Other Selection Parameters

The film thickness of rust-preventive compounds must be controlled in order to maintain both uniform corrosion protection and a prescribed level of efficient application. Films that vary in thickness do not permit an accurate forecast of storage life or material cost.

The film thickness of oil compounds is controlled by viscosity and surface tension. However, gravitational pull causes oil to seek its own level. Thus, all oil films are reduced over time, so the viscosity effect is temporary at best. Rust-preventive compounds based on solvent cutbacks are formulated to provide a specific film thickness when the solvent evaporates. This thickness is controlled by the percentage of solids contained in the compound. For petrolatum compounds, thickness is controlled by the application technique and temperature.

Fluid types that are applied by any method will drain off to a thin film. Varying viscosity levels will initially allow varying coating weights, but both high and low levels will seek the same level of coating weight because of the pull of gravity. Roll coating and electrostatic application, as well as a spray application followed by wiping or squeegee rolls, will lay down a consistent film. Oil- and solvent-based films will migrate and puddle in low areas. On plated products, these puddled areas will appear shiny, in contrast to areas that appear dull. In the dulled area, however, oil is present at a lesser coating weight. Because coiled or sheet metals cannot be produced perfectly flat, puddling will occur over time. Usually, a sufficient amount of oil is present to provide corrosion protection.

When compounds are applied by hot dipping or hot spraying, the temperature of the coating and the temperature of the part both influence film thickness. Thick coatings will congeal on room-temperature parts that are dipped and withdrawn quickly, so that there is no appreciable increase in part temperature. However, if a part is immersed until its temperature approximates that of the heated rust-preventive compound, then a thinner coat results. In general, the greater the difference between the temperature of the compound and the temperature of the part at the time of withdrawal, the heavier the coating.

Figure 2 shows the influence of the temperature of a petrolatum rust-preventive compound and the temperature of dip-coated panels on the film thickness obtained when the panels are dipped and withdrawn rapidly (1 s for immersion, 1 s for withdrawal). The curve shown is for panels at 27 °C (80 °F). Comparisons of film thicknesses are presented for panels at temperatures of 21 and 32 °C (70 and 90 °F), dipped in rust-preventive compound with a temperature of 85 °C (185 °F).

lODO

Dipping temperature ot petrolatum, *F HO 160 130 2Ö0

Dipping temperature ot petrolatum, *F HO 160 130 2Ö0

Cl

—i-

N

. Panel [empErsture, 27 ÜC

21 °C

32 °C

>-

Dipping temperature of pt-lroiatum, °C

Dipping temperature of pt-lroiatum, °C

Fig. 2 Effects of panel and compound temperatures on thickness of petrolatum film applied by dipping

The specific gravity of the compound also influences coating thickness. For a specific weight of compound, the film thickness increases as the specific gravity of the compound decreases. The results of a test that determines the influence of specific gravity on film thickness are shown in Fig. 3. The panels used in this test were 75 mm (3 in.) long, 50 mm (2

in.) wide, and 3.2 mm (1 in.) thick. They were at an ambient temperature of approximately 24 °C (75 °F) when they were 8

subjected to a 1 s dip into and a 1 s withdrawal from the petrolatum compound, which was heated to various temperatures to produce different film thicknesses. Five petrolatum preservatives were used, each of which had a different specific gravity.

Weigh! of rust preventive, Oi n

B 50

ra 6

Weigh! of rust preventive, Oi

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r -

i on

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Weigh) Df rl k t preventive, 5

Weigh) Df rl k t preventive, 5

01 c s ihj

Wehghi ¡>1 ruil pfiNeriive, Oi 003 0.Ö6 0.03 0.12 0.15

Spccifrcgrairitv 0.30 [ \ oe.\ ^d

T

0.95

3D yi

t OD

«e (a)

U 1 2

4 5

Weigh! al rysl plrevCn I ivt,-g

Weigh! al rysl plrevCn I ivt,-g

Fig. 3 Influence of specific gravity of petrolatum compound on film thickness for a specific weight of rust preventive applied to test panels. (a) Coating thicknesses up to 75 pm. (b) Coating thicknesses up to 750 pm

Tests also were conducted to determine the influence of four other variables on coating thickness obtained under laboratory conditions using a petrolatum rust-preventive compound. These variables were:

• Temperature of preservative

• Mass of metal specimen

• Duration of immersion

• Rate of withdrawal

Four conclusions were drawn from data obtained in these tests. First, as the temperature of the compound increased, its fluidity increased. Thus, progressively thinner coatings can be obtained with petrolatum rust preventives as temperature increases.

Second, the thickness of the panel (the mass of the metal specimen) influenced the thickness of the coating. With a greater metal mass, a temperature lag existed between the specimen panel and the heated preservative. The coating thickness was greatest with the greatest temperature differential. Thinner coatings developed as the temperature of the panel approached that of the compound.

Third, the duration of immersion had great influence on coating thickness. As immersion time increased, the coating first reached a maximum value, and then melted away uniformly as the panel temperature approached that of the compound.

Fourth, a slow rate of withdrawal left the surface of the coating smooth and regular, indicating uniform film thickness. Rapid withdrawal resulted in distorted, irregular surfaces with numerous small, shallow areas.

The curves in Fig. 4, which were obtained from these tests, show the influence of mass and withdrawal rate of the panel on film thickness for one petrolatum rust-preventive compound. For both curves, the immersion time was 10 s.

Fig. 4 Comparison of coating thicknesses obtained using the same withdrawal rate of panels from a petrolatum rust-preventive compound

The removability of rust-preventive compounds primarily depends on the thickness, hardness, and chemical characteristics of the protective film. Nondrying oil or grease compounds can be removed by simple petroleum solvents, preferably those with a flash point above 40 °C (105 °F). Vapor degreasing, hot power-spray washing with alkalies or strong detergents, and steam alkali or steam detergent blasting are also used to remove nondrying oil or grease compounds. Wiping can be used, as well, if the desired degree of removal will permit it. The solvent-cutback asphaltic or dry-film compounds usually require high-solvency organic solvents, vapor degreasing, or vigorous extended treatment with steam or hot spray cleaners.

Highly active rust-preventive compounds are difficult to remove. Therefore, when readily removable products are necessary, some corrosion protection usually must be sacrificed.

Scraping, blasting, or wire brushing may be required to remove hard-film preservatives that are completely or partially impervious to solvents, and damage to machined or polished surfaces can result. Compounds of hard film, viscous oil, or heavy grease, which are used to protect parts in either outdoor storage or highly corrosive atmospheres, will cause problems if they are not thoroughly removed, because their residues usually are incompatible with lubricating oils and greases. Examples of parts that can experience such problems are components of hydraulically actuated systems and precisionmachinetoolspindles. These rust-preventive compounds also can clog filters and screens in circulating-oil systems. Additionally, hard-wax rust preventives can cause serious problems in sliding bearing mechanisms if they are not completely removed.

Costs. Rust-preventive compounds are rarely selected on the basis of cost alone. Table 3 lists the typical duration of protection provided by four general types of compounds in three different storage conditions. In general, the cost per liter increases as the amount of protection increases.

Table 3 Duration of protection afforded by rust-preventive compounds

Type of compound

Duration of protection, method

Indoor

Shed

Outdoor

storage

storage

storage

Petrolatum

Not used

12-24

6-18

Oil

2-12

1-6

1-2

Solvent

<1-3

<1

Not suitable

Solvent cutback

6-18

3-12

1-6

Emulsion

3-12

Under 3

Not suitable

The quality control of rust-preventive compounds is necessary for maintaining established levels of efficiency and performance. The extent and frequency of the quality-control program depends on the compound being used and the degree of control required. It is often advisable to start with frequent checks, in order to develop a system history, and then to adjust to safe intervals.

The checks should include the concentration level of the active ingredients; contaminant levels for dirt and water; and degree of oxidation, as determined by some means of pH and neutralization numbers. The nature of the processing operations prior to the application of the rust-preventive compound may make it necessary to run other special quality-control checks, such as those that determine viscosity, flash point, copper strip corrosion, and infrared spectra.

The viscosity and specific gravity, or both, of oil and solvent-cutback rust-preventive compounds are checked as received, at a specific temperature. Standard values, against which the result of checks are compared, either are obtained from the supplier or are established from material that has performed satisfactorily in controlled salt spray, humidity, or field tests. The viscosity is checked using effluent cups, or instruments that measure the resistance of the material to a moving circular spindle or a falling ball. Methods for testing viscosity are described in detail in the article "Painting" in this Volume.

Specific gravity, sometimes used to indicate solids content, is determined by the correct hydrometer. This measurement is usually taken at a specific temperature of the preservative. The viscosity of the compound should be checked when processing begins, and then rechecked at definite intervals in order to correct changes that result from usage conditions, such as the replacement of solvent lost by evaporation. However, because some materials have approximately the same viscosity as the material used to dilute it, periodic checking of specific gravity or other characteristics is necessary for maintaining the proper balance between solids and solvents.

The flow characteristics of petrolatum compounds can be determined empirically by methods described in ASTM D 937. The results obtained by this method are of maximum value when correlated with the flow characteristics of similar materials.

Film Characteristic. The efficiency of the film ordinarily is determined by exposing a precisely coated part to either a salt-spray test (ASTM B 117) or a humidity test (ASTM D 1748). However, the accurate projection of test results to give an expected service life requires experience. Because storage areas undergo varied and changing conditions, it is difficult to predetermine actual protection data. If the product is held under static or ideal conditions, then correlations between lab tests and storage conditions would be more reliable.

The preparation of surfaces to be protected has an important bearing on the effectiveness of a rust-preventive compound. Prior surface treatment ensures freedom from incompatible processing oils, waxes, hygroscopic or chemically reactive salts, or random dirt. These materials could either modify the adhesion or thickness of the film or counteract the inhibitors in the film.

Precleaning during routine parts processing should occur under effective process control, in order to minimize variance from acceptable limits of surface cleanliness. When a temporary setup is used, extra precautions against contamination are essential. Contamination of the compound itself from careless handling or storage procedures should be prevented.

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