Using a widely available acid to etch mill scale off of hot rolled steel
What is Mill Scale?
How it easily can be removed
In case you where wondering what mill scale was, it is a form of oxidation on steel that form during hot working operations. It is a tough blackish grey coating composed chiefly of magnetite or ferroferric oxide that is nearly impossible to grind or wire brush off. It causes some metal workers to avoid cheaper hot rolled steel altogether. This not need be the case. You only need to soak the steel in an acid bath that is cheap and available virtually anywhere paint is sold.
Hydrochloric or Muriatic acid
In many home improvement stores, paint stores, and swimming pool suppliers, there is a chemical sold in 1 gallon jugs called muriatic acid. It is technical grade hydrochloric acid of about 30% strength and ranges in price from $5 to $8 per gallon. For removing mill scale, a solution made from 1 part acid and 3 parts water will suffice. A sturdy 5 gallon bucket with a tight fitting lid is highly recomended. This bath will clean the mill scale off hot rolled steel in roughly 20 minutes at 50 degrees fahrenheit or above. Slightly faster in hotter weather.
Few words on safety
Before you go off to the paint or hardware store, keep in mind that muriatic acid is very corrosive and quite toxic. The hazards will be explained on the bottles in full detail, READ THEM! Furthermore, do not store muriatic acid or its water solution around anything of value, especially things containing metal. The fumes alone can cause intense rusting of steel and patina copper and brass. I can corrode electrical connections making equipment inoperable. Best bet is to store the concentrated acid in its bottle outside with a large bucket or plastic bin turned upside down to cover it. And for sure make sure children or pets do not have access to it!
What to Expect
After you have placed your steel in the acid solution and let it sit for 15 to 20 minutes, you will notice that it looks as if your mill scale is still there. But, with a gloved hand, if you can rub the black smut off with your finger! Just use a scouring pad and cool running water and you will have grey steel ready to accept brazing! Keep in mind that once the mill scale is removed, the metal is activated and it will flash rust in minutes! But don’t worry the orange rust that does form easily comes off with a wire brush or 200+ grit sandpaper. Have fun and be safe!
Alternative Application of Muriatic Acid
If your work area is subject to heavy traffic or there is even a slight risk that children might be near, keeping a 5 gallon bucket of acid solution around is impracticle. There is an alternative that uses far less resources. This entails using an EMPTY bucket and applying the diluted acid by spray. Although you eliminate storage and disposal problems, you creat new issues by spraying acid. This must be done in EXCELLENT ventilation and preferrably with a safety mask. The ventilation problem can be mitigated by using the smallest portion needed. You would be surprised how much scale 1 ounce of acid will help remove.There are a couple of readily available tools to apply this solution. Believe it or not, a Preval Sprayer Kit, pictured below, can satisfactorly be used to apply muriatic acid! That is because the aerosol delivery system is constructed of chemical resistant polyethylene plastic. As long as you thoroughly rinse the power unit iwth clean cool water, you should be able to continue to use the power unit without affect.
The Spraymaster cost a little more but allows you to apply more volume if needed. Unlike most spray bottles, there is no metal springs in the valve to get corroded. But you should not store acid in the Spraymaster for extended periods. Also, the bottle shoots the liquid in a fast moving stream instead of a fine mist. This is helpfull to minimize ventilation concerns.