General Batch Galvanizing Procedures

Wet and Dry Galvanizing. Two types of conventional batch galvanizing practices currently in use are the wet process and the dry process. Dry galvanizing was developed and refined in Europe. Recent surveys indicate that 40% of galvanizing operations in North America are wet and the remaining 60% dry. The dry process is generally considered to be less energy intensive than the wet process, but it is more sensitive to surface preparation deficiencies.

Wet galvanizing involves a kettle-top flux blanket; dry galvanizing uses a preflux, and no flux blanket on the kettle. Sulfuric or hydrochloric acid pickling can be used with either wet or dry galvanizing. The choice is primarily based on disposal and reprocessing costs for the acids.

In the dry galvanizing process, after the material is degreased and pickled, the workpieces are immersed in an aqueous flux solution, dried, and then immersed in the molten zinc bath. In the wet galvanizing process, the work is not usually prefluxed after cleaning and pickling but is placed in the molten zinc bath through a top flux blanket on the kettle. However, an aqueous preflux may be used in conjunction with a top flux on the zinc bath.

Workpieces are handled either mechanically, using overhead hoists, or with hand tools. Small items such as washers, fasteners, and nails are handled in baskets. The baskets are usually centrifuged as the work leaves the molten zinc bath to remove excess zinc and to distribute the coating evenly. In all cases, workpieces must be handled properly until the coating has completely solidified, which is accomplished by either quenching or air cooling.

Surface Conditioning Requirements. Although degreasing, pickling, water rinsing, and other cleaning procedures remove most of the surface contamination and scale from iron and steel, small amounts of impurities in the form of oxides, chlorides, sulfates, and sulfides are retained. Unless removed, these impurities will interfere with the iron-zinc reaction when the iron or steel part is immersed in molten zinc.

In the wet galvanizing process, a flux blanket on the surface of the molten zinc bath is used to remove these impurities and to keep that portion of the surface of the zinc bath through which the steel is immersed free from oxides. The flux blanket floats on the surface, and when workpieces are immersed in the bath, their surfaces are wetted by the molten flux. The flux must have sufficient chemical stability to maintain a chemically active foam at the galvanizing temperature and to perform its cleaning function at a high rate of speed.

Zinc ammonium chloride is generally used to provide a flux blanket on the molten zinc bath. There are several procedures for preparing the flux blanket. One generic method consists of mixing ammonium chloride (sal ammoniac) and zinc oxide to form the monoamine of zinc chloride. In the ensuing reaction, hydrogen and nitrogen are released, and the flux takes the form of a foam. Today, most commercial kettles are operated using premixed fluxes, which can be obtained from a variety of suppliers.

To be fully effective and to attain minimal fume galvanizing conditions, the flux should not contain an excess of ammonium chloride. To prevent abnormally rapid chemical breakdown of the flux, glycerin and other organic substances are added to the flux in small amounts of 1 to 2 vol%. These substances increase the foaming action, markedly reduce the loss of ammonia, and serve as insulators. Because the boiling point of a mixture of zinc chloride and ammonium chloride decreases as the ammonium chloride content increases (Fig. 8), no more than 2 to 3% ammonium chloride will remain in the molten salt solution at a kettle temperature of 455 °C (850 °F). However, to function effectively, a zinc chlorideammonium chloride top flux must normally contain more than 5 to 10% dissolved ammonium chloride. To minimize fuming, it is necessary to reduce the top flux surface temperature, maintain an optimal level of dissolved ammonium chloride, and to disturb this equilibrium as little as possible.

Fig. 8 Zinc chloride and ammonium chloride phase diagram

For dry galvanizing, it was once common practice to take the work directly from the hydrochloric acid pickle, dry it, and then put it in the molten zinc. However, the practice of going from the pickle tank to drying and then to the molten zinc produces more dross than the procedures of rinsing, prefluxing, and drying.

A preferred method, widely used today, is to pickle, rinse, flux in an aqueous zinc ammonium chloride, dry, and dip in the molten zinc bath. By using a preflux step, better control of the fluxing is possible, resulting in a more consistent finish. Also, the material may be held for about 1 h before dipping, which gives the galvanizer some flexibility in work flow on the galvanizing line.

Galvanizing Bath. The molten zinc bath is operated at temperatures usually in the range of 445 to 454 °C (830 to 850 °F). At 480 °C (900 °F) and above, the dissolution rate of iron and steel in zinc is extremely rapid, and the effects of these temperatures on both workpiece and galvanizing tank are generally harmful. Within the conventional galvanizing temperature range, an increase in temperature:

• Increases the fluidity of molten zinc

• Accelerates the formation of oxides on the bath surface

• Heats the part to a higher temperature, thus increasing the time required for the zinc to solidify when the part is withdrawn

• Reduces immersion time, thereby increasing the kettle utilization factor

Each of these considerations has a distinct effect and may be used to control the galvanizing process.

An increase in the fluidity of the bath improves drainage and is desirable provided the bath temperature does not exceed the normal operating range. An increase in bath temperature produces a much sharper temperature gradient from the surface to the center of the part, which depending on shape may result in an increase in distortion.

Unless the bath contains aluminum or unless its surface is well protected by a foam blanket of flux, an increase in bath temperature will accelerate the formation of an oxide film (or ash) on the surface of the bath. Some of this oxide film may cling to the workpiece when it is withdrawn from the bath, interfering with drainage and contributing to the formation of a coating with less desirable aesthetic properties. The effects of these oxides are most apparent on parts of thin cross section and large surface area.

Depending on the chemical composition of the iron or steel, the bath temperature may have significant metallurgical effects on the galvanized coating. The temperature at which the iron-zinc alloy layers are formed affects the relative amounts of each iron-zinc phase formed and the depth or total thickness of alloy layer (Fig. 4).

In the hot dip galvanizing of fabricated articles, the thickness of the coating is controlled by immersion time. Although timing is to some extent dependent on ease of handling and must be established by trial for each design of part being coated, the duration of immersion is usually in the range of 3 to 6 min. The speed of immersion influences the uniformity of the coating, particularly with long articles for which the difference in immersion time between the first and last areas to enter the bath may be considerable.

The reaction between clean low silicon steel and molten zinc proceeds rapidly for the first 1 or 2 min after the work has been immersed, producing an alloy layer that continues to grow at decreased rate the longer the article is left in the bath. However, for steels containing silicon in excess of 0.05%, the coating weight increases linearly with respect to the time of immersion producing, in general, heavier coatings (Fig. 9) Therefore, it is important to minimize immersion time for silicon-bearing steels to prevent excessive alloy growth and coating weight.

Fig. 9 Effect of immersion time on galvanized coating weight for killed and unkilled steels. Galvanizing temperature, 455 °C (850 °F). Killed steel: 0.35% C, 0.26% Si, 0.46% Mn. Unkilled steel: 0.13% C, trace Si, 0.40% Mn

To provide a uniform coating of minimum thickness, work that is not subsequently to be centrifuged is withdrawn from the bath slowly and at a controlled rate, thus permitting maximum drainage. Two-speed hoists are usually employed, permitting the work to be immersed rapidly and withdrawn slowly. The rate of withdrawal, which partially determines the thickness of the unalloyed zinc layer left on the work (Fig. 5), varies according to the type of process being operated. The optimum withdrawal rate for most articles is about 1.5 m/min (5 ft/min).

With long articles, for which the withdrawal occupies a large part of the total cycle time, higher speeds may be necessary to maintain a reasonable rate of production. If possible, however, it is better to overcome this difficulty by using special jigs and carriers for dipping and withdrawing the work in batches. Provided the work is not withdrawn faster than the rate at which the zinc drains freely from the surface, the unalloyed zinc layer of the coating is uniformly distributed. With faster rates of withdrawal, the surplus zinc carried clear of the bath runs down the surface until it solidifies, and the resultant coating may be lumpy and uneven.

When withdrawal from the bath at a slow controlled rate of speed is not feasible or economical, as it generally is not for small parts, the withdrawal rate may be greatly increased, and drainage is accomplished by spinning the parts in a basket. The excess zinc is drained from the parts by centrifugal force. Mechanically wiping the excess zinc from the parts with tools designed for this purpose is an alternative method of forced drainage.

Cooling. Because of retained heat, the iron-zinc reactions can continue to occur even after the surface layer of zinc has frozen. This type of post-immersion reaction may occur if cooling is hindered by the stacking of parts in close proximity and by the heat capacity of the part. Some or all of the pure zinc layer may be converted to iron-zinc alloy, thus darkening the surface and altering its properties. Very slow cooling or holding of the material at temperatures over 190 °C (375 °F) may cause small voids in the coating due to interdiffusion between the coating and the base metal. The problem, known as the Kirkendall effect, may result in peeling failure of the coating.

To avoid delayed cooling, parts should be spaced adequately after immersion to permit the free circulation of air. Parts with large cross-sectional areas or parts fabricated of silicon-bearing steel may require forced cooling with air or water.

Galvanizing of Silicon-Killed Steels. The difficulties encountered in galvanizing silicon-bearing steels have been noted under the preceding discussions on conventional galvanizing practice. Many techniques have been studied in an effort to find a better way to control the iron-zinc reaction kinetics in the presence of silicon, none of which has proven to be fully acceptable over the silicon content ranges commonly being encountered. At temperatures not exceeding 460 °C (860 °F), aluminum additions of 0.02 to 0.04% to the bath may be of advantage for steels containing up to 0.05% Si. The most common method has been to use conventional techniques at a temperature not exceeding 440 °C (825 °F) coupled with a short immersion time. For light structural shapes, this is possible without any additional measures. Heavy structural shapes require preheating to reduce the immersion time to an acceptable level.

However, three technologies developed during the 1970s permit better control to be maintained in the galvanizing at reactive temperatures of silicon-bearing steels. One is a patented process using the zinc alloy Polygalva for galvanizing. The second is by galvanizing at high temperatures, 550 °C (1020 °F), instead of the temperatures used for the conventional process, 450 °C (840 °F). The third uses small amounts of nickel in the bath. All three methods are used to some extent in Europe. Polygalva is not available in North America, but the other two methods have seen limited use. Several North American galvanizers use the nickel addition method, and at least one uses the high-temperature method.

Polygalva Process. Polygalva is essentially a zinc alloy containing controlled amounts of aluminum, magnesium, tin, and lead. The aluminum is used to retard the formation of the intermetallic layer, and the other elements help to ensure continuity of the galvanized coating. As with the conventional process, thorough surface preparation prior to galvanizing is essential for good results and must include the following steps unless otherwise indicated:

• Degrease in an alkaline bath heated to 80 to 90 °C (176 to 194 °F)

• Rinse in running water

• Pickle in 50% hydrochloric acid with inhibitor

• Rinse in running water

• Pickle in 70% hydrochloric acid without inhibitor (in most situations this step is optional)

Routine maintenance of pretreatment facilities is important.

Also, a weekly zinc bath chemical analysis is required to ensure that the alloy composition is being maintained within the working range. During galvanizing operations, two master alloys are added to the bath to compensate for losses of aluminum and magnesium. By doing this, proper alloy balance is reportedly readily maintained.

As shown in Fig. 10, Polygalva is most effective in galvanizing steels with silicon in the range of 0.05 to 0.20%. Comparative micrographs of a steel with 0.08% silicon galvanized using conventional and Polygalva techniques are shown in Fig. 11. Above 0.20% Si, the Polygalva alloy reportedly loses at least part of its effectiveness.

\ fine \

ventional

Polygatva

zinc alloy

Fig. 10 Comparison of coating weight as a function of silicon content for conventional and Polygalva galvanizing processes

Fig. 10 Comparison of coating weight as a function of silicon content for conventional and Polygalva galvanizing processes

Fig. 11 Micrographs of a silicon-bearing steel (0.08% Si) galvanized (a) in a conventional bath and (b) in a Polygalva bath

High-Temperature Galvanizing. It has been found that when galvanizing is performed at a temperature of approximately 550 to 560 °C (1020 to 1040 °F), a coating weight to immersion time relationship is obtained for silicon-bearing steels which is much less sensitive than at conventional galvanizing temperatures (Fig. 6). The effect of immersion time at elevated galvanizing temperatures is shown in Fig. 12; the coating weight increases at a rate less than linear with time. Doubling the time of immersion from 4 to 8 min increases the coating weight by about 30%.

Fig. 12 Coating weight versus immersion time for three steels with varying silicon contents galvanized in a high-temperature bath containing 0.22% Fe. • , steel containing 0.02% Si; •, steel containing 0.22% Si; • , steel containing 0.42% Si

Because of the high reactivity between molten zinc and steel at these temperatures, a ceramic-lined steel kettle is used. While the available ceramic kettle technology is adequate for this application, the state of the art of ceramic kettle heating currently lacks the efficiency of flat flame burners used to heat steel kettles. The development of immersion heaters to heat the ceramic baths is proceeding and offers good promise of improved efficiencies. Pending development of an operationally suitable immersion heater system, ceramic galvanizing kettles are top heated. This technique is relatively inefficient and interferes with the handling of materials into and out of the kettle.

In the bath, the solubility of iron increases from 0.03% at 450 °C (840 °F) to 0.3% at 550 °C (1020 °F). By controlling the iron content, the coating weight can be controlled. As shown in Fig. 13, increasing the iron content from zero to 0.3%, the solubility limit at 550 °C (1020 °F) increases the coating thickness by a factor of 2. Controlling the iron content within the range 0.1 to 0.2% produces coating weights which meet specifications and are not excessively heavy.

Fig. 13 Coating weight as a function of galvanizing bath iron content for three steels with varying silicon contents. Galvanizing time, 3 min at 550 °C (1020 °F). • , steel containing 0.02% Si; •, steel containing 0.22% Si; • , steel containing 0.42% Si

In a high-temperature galvanizing bath, aluminum up to 0.5% does not appear to have any systematic effect on coating weight. However, above 0.3%, it produces a floating dross which can mar the coating appearance. An aluminum addition of 0.03% is sufficient to brighten the coating if this is desired.

There have been reports of occasional adherence deficiencies of high-temperature galvanized coatings. At this time, it is believed that this is the result of a lead-deficient bath. As a result, the bath lead level should be maintained at 1%. Therefore, the following bath conditions are considered ideal:

The coating has a light gray, uniform appearance that does not vary with silicon content of the basis steel. Brighter coatings can be obtained by the aluminum addition to the bath described above and by quenching instead of air cooling. Coating adhesion and ductility are equivalent to coatings galvanized at conventional temperatures. The metallographic structures of all high-temperature coatings are similar, the only variation being in the constituent proportions of some of the layers (Fig. 14).

Fig. 14 Electron-scanning image of a high-temperature galvanized coating

Mechanical property tests of the basis steel subjected to high-temperature galvanizing reveal no significant differences from the results obtained using conventional galvanizing techniques. These tests include tensile properties on plain, punched, and welded specimens, reverse bend tests for strain age embrittlement, ductility bend tests, and pulsating tension fatigue tests.

Because of the recent development of high-temperature galvanized coatings, their performance is not as well documented as that of conventional coatings. Based on a limited body of longer term exposure data and accelerated weathering tests in marine, urban, and industrial environments, it appears that the performance of these coatings is at least equal to that of coatings produced by conventional techniques.

Nickel Addition to the Bath. It has been known since the 1960s that the addition of certain alloying elements to the bath will minimize the reactive effect of silicon-killed steels.

European galvanizers have been adding nickel to kettles since the 1970s. The process was brought to North America in the early 1980s and has been used in Canada with some success; it has also been in limited use in the United States since that time.

In this process, nickel is added to the bath as part of the zinc, as a special additive similar to brightener bars, or as a powder. The bath nickel concentration should be 0.05 to 0.09%; experience has shown that maximum effectiveness is achieved in this range. This technique has maximum effectiveness for steels with silicon levels below 0.25%. For steels with higher silicon contents, it provides only minimal coating improvements.

Nickel generally decreases the reaction between the zinc and steel for all steels. It is possible that nickel additions may produce low coating thicknesses on some steels. Care must be taken to ensure adequate zinc thicknesses. Figures 15 and 16 show the effects of nickel additions in the galvanizing bath.

Fig. 15 Effect of nickel additions to the galvanizing bath. (a) Typical hot dip galvanized coating on mild steel. (b) Coating on silicon-killed steel, galvanized in bath containing nickel additions. Note the relatively thin delta layer and the thick, coarse zeta layer in (b). Both 250x. Source: Ref 6

Fig. 15 Effect of nickel additions to the galvanizing bath. (a) Typical hot dip galvanized coating on mild steel. (b) Coating on silicon-killed steel, galvanized in bath containing nickel additions. Note the relatively thin delta layer and the thick, coarse zeta layer in (b). Both 250x. Source: Ref 6

Fig. 16 Effect of nickel additions in the galvanizing bath for a steel containing 0.15% Si. (a) Galvanized in nickel-free bath. (b) Galvanized in bath containing 0.095% Ni. Source: Ref 6

Additions of vanadium, 5% Al, and rare earth metals have also shown positive effects on galvanizing of silicon-killed steels.

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  • gary steverson
    How to add nickel in galvanizing zinc bath?
    3 years ago
  • lisa
    Why zinc bath material gray after addition of nickel?
    2 years ago

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