Phosphate coating

Phosphate coating is a crystalline conversion coating that is formed on a ferrous metal substrate. It is employed for the purpose of pre-treatment prior to coating or 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 (P&O) to add anti-galling and rust inhibiting characteristics. The phosphating sequence is normally broken down into the following stages– degreasing and cleaning, derusting and descalling, activation, phosphating, and post-treatment. The phosphoric acid solution reacts with the surface of the metal to chemically form a mildly protective layer of insoluble crystalline phosphate. Phosphate coatings can also be applied to zinc, cadmium, aluminum, tin and galvanized steel, but are difficult to apply on material with high alloys which are often immune to the phosphoric acid. Factors to be considered in this type of coating are nature of the metal to be coated, shape and surface condition of the metal, and number of parts and uniformity of their surface state.

Another type of coating is the Manganese Zinc which can also be called as Manganese Zinc coating. Zinc and manganese phosphate coatings are the treatment of iron or steel by immersion in a dilute solution of phosphoric acid and other additives. In the resulting chemical reactions, the surface of the metal is chemically converted to an integral protective layer of insoluble zinc and iron or manganese and iron phosphate crystals. Depending on the physical characteristics of the substrate and the pre-treatment methods used, the translucent crystals appear black to light grey in color for zinc phosphate and black to dark grey in color for manganese phosphates. Material to be coated is cleaned by immersion in a hot alkaline solution that removes most oils and loose soil. If surface oxides are present, the parts are then stripped in an acid-cleaning step that undercuts the rust or scale, exposing the bare metal beneath.

The work is then rinsed thoroughly and coated in a chemically balanced hot phosphoric acid solution via an autocatalytic reaction. The temperature, time and chemical composition of this bath must be carefully controlled to produce consistent results. After coating is completed, excess acid is neutralized and a supplementary treatment is applied if required. Material to be zinc or manganese phosphated may either be racked or bulk-processed in barrels, depending on the specifications of the end user. Normally threaded parts, soft alloys, or parts which weigh in excess of 6 ounces are racked to avoid nicks, distortion, and coating damage which result from bulk handling practices. The purposes of this type of coating are for lubrication during cold forming, corrosion resistance, torque-tension requirements, bonding organic coatings to metals and to reduce break-in wear on adjacent moving surfaces. This coating is commonly used by hydraulic system manufacturer, military armament suppliers ,rod-by-coil manufacturers, nuclear component fabricators, cold-forming industries, sports equipment manufacturers, automotive fastener companies, aerospace industries, motorcycle specialty groups, marine equipment vendors, various stamping applications, diesel engine manufacturers, and electrical connector manufacturers.


#manganese-zinc-coatings, #phosphate-coatings

The Basics of Silver Plating

Gold and silver share a lot of similarities. Both have almost the same chemical and physical properties and both have soft, ductile and malleable characteristics and have melting points of 1,065°C as well as 961°C. Because of this, the process of Silver Plating is pretty much similar to Gold Plating, except for some minor differences.

Silver is considered a coinage metal. Because of this characteristic, it has become one of the most sought after metals since the ancient times. Much of the recorded history involves silver and they have since been considered the most precious metal used for monetary and decorative usage.

Early Application

Early application of the Silver Plating would require fire gilding and leafing especially if used mostly for decorative purposes. When the electronics and electrical industries have become popular especially in the 19th until the 20th centuries, various applications were created by means of Silver Electro Plating. It is due to its chemical inertness, conductivity, resistance to arcing, as well as low and stable contact resistance.

Early applications of gold plating and its predecessors, fire gilding and leafing, were almost exclusively decorative. The rise of the electrical and electronics industries in the latter half of the 19th and throughout the 20th centuries created applications for gold based on its chemical inertness, low and stable contact resistance, conductivity, and resistance to arcing.

Silver-Brightened Gold

Silver is the first commercially successful brightening agent that was used for gold. The early formulations of this system, including the similar formulations that employ antimony or tin, has made use of a large concentration of free cyanide, that has caused staining problems and have attacked some printed circuit board laminates when in use. The Silver Plating solutions were re formulated along with electrolytes that are similar to those with color flash baths. The Silver Electro Plating is based on phosphate and comes with free cyanide that was reduced to 0 up to 7.5 grams per Liter. The deposits that come with silver content that ranges from 4 to 9 weight percent are often more durable as a sliding friction, though it is still commonly used for slide wire as well as with rotary switch applications.

Silver Plating

Although the complex cyanides of gold is stable at a certain pH value that is low enough to allow the use of electrolytes through Silver Electro Plating, the complex cyanide of the Silver is unstable below the neutrality. It therefore hydrolyzes to release the insoluble AgCN which imposes a requirement to fully maintain at least some of the free cyanide within the system and set a minimum operating pH for the Cyanide Silver Plating solutions of at least 8.0 up to 8.5. Within the general class of the Cyanide silvers, there are several variations available.

Recently, a series of the silver complexes with the hydantoin as well as the substituted hydantoins are being prepared and are readily available for use. These types of Silver Electro Plating solution promise an improved pH stability as well as solution control, along with higher brightness and greater resistance to tarnishing.

#silver-electro-plating, #silver-plating