Black Oxide Problem

Q: We use a high-temperature black oxide to finish a number of different tools. A couple of these tools have joints with a tight fit. The tools are thoroughly rinsed and soaked in hot water an extended period of time. We find that 30–40% of the tools show salt bleed-out at the joints. Can you suggest anything we can do to solve this problem? J. W.

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A: I assume that the temperature of your black oxide bath is much higher than the temperature of your rinse water. What is most likely happening is, when your parts are removed from the hot black oxide process tank and placed in the first rinse tank, the temperature drop is drawing salts into the joint of the tools. Because of the tight fit, it is almost impossible to rinse out the salts.

I can think of two possible solutions. One solution is to consider using a low-temperature black oxide bath. There are vendors that sell this particular chemistry—go to www.pfonline.com/plate/suppliers.html and search under Black Oxide. These finishes have different properties than classic black oxide finishes. You will have to determine whether one of them may be a suitable replacement for the high-temperature process that you are using.

The second possible solution is one that Larry Durney suggested a number of years ago in one of his columns. He found that immersing the parts in hot rust-preventive oil after they had been thoroughly rinsed minimized salt bleed-out. This process sucks oil into the joint, preventing moisture from the environment from reaching the residual salts in the joint. The oil used was non-emulsified.