Low-cost nontoxic corrosion inhibitor for steel exposed to hydrochloric acid
An extremely wide variety of plant extracts have been shown to serve as corrosion inhibitors, including black tea, chamomile, orange peels, black pepper, mustard seed, neem, the list goes on an on. 

I have skimmed almost 100 papers so far and handfuls of review papers, and it is not clear to me that any of these "results" can be trusted. Most of these papers (all?) are from "PhD Factories" in developing countries, and zero of them give actionable insights. However, apparently almost every plant extract any author has ever tested offers protection against corrosion by a factor of roughly 5x in grams-per-liter concentrations.

A plant extract is generally made by taking about 10 grams of dried plant, and heating it in 0.5-1 liter of water (or some other solvent), and removing all but ~20ml/20g of liquid, leaving a very concentrated solution of plant compounds ("plant tea"). Or you can mix the dried material directly with the hydrochloric acid if you can then filter it.

Thus a food-grade hydrochloric-acid / "plant-tea" mixture might be an effective and cheap alternative to extremely expensive commercial products. 1 gallon of muriatic acid is \$10, producing 5 gallons of plant-extract/HCl descaling solution at a cost of \$2/gal. It may have comparable performance to commercial descaling solutions at more than \$50/gal.

However, these extracts are poorly understood and are extremely complicated bio-chemical solutions. My search continues for a single well-understood compound with low-toxicity and good corrosion inhibiting properties for steel in acids like HCl.