From a close friend of mine who is a chemist:
Of all the acids with which I’ve worked I dislike HCl most. The fumes not only take your breath away, but they hang around for what seems to be an indefinite period. The fumes deposit on “everything” and continue fuming albeit not in a copious cloud. This is the source of the rust and it will continue to rust if painted over, which you already know.
We used a scrubber to clean metal, which was nothing more than rollers with metal bristles un a water wash. We used caustic baths that contained wetting agents and corrosion inhibitors to remove surface oils and dirt. Phosphoric acid isn’t as aggressive on metal as HCl or nitric acids, but the benefit is that it polishes the surface of the metal. It requires longer treatment (soaking) but it leaves a smooth finish.
I’m guessing the HCl was 25 – 35%. I’m not sure how it was applied (concentrated or diluted), nor do I know how it was neutralized or for how long. Your description indicates you scrubbed it with a scotch pad. The metal “softens” due to the microscopic rust pockets in the pores. I’m not familiar with the terms “soda blasting” or “media blasting”. A better choice for removing rust would have been a salt vinegar bath or a commercially available rust remover such as Metal Rescue, but the damage has been done, so how to clean it up.
How long did you soak the leg in the acid and for how long did you neutralize it? Did you neutralize immediately after removing it from the acid (after rinsing) or did it dry out first with neutralization following later? Neutralization should be 1 – 2 times longer than the acid dip. Rinsing should take a while to ensure the removal of residual acid (unreacted) and neutralization salts. A second base dip and rinse couldn’t hurt. The part should have been thoroughly dried and then put before a fan. We would use drying followed by placement in a forced air dryer and subsequent placement in a desiccator. Without the desiccation, the part should receive a base protective coat to prevent rust. If you aren’t going to paint it, use an oil to coat the part for rust prevention. This could be removed with a caustic bath.
So, try a wire cup brush on the legs. Check with the guys at Lowes or on the website for which gauge wire is best. If your neutralization period was shorter than your acid dip, repeat neutralization, then dry and coat.
A photo of the leg would help as there are other scrubbing techniques.
Long answer, but I hope it helps. Sometimes it is easier to buy ready-made products for “consumer” metalworking because they have the wetting agents and corrosion inhibitors in the formulations.
I replied:
I used heavy duty Easy-Off and aircraft paint stripper to get the grime and paint off. Then I sprayed them with the Muriatic Acid undiluted from a spray bottle. 31.0-35.0% Source:
http://www.jasco-help.com/uploads/general/Jasco_Muriatic_Acid_GJMA220_MSDS.pdf It was on for about 10 minutes when I read that it could soften and basically melt the cast iron. I rinsed the acid off for a minute of two. Then I mixed about 2 cups of soda ash to 2 cups of water then scrubbed the legs with a green scotchbrite with copious solution. I kept stirring the solution to keep it from separating. I let the solution sit overnight. The next day there was a white powder on the concrete floor of the shop. I rinsed that off and then blasted the legs with a coal slag type of sand.
After that I scrubbed them down with a soapy water solution then rinsed them off. That was Saturday. They have been hanging since. I was going to primer them with a rust remover/converter then top coat with 2 coats of a Sherwin-Williams enamel.
Attachments