Getting rid of red flux stains on brass

[Brian Rupnow]

Whenever I silver solder brass parts, I use a paste flux recommended by the silver solder manufacturer.It is made by Harris, and is simply called "StaySilv white brazing flux". It works great but wherever it melts and runs to as a result of heat from the torch, which liquifies it, it stains the brass red. Not a vivid dark red, but still red enough to be quite visible. The attached picture shows two test parts immediately after silver soldering them together, and the reddish color around the soldered area is quite visible.

[Brian Rupnow]

Yesterday, I read on the internet that making up a cup of white vinegar and a teaspoon of salt, then heating it would yield a solution that, along with "brisk scrubbing" would remove the red stain. This is the result after a half hour soak. Although the parts are undoubtedly cleaner, the red stain is not removed, in fact doesn't seem to be affected at all.--Okay, so much for internet solutions!!!

[mklotz]

I believe the red "stain" is due to the zinc being leached/dissolved out of the brass.  The copper is left behind so what you're seeing is the native color of the copper.

We'll need a better chemist than I to suggest a solution.

[Brian Rupnow]

Marv--You are probably right. It sands off easily enough, however, I am attempting to find a chemical answer to the problem.---Brian

[b.lindsey]

As I recall it was Stan (sshire) that had a recipe of vinegar and hydrogen peroxide they he had used with good results. Maybe he will see this or you can send him a PM. I don't recall the proportions.

Bill

[mklotz]

Thanks for the reminder.  I had Stan's recipe bookmarked...

http://www.modelenginemaker.com/index.php?topic=3460.msg61586#msg61586

[Brian Rupnow]

I tried that and it didn't work for me.---Brian

[Nerdz]

I have used Baking soda and water (A paste), vinegar and salt, and toilet bowl cleaner (Make sure it says it has Hydrogen Chloride in it-This is Hydrochloric Acid!). Soda (in theory) should work as well, because it contains acid (Phosphoric acid) in small amounts.

There are a few mixes I would not use; Peroxide and Bleach, Vinegar and Bleach, and finally Hydrogen peroxide and Vinegar. The first two will get you chlorine or chloroform gas (I only know this because I wanted to rust iron on purpose). Mixing H2O2 with Vinegar will get you something called "Peracetic Acid" , which is a strong oxidizer. It doesnt sound as bad as chlorine gas though.

http://en.wikipedia.org/wiki/Peracetic_acid

[Brian Rupnow]

Okay---it's experiment time. I have been assured by a forum member that he has good results by using citric acid. Since I am a bit too chicken to mess with sulphuric or hydrochloric acid, we will give citric acid a shot. Good wife went out shopping today, so I asked her to buy me a small bit of Citric acid at the natural food store. It comes in dry crystalline form, and looks exactly like table salt. The test vessel is a 414 mll pepsi bottle (Thats about 14 ounces to North Americans) I have absolutely no idea of the proportions to mix this stuff in, so we will start with 3 level teaspoons of Citric acid mixed with half a bottle of water. I will submerge the two soldered test pieces in the mixture and see what happens.

[b.lindsey]

Looking forward to seeing what happens Brian. Did you happen to take a before picture?

Bill

[Steamer5]

Hi Brian,
 Watching with interest. I think your mix is a little weak, sorry cant remember what strength citic acid I've seen used for copper boilers, 3% seems to ring a bell, but even that seem a little low, as it's not as nasty as the others you mentioned, after all gin & tonic with lime on a sunny day......, it should be no problem ramping up the citric strength.
I had a look at Stan's link that Marv posted, thanks for that Marv, Stan didn't mention what strength hydrogen peroxide he used, from my dim distant days playing in labs it comes in different strengths so maybe that's why it didn't work for you. Hopefully Stan will pop up & let us know.

Cheers Kerrin

[Brian Rupnow]

Looking forward to seeing what happens Brian. Did you happen to take a before picture?

Bill

Bill---The "before" picture is the very first post.---brian

[Nerdz]

Tip: 100ml of water is 100 grams (some measuring cups are in ml).  As for the citric acid, well I would start with 1 tsp and record your results as such (ie 1tsp in 100mL of water, or 1tsp in 500mL of water, etc etc) . If you have access to a scale that would be even better.

[b.lindsey]

  Thanks Brian...I should have looked back before asking.

Bill

[Ian S C]

I think you can be fairly random with the measure of citric acid, and don't chuck it out when the job's done, just put it in a jar with a cap on it, even if you don't, and it dries out completely, just top it up with water and away you go again.
Next time you buy it, go to the Home Brew Shop, it's cheaper, you can also buy it(sometimes) at the garden shop by the pound or Kg depending where you are.
                                             Ian S C

[Stuart]

Brian
Put one drop of household bleach into you mix it will stop bacterial growth , as has been said keep it and reuse it's only lemon juice after all
It will take on a colour as it leaches the copper , it goes bluey green but still ok

Stuart

[derekwarner]

Guys...it's OK.....just don't drink it .... Derek

[Brian Rupnow]

After soaking for 10 hours in a solution of citric acid,I don't see any real difference to the very first picture at the beginning of this thread. I think I can safely say that citric acid (at the concentration I used) is not going to have much effect on the red stain. It may be a bit paler, but not enough so that you can't recognize it. I checked to see what I have for Hydrogen Peroxide (as purchased from the pharmacy) and it says that the solution I have purchased is 3%. I am now going to submerge the test pieces in a glass jar filled with Hydrogen Peroxide and see what that does.

[Graham Meek]

I have always used Citric acid for all my Silver Soldering, an example of the results obtained after pickling for about an hour is attached below. I use a ratio of 20 g of Acid to 1 litre of Water and I find the best results are obtained when the solution is warm.

My best regards
Gray,

[Brian Rupnow]

Graham--I just clicked on the picture you posted to enlarge it, and the thing in the picture is covered with the red stains I am trying to get rid of???

[Jo]

Brian: The red stains are caused by the leeching of the zinc out of the brass by the acid in the flux leaving pure copper. You need to remove the few microns of copper with an abrasive. Try a steel/brass wire wheel.

Jo

[Brian Rupnow]

Jo--Thanks--I am convinced. I have posted on three different forums on this issue, and received the same answer on all of them. There is no chemical answer. The reddish pink coloration will have to be removed by steel wool or sandpaper, or some kind of physical abrasion.---Brian

[Graham Meek]

Hi Brian,

I thought the red stains you were referring to were actually in the joint itself. I have subsequently cleaned off the residual staining from the flux by using an old tooth brush and some scouring paste intended for pots and pans. The trade name here in the UK is "Astonish", it leaves the surface with a dull matt finish.

My best regards
Gray,

[Jasonb]

A small bead blaster or abrasive gun will also work and give a nice satin finish

J

PS Gray the Fiat is progressing well

[Graham Meek]

Hi Jason,

Unfortunately work on the Fiat has ground to a halt. I have been side tracked on a pair of single acting steam engines. One is a 2 cylinder and the other is a 3 cyl version. They are based on Edgar T Westbury's Gemini High Speed Steam engine but with a toothed belt drive, with a reduced bore and stroke.

(My apologies to all for digressing)

My best regards
Gray,