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Crystal structure of nickel chloride strike
May 30, 2009
We are processing Inconel alloys through a sulfamate nickel plating process. The process includes a post-machining stress relief
(parts are 40RC) at 375F for 4 hours; wax mask; pumice; alternating current off-the-shelf alkaline electroclean; reverse current sulfuric-HF acid etch; nickel strike; sulfamate nickel plate; demask; embrittlement bake.
Problem: After the nickel chloride strike, one out of four parts processed batch-style AT THE SAME TIME on the same rack look like they are coming out of the strike with black spots on them. After closer examination with a microscope, it appears that the black spots are actually smooth crystal facets surrounded by proper dull nickel strike. The defect will cause the nickel sulfamate to de-bond during finish machining.
Items of interest: These are rework parts and the machine shop has not properly removed all the pre-existing nickel during pre-machine. But the machinists say they have been doing this for years with no issues. We invested in new chemical make-ups (because of other issues) and the problem is still occurring. A few spot checks have been used to check for magnetism, none detected as yet.
process engineer - Oklahoma City, OK, USA
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