Surface - coatings, cleaning and heat treatment

Surface Coatings

an application of a thin film of protective material to the surface of an object

Purposes of surface coatings

• Provide corrosion protection

• Enhance product appearance

• increase wear resistance and/or reduce friction of
  there surface.

• increase electrical conductivity

• increase electrical resistance

• prepare a metallic surface for subsequent processing

• rebuild surfaces worn or eroded during service

  
  

Types of Coatings

• Electroplating (electrochemical plating)

• Anodizing

• Porcelain enameling

• Hot dipping

• Painting

Electroplating

process of covering the surface of an object with metal
(plating)

Electroplating terms - Electric current

transfer ions of one metal on to the surface of
another material

Electroplating terms - anode

is generally made of the metal being plated

Electroplating terms - cathode

is the work part.

Electroplating terms - direct current

from an external power supply is passed between
the anode and the cathode

• Improve Appearance
• Increase Corrosion Protection
• Increase Wear Protection

Purposes of Electroplating

Anodizing

process to increase the thickness of a metal’s natural oxide surface

• It works best on Aluminum metal.

1. The aluminum is put into an acid electrolyte bath.
2. An electric current through the medium.
3. Oxygen ions from the electrolyte combine with the aluminum atoms
at the surface creating a protective layer of Al2O3

• Increased resistance to corrosion

• Increased resistance to wear

• Increased surface toughness

• Improved surface appearance

• Improved painting results

Anodizing Advantages

Porcelain Enameling

• is a material made by melting powdered glass on to a surface

• Powered glass is applied to a surface and then fired
  until the powder melts and then hardens to a smooth,
  durable coating (750 - 850 °C)

• Enamel can be used on metal and ceramics or any
  material that will withstand the melting temperature

1. Corrosion
2. chemical attack
3. physical damage
4. improve the appearance of the product.

Purpose of Porcelain Enameling

Hot Dipping

• process in which a metal substrate is immersed in a molten bath of a second meta

• The primary purpose of hot dipping is corrosion resistance.

• Coating thickness is typically 0.04 to 0.09 mm (0.0016–0.0035
  in)

• Bath temperature is maintained at around 450C (850F).

Even if the zinc coating is scratched, the exposed steel will still
be protected from corrosion by zinc forming a coating of zinc
oxide.

• This makes galvanization better than paint and powder coating

Advantages of Hot Dipping

1. Prevents corrosion
2. Low cost
3. Easy application
4. Long maintenance-free service life

Advantages of hot dipping (list)

Painting

• decorative

• corrosion resistance

Organic coatings

are polymers and resins, produced either naturally or synthetically, usually formulated to be applied as liquids that dry or harden as thin surface films on substrate materials

binders

are polymers and resins that determine the solid-state properties of the coating, such as strength, physical properties, and adhesion to the substrate surface most common binders are natural oils, resins polyurethan, epoxies, acrylic, and cellulosic.

Dyes

are soluble chemicals that color the coating liquid but do not conceal the surface beneath

Pigments

are solid particles of uniform, microscopic size that are dispersed in the coating liquid but insoluble in it. They not only color the coating; they also hide the surface below

Solvents

are used to dissolve the binder and certain other ingredients in the liquid coating composition

Additives

in organic coatings include surfactants (to facilitate spreading on the surface), biocides and fungicides, thickeners, freeze/thaw stabilizers, heat and light stabilizers, coalescing agents,
plasticizers, defoamers, and catalysts to promote cross-linking.

1. Dip coating (liquid)
2. Spray coating (liquid)
3. Powder coating (powder)
4. Applicator coating – brushes or rollers

Methods of paint application

Dip coating

Submerges the whole part in the paint to form a thin uniform layer coat on all surfaces.

• Used more used for a protective coat than decorative.

Spray painting

is a painting technique where a device sprays a coating of paint through the air onto a surface.

Powder coating

is a coating that is applied as a dry powder.

1. The powder is applied and is cured under heat to allow
it to flow and form a "skin".
2. The powder may be a thermoplastic or a thermoset
polymer.
3. It makes a hard finish that is tougher than conventional
paint.

• mainly used for coating of metals, such
  as appliances, hardware, automobile and bicycle parts

Surface cleaning

Almost all parts must be cleaned one or more times during manufacturing.

Processes used to clean the work surfaces:

1.Chemical cleaning methods: Use chemicals to remove unwanted contaminants from the work surface

2.Mechanical cleaning: Involves removal of contaminants by various mechanical operations

1.Prepare surface for processing, such as applying a coating or  adhesive

2.Remove contaminants that might react with the surface

3.Improve appearance of the product

Reasons Why Parts Must be Cleaned

Chemical Cleaning Processes

1.Solvent cleaning

2.Alkaline cleaning

3.Emulsion cleaning

4.Acid cleaning

5. Ultrasonic cleaning

Solvent Cleaning

1.Oil and grease are removed from a metallic surface by chemicals that dissolves them.

Common application techniques:

1.hand‑wiping

2.immersion

3.Spraying

4. Vapor degreasing.

Alkaline Cleaning

Alkaline cleaners contain strong bases like sodium hydroxide or potassium hydroxide.

Uses an alkali solvent to remove:

1.Oils

2.Grease

3.Wax

4.Metal chips

Acid Pickling

is a heavy duty metal surface treatment used to remove oxides, rusts, and scale.

Acid pickling is a deeper removal process compared to acid cleaning

Mechanical Surface Cleaning

Mechanical cleaning is the removal of soils, scales, or films from surfaces by using abrasives or similar mechanical action

 

Mechanical cleaning processes:

1.Blast finishing and shot peening

2.Mass finishing processes

Blast Finishing

•Is the high velocity impact of particles to clean a surface.

•The particles are projected at the target surface by pressurized air or centrifugal force.

•Well known is sand blasting using SiO2

•Alumina, and silicon carbide, nylon, and crushed nut shell can be used.

•In shot peening: Small cast steel balls

      Are used.

•Shot peening is used to improve

      fatigue strength

Mass Finishing

Many parts in are put in a container with an abrasive media

Mixing causes parts to rub against media and each other to achieve desired finishing action

Good for cleaning small parts