Perhaps no area of manufacturing has undergone more change than the critical area of metal pretreatment. Worldwide, environmental regulations have specifically targeted hexavalent chromium, a highly effective but toxic and carcinogenic material. For decades, "hex chrome," was the primary ingredient in passivating seal rinses, the mainstay of high-performance metal pretreatment.
The search for an alternative to hexavalent chromium has been lengthy. The greatest difficulty is creating a formula that offers the protective properties of chromium but without the toxicity, environmental hazards and the handling risks.
Hexavalent and trivalent chromium
All chrome sealing rinses are manufactured from blends of hexavalent and trivalent chrome salts. Hexavalent chrome has substantial oxidizing capacity. Because of its higher reduction potential, phosphated steel can be converted to a more passive state. Additionally, during processing highly insoluble chromium salts are deposited on the phosphated metal surface. These two actions retard the natural process of metal corrosion. Thus, substrates treated with chromium-containing rinses oxidize far more slowly than those that are not treated.
Chromate rinses are applied to phosphated metal surfaces in a precise cycle. They are highly reactive and require a misting with deionized water to remove the excess unreacted chrome salts. Left in place, these excess chrome salts would prevent proper coating adhesion.
Chromate final rinses present daunting problems for users. Hexavalent chromium, classified as a carcinogen, is absorbed through skin, and is toxic if inhaled. Disposal of used solution is also a problem. Because chromium is a hazardous chemical, final rinses containing the element cannot be discharged to sewers without extensive treatment. In addition, chrome- containing sludge must be buried in an approved hazardous waste disposal site, a procedure that is becoming ever more costly.
Coatings provide metal two benefits. They are decorative, and they provide a film barrier through which oxygen, water and salts cannot penetrate. However, studies during recent decades have demonstrated that micro voids exist in all coatings. Micro voids vary in size, but many are large enough to allow permeation of water molecules and ions of oxygen and salts. Since metals naturally oxidize in the presence of oxygen, permeation of the coating through micro voids greatly enhances the chance that corrosion will take hold.
|1. Typical five-stage pretreatment line.|
Alkaline spray cleaner
Rinse 6107 in
D-I. or R.O.
|1968.2 gallons||706.7 gallons||1413.1 gallons||706.7 gallons||706.7 gallons|
Phosphate pretreatment converts (passivates) the active metal surface, which readily oxidizes into a relatively inert surface that will not readily oxidize. This substantially retards the oxidation process. Because the phosphate layer is crystalline, the total surface area of the metal increases, and irregular shapes are formed. This enhances the mechanical adhesion of the coating.
When the coating is mechanically and chemically bonded to the inert, phosphate-coated metal surface, an anaerobic (oxygen-free) condition is created beneath the coating. If oxygen, water or salt permeate the coating, this anaerobic environment retards oxidation. If the phosphate treatment is insufficient or absent, an aerobic (oxygen) environment will be present beneath the coating, and oxidation or corrosion will occur quickly upon permeation of the coating.
The degree of corrosion resistance that a coating applied over phosphate-treated metal can achieve depends on several factors:
- Cleanliness of the metal substrate prior to phosphating;
- Density and size of the phosphate crystalline structure; and
- Reactivity of the final seal rinse.
Modern pretreatment lines include a cleaning stage prior to phosphating. Iron or zinc phosphates are then applied to the clean metal substrates. High-performance iron phosphates require a minimum of five stages (Fig. 1), including a final seal rinse or post rinse. Zinc phosphates require a minimum of six stages and as many as 11 stages for high-performance applications such as automotive or appliance components.
All stages in a pretreatment line serve important functions. The final seal rinse or post rinse significantly improves coating adhesion as well as corrosion resistance. Historically, chromated rinses have provided the best corrosion resistance for steel substrates and have been compatible with the widest variety of paint resin systems. The use of chrome seal rinses has been based on its electrochemical advantage over other treatments.
|Table I—Paint resin system known to react with the non-chrome rinse|
|1. Polyester TGIC resins
2. Polyester/epoxy hybrids
3. Acrylic/TGIC resins
(1 or 2 component)
In 1997, a new chemistry was introduced based on an organo-metallic polymer synthesized for the first time. The chemistry achieved a level of corrosion resistance comparable to that of a chrome rinse, and it was compatible with a variety of coating chemistries. Table I shows paint resin systems known to react with the rinse system.
The organo-metallic polymer in the rinse is bi-functional. One end of the polymer has an organic function that reacts effectively with liquid or powder paint resins containing hydroxyl, carboxyl or amino functionalities. The other end of the polymer reacts with the metal pretreatment, creating a reactive chemical bond between the metal and the paint.
Paint adhesion now becomes a tenacious chemical bond in addition to a mechanical bond. The result is a special combination of chemical and physical properties that can substantially benefit the manufacturers and users of metal products.
No post rinse required
The rinse is applied to the phosphated metal in the final stage of a pretreatment line. Because it does not react with the phosphate coating until it reaches the dry-off oven, it does not require a post rinse with water. This saves a step and reduces overall wastewater volume.
The rinse is not considered a hazardous material. It is safe to handle, and may be discharged to any sewer. The non-chrome seal rinse has run daily in production for four years at Hobart Brothers Company, Troy, Ohio.