Among the ingredients of simple green I think it comes down to these three. Sodium Citrate, Sodium Carbonate, and citric acid.
The Sodium Citrate was apparently used in brass plating in the past, not sure the particular reason.
Sodium carbonate solutions (concentrations up to 35%) are corrosive to aluminum, lead, and zinc and zinc brasses at 21 deg C.(22,25-27) Solid sodium carbonate is corrosive to aluminum at 100% relative humidity and normal temperatures.(27) It is not clear if moisture must be present for corrosion to occur with anhydrous sodium carbonate.(16) Sodium carbonate solutions are not corrosive to other common metals, such as stainless steel (types 300 series, 400 series and Carpenter 20Cb-3) (10% to saturated solutions), carbon steel (types 1010 and 1020), nickel cast iron, nickel and nickel-base alloys, such as Monel, Hastelloy and Incoloy, copper, copper-nickel, bronze, aluminum and silicon bronze, brass, Admiralty brass, titanium, tantalum and zirconium.(22,25-27)
Citric acid is pretty corrosive to most metals.
With the above chemicals it would be imaginable that some of the brass for example was dissolved into the liquid. Why it is then attracted to the bare metal screw I'm not sure.
The Sodium Citrate was apparently used in brass plating in the past, not sure the particular reason.
Sodium carbonate solutions (concentrations up to 35%) are corrosive to aluminum, lead, and zinc and zinc brasses at 21 deg C.(22,25-27) Solid sodium carbonate is corrosive to aluminum at 100% relative humidity and normal temperatures.(27) It is not clear if moisture must be present for corrosion to occur with anhydrous sodium carbonate.(16) Sodium carbonate solutions are not corrosive to other common metals, such as stainless steel (types 300 series, 400 series and Carpenter 20Cb-3) (10% to saturated solutions), carbon steel (types 1010 and 1020), nickel cast iron, nickel and nickel-base alloys, such as Monel, Hastelloy and Incoloy, copper, copper-nickel, bronze, aluminum and silicon bronze, brass, Admiralty brass, titanium, tantalum and zirconium.(22,25-27)
Citric acid is pretty corrosive to most metals.
With the above chemicals it would be imaginable that some of the brass for example was dissolved into the liquid. Why it is then attracted to the bare metal screw I'm not sure.