Electroless Nickel Plating on Stainless Steel
This substrate needs special preparation before the deposit can adhere. Stainless steel is a tough material to plate and once properly prepared, a hard, corrosion resistant, wear resistant coating is produced that can be applied to complex shapes.
Remove the Passive Oxide Layer First
The passive chromium oxide layer found on Stainless Steel surfaces must first be removed prior to the plating process. This initial stage is commonly known as surface activation. This activation process must take place prior to the plating process in order to ensure effective bond between the part and the Electroless Nickel Plating deposit.
Use a Wood’s Nickel Strike
Stainless steel can be processed in many different ways to create a surface for plating, with activation being the most critical step to achieve adequate adhesion between the substrate and the deposit. As with any other metal, stainless steel has to be prepared for plating and a Wood’s nickel strike is used as the most reliable method to create a surface for plating Electroless Nickel on stainless steel parts.
Wait – that em dash crept in. Let me fix that:
The most reliable way to activate a stainless steel surface is by using a Wood’s nickel strike. The part is electroplated for a short time in a high-chloride nickel bath. The surface is etched and a thin active nickel layer is deposited. This layer is then bathed in the Electroless Nickel Plating bath to produce the deposit. A mixed-acid activation is also available and is used by some applicators. However, for the majority of stainless steel parts, particularly those made of austenitic and duplex grades, a Wood’s nickel strike gives the strongest adhesion.
Match Phosphorus Content to Your Performance Needs
The amount of phosphorus in the Electroless Nickel Plating is another key factor. Low-phosphorus deposits (two to five per cent) are very hard and, therefore, suitable for wear applications. However, corrosion protection is low. High-phosphorus deposits (ten to twelve per cent) offer good corrosion protection. They are naturally amorphous and, therefore, very suitable for stainless steel parts that are used in aggressive chemical environments. For coating standards and for measurement guidance, refer to surface coating measurement and standards, published by the UK’s national measurement institute.
Plan for Uniform Deposit Thickness
Electroless nickel coating is by nature a uniform coating and is typically measured to determine the thickness across the part. The coating thickness on bore holes, recesses, and threads are the same as on the external surfaces. A typical thickness for an electroless nickel coating is in the order of 5 to 50 microns. Pre-plate dimensions for any part to be coated must therefore be reduced to allow for the desired coating thickness.
Specify Your Part Carefully Before Sending It
Notify the plating company of the alloy being plated, surface finish before plating and any areas to be masked. The rougher the surface finish before plating, the rougher the plated surface finish will be. Therefore, rough surfaces need to be polished before plating to the desired surface finish. Also, areas to be masked need to have clean sharp boundaries.
Electroless nickel plating on stainless steel, a preparation and a specification.
