Canadian Finishing & Coatings Manufacturing - Sept/Oct '23 - 35

PLATING AND ANODIZING: PRETREATMENT
Critical Step:
Surface Prep
Pretreatment cleaning technologies and troubleshooting
BY MAX GEORGE
P
lating processes can be difficult to maintain and
control on a clean surface. Poorly preparing your
parts for your finishing step will put you at a
severe disadvantage for controlling the quality and
consistency of your operation. Therefore, cleaning is the
critical first step of any finishing process and overlooking it
can come with costly consequences.
Good cleaning works in two ways: it removes all foreign
matter from the surface of the metal part and it maintains
the surface of the part. If we do not effectively remove
contaminants like drawing compounds, lubricants, and rust
preventives, these can act as a barrier on the metal surface
and cause your plated metal to blister or peel. Also, if the
cleaner is not appropriate for the substrate being cleaned, the
cleaner can degrade or erode the metal surface that can make
the surface inert.
Alkaline cleaners will be used in the majority of your
pretreatment processes to remove organic soils from your
metal that make up your most common metalworking fluids.
They do this with a variety of ingredients, with the most
important being alkaline builders. Sodium or potassium
hydroxide are the most alkaline of the builders and function
to saponify fats and oils while neutralising any acidic soils.
Other alkaline builders include carbonates, phosphates and
silicates. Silicates are particularly important when cleaning
nonferrous parts as these serve to inhibit attack of the base
metal. Most cleaners also contain surfactants which are
organic materials whose molecules have both a water-soluble
portion and an oil soluble portion. Surfactants lift the oil from
the part being cleaned and hold it either in solution or cause
it to split out and move to the surface of the cleaning bath.
Sequestering agents and chelators are also often included.
These ingredients work to tie up metal ions like calcium, iron,
and magnesium that are commonly found in water and can
interfere with the detergency of the cleaner.
In most plating processes, the first step will be an alkaline
cleaner. One method of application is spray cleaning, where
the parts are hung or placed on a conveyor and sprayed
with a heated cleaning chemistry. The spray produces an
impingement action when used in combination with the
chemistry and heat can quickly prepare your part for plating.
A more common method is immersion cleaning. This is a
cleaning process where the bulk of the work is done by the
chemistry of the cleaner and heat. This process is often used
Air Agitation
Electro Cleaning
Operation
September/October 2023
35
Canadian Finishing & Coatings Manufacturing - Sept/Oct '23

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