Another lesson ...

They were probably anodized. Sometimes its really hard to see,

Nope , these were cut from lengths of 2" angle and installed in the cabinets by me - I also built the cabinets , while working for a Memphis cabinet shop . They've been hanging around in my scrap/recycle pile for about 7-8 years . And most of that time they were outside on a pile with all kinds of stuff . I was going to melt them ...until I got the means to weld them into something useful without melting . Still got a big "melt pile" .

While almost all aluminium angle is sold in anodised condition, leaving aluminium out in weather for a few years will do much the same job as anodising it.

When anodising first the metal is cleaned then a layer of oxide is grown electrolytically. Leaving it in the air will grow such a layer, but much more slowly.

Then the flakes of oxide layer (which look like many-sided polygons) are swollen up into a aluminium oxide/hydroxide complex by immersion in boiling water, which closes up the gaps between flakes. Letting moisture in the form of rain get near will do the same job, but again more slowly.

However several years exposure would correspond to a fairly thick layer of anodising - in the 25-30 micron range - which is about the thickest layer done in commercial sulphuric acid anodising where layers range from about 5 to 25 microns (less for hard chromate anodising).

-- Peter Fairbrother