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Brazing Q & A
Q: We have been brazing 304L with BNi-2 at our facility for more than ten years. To the best of my knowledge, we have not had problems regarding corrosion resistance until we subjected an assembly to a mixture of salt fog, sulfur, nitrogen oxide, and aircraft exhaust. This mixture forms a highly acidic (pH 2.44.0) moisture film. Do you have any knowledge as to whether there are any electrolytic issues between 304L and BNI-2 braze material? A: Yes, I know about this problem; it is a result of interfacial corrosion. Under certain conditions during brazing, as the boron in BNi-2 diffuses out of the brazed joint, it combines with the chromium in the 304L, leaving the iron free to be attacked interfacially in the base metal, along the edge of the joint. (See Fig. 1, an etched micro of a 304L joint brazed with BNi-2. The dark, heavily etched area in the base metal is the area that will be attacked by the corrosive media.) In your ten years of brazing 304L, it would appear that no corrosive media have been encountered, as is usual in most of the brazed joints of this combination.
Fig. 1 - Microphotograph of an etched 304L joint brazed with BNi-2. In cases where there is sufficient base metal, the assembly can be held at the brazing temperature for longer diffusion times. The boron will then be diffused further into the base metal, leaving the chromium, which will reconstitute with the nickel and iron of the 304L base metal, preventing further interfacial corrosion. Unfortunately, your parts are sheet metal, and there will be insufficient metal to fully diffuse the boron away from the chromium at the base-metal interface. When it comes to the corrosion problem, I do not recommend the boron- containing brazing filler metals for brazing 304L, or any of the similar iron-based metals. I would recommend that BNi-5, BNi-12, which contains 25% chromium, or a more recent brazing filler metal, which is a Ni-Cr-Si-P filler metal, be substituted for the BNi-3. BNi-5 is a 19%-chromium brazing filler metal that could be substituted; however, it has a higher brazing temperature. The Ni-Cr-Si-P is a modification of the BNi-5 and has a lower brazing temperature. Another solution is to protect the fillets and edges of the fillets where the interfacial corrosion starts, by painting or by applying electroless-nickel plating or some similar materials. Since interfacial corrosion progresses from the edge of the fillet and base metal, protection of this initiating area will prevent the attack. I hope this answers your question about interfacial corrosion of 304L stainless steel and similar base metals brazed with brazing filler metals containing boron, when exposed to highly corrosive media. R. L. PEASLEE is Vice President Emeritus, Wall Colmonoy Corp., Madison Heights, Mich. Readers, may send questions to Mr. Peaslee c/o Welding Journal, 550 NW LeJeune Rd., Miami, FL 33126 or via e-mail to bobpeaslee@wallcolmonoy.com. |
