tinning steel

[AJH]

I was helping out a chap that collects military vehicles from the 70s. A coolant reservoir had developed a pin prick rust hole fairly near a seam. Initial inspection made me think it was basically a steel cylinder construction tinned after the joints and pipes had been attached. The tinning had corroded to allow some light rusting over the surface.

I chose to braze the hole shut. The interesting thing, to me was the adjacent joint was some sort of hard solder. It would have been far tooexpensive to be silver solder. Anyway I sealed the hole and the adjacent solder melted a bit but no damage.

I now need to address the surface rusting and was thinking of using hydrochloric acid and then wondering about re tinning, any suggestions for what to use? On another rusty tank I previously used zinc chloride flux and an old lead based solder, a heat gun and wiped it on but it was a bit patchy.

[Fredxx]

My first port of call would be plumbers flux. Though I guess they're not all equal.

I recall an instance when I attached a copper pipe to a club hammer I was using to support the pipe whilst soldering a joint.

[newshound]

Personally, I'd be tempted to use a gas torch and an active flux.

[John Walliker]

Phosphoric acid is good.

John

[AJH]

I've got some 80% strength what dilution? Why would it be better than hydrochloric acid?

I am thinking of unearthing a set of bodywork books from the 1930s that my wife's granddad gave me shortly before he died in the 60s. He was an old school pane beater and before resin fillers they would beat out the dents and use lead to get the final finish as a skim.

[newshound]

Phosphoric will convert the oxide (rust) to an adherent layer of iron phosphate. Good if you are going to paint, less helpful if you wanted to tin. Hydrochloric if you want to pre-treat (active fluxes are doing the same thing).

[John Walliker]

I have used concentrated phosphoric acid as a flux when soldering stainless steel. It can spit a bit, so wear eye protection. It makes really nice joints with tin/lead solder. I haven't tried lead-free solder. Hydrochloric acid or chlorine containing salts are more likely to leave corrosive residues as chloride ions catalyse rusting.

[Andrew]

Isn't that what 'Kurust' was intended for ?. That has been reformulated as a water-based compound and is nowhere near as good as the original product.