Stainless Steel Handles Brazing—Leaves Stain

Question:

We are manufacturing stainless steel mugs and the handles are attached to the mugs by a brazing process. As there is very little gap between the handles and the mug, post-brazing it is difficult for mechanical cleaning to happen. Also, there are quite prominent yellow stains around the brazing area. This entirely ruins the product's look. We are using silver rod and white silver flux for brazing. Can you recommend a solution to avoid/clean these yellow stains? R.A.

Answer:

It makes sense that you would first want to find a process that would avoid the creation of these stains altogether. That will simplify your process and eliminate the need for an additional manufacturing step. However, to do that, you will have to try some alternate fluxes, not all of which will be as effective as you are currently using.

The purpose of the flux is to remove the surface oxides slightly below the solidus/liquidus of your braze alloy. Based on your description, it sounds like you are using a flux that contains chloride. These are common and are generally considered a corrosive flux. They usually require a secondary step, post-braze for removal. Often a sulfuric acid cleaning is effective at removing these flux residues. You could start at room temperature with about a 10% by volume concentration and go from there. Increasing the temperature and concentration will increase the effectiveness of the process by providing more complete flux removal in shorter times.

Alternately, you may be interested in trying some non-corrosive fluxes. They do not contain chlorides. Their primary ingredient(s) are borates. They may be able to perform the necessary fluxing, but may require more application prior to brazing.

Additionally, to help either process, you may want to consider quenching the sample in room temperature or warm water immediately after brazing. This rapid temperature change will effectively remove a large percentage of your flux residue. This could eliminate the need for cleaning or at least minimize the amount of effort you have to spend on it.