Ok Russe I'm not in rush, and it will be a pleasure to read your experience.
Thanks
Yves,
Thanks for your patience. I’ve been very busy, but I wanted to share what I can in the hopes that it will prove helpful. Sorry this is so long, but I’m trying to be thorough without being overly technical.
I have a lot of experience soldering and brazing, but almost exclusively in the realm of repair and manufacture of musical instruments. I can’t guarantee that the experience applies directly, but I suspect it is a good starting point.
Let me start by differentiating a couple things. The term ‘silver solder’ is synonymous with ‘silver brazing’. I prefer the later term because it is less likely to be confused with ‘silver bearing solder’. Silver Brazing (like the product in your link) is a ‘Hard Solder’ and melts in the ballpark of 1500F (800C). Silver Bearing Solder (like the ‘Stay Brite’ Jeff mentioned) is a ‘Soft Solder’ and melts in the ballpark of 450F (230C). So, the two kinds of products are in completely different classifications and using them is significantly different. Using a soft-solder (including the silver bearing solders) to repair a structural failure of a brazed joint won’t be strong enough, and will fail again. But if the joint has structural integrity but leaks, it can be a viable repair option. Bear in mind, though, that if you introduce a soft solder into a brazed joint and then it fails, all the soft solder must be removed before the joint can be re-brazed.
I asked the folks at Harris (the makers of Stay Brite) about using it in this application. The most significant part of their reply was that they didn’t recommend it. (They said that if the operating temperature was up to 200F and there was vibration present, the joint may come apart.)
With regard to Silver Brazing, I haven’t used the exact product in your link, but I use a very similar silver-brazing alloy with 56% Silver on an almost daily basis. That percentage has the lowest ‘flow’ temperature of the cadmium-free alloys. It would be a good choice, although I’d use a small diameter (.032") wire with a separate flux rather than pre-fluxed rods. It’s fairly easy to use, but doesn’t flow quite as easily as the cadmium bearing alloys. An alloy with 45% silver with Cadmium is probably the easiest flowing one you can get, and has the best chance of wicking to the ends of the crack. The cadmium does add some health risk, but it’s a cumulative thing. You won’t have much exposure doing this one little job. And you’ll be following good personal-protection practices, anyway, right? (safety glasses,
ventilation, rubber gloves, respirator…)
Brazing is intolerant of contaminants in the joint. As Hussein mentioned, the chrome plating will have to be abraded away in the area of the joint (and will likely be discolored and possibly blistered by the heat in surrounding areas). You’ll also want to remove any trace of coolant that’s left behind in the crack. I think if you fill the tank with water and pressurize it to blow water through the crack that would be a good start. If you’ve got access to an ultrasonic cleaner, that would help greatly. (I’d suggest a diluted phosphoric acid solution in it.)
I have always thought the pre-fluxed rods (like the product you linked) have inadequate flux. Inadequate both in terms of quality and quantity. Superior 601 was my favorite until I switched to Cad-Free a couple years ago. Now I’m using Aufhauser’s ‘Black Silva Flux’, which has a higher limit on the upper temperature. Use liberally.
If you’ve got further questions, I’ll do my best to give good answers, and to reply as soon as I'm able.
Your car is so beautiful and your work so inspiring that it's a little bit intimidating to think I can give you suggestions. But your question addressed my experience so directly that I couldn't just let it pass by.
Best regards,
Brian Russell
