Post 571 –by Gautam Shah
Metals parts need surface treatments and cleaning for conservation, restoration, decoration, reflectivity (shine), dulling (removal of gloss), better hardness, surface integrity, prevention of corrosion and in preparation for the next treatment. Metal components get both organic and inorganic substances deposited from various lubricating oils, corroding environments and substances, and deposits from hard water etc. Metal surfaces need cleaning for removal of residual products from earlier processing, weathering, depositions during storage and transport and environmental contaminations. A cleaned metal is comparatively more active, and so may need, immediate next treatment, to prevent corrosion and contamination. So metal surfaces often receive simultaneously the processes of cleaning, surface activation and protection.
Metal surfaces needs to be smoothed, scrubbed or scoured to receive next set of treatments. Cleaning processes are employed to discharge electrical ions from the component. Cleaning processes are designed to serve several purposes, such as cleaning, scoring and protective coating. Cleaning occurs at a raw material stage, product formation stage, in preparation to other finishes, or just prior to a marketable finish. But there are two distinct stages, at manufacturing plant and on-site applications.
Alkali cleaning is typically done with caustic soda (NaOH) which removes all polar substances such as fats and oils. The vegetable and animal oils are converted to glycerol and soap, and the petroleum-based oils and grease are emulsified.
Acid cleaning, like alkali removes ionic deposits from the metal surface. Depending on the secondary products like salts determine the quality of acid to be used. Pickling process removes oxides from the surface by a dissolving liquid. For steel, a warm dilute sulphuric acid, and in some cases hydrochloric acid, hot or at normal temperature, is used. However, for some alloy steels nitric acid, is needed. Acid and alkali both processes require post treatment ph. balancing rinse or treatment.
Solvent cleaning: the articles are washed with a petroleum solvent or undergo cleaning (de-greasing) by vapour, in which a solvent such as tri-or tetra chloroethylene is heated in a closed system, and its vapours are condensed on the metal surface.
Emulsion cleaning: the metal parts are immersed in a warm mixture of kerosene, a wetting agent, and an alkaline solution.
Mechanical cleaning use force of air or water jet and sonar waves to remove partially attached particles. The process may include fine particles like sand, silica, metal grit etc. to blast the surface for the same purpose.
Electrolytic cleaning: the articles are immersed in an alkaline solution and a direct current is passed between them and the other electrode which is usually steel.
Ultrasonic cleaning is used for blind holes or gears packed with soils etc. Small particles embedded in crevices get detached due to ultrasonic waves.
Plasma treatments bombard a surface with electron ions to break the surface level chemical bonds and change the chemical composition of the surface. Plasma treatments may also remove variety of material types such as paints, polymers, glass and ceramics.
Metal cleaning Tools and Techniques
The metal products are small items like nails, rivets, long rolled sections, drawn items like wires, woven items like lattices, entwined items like ropes and barbed fencing, hollow pipes and tubes, or solid castings. Other products include assemblies like automobile bodies, boxes, cages, machine housings and structures like a truss. Metal components are single or multi-metal items, composites with polymers, ceramics, etc. Art restorers have to deal with metal objects of unknown composition, ageing, and coated with natural or applied substances. Art restorations require reversible processes, so if a treatment is found unviable it can be fully reverted. Metal products in continuous line productions are moved through several sets of electrodes, which, submerged in a cleaning liquid, electrolytically generate hydrogen gas at the steel surface for lifting residues off the material.
Hard substances like metal nodules or slags are removed by abrasion or high pressure air or water jet. Soft materials like soil etc. are removed by brushing. Brushing may be carried out dry or wet and with scrubbing and scourager agents. Brushes with natural hair, synthetic fiber, metal fibres and wires are used. Abrasive materials like A Carborundum, sand grit, metal particles etc. are blasted on the surface to clean up the surface. In some instances like manufacturing of nails, abrasive materials rotated in ball mills with nails, to achieve high polish. Ultrasonic sound waves are also used to remove particles.