Removing Rust with Citric Acid

[SuperNovaSS]

Paul(PxTx) recently started a thread in the Garage/Shops forum about using 5 gallons of Evaporust to derust a block. Evaporust is a good product. It is just pretty expensive, especially for larger parts. 5 gallons is around $80 if I remember correctly. Paul's idea about using a bag inside a larger container with water outside the bag is brilliant and effective. His post inspired me to try something I've been meaning to do for a long time.
Paul's thread can be seen here:

https://www.yenko.net/forum/showthread.php?t=155144

The idea of using water pressure to reduce the amount of evaporust used is pure genius. Unfortunately, I am lazy and cheap. I just want to be able to dunk my parts in a container, pluck them out to check on progress, then dunk them back in. I also want to be able too reuse the liquid for more parts if possible. I have been looking for something non toxic, cost effective, gentle on parts for a couple years. I have tried a few different options but nothing has wowed me.

Last year, firstgenaddict, posted in a thread that citric acid could be used for derusting and was cheap. He said about a pound of citric acid powder added to 5 gallons of water did the trick. Here is the thread:

https://www.yenko.net/forum/showthread.php?t=150352


I checked out Amazon and saw that 5 pounds of powder is $14 including shipping. Hmmm, about $3 for 5 gallons instead of $80? Sounds like its worth a try! So I bought the citric acid and it sat on the shelf for almost a year. I still had 5 gallons of evaporust and used that for small items that I needed to clean. I had larger items to derust but simply wasn't motivated and had other things to do.

When I saw Paul's thread, I had just brought home a bunch of engine parts that had sat outside for a long time. Seemed like a good time to give the citric acid a try. Yesterday, I went the the local Resourse Recovery Center, AKA the dump, and found a 55 gallon plastic barrel. I decided to try a few cylinder heads, a very rusty crank, and some rusty flywheels. It looked like the barrel would need to be about 2/3 full and the parts would displace a good bit of that volume. So 2/3 of 55 is about 37. The parts may displace 10 gallons? That leaves 27. I have 5 pounds of acid which would make around 25 gallons using firstgenaddicts formula. Sounds good to me.

So I dumped the acid in and poured water on top of the parts. njsteve has made many posts about Evaporust and has mentioned that heat and circulation is important. Last year, when I bought the citric acid, I also bought an aquarium heater and pump for this purpose. It was time to put them to use. I hooked up the heater and pump and let the parts sit there for about 4 hours. I checked the parts and things were certainly happening. The liquid was getting darker and I could wipe rust off with my finger. The citric acid seemed to be working just like Evaporust. The fluid was getting warmer but hadn't quite reached that sweet spot. I checked at 6 hours and there was a little more progress and the liquid was getting warmer. At 8 hours, the liquid was warm and the rust was all gone. Between 6 and 8 hours, the black oxide had started to form on the parts. I really didn't mind because I planned to glass bead these parts anyway. I bit closer eye or temp control may help with the black oxide.

The next morning(today), I took a look in the barrel. The liquid was pretty black at this point. I dipped my finger in the liquid and tasted it(yep, tasted it). It was still very sour so I figured there was still acid that could do its work. I had a rusty 396 block from the same group of parts and decided to toss it in the barrel. Again, I turned on the heater and pump and let it sit. I checked it after 6 hours and it was moving along nicely. There was no much black on the block so I let it go for another 2 hours. I pulled it out and it looked great. I hosed it off and most of the rust that was still on the block sprayed right off. There were still a few areas of light rust but those seemed to easily wope off with a finger or brush. I was happy with this and decided to spray this block down with WD-40 to keep it from flash rusting. I checked out the liquid again and it is very dark. I dipped my finder in the liquid and tasted it(yep, tasted it). It still tastes sour but not as sour as earlier. I had another block that was rusty and decided to toss it in. This time the liquid is already warm so I think it will go quickly. I am going to let it go overnight and see what it looks like in the morning. I will report back.

I posted a few pictures of the progress so far. There is a picture of the cylinder head before, after soaking, then after blasting to remove the black coating that formed. The blasting was very fast and would have taken hours if not derusted first.

Anyway, this seems to be a very cost effective way to derust parts. Once the liquid stops working, I will have to get rid of it. I imagine it is safe for the drain but will have to look into that.


Does anyone else have experience using citric acid or other methods to remove rust?

Jason