Preparing Brazed Metal

Question:

We manufacture aluminum heat exchangers and are planning to implement CAB brazing soon. What type of powder should we use and is there any pretreatment necessary? T.C.

Answer:

I have never heard of CAB brazing. I even checked with some welding sources, and they have never heard of it. However, all brazed parts will need some “special attention” if you intend to powder coat over them. Brazing of metals leaves residue from the flux and soot contaminants. These must be removed to ensure that the powder coating will adhere properly to the aluminum. These contaminants can be removed either mechanically (grit blasting, grinding, sanding, etc.) or chemically using pickling chemistries.

Often brazing materials will outgas during the cure of the applied powder coating. Sometimes preheating will eliminate this outgas, but selecting an outgassing powder formula may be your best bet.

Lastly, brazing sometimes leaves pinholes that will not be filled by the powder coating. In these cases, filling these pinholes before coating is recommended. Select a filler material that can both conduct the ground necessary to electrostatically apply the powder coating and withstand the heat of curing without outgassing.

Besides the cleaning of the brazing area, you must also remove both the inorganic and organic soils from the aluminum substrate to get proper powder adhesion to the substrate. Using a pretreatment (such as Alodine) will greatly improve the corrosion resistance of the product. This can be important when products are fielded in outdoor environments or in corrosive atmospheres. The type of powder coating you should select depends upon the appearance, mechanical, weatherability and corrosive properties you need. Discuss these issues with the powder suppliers listed in PF ONLINE to determine which formulations are best suited for your particular application. Without more information, I am afraid that I cannot be any more helpful than this.