标题: 一周年庆典－－HOT DIP GALVANISING and related techniques [打印本页] 作者: BMA004 时间: 2008-6-21 10:05 标题: 一周年庆典－－HOT DIP GALVANISING and related techniques HOT DIP GALVANISING and related techniques
ELECTROPLATING
Electroplating is the deposition of a metal onto a metallic surface from a solution by electrolysis. It is used for purposes of decoration and protection of the metal. Metals commonly used to plate surfaces are silver, nickel, chromium, cadmium, zinc, gold, and copper. In the case of copper plating, for example, electroplating takes place by means of the reaction in which a copper ion is carried to the metal surface to be plated, known as the cathode, from the source of the metal being plated, known as the anode.
The ion is forced to the cathode by an external source of electricity. The electrolytic solution is a salt of the metal being plated; in the case of copper plating, it is copper sulfate. This process is called electrodepositing.
The appearance, adhesion, porosity, and protection value of electroplated coatings depend on several things, including the type of base metal to be plated, the preparation of the metal for plating, and the electrodepositing process itself.
Base metal
The composition of the base metal is usually known and controlled. Applying high-quality coatings to metals of poor quality is not possible. The condition of the surface, including the degree and nature of the polishing processes and the presence of pores, cracks, or inclusions, may also have a direct bearing on the character and porosity of the coatings.
Preparation
Having the base metal surface chemically clean to secure good adhesion of deposits is necessary. The preparation usually involves cleaning, that is, removal of grease and foreign particles, and pickling in the case of steel, or dipping in the case of nonferrous metals, to remove oxides or other compounds, and sometimes, to etch the surface. Because of a thin, natural oxide film, aluminum alloys do not usually respond to the cleaning and surface preparation treatments employed for other metals; special techniques must be used for such alloys.
Electrodepositing
In the plating process many variables are involved that are subject to control. The composition and temperature of the electrolytic bath itself must be closely maintained. Agitation of the bath often permits an increase in current density. Filtration, either continuous or intermittent, is often used to keep the solution free of suspended matter. The average current density should be kept within desired limits for the sake of uniformity in thickness and quality of coatings.
Finally, the anodes should ordinarily be of such composition and structure as to maintain the metal concentration and the pH-value of the solution. Where insoluble anodes are used, the metal content is maintained by replenishment.
Most commercial plating is conducted with motor-generator sets. Usually with potentials of 6—12 V, and with current outputs, depending on the actual metal to be plated and the current density.
HOT DIP GALVANISING
By hot dip galvanizing we understand immersion of the object to be galvanized in a bath of molten metal. This metal must have a lower melting temperature than the object to be dipped.
Before immersion in the bath of molten metal, all areas on the object must be cleaned and degreased. Cleaning and degreasing of metal surfaces is of prime importance to the subsequent treatment methods, whether these are metallic or organic coatings like, e.g. paint, lacquers or powder. The metal substrate, which is to be coated usually, has a number of contaminants on its surface, and these contaminants must be removed. These contaminants are often complex and consisting of both organic and inorganic substances.
The organic contaminants may be mineral oils mixed with animal and/or vegetable oils, fats and grease or organic metal compounds. Emulsifiable cutting oils may contain tars and/or sulfonated oils. The inorganic contaminants may be metal particles melted onto the surface during machining, welding or grinding. Furthermore there may be sand, dust or remnants from pervious cleaning or pickling processes. Not all coating processes demand equally clean surfaces. Acidic or neutral plating baths demand a very high degree of cleanliness, whereas alkaline baths like, e.g. zinc baths, will tolerate a certain amount of contamination.
Emulsifiable degreasing
The most common types of emulsifiable degreasers are self-emulsifying. Emulsifiable degreasers consist of solvents with varying aromatics content to which emulgators and wetting agents have been added. Ideally an aromatics content of 80 – 90% and a boiling range of 150 – 200℃ should be used. The solvent will dissolve the oil and grease between the solid particles and the surfactants will attract grease, oil and the solid particles. After the contaminant layer is saturated by the degreaser, all areas with degreaser either are high pressure washed or flushed with water. The oily film and the solid particles are washed away with the water. Degreasing by means of emulsifiable, solvent-based products is a very effective method for getting rid of the major part of the contaminants and is often used as pre-degreasing and cleaning.
After degreasing there remains a thin oil film on the surface, which has to be removed before galvanizing. This is usually done by means of an alkaline degreaser.
Alkaline degreasing
Alkaline degreasing is conducted in large dipping baths or in specially constructed degreasing and cleaning plants where the degreaser is applied through nozzles and flush down the object. The degreasers are very different in composition and must be selected from the metal to be cleaned and the type of contaminants, which are to be removed. It is vital that the degreaser does not contain substances capable of attacking the metal substrate and damaging this.
Alkaline degreasers consist of phosphates, hydroxides, silicates and various types of emulgators and wetting agents. Alkaline degreasers for aluminum, zinc and cadmium consist mainly of alkali-phosphates but also inhibited hydroxides. The degreasing power may be increased through addition of pyro-phosphates. Alkaline degreasers for steel commonly have a high content of silicates together with phosphates and hydroxides.
The degreasers used for steel are usually too strong for aluminum, zinc and cadmium.
Acid pickling
Acid pickling comprises removal of mill scale and/or oxide film from the steel surface. This is of prime importance for the adhesion of subsequent coating systems. All oil and grease must be removed from the surface before acid pickling due to normal acids not dissolving oil and grease.
The pickling bath composition varies according to the base metal, the thickness and structure of the oxide film. Usually acids or aquatic solutions of acids are used, however, alkaline solutions are also in use for, e.g. aluminum. The pickling baths normally contain inhibitors in order not to have acid attacks on the base metal.
After pickling the substrate has to be passivated. This is normally done in a passivating bath of water corrosion inhibitors added.
Hot dipping
The cleaned and pickled object is immersed in a bath of molten metal, usually zinc as concerns steel objects, however, tin is used in some special cases. Coatings from hot dipping are usually ductile.
Hot dipping is usually specified when a thick coat without narrow tolerances as to film thickness and evenness is needed. The objects must not be too complex in shape and not contain pockets where the zinc is hindered from penetrating as such areas will not be properly protected and usually lead to premature corrosion.
The thickness of the zinc film may be varied by keeping the object immersed for shorter or longer periods. The speed of removing the object from the bath and the temperature of the molten zinc also have an influence on the film thickness.
Looking at the hot dip galvanizing process schematically we get the following:
Pre-degreasing and cleaning with emulsifiable solvents
Degreasing with alkaline degreasers
Acid pickling
passivating
Hot dipping
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Hot dip galvanizing is used for corrosion protection not only for large objects, but also for large amounts of small objects like, e.g. nuts and bolts, mounts and supports. The advantage of hot dip galvanizing lies in the process being relatively simple and low in cost.