Low-cost nontoxic corrosion inhibitor for steel exposed to hydrochloric acid

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? • I assume you mean for a hot water boiler system, not the hot water heater that would supply domestic hot water. – MaxW Mar 25 '19 at 23:41 • Hot water heater, as in your shower etc. Meaning preferably food-grade corrosion inhibitors and acids. Would also work for descaling boilers for heat. Mar 26 '19 at 0:55 2 Answers 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.

Much more practical to flush the water heater occasionally as recommended by the manufacturers ( in the US). Keep in mine a normal water heater also has a magnesium /zinc/aluminum anode that will dissolve in acids. However citric, formic and acetic acids are used to cleans steels with minimal damage. PS : Refineries will pay you a fortune for your HCl inhibitor when you find it, they have been looking for almost 100 years. PS #2; I flush a white product out of my water heaters ( city water) that is not acid soluble.

• not only will the anode dissolve in acid, in many cases it will generate hydrogen gas :) HCl-steel corrosion inhibitors exist and they are plentiful. Their job is to slow corrosion, not stop it. They are used extensively in boiler descaling / pickling, oil and gas activities, etc. The problem is finding one that is non-toxic. Apr 4 '19 at 8:16
• Properly operated boilers ( industrial and power) generally do not have scale problems ; water chemistry , inhibiters ,chealates and blow-down control scale. Calcium Sulfate scale is a problem in oil field heaters but is not removable with HCl. Apr 5 '19 at 16:20