Metal Finishing General Information
The following information is intended to serve as a reference for basic information concerning various metal finishing processes. The information is intended to be used as a brief overview of various coatings offered here at PECO. We hope you will find the information helpful and we welcome request for more comprehensive information if you desire.
Adequate cleaning of metals is the first step in preparing the material for finishing. All foreign substances, regardless of origin or nature, must be removed to a degree where the metal surfaces are substantially bare and free from substances tending to interfere with subsequent treatment or operations. most metals go through one or more fabrication or production processes. During these procedures, the metal can pick up foreign substances or start corroding by the end of the process. We run all materials through the appropriate chemical cleaning process along with media blasting, sanding, and polishing. We take the extra step to prepare the metal to insure the finishes durability.
Hard anodizing is a term used to describe anodic coatings with surface hardness and/or abrasion resistance as their primary characteristic. These anodic coatings are usually thick, greater than .001″ (50%-buildup & 50%-penetration), by normal anodizing standards, and they are produced using special anodizing conditions. Thick coatings (over .004″) will tend to break down sharp edges. Alloys with a very high copper or silicon content are less suitable for this process. The color of the natural anodic coating depends on the alloy and the coating thickness. e.g. 6061 has a tan or gray color which darkens to almost black at .003″; 6063 has an amber shade which darkens to bronze. Both are considered clear.
An acid tin electroplating solution is a mixture of water, organic acid, and stannous tin. To this is added a number of organic constituents that serve to regulate and distribute the delivery of tin ions to the surface being plated. The two basic organic brighteners are commonly referred to as the “brightener” and the “starter”.
A basic electroplating cell consists of a tank full of the above electrolyte with arrays of tin bars (or baskets of nuggets) arranged along two opposite sides. These bars are referred to as the anodes, and, as you might expect, are connected to the positive terminal of a current source. This supply must be capable of continuous sourcing into a near short circuit load (a typical tin/lead electroplating bath has an effective full load operating “impedance” that ranges between 0.015 Ohms and 0.035 Ohms). Situated halfway between these anode “banks” is the copperclad substrate that is to be plated. It is variously referred to as the cathode or the workpiece.
In the simplest terms, metal deposition occurs when an electrical potential is established between the anodes and the cathode. The resulting electrical field initiates electrophoretic migration of tin ions to the cathode where the ionic charge is neutralized as they plate out of solution. At the anode (in a properly maintained bath), sufficient tin erodes into the electrolyte, to exactly make up for the deposited material, maintaining a constant concentration of dissolved tin metal. As in all electrolytic solutions, there is a tendency of electrical charges to build up on the nearest high spot, thereby creating a higher electrical potential. This area of increased potential attracts more metal ions than the surrounding areas which in turn makes the high spot even higher. If this process were allowed to continue unchecked, the resulting plated surface would resemble a random jumble of tin instead of the smooth, bright surface needed for reliable resist action inside the etching tank. Inhibiting and controlling this nonlinear behavior is where the organic brighteners come in to play.
• Silver Plating
Silver plating offers the highest electrical conductivity of all metals. It is not a precious metal and will oxidize rapidly. Silver plating is best suited for engineering purposes for solderable surfaces, electrical contact characteristics, high electrical and thermal conductivity, thermocompression bonding, wear resistance of load-bearing surfaces, and spectral reflectivity electrical applications, good corrosion resistance, good solderability, and other applications.
• Cadmium Plating
Cadmium (Cad) is a bright silvery white metal deposit (as plated). Supplementary treatments for type II can be golden, iridescent, amber, black, olive drab, or clear. All enhance the corrosion resistance of the coating.
Corrosion resistance is very good, especially with type II finish. The cadmium plating is smooth, adherent, uniform in appearance, free from blisters, pits, nodules, burning, and other defects when examined visually without magnification.
Luster: Unless otherwise specified, the use of brightening agents in plating solution is prohibited on components with a specified heat treatment of 180 Ksi minimum tensile strength (HRc 40) and higher. Either a bright (not caused by brightening agents) or dull luster shall be acceptable. Brighteners may be used with alloys listed in paragraph 3.2.8 of the specification. Parts which have been machined, ground, cold formed, or cold straightened after heat treatment shall receive stress relief bake in accordance with table I or Ia of the specification prior to shot peening, cleaning or plating. All parts shall be baked within 4 hours of plating as specified in tables I of this specification. Baking on Types II and III shall be done prior to application of supplementary coatings.
Excellent for plating of stainless steels that are to be used in conjunction with aluminum to prevent galvanic corrosion.
Fasteners; Aircraft Components; Automotive Components.
Ceramic materials are inorganic, non-metallic materials that are processed and used at high temperatures. They are highly resistant to corrosive compounds.
Ceramic materials are harder; more resist to heat and frictions, last longer than other materials. To make it usable with other materials, ceramic materials are generally coated on. These coatings may be thick and thin depending on the functional application. For example, glaze is a simple ceramic coating which makes porcelain tableware usage healthy and resistant to scratching.
Nowadays there are many coating techniques for advanced ceramic materials where they are absolutely needed on other materials : Plasma Spraying, Dip-Coating, Electrophoretic Deposition, Chemical Vapor Deposition, Physical Vapor Deposition, Sol-Gel Coating, etc.
There is a nickel finish for almost any requirement. Nickel can be deposited soft or hard, dull to bright. The difference is dependent on process used and conditions employed in plating.
The hardness can range from 150 – 500 Vickers. Can range in appearance from matte, light gray (almost white) to a condition resembling stainless steel. Corrosion resistance is a function of thickness. Has a low coefficient of thermal expansion. Nickel plating is magnetic.
Phosphate coating is a crystalline conversion coating that is formed on a ferrous metal substrate. The phosphate process relies on the basic pickling reaction that occurs on the metal substrate when the process solution comes in contact with the metal. This coating is employed for the purpose of pretreatment prior to painting, increasing corrosion protection, and improving friction properties of sliding components. In other instances, phosphate coatings are applied to threaded parts and top coated with oil to add anti-galling and rust inhibiting characteristics.
Powder coating is a system that applies, to a substrate, finely ground charged particles of pigments & resins. This powder is heated to above its melting point causing the powder to flow & fuse, creating a continuous film. The result is a uniform, durable, high-quality, and attractive finish. Powder Coating is also [used] as a general term and usually non-specific, to extol the virtues of a durable painted finish on metal surfaces. Below are some powder coating specifics.
Epoxy Powder Coatings
Epoxy based powder coatings were the first thermosetting system to become commercially available and are still widely used today. Epoxy coatings are used primarily as functional coatings for substrate protection where inherent toughness, corrosion resistance, flexibility, and adhesion are required. The primary limitation of epoxy-based coatings is poor weather-ability. Typical applications include pipe coatings, electric meters, metal furniture, shelving, electrical parts, refrigerator liners, dryer drums, and high voltage switchgear
Hybrid Powder Coatings
Hybrid coatings share many characteristics with standard epoxy-based powder coatings. The primary differences are improved resistance to over bake yellowing during cure, somewhat softer films, and slightly improved weatherability. Like epoxies, hybrid coatings are not considered suitable for exterior applications, even though their deterioration and discoloration when exposed to sunlight is slower then pure epoxies. Hybrid coatings have similar characteristics associated with polyester formulations in terms of impact and bend resistance. Hybrid coatings are often used in the same applications as epoxies particularly when slight improvement in weathering and over bake stability is required. Typical applications are shelving, hot water heaters, office furniture, power tools, tool boxes, electrical boxes, fire extinguishers, under-hood automotive and oil filters.
Polyurethane Powder Coating
Urethane-cured polyester powder coatings combine very smooth, exceptional thin film capability, with excellent mar and chip resistance, and good weathering properties. Adhesion to properly prepared (pretreated) substrates will provide very durable coatings with long-term resistance to humidity and corrosion. Polyurethanes are typically resistant to many diluted aqueous acids, salts, aromatic and aliphatic hydrocarbons, grease, and oils. Typical applications utilizing polyurethane coatings include fluorescent light fixtures, lawn and garden equipment, electrical enclosures, playground equipment, range panels, air conditioners, automotive trim, and patio furniture.
TGIC Powder Coatings(Triglycidylisocyanurate)
Polyester TGIC coatings utilize the epoxy functional cross-linker TGIC (triglycidyl isocyanurate). Use of this low molecular weight, multifunctional cross-linker enables polyester TGIC formulations to contain 90% or greater resin within the binder system. Weathering of polyester TGIC powders is comparable to polyester urethane coatings. Polyester TGIC coatings typically offer faster or lower temperature (280-300 F) curing than polyurethanes. Unlike urethane coatings, TGIC’s maintain excellent mechanical properties at film builds above 3 mils with no out-gassing. Additionally, TGIC coatings provide superior edge coverage when sharp edges are present. Typical applications for polyester TGIC coatings include aluminum extrusions, automotive wheels, transformers, air conditioners, fencing, lawn and garden equipment, and gas cylinders.
•Teflon® PTFE (polytetrafluoroethylene) nonstick coatings are two-coat (primer/topcoat) systems. These products have the highest operating temperature of any fluoropolymer (260°C/500°F), an extremely low coefficient of friction, good abrasion resistance, and good chemical resistance. PTFE is available only in water-based liquid form.
•Teflon® FEP (fluorinated ethylene propylene copolymer) nonstick coatings melt and flow during baking to provide nonporous films. These coatings provide excellent chemical resistance. In addition to low friction, FEP coatings have excellent nonstick properties. Maximum use temperature is 204°C/400°F. FEP is available in water-based liquid and powder forms.
•Teflon® PFA: Like FEP, PFA (perfluoroalkoxy) nonstick coatings melt and flow during baking to provide nonporous films. PFA offers the additional benefits of higher continuous use temperature (260°C/500°F), film thicknesses up to 1,000 micrometers (40 mils), and greater toughness than PTFE or FEP. This combination of properties makes PFA an excellent choice for a wide variety of uses, especially those involving chemical resistance. PFA is available in both water-based liquid and powder forms.
•Teflon® ETFE is a copolymer of ethylene and tetrafluoroethylene and is also sold under the Tefzel® trademark. Although not fully fluorinated, ETFE has excellent chemical resistance and can operate continuously at 149°C/300°F. This resin is the toughest of the fluoropolymers and can be applied at film builds up to 1,000 micrometers (40 mils) to provide a highly durable finish. ETFE is available in powder form.
•Teflon®-S One Coat: These solvent-based liquid coatings are formulated with special blends of fluoropolymers and other high-performance resins to improve toughness and abrasion resistance. Because the film components stratify during baking, most of the fluoropolymer properties (such as low friction and nonstick character) are retained. The resins provide adhesion and abrasion resistance. These products can sometimes be applied to smooth, clean metal. Bake requirements vary, depending on the specific coating, from 163°C/325°F to 316°C/600°F.
•Teflon®-S Dry Lubricant: Dry lubricant coatings are special versions of Teflon®-S technology designed to provide lubrication under high-pressure/velocity (PV) conditions. These products are solvent-based, one-coat systems that are usually cured between 260°C/500°F and 371°C/700°F.
Zinc coating of steel components has been thus far considered the most economical and viable industrial finishing process for steel, where sacrificial type corrosion resistance is required. For most applications, zinc finishes afford from 24 hours to “white” rust and up to 240 hours to “red rust” in accelerated neutral salt spray testing, depending on zinc thickness, type of chromate and availability of organic top coat.
The process produces zinc alloys containing l5-25% iron. The alloy has good weldability, workability and can be adapted to commercially electroplated strip steel. The alloy composition and process can be varied to enhance weldability or adhesion of electropainting processes. Black chromating is the most suitable for this type of alloy.
Commercially available processes are similar to low ammonia or ammonia free acid chloride zinc baths. Some newer baths operate on the alkaline side. The deposit contains from 1-3% cobalt. The acid type bath has a higher cathode efficiency, and reduced hydrogen embrittlement, but its plating thickness distribution varies substantially between low and high current density areas. Table X – Typical bath composition. Chromate conversion coatings in iridescent, black and yellow are available.
There are two types of zinc-nickel processes currently available commercially. Acid and alkaline non cyanide types. Alloys deposited contain from 5 to 15% nickel. Corrosion resistance improves with nickel content up to 15-18% but the deposit becomes more noble and loses its sacrificial protection property. Chromate film formation was found to be at optimum in the 5-10% nickel content range. Above this range the deposit tends to be passive and chromating becomes very difficult.