Electro Cleaning Work

The purpose of this vital step is to remove surface contaminant of various types which would otherwise prevent proper adhesion of metal plating to the surface and/or cause the finish obtained by electroplating to lift oft completely.

The required mechanical surface preparation is needed such as machining, grinding, sanding and buffing, the metal may look perfectly clean but it isn't, to see the cleanness of metal should be chemically and microscopically clean.

The next step is electrocleaning work where we make an apparently clean part even cleaner. Without this step, plating problem such as bare spots, poor or no adhesion, or even no deposit may develop.

The process electrocleaning is simple enough, prepare one gallon container, mix 2 to 10 ounces of caustic soda or sodium hydroxide with one gallon of warm water. If you're operating from a pint sized stainless steel container, that's OK because the solution grows dirty in use and can be replenished from the handy pre-mixed plastic one gallon container of fresh solution.

Cathodic Cleansing of Tin, Brass, Aluminum, Magnesium, Nickel and Chrome

In this process of cathodic cleaning of tin, brass, aluminum, magnesium, nickel and chrome, the part will be suspended in the above mentioned alkaline solution at room temperature, connected to the negative DC supply terminal. The positive DC terminal will be connected to the stainless steel container.

Increase DC current through the both until the part is gassing freely (generating hydrogen). Maintain this for one to two minutes. This bubble scrubbing action will also assist in the cleaning process.

The DC current will be in the range of 1 A to 1 Amp/sq.in. For an applied voltage of 3 to 1.2 VDC-not critical, just enough current to induce free gassing at the cathode connected work-piece. Exercises current will cause localized surface burning, so a little restrain is called for when cranking up the current to obtain the free gassing.

Ventilation to remove gas hydrogen that generated is mandatory because this stuff has a well earned reputation of going off with a bang (exploding) if given the chance to accumulation anywhere. One further note on safety concern the poisonous/corrosive nature of many of the nasty chemicals used. Also disconnected the DC supply at the power controller before making or breaking electrical contact at the working end. This will avoid sparking around the bath and the possible accidental ignition of the gasses generated.

Hot Diffusion Processes

There are a number of processes in use on a limited scale in which steel articles are coated with another metal by being heated in contact with the metal or one of its compound in the absence of air.

1. Colorizing: This process is a means of obtaining on aluminum coating on heavy steel vessels and boxes, furnace parts and other pieces which are used in heat treatment or firing of ceramic goods, or exposed in some other way continually to high temperatures in air. The aluminum coating prevents the steel scaling and wasting away and thus materially prolongs its life. The process of colorizing is carried out by prolonged heating of the steel articles at about 500^oC in a box with a mixture of aluminum powder, alumina and ammonium chloride, air being excluded. If probably functions via the formation of aluminum chloride vapor and the reaction of this with the iron in the surface layers.
2. Shrerardizing: This is a means of zinc coating small steel articles such as nuts and bolts. The coating is extremely uniform and thus, for example, the functioning of screw thread is not impaired. The articles are tumbled of some hours in zinc dust at a temperature of about 370^oC, air being excluded. The coating contains a high proportion of iron-zinc alloy; it is dark-blue in color and brittle. A normal thickness is 0.5 0z/ft² (0.0005 in).
3. Chromizing: The chlorides of a number of metals (and some non-metals) are volatile and will react with steel at elevated temperatures to form iron chloride and deposit the metal in the place of the iron removed. Thus steel articles can be chromized by heating in chromic chloride vapor. An iron-chromium alloy is formed on the surface, which has corrosion-resisting properties similar to those of stainless steel; if carbon is present, the process may be used to effect a surface hardening. The process can also be used to confer resistance to heat oxidation onto steel articles.

Electroless Plating of Nickel

Metals can be recovered in metallic form from aqueous solution of their salts by means of reducing agents. Development of photographic film and the silvering of mirrors are examples of this. The more noble metals such as gold and silver are easily turned out of the combined form: the more electronegative metals require much more powerful reducing agents. It is also essential that the metal deposits on the solid surface as a coherent and continuous film, and highly desirable that it only coats the surface desired, and not the walls of the containing vessel as well. In the silvering of glass mirrors, for example, the silver deposits on the glass but also on the walls of the containing vessel.

A process has been devised using the very powerful reducing agent, sodium hypophosphite, no produce nickel coatings on metals, and has the peculiar and advantageous property of proceeding only on nickel itself or on metals of the platinum group. Once started, it is therefore generally known by the ugly term ‘electroless’ nickel plating. The solution contains nickel sulfate, chloride or acetate, sodium hypophosphite and an organic acid, such as nitric acid or lactic acid to control the change of pH value, which has maintained at about pH 4.0. The solution is operated at about 90 oC and deposit nickel at the rate of about 0.0008 in/h. In due course the solution become spent and must be renewed. The formation of the nickel coating on a different metal such as steel can be ensured by a prior treatment of the surface with a dilute palladium chloride solution; a thin film of palladium is formed by chemical replacement which catalyses the subsequent reduction of the nickel salt by the hypophosphate.

The metallic deposit has an appreciable content of phosporus. It is smooth and often lustrous, very fine grained and extremely hard (500 Vickers pyramid number and indentation measure of hardness), it is very brittle. Since the process is chemical and autocatalytic, the thickness of the deposit can be uniform even on the most irregular and re-entrant areas. The process is naturally rather expensive, but it is uniquely suited to coating the interior of complicated chemical plant, tubes, condensers, etc., even after assembly or installation.

Cathodic Cleaning

The method of Cathode Cleaning:





Cathodic cleaning results in the liberation of hydrogen; at a given current density twice as much hydrogen is evolved as is oxygen at the anode. The cathodic cleaning is assisted by the repulsion of the negatively charged particles of dirt, in these strongly alkaline solutions most colloidal particles carry a negative charge. The negatively charged work will also attract positively charged ions such as copper, zinc, or other metal ions which are discharged at the surface, forming a metal smut. For this reason different solution tanks should be used when nonferrous metals are being cleaned by both cathodic and anodic cleaning. The principle reaction taking place at the cathode is the liberation of hydrogen, which may penetrate the metal and become occluded or absorbed by it. Under these conditions steel may become very brittle. Nickel surfaces and nickel base alloys must be electrocleaned cathodically to avoid passivation of the surface. The passivated surfaces will not plate satisfactorily.

Anodic cleaning results in only half the gas liberation that is obtained with cathodic cleaning at the same current density. While it charged anodically, the work will rapel metallic ions. In fact, smuts formed during cathodic cleaning are dissolved or dislodged during an anodic cycle. This action contributes to the efficiency of anodic electrocleaning. Copper and copper base alloy should be cleaned anodically for only short periods unless specially inhibited cleaners are used; otherwise corrosion or an oxidized or tarnished surface will result.

The use of periodic reverse current with alkaline derusting compounds for the removal of rust and scale is widely practiced. It eliminates the use of acid descalers which is not derirable for machined surface or where bleedout after acid descaling can cause problems. Oxides can be removed without etching or smut formation. Another cleaning process are Electroless Plating.

Other treatment of Cathodic Cleaning:

Ultrasonic Cleaning and Electrocleaning

Ultrasonic Cleaning
The application of energy to a cleaning solution such as an alkaline soak cleaner, a surfactant type of buffing compound remover, or a solvent, in the form of high frequency sound wave above 20,000 Hz in the inaudible range, has proved very effective for removing hard-to-remove soil from inaccessible places as crevices, blind holes, and gear teeth. Each application of ultrasonic cleaning has to be engineered and applied for the job at hand. Ultrasonics is a tool for making a good cleaner work better.

Machine Cleaning
Most rapid alkaline degreasing is accomplished by spray washing in an automatic or semiautomatic machine. In order to justify the expense of such an installation, the volume of work must be large. In spray washing machines the mechanical force of the spray in addition to the chemical and physical action of the cleaning solution speeds up the cleaning process. The important use is for precleaning of metals to remove large excesses of soils.

Electrocleaning
Although it is possible to clean work effectively in soak tanks, electrocleaning is the most reliable method of preparing surface for electroplating. In electrocleaning the work is made either cathode (direct current) or anode (reverse current); in either case gas is generated at the metal surface and assist in dislodging the soil, and at the same time brings up fresh cleaning solution.

Alkaline Soak Cleaning

The parts are immersed in tanks of hot alkaline cleaning solution. The concentration of the cleaning solution and the temperature should be maintained as high as is safe for the metal being cleaned in order that the time for complete cleaning may be reduced to the minimum. Agitation of the cleaning solution is desirable, because the cleaning action in soak cleaners depends on the wetting and gradual emulsification of oils and greases. Sometimes saponification of the soil plays a role. Dirt and other solid particles are bound to the metal surface by oils and grease and are removed when the binding material is dislodged from the surface. Heat and agitation speed up the wetting, emulsification, and saponification. The mechanical force of the moving solution is helpful in dislodging the soil; the newly formed emulsions and metal soaps are washed away from the surface which is thus constantly exposed to fresh cleaning agents.

The design of cleaning tanks is described by lux and linford and Myers. The principal points of design are as follows:

1. To improve agitation of the solution, a sheet metal shield should be installed I front of the steam heating coils to make use of convection currents. In addition, either motor driven propellers or movement of the work should be provided to ensure adequate agitation.
2. In order that there may be a quiescent layer of solution at the bottom of the tank to provide for settling of sludge, the bottom of the lowest heating coils should be at least 75 mm from the bottom of the tank.
3. An overflow dam should be provided so that oils and greases floating on the surface will be removed each time a load of work causes some of the solution to overflow into the drain. This is necessary to ensure that the cleaned work is removed through a clean surface.
4. A mesh grid false bottom is desirable to make it easy to recover parts that drop into the tank.
5. A means should be provided to control and either indicate or record the temperature.
   

Hand Cleaning

Hand Cleaning

Parts that will treatment by electroplating can be cleaned by hand cleaning method. This method if the part is not even and need special treatment by hand in order the dirt on the gap sometime can not be cleaned by chemical cleaning.

This method is usually resorted to when the volume of the work to be cleaned is small or the pieces are too large for easy handling in other types of cleaning equipment. There is no set procedure for hand cleaning; the solution is applied with brushes, swabs, or cloths. The operator should be properly protected by means of suitable goggles, gloves, and other protective clothing. The use of toxic or flammable solvents should not be permitted in hand cleaning operations.

When a shop has plating failures, one method of isolating the trouble is to hand clean a part thoroughly and then acid dip and plate it. If it plates satisfactorily, the trouble is in the cleaning cycle. If it does not plate well, the problem is in the plating bath.