Chromating By Herbert Brumer, Chief Chemist Heatbath Corporation Springfield, Massachusetts Visit PF's Plating Zone PF's Tools Chromate films are chemical conversion coatings. The substrate metal participates in the coating reaction and becomes a component of the coating; and it has a profound influence on the properties of the coating. Among the metals commercially chromated are zinc and cadmium electroplates; zinc die castings; hot-dipped galvanized steel; aluminum (in almost every conceivable form); and sometimes copper and silver alloys. Chromate coatings improve corrosion resistance and appearance of metals and adhesion of organic topcoats. The chemistry involves reaction between the metal surface and an aqueous solution containing chromates and certain activator ions. Activators include sulfates, chlorides, fluorides, phosphates and complex cyanides. Normally a given chromate is designed to work on a particular metal; but in a few cases there are solutions that will work on two or more. For example, certain chromates for zinc electroplate will also coat cadmium and may be suitable for zinc die castings. The solutions for chromating are acidic. A simplified reaction proceeds along these lines: Metal at the interface is dissolved by the acid and enters solution as metal ions. There is a local rise in pH (lowering acid content) in the immediate vicinity of the interface. Basis metal ions combine with chromate ions to form a compound that is insoluble at the local (higher) pH. This compound precipitates on the metal surface as an adherent coating. Reaction by-products enter the main solution. Coatings formed by the reaction are of necessity built up from the inside out. Since coating can only be produced by the interaction of the solution with the metal, solution must diffuse through prior layers of coating and reaction products must diffuse outward. The outermost layer of visible coating is that which was formed initially. This inward-outward diffusion of solution can have a detrimental effect on thicker coatings. They may be powdery and loosely adherent. And solution trapped in the coating interstices can lower final corrosion resistance. Another implication of forming coatings by chemical reaction is that the solution chemistry is continually changing. The chemistry has to be carefully balanced so that uniform coating properties can be maintained over the entire working life of a chromating bath. Zinc and Cadmium Electroplate A typical cycle used to chromate electroplated work follows these steps: Electroplate. Rinse (multiple rinses may be employed). Optional dip in 0.25-0.50 pct nitric acid. Chromate. Cold rinse. Leach (if required). Cold rinse. Warm-air dry off. Plated parts are chromated on racks or in stainless steel baskets. The selection of a particular chromate depends on the corrosion resistance required. Appearance also is a consideration. Plate parts to be chromated with a minimum thickness of .25 mil (0.006 mm). An average of .05 mil is consumed by the chromating process, leaving .2 mil, which is required for satisfactory corrosion resistance. Clear Bright. One of the earliest types of chromating solutions was a concentrated (100 g/liter chromic acid) solution that did a certain amount of chemical polishing. The solution consisted of chromates, sulfates and buffers such as borates or ac-etates. It deposited a gold film, which was then removed by a subsequent immersion in dilute alkali (leaching, sometimes called bleaching). This coating was once very popular for wire goods such as refrigerator racks since the polishing of the chromate produces an appearance similar to that of chromium. For use on zinc plate, this older two-step process has now been largely replaced by more dilute, single-dip processes. Reasons: the cost of maintaining a highly concentrated solution and the cost of waste treat-ing the spent chromate. The double-dip process, i.e., chromate plus alkali leach, is still often used on cadmium plate. Blue Bright. Single-dip solutions for passivating zinc plate and providing a degree of stain resistance are used in both rack and barrel plating. Plated fasteners, steel stampings and wire goods are now commonly chromated in these solutions. The baths consist of chromates, fluorides and large amounts of nitric acid. The concentrated proprietary products are available in liquid form (which contains nitric acid) or as a granular powder (to which the user adds nitric acid). Working solutions will contain only about one g/liter of chromic acid (or even less), about one pct of the amount in the former baths. Naturally, the cost of preparing these and of subsequent waste treatment is considerably lower. Chromate films from this type of bath can be dyed a variety of colors with organic dyes. This can serve as identification, especially for screws, nuts and small parts. These solutions are specific for zinc electroplate and will not work on cadmium or other types of zinc. Gold Films. Solutions containing chromates and sulfate or chloride activators produce gold colors. The films contain considerably more hexavalent chromium than clear films, accounting for their color. Corrosion resistance is better. Baths of this type can be used for zinc, cadmium and some zinc die castings. Olive Drab. These coatings have exceptional corrosion resistance and are applied to zinc and cadmium electroplates from baths similar to those used for gold coatings. The inclusion of an organic acid modifier is responsible for the olive drab color and the enhanced corrosion resistance. The color is not especially pleasing, however, and coatings of this type are functional rather than decorative. Black. Incorporating a soluble silver salt into a gold chromate solution produces a deposit of black silver chromate. This coating has excellent corrosion resistance and a jet black matte appearance, with good light absorbance. These products should find some applications in coating parts for solar collectors. The use of a silver salt tends to make the process expensive and sensitive to chloride contamination. The latter becomes an important consideration if electroplates from chloride baths are to be chromated. Zinc Die Castings Zamak castings are extensively used in decorative and functional automotive applications. Parts such as carburetors and fuel pumps are highly susceptible to the ravages of water-gasoline corrosion, which is a particularly aggressive combination. Without chromate protection, a gelatinous precipitate of zinc hydroxide quickly fouls small orifices and venturis. A gold chromate deposit prevents this problem. A typical cycle for chromating Zamak die castings is as follows: Deburr (mechanical or thermal). Alkali clean. Cold rinse. Acid activation (dilute sulfuric or phosphoric acid, often with included fluorides). Cold rinse. Warm rinse. Warm-air dry. Hot-Dipped Galvanized Steel Hot-dipped galvanized steel coils are often chromated at the mill to prevent white corrosion products from forming during storage. Or they can be chromated on high-speed coil-coating lines as a pre-paint treatment, to provide corrosion resistance and excellent paint adhesion. Unfortunately the first process is no substitute for the second. The mill process is intended only for passivation and is an inferior adhesive coat for paint. It is also extremely difficult to remove, so that galvanized steel that is ultimately to be painted should be specified without mill chromate passivation (it should be oiled instead). Mill Chromate. Two types are used. One has a fluoride activator, while the other is based on reduced (trivalent) chromium. Both are flooded on the galvanized coil directly as it comes from the galvanizing pot (after water quench). The chromate solution is dilute (1 g/liter) and is not rinsed, but squeegeed off. A thin film remains, it reacts with the zinc and the coating is dried in place. Coatings are clear, colorless and intended to provide protection until sheet is fabricated, often under outdoor storage conditions at construction work sites. Paint-Base Chromate. Applied on high-speed coil-coating lines, these are chromate solutions of moderate concen-tration (10 g/liter) with complex fluoride activators. New coil-coating lines can handle widths of metal up to 60 inches wide and run at 300 feet per minute. Coil is painted in-line by reverse-roll-coat methods and subsequently fabricated into residential siding and architectural building panels. Gold Chromate. Gold chromates of this type used for zinc plate and Zamak are sometimes used on galvanized steel stampings and hardware. Aluminum Chromates for aluminum fall into two categories: chrome-phosphates, primarily those used on architectural aluminum extrusions to provide a paint-bonding coat, and chrome oxides applied to almost every type of aluminum, i.e., sheet, coil, stampings and castings. Purposes: paint bonding and corrosion resistance. Chrome Phosphate. Baths for apply-ing chrome phosphate consist of chromate ions, phosphates and fluorides. Although the solution contains chromium in the hexavalent form (+6 chromium) chemical reduction takes place on the metal at the point of application, so that the chromium in the coating is essentially all trivalent chromium (+3) phosphate. Most lines for applying this type of chromate spray it on cleaned aluminum extrusions, but some dip lines are in operation. Since the chromate coating itself contains no hexavalent chromium, it is common practice to use a passivating hexavalent chromium rinse as the final step in the treatment. A typical cycle would be: Alkali clean. Rinse Chromium-phosphate. Rinse. Hexavalent chromium rinse (seal). Paint is customarily applied in-line by electrostatic spray and then baked, pro-ducing a finished extrusion ready for assembly into window casings and such. This process produces remarkable adhesion. Corrosion resistance is also excellent if the final seal and paint are properly selected. The coating has an almost colorless to pale green appearance, making it very suitable for the application of white and pastel colors (no bleed through). Chrome Oxide. Chrome oxide treatments deposit a chromate film ranging in color from nearly colorless through gold to brown. Baths contain chromates, fluorides, nitrates and an accelerator, which may be ferricyanide or a metal from group 7 or 8 of the periodic table. Ferri- cyanide-accelerated baths are fastest, developing the most coating for a given time. They are also quite easily controlled and rejuvenated. The main objection to ferricyanide baths is the complex cyanide radical in the subsequent rinse water, which is difficult to remove or destroy, and decomposes to cyanide in the presence of ultraviolet light. Gold chromate films are used both with and without paint. Unpainted, they give corrosion protection on aluminum stampings, tubular products, extrusions, die castings and aluminum heat exchangers. As a base for paint, the gold chromates afford both adhesion and corrosion resistance to coil-coated sheet used for residential siding and building panels. Unlike the chrome-phosphate, chrome oxide coatings are rich in hexavalent chromium and need no additional seal to develop their maximum corrosion resistance. Process Parameters In general the operation and control of a chromate processing solution are simplicity itself. Treatment times, whether by spray or dip, are relatively short, ranging from a few seconds (for a bright dip on zinc plate) to a maximum of five minutes (for the heaviest coatings on aluminum). Temperature requirements are also moderate: most solutions operate at or near room temperature and few require temperatures above 130F. The solutions are aggressive, especially those that contain fluorides. Satisfactory acid-resistant materials are stainless steel (316 ELC), polyethylene, polypropylene, PVC and rubber. The usual chemical control of a chromating solution consists of a titration to determine hexavalent chromium, and pH measurements to regulate acidity. Titration of total acid is sometimes used, but the build-up of reaction products makes pH a more reliable choice. Chromate films should not be dried at elevated temperature, since they rupture (mud cracking) and lose protective value. For films on zinc, drying temperature should be below 130F, while aluminum can tolerate temperatures to about 180F with no adverse effects. Testing All chromates are intended to increase the service life of the finished article and this implies improved corrosion resistance. Accelerated tests have been developed in an attempt to predict service life and to compare various treatments. The most common of these is exposure to five pct salt fog (ASTM B-117). Use caution both in performing the test and in the interpretation of results. When evaluating results, comparing results between different test sites or recording results, give particular attention to the following: Use of good quality sodium chloride in the test solution, and pH adjust. Measure atomization rate (amount of salt solution sprayed). Specify what constitutes a significant surface. A significant surface is customarily exposed at an angle 15 degrees from vertical. This may be particularly difficult in the case of odd-shaped parts or small screws. In these cases, the angle or position of exposure must be agreed upon, since it is of paramount importance and results are otherwise not comparable. Companies or trade associations may specify how parts are to be positioned. Specify what constitutes corrosion, e.g., a white pinpoint, heavy white corrosion products or red rust. If possible, have photographs or examples to illustrate degrees of corrosion. Specify if parts are to be observed at periodic intervals (e.g., 24 hours) and tested until failure, or exposed for a certain time (e.g., 168 hours), then evaluated and rated after completion. These points are directed to the salt spray test but are valid for other corrosion tests as well. All corrosion tests are subject to interpretation. Outdoor exposure tests are among the most reliable, but even these really measure only what happens under a very specific set of conditions in a single environment; they should not be interpreted as "real world performance." OSHA Chromating chemicals do not present extreme hazards to workers if they are handled properly. Workers must be instructed in safe handling. All chromates are acidic and will attack living tissues. Damage to eyes can be especially severe. Skin burns may be slow to heal and secondary infections can result. Materials high in fluoride can cause sores that are even more resistant to healing. Follow these precautions: Wear protective clothing, i.e., rubber gloves, safety goggles or face shield (in the case of stronger concentrates). Immediately wash chemicals from skin whenever contact occurs. Use plenty of water, then wash with soap and water. Have a safety shower and eye fountain nearby, in case of accidental contact. Do not eat or smoke when handling chemicals; wash well before doing so. Launder all contaminated clothing before re-use. In addition to their acid properties, chromates are strong oxidizers and may cause spontaneous combustion if the concentrated products are mixed with other substances, especially paper, rags, wood or flammable solvents. Do not put spilled chemicals in waste containers. EPA Chromium, whether in hexavalent or trivalent form, is classified as hazardous waste by the EPA. Chromate-containing effluents are also regulated on the federal, state and local level. Rinse waters and spent solutions must be treated to remove chromium before discharge. Most commonly, hexavalent chromium is reduced to the trivalent form with sodium metabisulfite. The trivalent chromium is then precipitated by pH adjustment with alkali such as liquid caustic or lime. The precipitated solids containing the chromium are mechanically separated from the liquid by settling or filtration. They can then be removed (by a certified waste hauler) to an approved landfill. PFD TABLE I—Common Chromate Treatments Metal Type of Chromate Accelerators & Modifiers Purpose Zinc Electroplate Cadmium Electroplate Zinc Die Casting Galvanized Aluminum Aluminum Clear Gold Blue Bright Olive Drab Black Clear Gold Clear Clear Clear Gold Clear to light green Sulfate, buffers Sulfate, buffers chloride Fluoride, sulfate Organic acid Silver Sulfate, buffers Sulfate, buffers Trivalent chromium, phosphate Hexavalent chromium, fluoride Fluoride, sulfate Fluoride, ferricyanide Fluoride, phosphate Protective decorative Protective decorative Protective decorative Protective Protective decorative Functional, protective decorative protective Mill passivation Mill passivation Paint base (coil stock) Protective decorative paint base Paint base(architectural)