Supplemental Oil Coatings

Unless they are to be painted, parts usually receive a supplemental coating of oil after being phosphated. This coating is applied to increase corrosion resistance, and it also neutralizes any residual acid that might remain on parts from the phosphating bath. The type of oil used depends on the degree of corrosion protection desired, subsequent operations to be performed on phosphated parts and the handling involved in these operations, appearance requirements, and compatibility of the oil with other lubricants in assemblies. Materials commonly used are water-soluble oils, nondrying oils, non-hard-drying greaselike materials, and oils that are dry to the touch.

Water-soluble oils provide both short-term and long-term protection against corrosion, depending on their composition. Water-soluble oils offer the advantage of allowing parts to go into water-soluble oil in a wet state. An additional advantage of the water-soluble oil is that it eliminates a fire hazard from the operation.

Figure 12 shows an immersion tank used in the coating of lightweight parts with soluble oil for applications in which subsequent handling or assembly requirements require a virtually dry part.

MilrJ Steel plaie egi

MilrJ Steel plaie egi

Fig. 12 Immersion tank for coating lightweight phosphated components with a soluble oil. Skimming trough removes floating globules of oil that might cling to parts.

Flash points of water-soluble oils are sometimes lower than those of petroleum-based oils or synthetic organic oils. However, after water-soluble oils are mixed with water to the 5 to 25% concentration range, little fire hazard attends their use.

Nondrying oils vary in type and viscosity and are selected on the basis of requirements of in-process handling or ultimate service. An advantage of this type of material is its ability to self-heal any scratches that may occur in bulk handling. Corrosion protection may be increased by adding a commercially available rust inhibitor that is compatible with the oil. Petroleum-based oils can be reduced with petroleum solvents to form a thinner film, if desired. If parts are not completely dry before the application of oil, water-displacing additives may be used.

Non-hard-drying materials are greaselike substances that have melting points above room temperature. These materials may be applied by dipping, spraying, or brushing. When necessary, these materials are readily removed by petroleum solvents. Figure 13 shows a tank for dip application of greaselike materials. The tank is provided with facilities for heating and cooling to maintain temperature control of the coating material.

Fig. 13 Tank used for dip coating of phosphated parts with greaselike materials that require temperature monitoring

Decorative Stains (Ref 7)

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Fig. 13 Tank used for dip coating of phosphated parts with greaselike materials that require temperature monitoring valve valve

Depending on the process and the substrate used, heavy phosphate coatings on steel vary from near-black to medium-gray. Manganese phosphate coatings are generally the darkest.

Many applications require a definitive, reproducible color, most often black, for decoration or identification purposes. This can be achieved by applying an alcohol-based stain, usually with a shellac or manila resin binder, and an original dye of the color desired. Because the stain has limited protective value, a light oil can subsequently be applied to help prevent corrosion and enhance appearance (see the section "Supplemental Oil Coatings" in this article). In heavy-duty applications, pigmented finishes can be used, but they produce thicker coatings that can cause problems with dimensional buildup on threaded components.

Figure 14 shows the corrosion resistance of black stain, corrosion-preventing oil, and black stain plus oil, relative to the type of phosphate coating applied.

Figure 14 shows the corrosion resistance of black stain, corrosion-preventing oil, and black stain plus oil, relative to the type of phosphate coating applied.

Fig. 14 Corrosion resistance of selected metal finishes relative to type of phosphate coating applied. (a) Black stain. (b) Corrosion-preventing oil. (c) Black stain and oil. Source: Ref 7

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