Electroless Nickel Coating Adhesion and Bond Strength

A part that looks perfect after plating can still fail if the nickel layer does not bond properly to the underlying metal. The adhesion is fixed before the part leaves the tank and can be tested afterwards to check if it did work as intended.

Start With a Genuinely Clean Surface

Any surface residue of oil, grease, cutting fluid, etc. as well as any natural surface oxide film must be removed prior to Nickel Plating to enable keying of the Ni deposit to the surface. Degreasing and cleaning of parts prior to plating is not a simple matter of giving the part a quick once over with a cloth to remove any surface dirt etc. prior to processing and can be the subject of a complete process stage prior to plating using alkaline cleaning agents followed by an acid etch to remove any residual surface oxide films prior to plating.

Match the Pre-Treatment to the Metal

Typically, steel requires activation of the surface to be coated before it can be plated using Electroless Nickel Plating. Aluminium requires a zincate solution to be used to create a coating on the surface of the part to be plated, prior to coating as it very quickly oxidizes once cleaned. Stainless Steel is possibly the hardest metal to successfully coat using Electroless Nickel Plating as it has a passive Chromium Oxide layer on the surface which repels nickel solution until this layer is ‘broken’ or activated in some way. This is often achieved with a nickel strike coating.

Give the Deposit a Mechanical Key

As already mentioned, chemical bonding alone is not sufficient, in particular for harder or smooth surfaces. In such cases, the electroless nickel coating can be improved by light acid etching or by producing a rough surface by mechanical means. The etching has to be carefully controlled in order to avoid pitting which could reduce the strength of the part. However, even light etching can produce excellent results and, once again, etching time and acid concentration are the critical parameters to be controlled.

Keep the Bath Tightly Controlled

Bath temperature, loading and pH are important process variables that control the microstructure of the deposit. These variables must be strictly controlled to produce a solid interface with the base material. Most plating baths operate optimally within a specific temperature range and must be checked regularly for pH to control the phosphorus content of the deposit and to ensure adequate bonding with the base material. Local ‘starvation’ of components in a heavily loaded plating tank can also result in variable quality of the plated deposit.

Heat Treat to Strengthen the Interface

The part is then heat treated by placing it in an oven and heating it up to around 190-200°C. The heat treat process serves two main purposes: 1) to diffuse the interface between the nickel and the base metal to increase the bond strength; and 2) to relieve any internal stresses that may have developed during the plating process. Most specifications that require the use of Electroless Nickel also require heat treatment of the part(s) after plating.

Prove It With Recognised Tests

Bend tests, quench tests, adhesive tape tests and grind tests are used to test for adhesion. All of these tests assess the bond of the coating to the base material. The most common tests for bond strength are the bend test and the quench test. These tests place stress on the coating/substrate interface. A tape test is also very simple to use and indicates failure of the coating if it lifts from the tape. It is however only good for indicating obvious failures.

Know the Usual Causes of Failure

There are three main reasons why adhesion fails on real parts. Oxide re-forming between process stages, contact marks from racks or jigs used to support the part and drag-out contamination of process solutions between tanks are the most common causes of failure. To prevent these types of failures it is necessary to control the handling of parts between the various stages of the process and work to recognised standards for metal finishing processes and surface finishing industry guidance.

Reaching adhesion with electroless nickel coatings is not just a matter of one process step. All process stages must be performed correctly. A good starting point for a coating with good adhesion is essential. Correct pre-treatment of the base metal, proper control of the plating bath and verification tests are also required to produce coatings that will not lift.