It cannot be over-stressed that the quality of an ESN coating is entirely dependent on the pre-treatment of the part to be coated. A few minutes improper preparation can result in a coating which is full of blisters, which peels off or which has an uneven appearance even when the coating chemistry has been properly controlled.
Degreasing
Contamination on parts can come from a variety of sources – oils from machining, handling contamination, etc. All of these must be removed prior to coating. Degreasing is typically done using an alkaline solution, but solvent cleaning can also be effective in removing surface contaminants.
Mechanical Cleaning
The roughening of surfaces can be achieved by way of abrasive blasting or by light grinding/linishing. This process is particularly important on the harder surfaces to create ‘grips’ for the deposit to bond to. Chemical cleaning alone is not sufficient to remove surface oxide and old corrosion staining, although it will remove surface contamination.
Acid Etching
An acid etch, typically specific to a substrate, will remove surface oxide(s) and slightly open up the surface at a microscopic level. An etch will activate a surface in preparation for the Electroless Nickel Plating process. Typically aluminium, steel and copper alloy surfaces require different etch solutions and contact times.
If you are looking for Electroless Nickel Plating, see https://www.poeton.co.uk/surface-treatments/plating/electroless-nickel-plating.
Catalytic Activation
An Electroless Nickel Plating surface needs to have a catalytic seed, typically made of a palladium-based treatment, in order for the chemical reduction reaction to create the Electroless Nickel coating. If this stage is not done correctly, the nickel plating may not start at all or it could possibly start to plate unevenly.
Rinsing Between Stages
Every stage must be thoroughly rinsed between to prevent drag-out of previous chemistry in the tank. For example a degreaser brought into the acid etch solution can cause problems with the subsequent activator and subsequent coating, and similar problems can occur if etch solution is brought into the activator. Most seemingly unexplained coating adhesion problems are caused by poor rinsing between stages.
Pre-treatment needs to be carried out in the correct manner to allow coating to last and not fail under stress.