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Table of Contents
An Overview of Hot Dip Zinc Galvanizing

Prepared by J.C. Birdsall, P.E. - GTI Engineering

 

Overview

 

The protection of steel from corrosion by the art and science of hot dip zinc galvanizing has been practiced for almost 200 years, and the technique is still unequaled for the long term protection of structural articles that may be subjected to physical abuse during shipping and installation. This is because of the nature of the alloy coating formed when chemically clean steel is immersed in molten zinc.

The coating formed is an alloy between iron in the base metal and zinc, and it is therefore much more resistant to mechanical damage than surface coatings such as paint, electroplating, anodizing or flame-spray overlays. Further, poorly accessible areas are completely protected, and even if small areas are left uncoated or are scratched by handling, these areas are still protected. This is because of the electrochemical difference between iron and zinc which means that the zinc is consumed by oxidation preferentially to iron or is "sacrificed" to save the base metal.

If, for reasons of appearance or extremely corrosive environments, a paint coating is desired, the surface of the galvanized article can be treated to enhance paint adhesion by dipping in solutions that are generally based on phosphoric acid.

Coating life is dependent on the environment and the rate of corrosion of the zinc layer is approximately 0.1 to 0.2 mils per year ( 1 mil = 1/1000 inch ). This means that a normal 3 mil coating weight which is equal to 2 ounces of zinc per square foot of surface area can be expected to give a 20 to 30 year product life. Painted galvanized structures can have much longer life, and the paint coating can be renewed to give indefinite life.

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Process Description

In order to obtain the alloying reaction, the article to be galvanized must first be clean. This can be accomplished by mechanical means such as grit blasting or by chemical process which is the most commonly used, particularly for large articles. At times, a combination of the two methods is used, such as the removal of weld slag by blasting before chemical cleaning. Grease and paint are first removed by soaking in a hot alkali solution such as caustic soda combined with an emulsifying agent and a detergent. Several companies market proprietary mixtures, or the galvanizer may blend his own. Solution temperature is usually 180 F, and chemical concentration is maintained at approximately one pound per gallon of water. Little maintenance is required of this bath except the periodic addition of chemicals to maintain solution strength, and the daily addition of water to replace evaporation loss. Once or twice a year, sludge is removed from the tank bottom.

Water rinsing follows, and the use of a dual rinse system is far more effective than a single tank. Fresh water is introduced into the second rinse tank which overflows to the first rinse tank which in turn flows to the alkali tank. Water flow is regulated to be equal to the evaporation rate so that no waste has to be treated. This system helps to prevent the carry over of emulsified oil to the acid tanks which in turn causes product quality problems such as black spots and ungalvanized areas.

Acid pickling next removes oxidation (rust) from the metal surface. Hydrochloric acid and sulfuric acid are commonly used for this purpose, and both have their pros and cons which will be discussed later. In either case, the work should be thoroughly rinsed after pickling to prevent contamination of the next chemical bath.

In order to promote uniform wetting of the steel surface by molten zinc, a wetting agent or flux is used. This may be done by dipping the work in a solution of zinc ammonium chloride, or by using a layer of this chemical which floats on the surface of the molten zinc bath. Although both systems have their proponents and special cases may require one or the other, the flux solution method generally results in a better and more consistent finish. This method is referred to as "dry galvanizing" as opposed to "wet galvanizing" in which the "wet" flux floats on molten zinc.

Finally, the prepared article is dipped into a bath of molten zinc which is kept at a temperature of approximately 840F. Other metals are mixed in with zinc to perform certain functions such as aluminum to improve coating uniformity and reduce zinc loss through oxidation. A small amount of lead is alloyed with the zinc to promote drainage, and other metals are present as impurities in purchased zinc. None of these have been found to create a hazardous environment for plant personnel in the quantities present.

After dipping, the product is cooled by quenching in water or allowed to air cool depending on product type, inspected for defects, weighed, and stored until ready for shipment or use. It is important that articles should not be stacked closely and stored wet because oxidation or "white rust" can occur and will rapidly destroy the zinc coating. To minimize the risk of this occurrence, a treatment with a surface passivation chemical can be used.

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Environmental Considerations

Air Quality - The primary concern in the operation of a hot dip galvanizing plant is the particulate emission (smoke) which escapes from the surface of the molten zinc bath as the article to be galvanized is dipped. The emission is caused by the volatilization of flux and is primarily ammonium chloride although zinc oxide is also present (See EPA AP-40). Analysis of the air around the kettle has shown that these fumes do not present a health hazard to personnel, but even though the rate of emission is low ( usually 1 - 3 lbs/hr ), the characteristic light blue haze is source of complaints.

Pollution control agencies in general have ruled that these fumes must be collected using the best available technology. This is done by using a tightly enclosed fume hood around the molten zinc bath ( galvanizers refer to this bath as the "kettle" ) and a type of air filter known as a baghouse. This filter is equipped with a powerful suction fan and cloth bags through which the air is filtered, and it may be thought of as a very large vacuum cleaner. The fume hood also makes a significant contribution to personnel safety by containing the splatter of hot zinc that sometimes results when work is dipped.

The combination of fume hood and baghouse will capture most of the particulate emission. In the last few years, we have constructed several new plants, some of which are in sensitive areas and in complete compliance with the applicable permits.

Emissions from the pickle tanks are not generally a problem since they are mostly water vapor from the heated tanks. Some acid is entrained with the vapor, and can be damaging to the plant building and equipment. Normally, in warmer areas, the building would be naturally ventilated, and if construction materials are properly chosen, damage to the building and equipment can be minimized. Colder locations require that attention be given to ventilation and fume collection. These emissions have not been found to be a health hazard in the quantities present.

Liquid Waste - The primary liquid waste is spent pickling acid, and for reasons that will be discussed later, it is recommended that hydrochloric acid be used for this facility. As the articles to be galvanized are pickled, the acid solution becomes saturated with iron from the rust that is dissolved and is no longer useful. While it is possible to completely neutralize this spent acid, filter the resulting sludge and discharge the treated water to the sewer, it is recommended that the services of a waste disposal company be engaged; at least during the first year of operation.

Solid waste in the form of sludge from the neutralization or acid rinse water or spent acid must also be disposed of as a hazardous waste. Solid waste from the baghouse used for air quality control is also in this category. These wastes can usually be tested and then "delisted" which means that it can be placed is a general purpose landfill.

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Personnel & Safety

As mentioned above, galvanizing plants are similar to fabrication shops, and a good manager will have a background in materials handling as well as the supervision of operating personnel. Of course, experience in the art of galvanizing is desirable, but not essential because an experienced galvanizer can be employed to train other employees in the art. Quality control of the finished product, maintenance of the pickling solutions at proper concentration, and supervision of the waste disposal operation could be done by a relatively inexperienced chemist.

Maintenance personnel should have good experience in mechanical equipment repair, and some knowledge of electrical control circuit troubleshooting is required. Operating employees need not be skilled and should be capable of learning how to operate bridge cranes, monorail hoists, and simple hand tools in the performance of their duties.

Burns from touching galvanized work before it has cooled, and mashed fingers and toes are the most common injuries. Chemical burns are less common and can be prevented by wearing eye protection and protective clothing such as rubber gloves and slickers. Burns from molten zinc splatter do occur, but the fume hood enclosure is the primary means of preventing these burns. The galvanizers should also wear eye protection and burn resistant long sleeve clothing.

Fortunately, serious injuries are not common, but can be caused by improper preparation of the work to be galvanized. For example, failure to properly vent tubular work to allow the escape of entrapped moisture can cause an explosion when the work is immersed in molten zinc. It is in these areas that an experienced galvanizer can be used to train new employees.

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Energy Conservation

Natural gas will be the greatest energy expense in the plant, and proper furnace design will be used to reduce this expense. A modern furnace is approximately 65% efficient which means that 65% of the heating value in the purchased natural gas does useful work. This is accomplished by the selection of insulating materials that virtually eliminate heat loss from the furnace walls, and proper application of burners that convert the gas into heat energy.

Energy for the heating of pickle solutions is the other main consumer of natural gas. The caustic tank is heated to 180F to insure removal of oil, and the rinse tanks are operated at 100F to allow the work to be moved into the pickle tanks while warm. This improves pickling rate.

The choice of pickling acid also has an impact on energy cost. Sulfuric acid must be heated to a 140 - 150F in order to pickle properly, and in a plant of this size amounts to an operating cost of $10 per hour if efficient tank heating means are used. Since these tanks are kept hot all the time, a cost per year of some $20,000 results. It is for this reason as well as others discussed below, that the use of unheated hydrochloric acid is recommended.  .

Electrical energy is a less significant cost, primarily being lighting for the building and energy for cranes, monorail hoists, air compressor, and baghouse fans. A total connected load of 50 horsepower would be typical for a medium sized plant.

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Raw Materials

Zinc - Zinc is traded as a commodity on the world market, and most of the zinc consumed in the USA comes from this hemisphere, with Canada and the USA as leading producers with some metal from Mexico. The market price is governed by the supply-demand relationship.

The pickup of zinc on galvanized structural items  is approximately 7% by weight (including allowance for losses ) so that an increase of 10 cents per pound results in an increase of only 0.7 cents per pound in the cost of galvanizing. Some zinc is lost through the formation of a waste alloy with iron which is called dross. This material is formed by direct reaction of molten zinc with the work to be galvanized or, to a lesser degree from reaction when iron salts are introduced into the kettle because of improper pickling. It is sold to a reclaimer for approximately 50% of the worth of the zinc it contains. It is very important that this dross be removed from the kettle on a regular basis because it can cause localized overheating and subsequent damage to the kettle.

Sodium Hydroxide - Otherwise known as caustic soda, this basic chemical is used in combination with other basic alkalis for grease and paint removal. It is a commodity chemical and a stable market.

Ammonium Hydroxide - This chemical forms the basis for the flux system, and is sold as a proprietary mixture of the triple salt with zinc.

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Pickling Acids

Both sulfuric and hydrochloric acid are in common use for the pickling of steel to be galvanized, but hydrochloric acid offers several advantages and is recommended for use in new plants. There are several reasons, and some are outlined below:

1. Energy consumption - As mentioned above, sulfuric acid must be kept at elevated temperature while hydrochloric is used at ambient temperature. ( hydrochloric must be 70F or above to pickle at an acceptable rate so that some heating may be required in the winter months )

2. Operator Skill - Because of its nature, sulfuric acid, even when used with an inhibitor, will work its way under the surface rust or scale and attack the base metal of the article being pickled. On the other hand, hydrochloric with the proper inhibitor will remove rust or scale and not attack the base metal. This means that an article may be left in hydrochloric acid solution for extended times without damage and therefore is easier for an operator to use.

3. Zinc Pickup - As mentioned above, sulfuric acid readily attacks the base metal and generally results in a rougher surface on the pickled article. This rough surface results in more ounces of zinc being used per square foot of surface area than the relatively smooth surface produced by hydrochloric acid pickling. Data exists in papers presented at American Galvanizers Association technical meetings, and in private communications to show that this increase in zinc consumption may range from 5 to 20% depending upon operator skill.

4. Water Treatment Costs - In order to prevent contamination or the flux solution, articles pickled in sulfuric acid must be thoroughly rinsed before immersion with the attendant cost of treatment and disposal of the rinse water. With the use of hydrochloric acid, while rinsing is recommended, it does not have to be as thorough, and the flux bath can be easily adjusted chemically if carry over of acid results.

5. Personnel Safety - While chemical burns are not frequent, sulfuric acid at 150F is definitely more hazardous than is hydrochloric at room temperature, a fact to which the writer can attest after several years of experience.

Hydrochloric acid is not without disadvantages, for example, more tankage is required because the pickling rate is 1/3 slower, and the price of the acid per ton pickled is higher than sulfuric. Also, spent sulfuric acid can be reclaimed by refrigeration based processes whereas no current process effectively recycles hydrochloric. However, the true cost of using either acid is a compilation of the factors mentioned, and a savings in energy and zinc pickup has already been discussed.

Another much discussed disadvantage of hydrochloric is its effect upon buildings and equipment. This may be true in an unprotected steel structure, but if proper precautions are taken as in the proposed plant, the structure can be made resistant to this effect.

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Marketing & Trade Associations

AGA ( American Galvanizers Association ), ILZRO ( International Lead & Zinc Research Organization ), and the Zinc Institute are the organizations of primary interest to the galvanizer. It is recommended that you become a member of AGA because the organization provides a forum for the interchange of information between galvanizers and serves as a marketing force for galvanizing.

The other organizations are primarily of interest to zinc producers, but carry out work that is of benefit to the galvanizer. AGA also serves as a representative for the industry in environmental issues and a clearing house of information concerning the workings of regulatory agencies.

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