Removing mill scale is a time consuming, laborious process. Usually, I use an abrasive process like a wire wheel, flap disk, or a 3M plastic stripper disk on my 4-1/2" angle grinder. However, as a person who shares back issues with a number of other builders on LocostUSA, I decided it was time to work smart, and save what's left of my back if I can.
I'd seen videos on YouTube about ways to remove mill scale, and decided to give some of the chemical methods a go. I planned the experiment last week, but spent most of yesterday helping a neighbor clear a common drain, and boy do I need it to work this time. I'm really hurting today, and pain killers aren't touching the back pain. So, I'm taking a break and posting this.
I bought two chemicals to try: muriatic acid; and an industrial grade vinegar cleaner. That last one sounds weird, but white vinegar is one recommended method on YouTube. I couldn't find a gallon of white vinegar, but could find the ZEP industrial product at Home Depot and decided to try it instead.
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Sunday night, I put some 2"x3"x0.120" mild steel pieces into a cheap plastic container,a and covered the parts with the ZEP vinegar.
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I'd planned to work on them Monday morning, but due to the drain issue, put it off until today, Tuesday. So, I guess they've been in solution for around 36 hours. Here's what was achieved in that time.
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I washed the off with a stream of water while brushing them with a small stainless steel brush. They really cleaned up well. All the rust, mill scale and paint came off with moderate effort. They need a little wire wheel time, but not much.
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In the photo above, the last one brushed (foreground) is bright and shiny. The big one lying down has a slight orange cast to it. The first one done on the left has a discernible orange cast to it. I'm not sure why this is so. There wasn't enough time for oxidation to form. We're talking 15 minutes time between the 1st and 3rd piece.
I assumed it was some kind of chemical process caused by the vinegar. I thought I'd try neutralizing the vinegar (acid) with some baking soda (base) in solution and see if that stopped it.
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After I post this and have some coffee, I'll go out and wire brush the neutralized pieces. If they spend overnight in the garage, and don't turn orange, I'll consider the case closed. I'll post again when I have those results.
There is one more thing. The two big pieces had to lie flat on their longest side to fit in the container and stay covered with vinegar. The sides lying down against the plastic container did not turn out well. So, I've placed them in solution again with the mill scale side up. I guess the not enough chemical got to the bottom side to react with it.
Cheers,