This topic has surfaced again in one of my client’s brazing shops, as brazing personnel encountered a brazing problem actually caused by the misuse of a brazing “Stop-off” in their braze-prep area. As the name of this product-type indicates, a brazing “stop-off” is supposed to be a paintable product that when applied to a metal surface, such as shown in Fig. 1, will “STOP” a molten brazing filler metal (BFM) from flowing into areas where it is not supposed to be, thus keeping it “OFF” any critical surface that is supposed to remain free from the presence of any BFM.
First of all, it is very important that the reader should understand that molten brazing filler metals (BFMs) do not like to bond to (or flow over) oxides, dirt, or lubricants. The presence of any of these contaminants on the surface of parts to be brazed can literally prevent the BFM from alloying with (i.e., bonding to) any surfaces on which any of these contaminants are found, and can prevent any capillary action of BFM from occurring.
Because oxides are so good at stopping the flow of molten BFMs, brazing stop-off compounds are made as blends of a variety of metallic-oxides and are packaged in a variety of forms: liquids, pastes, powders, sprays, or tapes (to name just a few). Fig. 2 shows some typical containers of brazing stop-off, which can be supplied in spray-cans, or containers (plastic or glass) ranging in size from very small to very large.
As just mentioned, a typical brazing stop-off will be a mixture of metallic-oxide powders (such as aluminum oxide, titanium oxide, yttrium oxide, magnesium oxide, etc.) that is mixed with a liquid carrier solution to form a slurry or suspension (that looks similar in consistency and viscosity to thin house-paint), which can be applied onto a metal surface by using a small brush, or by dipping, or by spraying.
Usually, a stop-off is applied as a thin, continuous line around critical areas on a metal’s surface where BFM must not be allowed to flow. This can be done using a small paintbrush, as shown in Fig. 1, or perhaps as a marker-pen, such as shown in Fig. 3. Since BFMs do not like to bond to or flow over oxides on metal surfaces, the presence of an adherent line of stop-off on a metal’s surface can effectively tell the BFM: “This far, but no further”.
Colors of Stop-Off. All the readily available metallic-oxide powders used in the manufacture of brazing stop-offs are essentially white in color. Thus, all commercial brazing stop-offs would look alike if no steps were taken to differentiate them in such a way as to make one manufacturer’s product look different from the others. So, to make each brazing stop-off uniquely different from the others that you could buy, manufacturers of stop-off started adding different colors to their product so that it would be more recognizable as their own! Please understand, therefore, that color has nothing to do with the type of metallic-oxide in the stop-off, but is merely a pigment added to the stop-off blend to make it look unique!
So, most commercially available brazing stop-offs today are specified by their color, such as pink, red, green, yellow, blue, etc., and few manufacturers use colors already being used by another manufacturer. Thus, if you know the color of the stop-off product you wish to use, that will usually also tell you who the manufacturer of that product is.
“Poor-man” Stop-off. Please note that milk of magnesia, readily and cheaply available from many drug stores and food-stores, is actually a decent stop-off that may prove effective for many general-purpose needs for a stop-off. Fig. 4 shows a typically available, inexpensive milk of magnesia product that many people can use for their brazing stop-off needs.
So, do some experimentation, and find out what kind of stop-off works best for you.
But —- VERY IMPORTANT — don’t let the easy availability of stop-offs lull you into carelessness in the brazing process, as discussed below!
1. Over-use. Because of the effectiveness of many stop-offs, a number of brazing shops tend to discount the need to control the amount of BFM they apply to parts, thinking: “Oh, don’t worry about how much BFM is used, just put on more stop-off”. That is a VERY bad idea! Applying only the correct amount of BFM is ALWAYS important!
STOP-OFF SHOULD NEVER BE USED AS AN
EXCUSE FOR USING TOO MUCH BFM!
Unfortunately, I see too many shops using too much stop-off, rather than properly training their people to back-off from such overuse, and training their personnel, instead, to use only the correct amount of BFM that is needed to make a good brazement. When the proper amount of BFM is used (the volume of BFM used — when it completely melts and turns to a liquid — should be approximately 150% of the volume of the joint to be filled), this amount of applied BFM may then justify using only a very small amount of stop-off, or it may permit them to get away from using stop-off altogether.
2. Correcting stop-off “mistakes”. It is not uncommon for persons applying the stop-off to make a mistake, and accidentally apply stop-off onto a surface that needs to be brazed. I’ve heard some folks very softly say: “Uh, oh…” as they’re putting stop-off onto parts, and then, when I look in their direction to see what they just did, I see them taking a cloth and dipping it into a solution such as acetone or alcohol, with the intent of wiping off the stop-off from the surfaces where the stop-off is not supposed to be. THIS DOES NOT WORK! Please understand that the following saying is very true: “Once stopped-off, always stopped-off”. You cannot merely wipe the stop-off from the surface and then expect that the surface will then become brazeable once again. That is incorrect thinking! Residues from that stop-off are still there in the nooks and tiny crevices of the “surface roughness” of that component, and these tiny residues of stop-off can actually prevent the BFM from properly wetting that surface.
The only way to effectively remove stop-off from surfaces is to either thoroughly ultrasonically clean that surface, or machine off that top surface layer that was contaminated.
BETTER YET! Find ways to eliminate the use of stop-off altogether. Too many shops plan on the use of stop-offs on too many of their parts, believing that it is always wise to use stop-off. No, it is not.
CONCLUSION: Brazing stop-offs can be a helpful tool for a brazing shop, but it should be used sparingly, and only when absolutely needed. Otherwise, overuse of stop-offs can actually result in scrapping parts unnecessarily or create a lot of extra work to clean off surfaces for subsequent brazing. Stop-offs can be an effective tool for brazing when used sparingly and wisely.
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Dan Kay – Tel: (860) 651-5595 – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 47-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Dan can be reached via e-mail at email@example.com, and his website can be visited at http://www.kaybrazing.com/
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