It seems that many commercial descaling solutions designed to remove scale (calcium carbonates, etc that form on water heaters) are a simple combination of an acid and an acid corrosion inhibitor for metals.
The acid renders the scale soluble in water, while (some) inhibitors coat metal surfaces preventing the acid from corroding the metal.
The problem is that these descaling solutions, despite their simplicity, are often \$30-\$100 per gallon (at just 5% HCl concentration!). Furthermore, most corrosion inhibitors are toxic.
A smart cost-sensitive consumer would obtain hydrochloric acid at a low cost (\$10 for 1 gallon of 35% HCl), and obtain or make a non-toxic, low-cost corrosion inhibitor.
Is there a non-toxic, low-cost, and reasonably well-established corrosion inhibitor that can be reliably obtained by a consumer for use as a descaling agent in combination with HCl?