Preparing electroless nickel surface for better adhesion
I am trying to prepare a surface for adhesive bonding using an epoxy resin. The surface is an electroless nickel coating with a mild steel substrate. This part will be joined to a larger (similar) part making up an assembly which will ultimately sit inside the cab of a road vehicle.
The epoxy manufacturer recommends a conc. nitric acid dip for 5 seconds. This appears to work really well. It leaves behind a bright nickel surface which adheres well to the epoxy. I later subcontracted the plating and the acid dip, only to find that the electroless nickel has gone black. I have discovered that the subcontractor uses a low-med phosphorous electroless nickel process, whereas I have used a high phosphorous process.
I assume that the black surface is nickel phosphate/phosphide? My question is this, how will the black finish fare when exposed to ambient atmospheric conditions, such as would be found in the cab of a road vehicle. Also how will this compare to the bright nickel finish obtained earlier under the same environmental conditions? i.e. which finish would give the best corrosion resistance.Victor Arghyrou
Materials Engineer - Plymouth, Devon, England (UK)
You have discovered the nitric acid test for EN coatings. The test is often used to distinguish between high phosphorus coatings and lower phosphorus coatings. Generally coatings containing less than 10 percent phosphorus will turn black in less than 30 seconds. High phosphorus coatings will stay bright for several minutes.
The black color is due the formation of nickel/phosphorus oxide AND to the formation of etch pits in the coating. Even without the blackening treatment, mid phos coatings are much less corrosion resistance than high phos coatings. After blackening (etching) the difference will be even greater.
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