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Surface preparation

Dr. Dmitri Kopeliovich

Surface preparation is a series of cleaning treatments of the substrate surface prior to the coating operation intended for ensuring strong and uniform adhesion of the coating to the substrate.

Adhesion is a state of bonding a coating to the substrate surface.

Bonding strength (adhesive strength) is a tensile stress (tensile load applied to the unit bonding area) required to peel the coating from the substrate surface.

Contaminants to be removed from the part surface

  • polishing and buffing compounds;
  • miscellaneous solid particles (dust, abrasive grits, chips);
  • oxides, scale, smut, rust.

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Technologies of surface preparation

1. Mechanical cleaning

  • Ultrasonic cleaning – scrubbing action produced by numerous small vacuum cavities forming as a result of high frequency (20 – 45 kHz) sound waves traveling in a fluid, in which the part is submerged.

As a cleaning fluid aqueous solution of alkaline detergents at room temperature is used.

Ultrasonic cleaning is highly effective in removing solid particles, dirt and smut.

  • Spray cleaning – cleaning by aqueous solutions (alkaline or mild acids) delivered to the part surface through spray nozzles at high pressure.

2. Soak cleaning

  • Solvent cleaning – removal of organic soils (mineral oils, fingerprints etc.) by dissolving them in a solvent.

The solvent may be used in the liquid state, when the part is either immersed into the solvent or sprayed by it.

The solvent may also be used in the gaseous state (vapor degreasing) when the soils are dissolved by the solvent vapors.

Use of some solvents like trichloroethylene and trichlorathane (chlorinated hydrocarbons) is forbidden or restricted in most of the world due to their adverse effect on the human health.

  • Emulsion cleaning – removal of soils by organic solvents dispersed in aqueous solution, containing emulsifiers (surface active substance preventing coalescence of the dispersed component).

Emulsion cleaning is followed by alkaline cleaning to remove the organic components of the emulsion from the part surface prior to coating.

  • Alkaline cleaning – removal of organic soils (mineral oils,fingerprints, wax etc.) and some solid particles by hot alkaline solutions.

The application methods are either immersion or spraying, followed by water rinse.

The work temperature of alkaline solutions is 130-190°F (50–84°C).

3. Electrocleaning

Electrocleaning is an electrolytic process conducted in an alkaline electrolyte, through which a direct current is passed.

The cleaned part is connected as either the anode or the cathode.

Electrocleaning combines effects of the soak alkaline cleaning and mechanical cleaning provided by gas bubbles, which form on the part surface as a result of the electrochemical reaction.

Electrocleaning is able to remove organic soils impregnated into the part surface, solid particles adhered to the surface and oxides.

Electrocleaning is followed by water rinse and then acid activation.

The following electrocleaning methods are used:

  • Anodic electrocleaning (reverse electrocleaning).

In this method the part is connected to the positive side of the rectifier.

The following reaction takes place at the anode surface:

4[OH]ˉ =2H2O + O2 + 2eˉ

Oxygen bubbles are liberated at the part surface assisting cleaning effect.

  • Cathodic electrocleaning (direct electrocleaning).

In this method the part is connected to the negative side of the rectifier.

The following reaction takes place at the anode surface:

4H2O + 4eˉ = 4[OH]ˉ +2H2

Hydrogen bubbles (in a quantity twice as much as oxygen quantity)are liberated at the part surface.

Cathodic electrocleaning is more effective than anodic electrocleaning due to the more intensive gas liberation.

Disadvantages of the cathodic electrocleaning are: possible impurities deposition on the part surface and hydrogen embrittlement of the part, caused by diffusion of hydrogen.

  • Periodic-reverse electrocleaning.

In this method the part is alternatively connected anodically and cathodically at controlled time intervals.

The method combines the advantages of the both: anodic and cathodic electrocleaning.

4. Acid activation.

Acid treatment is used to remove oxides, scales and activate the metal surface of the part prior to coating.

Mild acids (citric, phosphoric) are used for activation of aluminum, zinc castings and other sensitive alloys.

Strong acids (hydrochloric, sulfuric and nitric) are used for activation of steels and stainless steels.

Acid salts are used for more controllable and safe activation process.

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surface_preparation.txt · Last modified: 2014/05/03 by dmitri_kopeliovich
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