boiling for a half hour or so. What you're going to want to see is a distinctly dark coating, which is the aluminum oxide you want to create that will resist the hot acidic wort and block aluminum from dissolving into your young beer.

Once you get that dark patina established, you want to keep it there. Don't use abrasives to clean the kettle thereafter...

Cheers!

Just some info on aluminum... Season it, and be careful how you clean it

Per john palmer:

Metallic flavors are usually caused by unprotected metals dissolving into the wort but can also be caused by the hydrolysis of lipids in poorly stored malts. Iron and aluminum can cause metallic flavors leaching into the wort during the boil. The small amount could be considered to be nutritional if it weren't for the bad taste. Nicks and cracks ceramic coated steel pots are a common cause as are high iron levels in well water. Stainless steel pots will not contribute any metallic flavors. Aluminum pots usually won't cause metallic flavors unless the brewing water is alkaline with a pH level greater than 9. Shiny new aluminum pots will sometimes turn black when boiling water due to chlorine and carbonates in the water.

The protective (grayish) oxides of aluminum can be enhanced by heating the clean pot in a dry oven at 250°F for about 6 hours.

Do not clean aluminum shiny bright or use bleach to clean an aluminum brewpot because this removes the protective oxides and can result in a metallic taste. This detectable level of aluminum is not hazardous. There is more aluminum in a common antacid tablet than would be present in a batch of beer made in an aluminum pot.

With that said... If for any reason you want to clean it shiny, starsan left in the pan for 24hr will make it look brand new. So be careful if you use the pot to soak bottles or gear in starsan.

well this was probably about my 6th batch of beer in this kettle and yeah, it's a little brown as you can kind of see, but didn't get any of these darker spots until i left that half pot of water just sitting overnight in there... so do i need to fill it all the way up and just boil with plain tap water? and letting it sit in there overnight, did that contribute anything also, for good or bad? will that change anything now that i've mashed and boiled wort (doing BIAB) 6 times in there.

this is all great info! thanks guys!