Question: We plated the mating surfaces of our Inconel 738 components that were going to be nickel brazed, and the parts failed in service prematurely. An analysis showed that our brazing was OK, but the plating had peeled from the base metal causing the premature failure. How can we prevent this in the future?
Answer: In my experience the probable cause is related to the plating process itself, in which the surfaces to be plated may not have been properly cleaned prior to plating. Not only should you be sending clean parts to your plater, but it is also the plater’s job to be certain that the surface is thoroughly cleaned, sufficiently enough to allow the plating to adhere/bond properly to the surface so that no separation in service would ever occur.
Figure 1 shows what typical blistering may look like when heated. What’s happening is the contaminants on the surface that were not properly removed prior to plating have now volatilized, forming a gas bubble under the plating. This can then become a leak path or a weak spot when the part is put into service. Such a thing should never happen if the plater properly cleaned the surface prior to plating the part.
To ensure that plating is done properly, you (as the plating customer) should always institute a “blister test” to ensure the soundness of the plating.
All metals, when heated up to brazing temperature, will want to react with any oxygen present in the atmosphere around the part, which will cause an oxide layer to form on their surface. Unfortunately, brazing filler metals (BFMs) do not like to bond to, or flow over, oxides. Thus, oxides must be prevented from forming during the brazing process. This is especially true for base metals that contain even small amounts of titanium or aluminum, such as an Inconel 738. Such oxides, once formed, cannot be “reduced” (i.e., removed or dissociated during any brazing process). What to do?
The best “safety procedure” to use is to prevent those strong oxides from forming at all. This can be accomplished by nickel plating the surfaces prior to brazing. Nickel plating is compatible with most metals, and BFMs will readily bond to, and alloy with, the applied nickel plating.
Plating Houses are Responsible for Integrity of the Plating
Any good plating company knows the importance of having clean surfaces on which to plate the nickel. They will normally take time to verify the cleanliness of the surfaces to be brazed and will clean them as needed so that the applied nickel plating will stick.
However, not all plating companies clean the base-metal surfaces as thoroughly as they perhaps should. Therefore, there will be times when the plating does not properly bond to the surface of the parts as well as it should. This means that during a high-temperature brazing application any oils/lubricants or surface contamination on the surface of the part that was not completely removed prior to plating might (as mentioned earlier) volatilize during the brazing process. This will cause blisters to form between the plating layer and the metal surface.
Then, during actual brazing, the molten BFM may bond very well to the top of the Ni plating, but the plating itself may not be well bonded to the substrate metal below it. This happened to parts in my brazing shop on a couple of occasions. My customer complained to me that the parts we brazed for them did not perform properly (the brazed assembly either leaked or it failed prematurely from a strength perspective). We then examined the part when it was returned to us, and we found that our brazing was indeed perfect, but the plating had failed (i.e., the plating was not done properly and had actually peeled off the underlying substrate to which it had been plated). This caused the leak or tensile failure.
Next time, in part 2, we will talk about blister testing.