Electroplating, also known as electrodepositing, is by far the most common form of coating parts. In electroplating, a thin metallic coat is deposited onto the surface of the part by means of ionized electrolytic solution. The workpiece is negatively charged (cathode) and submerged into a solution that contains a positively charged metallizing source (annode). When electrified, the positive ions migrate from the annode to the cathode, and in the process a part is plated.

The actual process is a little more complicated. The plating system involves several tanks which clean, degrease, and de-ionize the parts in advance. There are two main type of electroplating. Barrel plating is a bulk process used for bulk parts that are tolerant of rougher handling. Rack plating is more expensive because of the cost of racks and set-up, but is used for longer parts, or parts with delicate features.

Immediately following and coincidental with electroplating is the process of chromate conversion, which is used to color parts and provide additional protection. Chromate conversion chemically alters the surface of a workpiece through the use of an acidic (chromic acid) solution which decomposes the surface and creates a protective film of chromium compounds. The process is a dipping process into a heated tank, which is often completed in-line to an electroplating line.

The most common chromates are clear, which has a silver appearance; blue, which has a deeper silver color with a very slight blue tint; yellow, which has a golden color; and olive drab which is used in military applications. Cadmium plating is a similar electrolytic process that offers superior corrosion resistance to zinc, but cadmium is considered an environmental risk.

Environmental requirements are an important consideration in electroplating. The European Restriction of Hazardous Substances initiative, known as RoHS (ROH-Hoss) has stimulated a general movement from hexavalent chromium (six atoms) to trivalent chromium (three atoms). Trivalent chromium is less stable and less reliable as a coating, but is less damaging to the environment. 

Hydrogen atoms bonding to the steel

Heat applied to release trapped hydrogen atoms

 

One other consideration in electroplating is to verify if coated parts have been heat treated. If so, the heat treating process can introduce hydrogen into the metal, a condition known as hydrogen embrittlement. At elevated temperatures, hydrogen solubility is increased and the hydrogen molecules will diffuse into the surface. Once plated the hydrogen molecules fill the voids in the metal, which can make parts brittle. To relieve this condition, electroplated parts must be baked within thirty minutes of plating, so that the heat causes the hydrogen molecules to diffuse back out of the part (similar to evaporation).

Electroplating is generally specified by process description (as in “electroplate per ASTM B633 with yellow dichromate”). Quality is audited either by thickness (“.0002” minimum thickness”) or performance (“must meet or exceed 72 hour salt spray test to white rust”). A salt spray test consists of placing the part in a chamber and a salt spray is applied to the part. Visual observations are noted at established durations. Corrosion (iron oxide, where oxygen in the air combines with iron in the part) begins as a white crusty rust, and proceeds to a red rust thereafter.

Mechanical plating, also known as “ping plating,” is an alternate coating method to electroplating and is used on heat treated parts so as to eliminate any opportunity for hydrogen embrittlement. In this method, parts are placed into a tumbler or vibrating device with zinc nuggets. The parts are vibrated and in the process, the zinc is “pinged” into the surface. The absence of water and heat in the process does not enable hydrogen atoms to absorb into the parts. Mechanically plated parts have a dull finish and are not as bright as electroplated parts.

Passivation is a process of de-staining stainless steel by cleaning parts with sodium hydroxide and citric acid, followed by nitric acid and a complete water rinse. The process restores a light film and removes metal particles and dirt from the surface. The net result is a bright finish. Anodizing is a similar process that is performed on aluminum. Anodizing can introduce coloration through a chromate-conversion process so that aluminum parts can be color coded or decoratively enhanced.

Black oxide and phosphate are rust preventatives that retain a plain-finish, but shiny appearance. They are sometimes used to protect ocean-transported plain parts, and on parts that are heat treated.

LPS (Leadership Performance Sustainability) is an in-house secondary process where a high-intensity rust inhibitor is applied immediately prior to shipment. The process adds costs but will also enhance shelf life of plain finish parts in corrosive environments.

Dykem is a stain that is available in a number of colors. Its primary use is in machining and layout for color coding. Suppose, for example, that parts are to be ground on both sides. Dykem can be used to stain the parts, where the stain is removed by grinding. A machine operator could now orient parts to know which side of the workpiece is not processed. Dykem is also used as decorative coloring, or for field applications for color coding (a red key is hardened, a green key is not).

G.L. Huyett has inkjet printing and engraving capabilities. The inkjet process is an automated conveyor line frequently used to mark the size and identification on keystock. We can also mark your name or part number on your parts to suit your needs.

Conversion coatings are thin, adherent chemical compounds that are produced on metallic surfaces by chemical or electrochemical treatment. These coatings are insoluble, passive, and protective, and are divided into two basic systems: oxides or mixtures of oxides with other compounds, usually chromates or phosphates. Conversion coatings are used for corrosion protection, as an adherent paint base; and for decorative purposes because of their inherent color and because they can absorb dyes and colored sealants.

Conversion coatings are produced in three or four steps. First there is a pretreatment, which often involves mechanical surface preparation followed by decreasing and/or chemical or electrochemical cleaning or etching. Then thermal, chemical, or electrochemical surface conversion processes take place in acid or alkaline solutions applied by immersion spraying, or brushing. A post treatment follows, which includes rinsing and drying, and may also include sealing or dyeing. If coloring is the main purpose of the coating, then oiling, waxing, or lacquering may be required.

Thin black oxide coatings are applied to steel by immersing the parts to be coated in a boiling solution of sodium hydroxide and mixtures of nitrates. These coatings serve as paint bases and, in some cases, as final finishes. When the coatings are impregnated with oil or wax, they furnish fairly good corrosion resistance. These finishes are relatively inexpensive compared to other coatings.

Phosphate coatings are applied to iron and steel parts by initiating reactions between them and a diluted solution of phosphoric acid and other chemicals. The surface of the metal is converted into an integral, mildly protective layer of insoluble crystalline phosphate. Small items are coated in tumbling barrels; large items are spray coated on conveyors.

The three types of phosphate coatings in general use are zinc, iron, and manganese.

Zinc phosphate coatings vary from light to dark gray. The color depends on the carbon content and pretreatment of the steel’s surface, as well as the composition of the solution. Zinc phosphate coatings are generally used as a base for paint or oil, as an aid in cold working, for increased wear resistance, or for rustproofing. Iron phosphate coatings were the first type to be used; they produce dark gray coatings and are generally applied as a paint base. Manganese phosphate coatings are usually dark gray; however, since they are used almost exclusively as an oil base, for break in and to prevent galling (wear caused by adhesion between sliding surfaces), they become black in appearance.

In general, stainless steels and certain alloy steels cannot be phosphated. Most cast irons and alloy steels accept coating with various degrees of difficulty depending on alloy content.

In the anodizing process, the aluminum object to be treated is immersed as the anode in an acid electrolyte and a direct current is applied. Oxidation of the surface occurs, producing a greatly thickened, hard, porous film of aluminum oxide. The object is then immersed in boiling water to seal the porosity and render the film impermeable. Before sealing, the film can be colored by impregnation with dyes or pigments. Special electrolytes may also be used to produce colored anodic films directly in the anodizing bath. The anodic coatings are used primarily for corrosion protection and abrasion resistance, and as a paint base.

The following is a list of military plating and coating specifications:

Anodize (Chromic and Sulfuric), MIL-A-8625F: Conventional Types I, IB, and II anodic coatings are intended to improve surface corrosion protection under severe conditions or as a base for paint systems. Coatings can be colored with a large variety of dyes and pigments. Class 1 is nondyed; Class 2 dyed.

Type I and IB coatings should be used on fatigue critical components (due to thinness of coating). Type I, unless otherwise specified, should not be applied to aluminum alloys with over 5% copper or 7% silicon or total alloying constituents over 7.5%. Type IC is a mineral or mixed mineral/organic acid that anodizes. It provides a non-chromate alternative for Type I and IB coatings where corrosion resistance, paint adhesion, and fatigue resistance are required. Type IIB is a thin sulfuric anodizing coating used as non-chromate alternatives for Type I and IB coatings where corrosion resistance, paint adhesion, and fatigue resistance are required.

Black Oxide Coating, MIL-C-13924C: A uniform, mostly decorative, black coating for ferrous metals used to decrease light reflection. Provides very limited corrosion protection under mild corrosion conditions. Black oxide coatings should normally be given a supplementary treatment. Typically used for moving parts that cannot tolerate the dimensional change of a more corrosion resistant finish. Use alkaline oxidizing for wrought iron, cast and malleable irons, plain carbon, low alloy steel, and corrosion resistant steel alloys. Alkaline-chromite oxidizing may be used on certain corrosion resistant steel alloys tempered at less than 900°F. Salt oxidizing is suitable for corrosion resistant steel alloys that are tempered at 900°F or higher.

Cadmium, QQ-P-416F: Cadmium plating is required to be smooth, adherent, uniform in appearance, free from blisters, pits, nodules, burning, and other defects when examined visually without magnification. Unless otherwise specified in the engineering drawing or procurement documentation, the use of brightening agents in the plating solution to modify luster is prohibited on components with a specified heat treatment of 180 ksi minimum tensile strength (or 40 Rc) and higher. Either a bright (not caused by brightening agents) or dull luster is acceptable. Baking on Types II and III needs to be completed prior to the application of supplementary coatings.

Type I is to be used for plating. Types II and III require supplementary chromate and phosphate treatment respectively. Chromate treatment required for type II may be colored iridescent bronze to brown including olive drab, yellow, and forest green. Type II is recommended for corrosion resistance. Type III is used as a paint base and is excellent for plating stainless steels that are to be used in conjunction with aluminum to prevent galvanic corrosion. For Types II and III the minimum cadmium thickness requirement must be met after the supplementary treatment.

Lubrication, Solid Film MIL-L-46010D: The Military Plating Specification establishes requirements for three types of heat cured solid film lubricants that are intended to reduce wear and prevent galling, corrosion, and seizure of metals. Solid Film lubricants are intended for use on aluminum, copper, steel, stainless steel, titanium, and chromium, and nickel bearing surfaces.

Types I, II, and III have a thicknesses of 0.008-0.013 mm. No single reading less than 0.005 mm or greater than 0.018 mm is acceptable.

Type III is a low volatile organic compound (VOC) content lubricant. Color 1 has a natural product color and Color 2 has a black color.

Nickel, QQ-N-290A: There is a nickel finish for almost any need. Nickel can be deposited soft, harddull, or bright depending on the process used and conditions employed in plating. Thus, hardness can range from 150-500 Vickers. Nickel can be similar to stainless steel in color, or can be a dull gray (almost white) color. Corrosion resistance is a function of thickness. Nickel has a low coefficient of thermal expansion.

All steel parts having a tensile strength of 220,000 or greater should not be a nickel plated without specific approval of the procuring agency.

Class 1 is used for corrosion protection. Plating needs to be applied over an underplating of copper or yellow brass on zinc and zinc based alloys. At no time should the copper underplating be substituted for any part of the specified nickel thickness. Class 2 is used in engineering applications.

Phosphate Coating: Heavy, DOD-P-16232-F: The primary differences between Type M and Type Z coatings are that Type M is used as a heavy manganese phosphate coating for corrosion and wear resistance and Type Z is used as a zinc phosphate coating.

Type M has a thickness from 0.0002-0.0004″ and Type Z, 0.0002-0.0006″. Class 1, for both types, has a supplementary preservative treatment or coating as specified; Class 2 has a supplementary treatment with lubricating oil; and no supplementary treatment is required for Class 3. For Type M, Class 4 is chemically converted (may be dyed to color as specified) with no supplementary coating or supplementary coating as specified. For Type Z, Class 4 is the same as Class 3.

This coating is for medium and low alloy steels. The coatings range from gray to black in color. The “heavy” phosphate coatings covered by this specification are intended as a base for holding/retaining supplemental coatings which provide the major portion of the corrosion resistance. “Light” phosphate coatings used for a paint base are covered by other specifications. Heavy zinc phosphate coatings may
be used when paint and supplemental oil coatings are required on various parts or assemblies.

Zinc, ASTM-B633: This specification covers requirements for electrodeposited zinc coatings applied to iron or steel articles to protect them from corrosion. It does not cover zinc-coated wire or sheets. Type I will be as plated; Type II will have colored chromate conversion coatings; Type III will have colorless chromate conversion coatings; and Type IV will have phosphate conversion coatings.

High strength steels (tensile strength over 1700 Mpa) are not to be electroplated.

Stress relief: All parts with ultimate tensile strength 1000 Mpa and above baked at minimum 190°C for three hours or more before cleaning and plating.

Hydrogen embrittlement relief: All electroplated parts 1200 Mpa or higher shall be baked at 190°C for three hours or more within four hours after electroplating.