I've spent a couple of hours googling various strings of key words and eventually got hot onto a trail. I now believe that a $4 jug of muriatic acid has caused almost $5 grand of damage to the inside unit of my heat pump. Actually, that's not entirely fair. It could be a combination of a leaky old main trunk line and the acid vapors, since both moisture and acid highly contribution to zinc oxidation.

Galvanized metal has a nearly 100% zinc coating.

Zinc metal is very reactive with acids, esp. strong acides, such as hydrochloric (muriatic) acid.

Zinc [zinc oxide] is susceptible to WHTE RUST and often appears as WHITE POWDER or powdery white deposit.

White rust is simply the chemical compound, zinc hydroxide which forms when zinc is in contact with moisture or other chemical agents.

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This really explains it all, from the white flakes on the galvanized main trunk line (white rust) to the white powdery deposits on some of the joists.

Since the ducts were so filled with black mold, it could be that the moisture--with or without vapors escaping from a muriatic acid jug--caused the chemical reaction. What's interesting is the tons of white powder *inside* the evaporator coil and in the old insulation that lined the main trunk line.

I just got off phone with HVAC guy and he is testing the system now. All the work is done, and tonight I will be breathing the highest quality indoor air I've probably ever breathed. I told him of my discoveries and he's right on board. At least we know what it is now, and the acid will never again be inside there. With him working just on labor and saving me thousands of dollars I'm also going to invest in a direct line dehumidifier this weekend.

Now, where to store the acid? Outdoors in the summer heat, behind the pool house or shed? Or contained in a plastic bin with a lid in the heat outdoors?