PURPOSE OF METAL FINISHING
- decorative; for example, to provide lustre and aesthetics to products
- Ion Deposition
- protective; for example, to protect products from abrasion and corrosion
- functional; for example, to augment the product surface with solderability and lubricity
VARIETY OF METAL FINISHING
- Vapour Deposition
- Ion Deposition
- Electro-Deposition ( ElectroPlating)
- Electroless Plating
- Chemical conversion Coating
- Thermal spraying
MECHANISM OF ELECTRO-DEPOSITION
- Metal ion in solution are reduced through an applied potential on the surface of the product to deposit as solid metal on the surface.
- Metal ions have a position charge, written generally as Mn+
- The surface of the product is charged with a negative potential, becoming the cathode. The Mn+ will receive electrons from substrate and deposit onto the surface as solid metal.
- At the same time, the anode will lose an electron from its surface and dissolve into the bath as an ion.
MECHANISM OF ELECTROLESS PLATING
- Metal Ions are activated through chemical means. This activated form reacts with the surface, depositing metal onto the surface.
- For electroless nickel, some metals have activated surface such as Fe, Al, Zn. Some have not, such as Cu and stainless steel, which need an active coating before EN-Ni. Other types of electroless deposition have similar conditions.
- Ni itself is active for EN-Ni, so electroless nickel plating is also named as self-catalytic nickel deposition.
MECHANISM OF ANODIZING
- A positive potential is applied to the aluminium substrate, making the product an anode, and a porous oxide film grows on the surface. This film can then be dyed in a variety of different colours, and then is typically sealed afterwards.
- Aluminium metal will lose electrons on anode and reacts with the surround water molecules to become aluminium oxide.
- Surrounding acids unevenly dissolve the aluminium oxide, which then redeposit in an adjacent area due to the anodic potential. This creates a porous layer of aluminium oxide, which can be grown thicker.
- These pores can be closed off by converting the aluminium oxide to aluminium hydroxide in a process called sealing. Aluminium hydroxide takes up more volume than aluminium oxide. Dying the parts is accomplished by having dyes absorb into the pores before sealing.
- Variances in the thickness of the aluminium oxide layer, along with various sealing methods and additional oxidants in the bath makes for many different types of anodizing.
- Includes all cleaning and surface preparation for metal finishing.
- Blasting: artificial sand is blasted in a stream of compressed air to mechanically remove surface contamination, and to create roughness on the surface on a micrometer level.
- Alkaline Cleaning: Alkaline cleaning solution saponifies organic substances, and dissolves them into the bath. Aluminium parts can only be cleaned with mild alkaline cleaner.
- Electro-Cleaning: Same principle as alkaline cleaner, with the addition of hydrogen gas being generated at the surface of the product, to further increase lifting power.
- Acid dip, etch and deoxidizing solution: Dissolves metal oxides and prepare a micro rough surface for new coating to bond.
- Enhances the function of the recently applied metal finishing, and reduces the side-effects of processing on substrate.
- Chromate: complex chemical reaction to produce a coating of mixing oxides, including hexavalent chromium, on the surface to increase corrosion resistance.
- Sealing: make anodizing coating more compact via hydrolysis. After sealing, the coating will contain more aluminium hydroxide, nickel hydroxide or chromate compounds, which possess much more volume than aluminium oxide, and increasing its physical properties.
- Baking: Surface treatments produces stress on the product, as well as induces hydrogen in its crystal structure. Baking the part afterwards helps realign crystal structure and have hydrogen diffuse out of the part.
- Baking at 375° F for 8 hour before plating for high tensile ( strength, hardness) steel to release the stress that may created during machining. This will increase adhesion from plating and surface treatments, as well as prevent hydrogen embrittlement.
HYDROGN EMBRITTLEMENT RELIEF
- Enables absorbed hydrogen to leave the part, greatly reducing embrittlement issues due to absorbed hydrogen.
- Because the hydrogen atom is much smaller in size than the metal of the deposit, it is able to migrate into the crystal lattice of the basis metal, and reside interstitially between the individual metal atoms. The interstitial hydrogen can greatly amplify the stress of applied forces within the basis metal which can produce catastrophic fracture at loadings much lower than the typical yield strength of the material.
Plating has metal in the solution directly deposit onto the surface, where as anodizing turns the substrate into the coating. An analogy would be that plating is like painting a piece of wood with black paint, and anodizing would be like charring the piece of wood. Anodizing can be categorized into as a type of conversion coating.
For plating, you can strip the coating away without change the size of substrate provided the stripper does not corrode the substrate. For conversion coating, if you strip the coating, you take some base material away and the size of original part will be changed. The thicker the conversion coating, the more material will be taken away, because more of the base material was converted.
Besides anodize coating, chem-film (chromate on aluminium), chromate on cadmium plating, phosphate on steel and passivation on stainless steel are conversion coatings. But those conversion coatings are chemical conversion coating which are created by chemical reaction on the surface of the substrate. And because this is the case, these coatings are different from anodizing in that they cannot be grown to be thicker, and do not need to be sealed. Normally the changes in dimension from these conversion coatings are negligible.
Passivation is mainly to clean foreign metal materials off stainless steel parts. At the same time during passivation, nitric acid will react with stainless steel to create a very thin oxide film to protect stainless steel from further corrosion. This thin passivation film (10 nm) cannot be easily established nor maintained in the presence of certain ions which are usually referred to as “aggressive”. The most commonly encountered ion within this category is the chloride ion. This is why cotton gloves are required while handling passivated parts.