This Quality Metal Finishing Guide deals with pretreatment of surfaces
for metallic coatings. In this context, pretreatment is defined
as the process of substrate preparation to receive a specific metallic
coating. Generally, the objective of applying a metallic coating is
one or several new properties to a basis metal. Coatings include decorative
finishes such as copper or brass on zinc diecast, and functional coatings
such as hard chrome or electroless nickel on aluminum,
or corrosion resistant zinc alloys on steel.
Regardless of the function, these coatings all must adhere completely
to the basis metal if they are to provide the decorative or functional
properties required. Plate adhesion must exceed the cohesive
strength of either the substrate or the electroplate. In other words,
an attempt to peel the metallic coating from the substrate should
a layer of the basis metal without separation of the coating from
the substrate at the boundary. In plating, this level of adhesion
between the atoms of the substrate and the plated metal.
Whatever the mechanism, adhesion depends on the condition
of the surface being plated. Pretreatment, therefore, consists of
steps used in the removal of any undesirable contaminants that might
interfere with the inter-atomic adhesion. Typical contaminants are
such as oils, grease, fingerprints, oxides and scale. Pretreatment
affects other coating properties. Even if adhesion is secured initially,
blistering, spotting out, stress in deposit, and failure due to hydrogen
embrittlement occurring at a later date may be related to surface
preparation. Premature failure resulting from corrosion can be avoided
by perfect surface preparation prior to complete coverage by the specified
coating. Automotive and other industries rely on the integrity of
components in providing long-term product-performance warranties.
The primary selection of a given pretreatment process is dictated
by two factors.
1. THE METAL BEING PROCESSED AND THE CLEANING SOLUTION
Generally, steel and copper can withstand strongly alkaline cleaners
(pH 10-14), and acid pickles such as 50% hydrochloric acid, with
or no attack of the basis metal. Bronze and brass require mild cleaners
(pH 9-13) and lower temperatures to prevent the selective removal
of the zinc from the alloy (dezincification). Zinc and aluminum
milder inhibited alkalis (pH 9-12) and mild acids. Proprietary formulations
are available to meet these needs.
2. THE TYPE OF SOILS TO BE REMOVED
Soil is any substance on the surface of the metal that will interfere
the metal-finishing process being performed. Soil may be the result
of intentional application to the metal of a material that facilitates
working, handling, and exposure to high temperature or the atmosphere.
Generally speaking, the types of soils can be broken down into two
categories: organic and inorganic.
animal and vegetable oils.
mineral oils and waxes.
contaminants either formed in situ (metallic soaps formed in the buffing
operation), or inhibitors from certain acid pickling solutions redepositing
on the metal.
and smut, oxide and metallic residues.
compounds, abrasive, grinding and polishing residues or grits.
shop dusts and soldering fluxes.
These are only a few of the soils encountered. Some of the others
paints, cleaning residues such as water-in-oil emulsions, fingerprints,
inorganic coatings such as phosphates, chromates and rust preventatives.
Many of the lubricants, coolants and corrosion preventives used
are soluble or emulsifiable. These soils are best cleaned in a mild,
non-caustic-soda-containing alkaline cleaner at low temperatures.
many water-soluble lubricants can be removed partially by a warm
pre-rinse, thereby reducing cleaner contamination and minimizing
problems. Particulate matter is also more readily removed by mild,