My recommendation is that you institute some QC tests on the plated parts that you receive back from your plater in order to evaluate the quality of that plating. The so-called “blister test” was the most important and valuable one we developed, and it should NEVER be done by the plating company. It should be performed by YOU. Otherwise, it’s like asking the fox to guard the chicken coup.
Here’s what I recommend for blister testing:
- Sample sent with parts. You should always send a “test-sample part” along with the parts you are getting plated, and that sample part is what you will subject to the blister test. The sample MUST always have the same approximate cross-sectional configuration and cross-sectional mass as the parts that you are having plated. Thus, if the parts being plated were heavy and had deeply curved surfaces into which the plating had to go, then the sample you send along with that load should also be somewhat heavy and have a similar deeply curved surface. In other words, the sample that you will blister test should closely represent all the variables evidenced in the production parts themselves. If not, that plated sample would not be very representative of your plating needs. Therefore, never send a small, flat test coupon to be plated for subsequent blister testing if the actual load of parts to be plated are heavy and curved.
- Test temperatures. We usually ran our plated test samples in a hydrogen atmosphere at about 1000°F (500°C) to see if any blistering of the plating would occur. The time at temperature is NOT important at all. A thermocouple was placed on the test piece, and as long as the sample reached about 1000°F, that was sufficient to reveal the quality of the plating. Similarly, the actual atmosphere used is not that critical. These tests will also work in vacuum or in an atmosphere of argon, nitrogen, etc., since the volatilization of any surface contaminant can cause a blister to form, irrespective of the type of atmosphere used. Bear in mind that there is a sufficient amount of dew point or atoms of air/moisture in any of these atmospheres so that any surface contaminant under the plating would volatilize and/or react with any given atmosphere’s constituents to form a blister.
- Results. If the sample part did not show any blisters, we then accepted the entire plated lot and put those parts into our brazing production. If any blisters showed up on the plated surface of the test sample, however, we rejected the entire lot of plated parts. Parts were sent back to the plating company so that the plating could be stripped off, the surfaces re-cleaned properly and then re-plated. Again, another test piece should be sent along with the load that is to be re-plated. This piece will be blister tested when the re-plated parts come back to verify that the re-plating was done properly and so on.
Nickel plating can be a reliable process for many companies in order to provide brazeable surfaces on metals that contain small additions of Ti, Al, etc. in their chemistries, as you have seen in your production load of Inconel 738. But the quality of that plating should ALWAYS be verified by running a “blister test” on a representative sample of the parts that you plan to braze. Only after the test piece has passed the blister test should you allow the load of plated components to be released for brazing.