Hardness Test of Coating Plate

Steel coated with chrome plating or with nickel plating will have different hardness and looking. After coating with chrome or nickel then the surface become more interesting and stand to corrode. This is the basic purpose of steel coating. The stand to corrode and how long it can be stay on the surface depend on the dept of coating and the strength of coating material on the plate surface. One of how to test the strength of this using hardness testing. This tool work is use a fixed forced and given identifier, that show the harder of surface material.

The value of hardness test is identified by measuring the depth of the area of the surface by using several methods. One method to identify of hardness is use Rockwell hardness test method, this method is the most commonly used to test hardness of steel surface.

The standard of hardness is illustrated by the most hardest material is diamond which have hardness value of 10, other material will have below this.

Soft: scale value = 1 – 3
Medium hard: scale value = 4 – 6
Hard: scale value = 7 - 10

Metal Coating Applications

Metal coating are applied in automotive area and motorcycle like on car accessories and parts of car like window handle, dashboard handle, and in many button. This coating is also apply on radio button, and other accessories parts that completed view of car. Large vehicle parts can be coated in manual varnishing cabins.

Chemical and Electrolytic Brightening

Chemical and Electrolytic Brightening of Metal

Most metals can be superficially by chemical or electrochemical attack in specially formulated solutions; this process is widely used for providing the final lustrous flat surface on metal objects. However, it has only achieved industrial importance in the treatment of brass, stainless steel and, particularly, aluminum, but don't use anodized aluminum because this metal already coated on their surface. Although these processes are frequently referred to as chemical or electrolytic polishing, this is a misnomer. They will not remove even minor irregularities of surface level or scratches in the way that mechanical polishing will. The actual effect is to convert an already smooth surface which is matt into a brilliantly lustrous one. Even so, unless the metal alloy is very homogenous, some parts become more severely attacked than others and the reflectivity of the surface after brightening may be lustrous, but will not yield sharp reflected images.

A simple process of this kind has been for brass article under the name of bright dipping. In this the degreased articles are dipped in a mixture of nitric and sulphuric acids containing a small but critical addition of hydrochloric acid. Cuprous chloride is relatively insoluble in this solution, so that when the brass is vigorously attacked by the nitric acid a very thin resistant film of cuprous chloride is formed on the surface, particularly in hollows. This delays the attack there, so that the surface becomes leveled on a micro scale. i.e. it becomes lustrous. Greatly superior results are obtained from solution containing sulphuric, nitric and phosphoric acids; the most effective formulations are different for brass and for aluminum. Since these solutions are protected by the licenses, under the trade name of Phosbrite. The solutions are used hot; they are very vigorous in action and evolve unpleasant fumes, so that fume extraction is necessary. The solutions become exhausted fairly speedily in use and are relatively expensive, because of the phosphoric acid content. Nevertheless, in the course of a 2 to 5 m dip they provide a very satisfying luster on sheet material, pressings and simple assemblies, thereby obviating expensive mechanical polishing.

See more about chrome plating process.

Specific Treatments to Aluminum Alloys

Aluminum and its alloys owe their excellent resistance to corrosive by the atmosphere, waters, foodstuffs, etc. to an oxide film on the surface of the metal, which is very rapidly reformed when the naked metal is exposed by cutting or deformation. This film is very protective and relatively pore free, but it is very thin about one millionth of an inch. It is therefore logical to devise means of artificially thickening it to provide even better resistance to corrosion. There are two main type of methods for doing this, one of which is purely chemical, the other electrochemical.

MBV Treatment
The modified Bauer Vogel process, known as the MBV process, consists of immersing the aluminum alloy parts for10 to 30 m is an almost boiling aqueous solution of 5% sodium carbonate and 1% sodium chromate. The oxide film so formed is light grey to almost black in color, depending on the aluminum alloy, but is smooth and glossy; it is only a few hundred thousandths of an inch thick. Nevertheles it has considerable corrosion resistance, and is absorbent to oil or paint so that it is an excellent preparatory process for painting aluminum alloys.

Chromium Process
Another chemical process for increasing the thickness of the oxide film employs an acid solution containing phosphoric and chromic acids and sodium floride. It is used at more moderate temperatures, (40 to 50oC) and for shorter times than the MBV process. The coating is green, brown or golden in color, quite thin and rather soft. It is principally employed as a preparatory process for painting aluminum alloys. Owing to the chromic acid content it should not be used on articles intended for use with foodstuff. This type of process is mostly used used proprietary names, such as alodine in the USA, and in GB Alocrom.

Previous Treatment: Polishing

Phosphating Step

In the operation of the process, the work must first be descale and degreased, but strongly acid or strongly alkaline cleaners leave the surface in a condition which encourages a coarsely crystaline phospate deposit, so these are usually avoided. The articles are then arised and hung in the phosphating solution. As the process is non electrolytic, close spacing is possible, and small parts can be treated in baskets or perforated containers. The solution is usually operated at 80 oC or above; treatment times of 3 to 20 m are usual. After phosphating the parts are rinsed, but residues of the solution inavertently left in joint, erevices, etc., are not corrosive. A short further dip in 0,5 % chromic acid solution is often given for further passivation, with or without a final rinse.

Since very considerable numbers of parts are processed, and these are often very large, e.g. complete car bodies, or very small, phosphating plants are frequently mechanized. In the motor car industry "Rotto-Dip" Plants are used in which complete car bodies, each skewered on a horizontal shaft, are passed through an elaborate automatic phosphating plant, and finally dipped in a bath of priming paint, suitably drained, and then stoved. The bodies are rotated continuously to ensure that every area is treated and that solution is not trapped in pockets.

Although the design and formulation of the phosphating process is complex, its control can be achieved by simple titrations with standard alkali solutions, using different indicators. The results are usually expressed as 'points', and additions of an acid phosphate mixture are made, as necessary to restore these to some determined value.

Phosphating Process

Phosphating process are used very widely in industry for three distinct purposes, for each of which a somewhat different formulation is necessary. These end uses are:

1. Application of a simple and cheap corrosion preventative for machine and mechanical steel parts. As formed, the phosphate coating is dull matt and grey black; it is unattractive in appearance and has only slight protective properties. It is, however, moderately rough and absorbent. It is therefore usually treated with an aqueous solution of a black dye and then, after drying with hot oil.
2. Rather thicker phosphate coatings are used alone or impregnated with oil to provide an exceptionally good lubricant in the processes of heavy pressing, drawing, forming and even extrusion, of steel. Similarly, they are often applied to gears and other sliding parts to reduce friction. The coating not only holds the oils, but itself prevents the metal-to-metal contact which is the cause of scoring and seizure when metal works on metal.
3. The most widespread application of phosphate coating is as a basis for industrially applied paints, lacquers and enamels. The phosphate coating greatly increases the adhesion of the organic coating to the base, and also largely prevents the lateral extension of the corrosion from inevitable points of damage, which would otherwise cause blistering of the adjacent paint film.
4. Phosphate coatings are sometimes applied to zinc-plated or galvanized steel, the zinc coating supplying even more assurance against rusting. Phosphate coatings are also applied to zinc base die-castings as an aid to paint adhesion. Thick coatings are in the range 700 to 2000 mg/ft2; light coatings are 150 to 400 mg/ft2. A process using a solution containing manganese gives thicker coatings, and is preferred for the first two application. A zinc process is more rapid, and can even be applied by spraying the liquid onto the work.

Phosphating

What is Phosphating:





There are a multitude of separate processes designed to produce thin, crystalline, adherent films of phosphates of iron the surface of steel articles; these often have an admixture of zinc and manganese phosphates. These film absorb oil readily and the oiled film confers a considerable measure of corrosion protection quite cheaply on iron and steel goods where appearance is not important. Phosphate coating are also an excellent pre-treatment for paint or stove enamel on steel, ensuring good adhesion and discouraging lateral corrosion under the paint film from points of damage. Basically this phosphating process consists of immersing the steel articles into a hot solution containing ferrous phosphate with some free phosphoric acid, and with additions of zinc and/or manganese phosphates. Phosphoric acid is tribasic, i.e. it has three replaceble hydrogen ions. The acid phosphates, in which only one or two of the hydrogen atoms are replaced, are fairly soluble in water, but the full phosphate, in which all three hydrogens are replaced by iron, is insoluble. As the phosphoric acid in the solution attacks the steel, the local acidity is reduced and a crystalline film of iron phosphate precipitate onto it.

The simple process described above is rather slow, taking up to an hour to provide a suitable coating; addition of zinc or manganese phosphates accelerates the process and manganese phosphate ensures a thicker coating. But the action can be speeded up more fully by the addition of accelerators which are slightly oxidizing chemicals such as nitrates or nitrites, or complex organic nitro compounds. The range of possible formulations and types of coating is very wide, and their chemistry complex, so that the different processes are mostly serviced by specialist companies under proprietary names, such as Coslettizing (the original, now obsolete, process). Bonderising, Granodising, Parkrising, Walterisation, etc. These companies sell the mixed salts or solutions with full instructions for their use, and often provide a supervision and chemical control service. In some cases they supply plant as well. In these circumstances the user is relieved of much of the technical difficulty and control work, and he can integrate the process into his metal manufacturing operations.

Metal Coating with Chromium

It is convenient to deal next with chromium plating because it is almost always a dirrect sequel to nickel plating, and also because the methods of chromium plating are so radically different from all other types of electroplating.

Chromium is a hard, brittle, silvery-white metal, with a faint blue color, which is not otherwise used unalloyed in industry. It is important constituent of stainless steel, both of the cutlery type (13%), and of the Austenitic ductile type (18:8 Nickel : Chromium). Chromium confers passivity to corrosion to these alloys because it automatically form a protective oxide film on their surface. In an alloy with nickel it gives resistance to oxidation at high temperatures, and is thus used for electrical heating elements. It is also an important minor constituent of many special steels and non ferrous alloys.





Although it would appear from the position of chromium in the electrochemical series that this metal is about as reactive as zinc, in practice it automatically covers itself with a protective oxide film, and then behaves like a noble metal, such as gold. It is quite unattacked by atmospheric exposure, by most oxidizing chemicals and by acids and waters.

Attempt were made for years to electroplate chromium could be electroplated satisfactory from chromic acid, if a small, but critical, proportion of chromic sulphate were also prevent. It was soon found that it was the sulphate that was critical, although chromium sulphate was indeed formed during the plating process. The surprising solution composition has reminded the basic of all chromium plating in spite of considerable research to find place that the chromium in chromic acid solution is present in the negatively charged anion, and would not be expected to be discharged at the negatively charged cathode, and in the second place, since electroplating is chemically a reduction process, it is unexpected that it should operated with the strongly oxidizing chromic acid.

Metal Coloring Processes

Although metals dull and tarnish in time, some of the patinas or surface effects so produced are considered to be visually pleasant, especially where patterns of clean and tarnished metal appear side by side due to constant handling. There is extensive literature on methods for the artificial production of other hardware. The erroneously termed oxidized finish on copper and silver articles is the best known in this class. The mauve, brown or black tarnish which appears in time on copper or silver articles exposed indoors is actually a supplied film. It can readily be produced artificially on the clean metal surface by immersing it in an aqueous solution of sodium or ammonium polysulfide, known in the trade as liver of sulfur. The simulate effect of constant handling of the coatings are then locally removed or ‘relieved’ by local rubbing with a cloth or brush loaded with pumice powder. All such tarnish film are neither stable nor wear resistant, so they are invariably further protected by a film or clear lacquer, usually or air drying nitrocellulose lacquer.

In the hardening of high carbon steel, it is first quenched from a high temperature into water or oil, and then tempered to improve the toughness by heating to a rather exact and moderate temperature, and thus the degree of temper, was judged by the interference color formed on the cleaned steel during tempering. When the desired color was attained, the parts were again quenched. Some of these colors are attractive in appearance, such as the peacock-blue color suitable for springs; they have moreover become associated with the properties of a particular grade of temper. The colors are caused by interference of light in oxide films of precise thickness determined by the temperature. These films are very thin, but when saturated with oil they have a slight protective value. They are therefore sometimes artificially produced by other chemical oxidation processes, such as heating in a melt of mixed sodium and potassium nitrates at the appropriate temperature. Such films are used, after oiling with e.g. linseed oil, in tool, watch and instrument manufacture.

The traditional attractive brownish black coating on firearms and weapons is also an oiled oxide film, but is much thicker and is a mixture of ferric and ferrous oxides. The traditional method of formation is lengthy and tedious; it involves repeated treatment with a complex oxidizing solution and successive heating. A simpler process of oxide blackening is widely used for sprongs, clips and similar small steel parts. They are immersed in a very concentrated solution of caustic soda containing sodium nitrate. Owing to the high concentration of caustic soda, this solution can be operated at 150oC. An oxide coating of 0.001 to 0.002 in thickness is obtained in 20 to 30 minutes. This is somewhat porous, but if keep oiled, it provides a fair protection from rusting.

Others Non Metallic Coating

The more important of non-metallic coating for metal are vitreous enamel and the various type of organic coatings such as paint, enamel and laquer. Nevertheless there are a number of other processed for producing non-metallic coatings which it is convenient to mention first, since they are either skin to electroplating processes or are preparatory to painting processes.

This further process of metallic coating have purpose to produce that considered to be visually pleasant or more bright, especially where patterns of clean and tarnished metal appear side by side due to constant handling.

The kind of further metallic coatings are as follows:

1. Metal coloring processes
2. Phosphating
3. Treatments specific to aluminum alloys
4. MBV treatment
5. Chemical and electrolytic brightening of metals
6. Bright anodizing of aluminum alloys for bright trim

The detail of those process will discussed more detail on the next blogs, so continuous read and follow every new articles of my blogs post.