How Electroless Nickel Coating Is Stripped and Replated

Parts that have been used and have worn or developed defects can be stripped and re-plated rather than being scrapped.

Table of Contents

Stripping the Old Coating

The old deposit is removed from the part using a solution of Nitric Acid. This solution selectively dissolves the nickel, leaving the Steel or most Aluminium substrates unaffected. The part is then rinsed to remove all traces of the old solution. It is then important to check that the correct solution and dwell time were used for the part being stripped, as over exposure can etch the part’s base metal.

Inspecting the Bare Surface

After stripping the part off, it is very important to inspect the bare surface of the part for any pitting, corrosion or mechanical damage that the coating may have masked. If the part has any cracks or deep pitting, it will not hold a good deposit of electroless nickel, and should be scrapped.

Preparing for Replating

This part would be degreased, etched and activated for a nickel plating process. For aluminium parts, a zincate treatment is used to activate the surface so that the nickel will bond to the aluminium substrate rather than the oxide.

Managing Dimensional Tolerances

With each cycle of strip and replate, material is removed and then added back. This means that for parts that require bores or shafts to be within very close tolerances, the amount of allowable material for the rework will decrease with each cycle. The amount that Electroless Nickel Coating will deposit on a part is predictable, typically 12 to 25 microns per surface. This is one of the characteristics of the process that makes it so very popular. But this same predictable nature has to be taken into account when strip and replate is required. So it is very important to establish in advance the part’s allowable material for the rework, and to specify the required thickness on the job sheet.

Confirming the New Deposit Meets Spec

When inspecting items that have been re-plated, the thickness, hardness and especially the adhesion of the new coating are critical. These can be compared with the specifications for electroless nickel coating as set out in ASTM B733.

Remember that every time a part undergoes a strip-and-replate cycle, there is a chance for material to be added or removed from the part. Therefore, it is wise to check the remaining substrate dimensions of a part that has gone through several cycles of stripping and replating against the original drawing before you begin another round of reworking a part.