December 2011 Archives

This cleaning method utilizes the chemical reactions of organic and inorganic chemicals to dissolve and remove soiling on metal materials.

1) The subject soils are the acid/alkali soluble type as follows.

1. Oxidation scales resulting from high temperature processes.
2. Rust removal as a pre-process for metal surface treatment.
3. Pre and post cleaning processes for welding, soldering and brazing.
4. Coating removal of passivated aluminum, magnesium and stainless steel.
5. Burr and metal chip removal.
6. Sludge and hard water scale removal of boilers and heat exchangers.

2) Acids and salts used

1. Acids:
   The acids react with oxides and sulfides turning them into water soluble salts. Sulfuric acid, hydrochloric acid, nitric acid and chromic acid are used. In order to promote moisturizing, foaming and to prevent Corrosions add surface active agent
2. Alkali:
   Alkaline rust removers such as gluconic acid, citric acid, thioglycolic acid salt, ethylenediaminetetraacetic acid (EDTA), and etc. containing chloride complexing agents.
3. Methods
   Since the process relies on chemical reactions, the agents are used in various water solution forms in submerged, sprayed ([Fig.1]), tumbled (in barrels), electrolytic cleaned (anodic/cathodic electrolysis), wiped on, ultrasonic cleaned (ultrasonic vibrations used while submerged), and etc. In order to accelerate the chemical reactions, the solutions are heated (to 40~60°C).
4. Characteristics
   
   1. Alkali cleaning and anodic electrolysis cleaning do not cause hydrogen embrittlement.
   2. Less metal losses compared to sand blasting.
   3. Widely applied as pre-processes for surface treating.
   4. In order to prevent excessive metal dissolution, addition of corrosion inhibitors is required.

1) Solvent types

1. Aliphatic hydrocarbons such as petroleum solvents Benzene, Toluene, Kerosene.
2. Aromatic hydrocarbons such as coal tar solvents
3. Non-combustible solvents such as Chlorinated hydrocarbons, Fluorinated chlorinated hydrocarbons and trichloroethylene
4. Polar solvents such as Ketone, Alcohol, and Phenol
5. Emulsification solvents, biphasic solvents, emulsion cleaners, etc.
6. Others

2) Application methods

1. Immersion (immersion cleaning, solvent heated and used as warm liquid)
2. Immersion and mechanical method combined (work being rotated/vibrated, solvent flowed
3. Spray cleaning
4. Steam cleaning (Dissolving power is increased when the solvents are vaporized but will require vapor recovery system) Shown in [Fig.1]
5. Ultrasonic cleaning (An oscillator is placed in the immersion tank and the solvent is vibrated)

3) Applicable areas

1. Rough cleaning of heavy soiling
2. Pre-cleaning for pre-processes
3. When water based cleaning solution cannot be used
4. When hydrophilic surfaces is not needed
5. When rapid cleaning is needed
6. Precision cleaning

4) Characteristic

This method has high penetration ability on soiling and effective even at low temperatures. But since the organic solvents are inflammble, safety measures for the operators and environmental concerns of volatile organic compounds will increase the costs.

1) There are following mechanical metal cleaning methods.

1. Wiping
   Manually or mechanically wipe off with cloth or buff material
2. Grinding and Machining
   Grind with buffs or belts made of cloth or paper with abrasives. Mill surfaces of cast products.
3. Blasting
   Silica sand, steel grits and glass beads are air sprayed or projected with rotating wheels (Centrifugal projection)
4. Hydro blasting
   Alumina and silicon carbide particles mixed in water are sprayed ([Fig.1])
5. Other, Tumbling barrels, rotary wire brush, etc.

2) Applicable areas

1. Scales from high temp. processes
2. Rust and sand removal from cast surfaces
3. Removal of high temp. baked paints
4. Various burrs and contaminants on casting molds and dies
5. Removal of rust and paints from repaired parts
6. Preparations for painting, thermal spraying and Teflon coating, decorative surface roughing
7. Partial repairs of other surface finishing processes

3) Characteristics

This method is characterized by its ability to: completely process, wide applicability, partially applicable, and fast processing speed. Other than cleaning, the process can also be applied to removal of surface defects and decorative processes.