Consequences of Silver Soldering

[runner42]

I assume that applying sufficient heat to a copper boiler to enable silver solder to flow has a consequence that the copper (at least that not protected by flux) will become blackened, resulting in oxidisation of the copper. This process is absorbing oxygen from the atmosphere and results in a net gain, in theory the boiler is heavier than before. However, the subsequent process of cleaning the boiler afterwards appears to cause separation of the oxidised layer from the copper and apparently some small degradation in the copper itself. This is evident by noting the black particles and also the copper coloured silt residual in the citric acid bath. To minimise this effect is is preferable to limit the number of times that the process of heating and cleaning is performed. My boiler uses 3 mm copper sheet throughout and the tubes and flues much less thickness. I was wondering how much thinning would occur at each heat up, in my opinion going by the amount of combined dross in the citric acid bath, it is something to consider. BIs in my experience rely of certification that you have used the correct thickness material, but are not aware of the number of times you had to apply the process of heating and subsequent cleaning. I have had to apply this process to the firebox stays a number of times. The difficulty for me was the silver solder ring around the stay would readily melt on the firebox wrapper, but because the stay wasn't at the correct temperature a miniscus was not formed and wicking through not obtained. Because of the shape of the Belpaire boiler it was not visible unless and until the boiler was cleaned. Also, because of only the stays closest to the foundation ring are visible from the bottom of the firebox and heat can be directed into the small gap between the inner and outer wrapper, these are readily soldered correctly by seeing that wicking through has occurred. The rest is suck it see.

I am not concerned that the thinning has occurred to a sufficient degree to degrade the safety of the boiler and this is an academic exercise, but at what point does it become critical?

Brian  
    

[runner42]

I suppose the lack of interest in this post is due to the fact that boiler builders see this thinning as very small and nothing important to consider in the scheme of things. However, my experience may be more significant than usually met. My choice of heating is with a 114 kW propane burner which is somewhat too large, apart from being a gas guzzler (5 x 25 kg propane bottles used so far), it has a large flame both in diameter and length. This makes focusing the flame to the point of interest very difficult. One can't use the propane burner to bring the copper to temperature and then touch the area with the silver solder rod, because this large flame one needs an asbestos jacket to enable the silver solder rod to be placed where required. The silver solder has to be placed in the required position before starting. This large flame covers a greater area of the boiler so more copper is heated than would have been otherwise heated with a smaller flame or with oxy-acetylene. 

Anyway, I think that I have nothing to be concerned over. 

Brian 

[John Baguley]

Hi Brian,

I agree that you shouldn't have anything to worry about with copper loss but it would be interesting to do an experiment on a known thickness of copper and measure it's thickness after a large number of heatings and picklings.

I think I would be more concerned about the effect of the repeated heating on the previous soldered joints. I'm sure that the repeated heating must have some effect on them, especially if they get hot enough to remelt which can cause pinholes to appear in the joints and materials to boil off. How often do you hear/read of someone reheating to cure a small pinhole in say a stay joint only to then discover they then have several new ones! That's one reason that I am not a fan of trying to re silver solder pin hole leaks unless you can use oxy fuel to localise the heat. You can finish up chasing leaks forever.

John

[millman]

Brian you worry about how much copper is dissolved off of the boiler when placing it in the acid, a well known boiler making company in the UK used to finish off their boilers using a small hand held belt sander, I am sure that this removed an awful lot more copper than a dip in the pickle bath could ever do. I don’t think you have anything at all to worry about

[runner42]

Jul 30, 2021 10:42:46 GMT John Baguley said:

Hi Brian,

I agree that you shouldn't have anything to worry about with copper loss but it would be interesting to do an experiment on a known thickness of copper and measure it's thickness after a large number of heatings and picklings.

I think I would be more concerned about the effect of the repeated heating on the previous soldered joints. I'm sure that the repeated heating must have some effect on them, especially if they get hot enough to remelt which can cause pinholes to appear in the joints and materials to boil off. How often do you hear/read of someone reheating to cure a small pinhole in say a stay joint only to then discover they then have several new ones! That's one reason that I am not a fan of trying to re silver solder pin hole leaks unless you can use oxy fuel to localise the heat. You can finish up chasing leaks forever.

John

John,

this is not what you proposed, but I collected the residue from the citric acid bath and dried it thoroughly so it was solid matter and not including water. I weighed the solid matter and it came in at 6 grams. The boiler weighs approximately 25 kgs.  We can estimate that the reduction is 6/25000 X 100%. A whooping 0.024% loss. But wait.

The black particles is the copper oxide due to heating the copper which forms the greatest part of the residue and the copper coloured silt is probably due to delamination of the copper. Copper oxide contains one copper atom and two oxygen atoms so this meant only 2 grams of the residue was copper loss (this is a naive assumption on my part and is probably incorrect).  But what was not considered is the production of copper citrate due to the cleaning in the citric acid bath and this is soluble which turns the water blue. How much copper was used to make copper citrate was not considered. It has got too complicated for me, so it's best I leave it there. 

Brian

[chiptim]

Hi Brian,

it's an interesting question that has drawn three valid and interesting replies.

In my very limited experience as a boiler maker, I too experienced some of the problems you did in your second post regarding trying to apply the silver solder whilst whielding a massive propane torch/flame and being dressed in some seriously thick heat protective clothing that made picking up the silver solder rods hard enough, let alone being able to get near enough to the boiler to apply them with the required precision.

When the rod is new and long the end flaps around violently fuelled by the adrenelin shakes of finally reaching melting temperature while the flux is still active. When the rod is well used and a short stub you can't get it close enough due to the heat and flames without (in my case) setting my gloves on fire.

It's all part of the "fun" of boilermaking :-)

I found it best to spend time in set-up and assembly so the solder was always in place on the other side of the joint to the flames. In this manner it is naturally drawn through the joint once the melting temperature is reached and avoids so may of the problems I've described.

Tim