Brass parts are different from steel or aluminium parts when it comes to plating. Ignoring these differences can lead to problems with adhesion and coating life.
Surface Activation Before Plating
Brass, a copper-zinc alloy, rapidly forms a surface oxide after cleaning, which can cause coating to detach from the brass surface. A specific activation process is required to remove the surface oxide prior to plating. A simple etch using a mild acid, or specialist brass activator solution are typical pre-treatments used prior to depositing a coating onto brass parts.
Galvanic Compatibility With Brass
On the galvanic series, the zinc-nickel coating is situated near to the brass substrate, with little chance of corrosion occurring between the coating and the brass. The galvanic compatibility of copper alloys ranks substrate/coating combinations in terms of susceptibility to corrosion, with brass coatings rated lower than steel coatings. Exposed cut edges and other exposed features are still susceptible to corrosion and require a good passivation to protect.
Thickness and Tolerances on Close-Fit Parts
The tolerance for thickness of Zinc Nickel on Brass parts can also affect the fit of certain parts. Fine pitch threaded components and parts with press fit bores will require coating to be masked off in certain areas or to be built to the lower end of the corrosion protection thickness range. These requirements must be clearly flagged on the parts list and noted on the relevant technical drawing.
Passivation and Sealing Choices
As with all parts, it is important to select a suitable passivation solution and topcoat sealant for parts to be used in humid environments or outside. A useful reference for Zinc Nickel Plating is www.poeton.co.uk/surface-treatments/plating/zinc-nickel-plating.
Communicating Requirements to Your Supplier
Set out the required surface finish and special masking requirements on the drawing. Reference the required plating specification, e.g. ISO 12683 or customer specific. Identify the base material i.e. brass and, where applicable, the alloy. Lead containing brasses may require special pretreatment processes.
Get the specification right first and avoid costly rework.
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