discusses criticalities associated with surface treatment pre -rocesses of

1. ahmadkhan_99@hotmail.com
1

2. Sequence
 Mechanical Preparation
 Blast Finishing
 Shot Peening
 Mass Finishing
 Polishing
 Buffing
 Chemical Preparation
 Pre-process Baking
 Preheating
 Effects of Discontinuity in process
 Effects of Poor Cleaning process

3. Mechanical Preparation
 Physical removal of soils, scales, or films from the work
surface by means of abrasives or similar mechanical
action
 It serves other functions such as deburring, improving
surface finish

4. Mechanical Preparation
 Processes
– Blast finishing High velocity impact of particulate media
(Hard: Al2O3 and SiC and Soft: nylon beads) propelled by
pressurized air or centrifugal force to clean and finish a
surface
• Most well-known method, sand blasting, uses grits of
sand as the blasting media
– Shot peening High velocity stream of small cast steel
pellets (called shot) is directed at a metallic surface to cold
work and induce compressive stresses into surface layers
• Used primarily to improve fatigue strength of metal parts
surface cleaning is accomplished as a byproduct of the
operation

5. Mechanical Preparation
 Processes
– Mass finishing processes Finishing parts in bulk
by a mixing action in a container with an abrasive media
for the desired finishing action. Usually for small parts
– Polishing It involves removal of small amounts of
metal by means of abrasives. It produces a surface that is
free of larger imperfections left by grinding etc and is
preliminary to buffing
– Buffing It is similar to polishing except it employs
finer abrasives and can produce extremely smooth, even
mirror surface

6. Chemical Preparation
 Parts as received in plating section simply cannot be
introduced into an electroplating solution without
pretreatment of some kind
 The electroplat (finish) is expected to be adherent to the
substrate, and such adhesion will not be attained unless
the parts are reasonably cleaned before entering the
plating baths

7. Chemical Preparation
 Parts must be cleaned chemically/mechanically many
times to remove films, oil, dirt and contaminants during
production
 Reasons for cleaning
• Prepare the surface for subsequent processing
(coating or adhesive bonding)
• Improve hygiene conditions for workers and
customers
• Remove contaminants (shop soil and oxide films,
quenching oils, heat scales, stamping & die lubricants)
that may react with the surface
• Enhance appearance and performance of a product

8. Chemical Preparation
 Important Factors in Selecting a Cleaning Method
• Contaminant to be removed
• Degree of cleanliness required
• Substrate material to be cleaned
• Purpose of the cleaning
• Size and geometry of the part
 The design of a process is linked with / dependent upon
various pre & post activities
THUS modifying / Omitting any step in the process
can have adverse effects on entire process

9. Chemical Preparation
Contaminants’ Origin
Various contaminants build up on part surfaces either due
to previous processing or factory environment
• Principal surface contaminants are found in the
factory:
− Oil and grease, e.g., lubricants in metalworking
− Solid particles such as metal chips, abrasive grits,
shop dirt, dust, etc.
− Residual of buffing and polishing compounds
− Oxide films, rust, and scale present in shop
environment

10. Chemical Preparation
Chemical Cleaning Processes
• Alkaline cleaning
• Solvent cleaning
• Acid cleaning
• Emulsion cleaning
• Ultrasonic cleaning
Ultrasonic cleaning uses high-frequency mechanical
vibrations combined with chemical

11. Chemical Preparation
Alkaline Cleaning
 Uses an alkali to remove oils, grease, wax, and various
types of particles (metal chips, silica, light scale) from a
metallic surface
 Cleaning methods: immersion or spraying, usually at
temperatures of 50-95 C (120-200 F), followed by
water rinse to remove residue

12. Chemical Preparation
Solvent Cleaning
 Organic soils such as oil and grease are removed from a
metallic surface by means of chemicals that dissolve the
soils
• Common application techniques: hand-wiping,
immersion, spraying, Vapor degreasing (discontinued)

13. Chemical Preparation
Acid Cleaning
 Removes oils and light oxides from metal surfaces using
acid solutions combined with water-miscible solvents,
wetting and emulsifying agents
• Common application techniques: soaking, spraying,
or manual brushing or wiping carried out at ambient or
elevated temperatures
• Cleaning acids include hydrochloric (HCl), nitric
(HNO3), phosphoric (H3PO4), and sulfuric (H2SO4)

14. Chemical Preparation
Acid Pickling
 A more severe acid treatment to remove thicker oxides,
rusts, and scales
• The distinction between acid cleaning and acid
pickling is a matter of degree
• Generally results in some etching of the metallic
surface which serves to improve organic paint
adhesion

15. Chemical Preparation
Emulsion Cleaning
 Uses organic solvents (oils) dispersed in an aqueous
solution
• The use of suitable emulsifiers (soaps) results in a
two-phase cleaning fluid (oil-in-water), which functions
by dissolving or emulsifying the soils on the part
surface
• Can be used on either metal or nonmetallic parts
• Must be followed by alkaline cleaning to eliminate all
residues of the organic solvent prior to plating

16. Chemical Preparation
Ultrasonic Cleaning
 Mechanical agitation of cleaning fluid by high-frequency
vibrations (between 20 and 45 kHz) to cause cavitations
(The sudden formation and collapse of low-pressure
bubbles in liquids by means of mechanical forces) that
scrub the surface
• Combines chemical cleaning and mechanical
agitation of the cleaning fluid
• Cleaning fluid is generally an aqueous solution
containing alkaline detergents
• Highly effective for removing surface contaminants

17. Chemical Preparation
During Cleaning process design, the substrate material is
critically considered, so that damaging reactions are not caused
by the cleaning chemicals
− Examples:
- Aluminum is dissolved by most acids and alkalis
- Steels are resistant to alkalis but react with virtually
all acids
“Don’t clean because Mom told you to
− Unless Mom runs the plating shop”
CLEANING IS A PROCESS

18. Pre-process Baking
Baking is carried out as a pre-process so that the
induced stresses (due to earlier operations of machining,
grinding etc) are relieved
These stresses, if not removed, can reach to part’s
critical stress level due to further stress aggravation
during plating processes (the embrittlement
phenomenon)

19. Preheating
Preheating, high temperature cleaning, hot water
rinse/dip is also a critical prior to certain plating baths
which have high temperature solution
Many platers are not aware of how important preheating
can be to the quality of the final deposit. If parts enter the
high temperature plating bath not at approximately the
same temperature; non-adhesion and many other defects
can result.

20. Effects of Discontinuity in Process
 The cleaning and activation steps must account for the fact
that surface oxide re-forms at different rates on different
metals
 Specifically, in case of iron or nickel the oxide re-forms
slowly enough that the part can be transferred from a
cleaning solution to a plating bath at a normal rate
 In case of aluminum or magnesium the oxide re-forms very
fast such that special processing steps are required to
preserve the metal surface while it is being transferred to
electroplating

21. Effects of Discontinuity in Process
 Discontinuity in Intra-cleaning, Inter-cleaning, Cleaning and
Plating processes has aggravated effects on metal
substrate
 During Mechanical cleaning (buffing, polishing),
residual polishing wax, adhesives' layer remain
available on the substrate, if not subjected to next
cleaning process within certain period of time, the
residue layer will form oxides or hard scales in case of
prolonged discontinuity e.g Indium plating of rotor


22. Effects of Poor Cleaning Process
 Plating defects
 Non-adherent deposits
 Discolored deposits
 Pin holes, peeling off coating film
 Contamination of subsequent process step
“When trouble arise in a plating operation, the
experienced Electro-plater always tend to suspect that
the root of problem is Pre-plating Cycle i-e Cleaning”

23. Effects of Poor Cleaning Process
 It is expensive and wasteful to find out the reasons, only
after the whole plating sequence has been accomplished
 Cleanliness tests which can be applied before the part
enters plating sequence are helpful
 Water Break Test
Clean metals are hydrophilic and will shed water in an
unbroken sheet, whereas if traces of soil remains on the surface,
water will run off, leaving un-wetted areas, which are easily
observed
 Wiping Method
 The surface is wiped with a clean white cloth, amount of
soil absorbed by the cloth is observed; Non-quantitative but easy