Platinum Plating Process
(Electroplating)
Scope: Platinum (Pt) is used to increase the oxidation capability over a simple aluminide
coating. Platinum, in a platinum aluminide (PtAl), slows down the rate of oxidation compared
to a simple aluminide coating. Platinum also helps in surface smoothness, especially when
simple aluminides form grain boundaries (Photo 1), that during the oxidation process are the
pathways for oxidation creating a rumpling surface. (Photo 2) Platinum keeps the oxidation
rate even and does not form the grain boundaries that a simple aluminide does. Platinum
aluminides are used commonly for the bond coat for Thermal Barrier Coating (TBC) top coat.
The slower oxidation rate, the even oxidation scale, and higher temperature capability makes it
a very good bond coat for TBC’s. In aircraft, platinum is used for this reason, also, that weight is
a big factor, a thermal spray coating can be used to have similar benefits, but being an over lay
coating, it is thicker and heavier than a diffusion coating.
Platinum (Pt) is applied to a part by electroplating typically 0.00015” – 0.00040” (3 – 10
um, 0.15 – 0.4 mils) of platinum to the required area of the part. The platinum is then diffused
to the part in a thermal cycle, typically 1900F (1038C) 1hour, for adherence and handling. The
part is then aluminide coated to form a platinum aluminide coating.
Photo 1 –Aluminide Grain boundaries as coated (etched to show grains)
Photo 2 – Depletion at grain boundaries during FCT (Furnace Cyclic Testing)
Prep: Water Wash Typically parts with internal passages are washed before plating
because of previous processes (FPI, Machining..etc..) can leave contaminants, that need to be
cleaned before going into the platinum bath. Also, helps keep the platinum bath clean of
contaminants.
Prep: Grit Blast To clean the external surface of the part to be platinum plated,
aluminum oxide grit media (Al2O3) is used, typically 220 mesh or finer. The grit blast is to
remove any contaminant on the surface, including finger prints (oil from handling).
Note: after parts are prepped at this point, parts are handled with gloves (cotton,
plastic…etc..) to keep from contaminating the surface to be plated.
Plating: Serial Number / Pre - Weight Gain Serial numbers are recorded and parts are
weighed on a 2 decimal place scale. Parts are recorded by serial number so parts can be
tracked pre and post weight gain to verify platinum thickness. This is also part verification for
amount of platinum plated on each part and is documented for traceability by Meyer Tool and
Customers.
Note: During qualification, parts are plated to the low and high end of plating
specification thickness, this is verified to actual part metallography. This is used to define upper
and lower weight gain limits for each component.
Plating: Masking (if needed): Parts that need masking for areas of “No Coating” or
areas that do not need to be plated, require that area to be masked. This is usually done with
boots to cover the area of no coating, but boots have a lead time to make. So, if parts are
needed in less time than can get boots or permanent masking, a high temperature lacquer is
needed to paint on the areas of no coating. The lacquer is a paint or dip type method. It needs
time to cure, also when fixturing the parts, thought should be taken to minimize the amount of
the lacquer in the platinum plating bath. Lacquer over time will break down and put
contaminate into the bath. Lacquer is removed by dipping in acetone to break down and
remove the lacquer from the part.
Plating: Power Supply (amps) Each component is measured for the surface to be plated
for surface area. The area squared (X2) the thickness of the plate, typically 0.0002” (6um). This
calculation is used to determine amount of platinum per part and amps per part to be applied.
(in3 / cm3). This is used to program by part number the total amps of all parts in a batch.
Note: A Power Supply carries the amps/volts to the parts and bath combination. Parts
are hooked up to the negative (-) cables (cathode) and the screens are hooked to the positive
(+) cables (anodes). The platinum in the bath is transferred from bath to the screens (anodes)
to the cathode (parts). Sometimes individual screens are used, this is to make sure the transfer
of platinum is distributed evenly. Ideal would be to plate a flat part, easy to keep even..but the
more twist in a part the more uneven the platinum is transferred. The ion (platinum) transfer
builds up on a trailing / leading edge of a part if not placed evenly to the anodes. If can not be
evenly placed, then an individual anode screen is placed in front of the part to take the place of
the large anode screen to evenly distribute the platinum.
Plating: Process The parts are ready to be platinum plated. Parts are fixtured, then
placed in the DI water (cold) tank to rinse the parts of any loose contaminants that may have
occurred during previous steps since grit blast. The parts are then placed into the heated
platinum solution. The connection clips for the power supply are attached per instructions.
Power is turned on and timer started. A visual small reaction, bubbling, next to the parts will
take place where the parts are in the bath. This is a visual that the power is connected to each
part. If part or parts are not bubbling, then a connection may be bad. After the timer is
complete, parts are platinum plated, the parts are placed in the hot DI water to rinse the
residual platinum off. As parts are pulled from hot DI water, the parts will dry quickly because
of the water temperature. Lightly compressed air dry if parts have internal passages.
(Plating) Temperature: If temperature is too high or too low the part will not plate
efficiently or plate at all.
(Plating) pH: If the pH is too high or too low the part will not plate efficiently or plate at
all. Out of range pH can cause spotty plating results on the part.
De-Mask: Post Platinumm Components are visually checked to verify they have been
plated. Then, the components are taken off fixture and if masked prior to plating, De-Mask
post plating.
Plating: Serial Number / Post - Weight Gain Components are weighed by serial
number post plating. The Final weight will be recorded in the serial number list. This is used to
document the part has the correct amount of platinum plating. Parts that are above or below
the weight gain window, will be placed on a rework routing to achieve acceptable thickness.
Too Low – add platinum to be in weight gain window by re working the part back
through the plating process. Adjusting time to only add platinum desired to meet specified
weight gain.
Too High – depending on how much the platinum weight Is over will determine the re
work. The part can be accepted on deviation or the part can be re worked by grit blasting the
platinum off that part before diffusion heat treat cycle.
Post Plating Diffusion: Place parts in trays and diffuse at required temperature / work
instruction. Typical platinum diffusion cycle is 1900F 1 hour in a vacuum furnace.
Coating Microstructures: