Introduction

The justifications for utilizing carbon steel in the manufacture of tubular products and shapes are numerous. Its strength, relative ease of fabrication, versatility and low cost make it the material of choice for many applications. Though however versatile and widely utilized carbon steel is, there are also significant limitations associated with its use. One of the most prominent limitations of carbon steel in tubular or shaped form for general applications is its susceptibility to corrosion. This article is structured to provide a review of process technologies associated with both galvanizing and coating of tubulars and shapes.

Fundamentals of Corrosion

Corrosion is basically an electrochemical process associated with the flow of current between surfaces having different electrochemical potential. Various metals and alloys by nature of their composition and structure will exhibit varying degrees of electrochemical potential. Metals which possess strong electronegative characteristics are referred to as cathodic or protected metals. Metals possessing strong electropositive characteristics are referred to as anodic or sacraficial metals. (see figure 1)

When dissimilar metals are subjected to an environment conducive to corrosion, the electrochemical process attacks the metal possessing the higher anodic value, allowing this metal to breakdown and sacrifice itself to the metal having the greater cathodic value.

Galvanizing as a means of protection

Galvanizing may be defined as the practice by which zinc is metallurgically bonded over an iron or steel substrate to provide protection to the base metal. The zinc coating provides protection to the substrate, in two distinct ways. One by shielding the base metal to the atmosphere and two, by providing anodic protection to the base metal. Corrosion resistance continues to be the primary reason for galvanizing tubular products and shapes. In general, unprotected steel surfaces will corrode at a rate of approximately .002 to .005 inch per year when subject to an arid environment. The rate of corrosion increases dramatically as precipitation, humidity or other detrimental factors are introduced to the surrounding environment. While surface oxidation will begin to form almost immediately on an unprotected substrate, a surface that is protected by a properly applied barrier of zinc corrodes at a rate of 3 to 5 percent of that which can be generally expected from unprotected steel. One unique characteristic of zinc as a means of corrosion resistance, and the prime benefit of protecting tubulars and shapes through its use, is zincs "self healing" properties. Unlike other non-metallic coating systems, even when the surface of a galvanized product is disrupted or lightly abraded, the electropositive or anodic characteristic of the zinc coating will continue to provide sacrificial protection to the exposed base metal.

Application of zinc to the surfaces of tubular products and shapes intended for use as electrical conduits, fence tubes, greenhouses and agricultural structures has for many years been the accepted industry standard for corrosion resistance. However due to the markets demand for a level of corrosion protection which cannot be met using other production methods, a wide range of mechanical and severe duty products are now utilizing galvanizing and hybrid coating processes to meet and exceed market demands. It is this emerging market segment which is driving force behind innovation in process technologies and coatings development.

Methods for Galvanizing Tubulars and Shapes

At present there are four production methods being employed to produce galvanized tubular products. Three of these are well known and only a brief description of each is provided. The fourth continuous process not being as widely known is presented in more detail.

Pre-Galvanized Strip with Metallized Weld Seam

This method utilizes pre-galvanized strip in lieu of un-coated steel strip. As the zinc coating is melted away at the weld seam during the welding process, a metallized spray is directed over the seam to provide corrosion protection. The inability to provide corrosion protection to the weld seam equal to that of the pre-galvanized strip makes this product susceptible to premature failure and is thereby practical for limited environments and usage. While this process requires minimal capital investment this is typically offset by the high cost of raw material which can be quite unstable do to wide fluctuations in balance of market supply and demand.

Electro-Deposition or Electroplating

This method transports tubulars or shapes produced off-line through the electroplating operation using a series of conveyors. By being a non-integrated system, this transport is commonly met with a number of transfers or pauses in material flow. Traditionally systems are designed to pass the product through a degreasing bath, rinsed, immersed in a pickling solution of either sulfuric or hydrochloric acid then rinsed once again. After the product surface has been sufficiently cleaned it passes by continuous transfer through the electroplating unit. This plating station includes a bath solution that acts as a transfer agent to enhance plating solution conductivity. Also contained with the plating unit is a series of anodes which when electrically charged by means of a high amperage, low voltage current allows the flow of zinc from the anode to the cathode, which in this case is the product to be coated. After exiting the electroplating station, coated tubing is again rinsed to remove residual plating solution and then dried in most cases by utilizing hot air. The semi-finished product is then ready for any subsequent surface coating operation.

Hot Dip Process

Hot dip galvanizing is similar to electroplating in that they are both non-integrated processes, but this is where the similarities end. While the electroplating process is a cold process, the hot dip process utilizes molten zinc for coating. With this method, formed and welded finished product lengths are transferred through cleaning and pickling operations prior to immersion into the molten zinc bath. The product is passed through and out of the bath via a star wheel, screw conveyor or magnetic roll unit. After exiting the bath, the product is subject to a series of air wipers which control coating thickness and surface condition on both the interior and exterior of the product. The coated length is then quenched and readied for further surface treatments.

Continuous or In-line Galvanizing Process

Today the most efficient, cost effective and versatile means of producing galvanized tubulars and shapes is accomplished utilizing the continuous or in-line process. As its name implies, the continuous process incorporates the galvanizing operations into the tube or pipe production line. Through this integration a manufacturer can transform coiled raw material to a galvanized finished product within seconds and completely eliminate work in process inventories and handling associated with other non-continuous or interrupted methods. For a summary comparison of the various processes used in the production of galvanized tubular products and shapes please refer to figure 2.

Figure 2 - Characteristics and Comparison of Galvanizing Processes

Common to all continuous galvanizing processes are those components which comprise traditional tube and pipe production lines. Equipment related directly to the preparation, application and post treatment of the galvanized surface will vary in design dependant upon raw material usage and finished product requirements. However the basic layout and task performed by each sub process remains fairly consistent from line to line. The first component unique to continuous galvanizing is the strip cleaning system. As with any coating operation, the condition of the substrate material is of prime importance in assuring acceptable product performance and aesthetics. This system is provided to remove any contaminants which would adversely effect surface condition or adhesion of zinc. Inconsistencies associated with the surface quality of raw materials produced by a cross-section of steel mills ranging from third world to world class magnifies the importance of this component. Located immediately downstream of the forming and welding operation is the surface preparation system. The primary responsibility of this system is to remove any surface oxides or other contaminants which remain on the surface of the formed and welded tubular or shape which will again effect coating adhesion or product acceptability. Upon achieving a surface which is suitably clean and ready for coating, the material must be heated to promote the formation of the zinc iron alloy layers which provides the metallurgical bond of the molten zinc bath to the steel substrate. This is accomplished by passing the product through an induction type heating unit prior to immersion into the zinc bath. The application of zinc is performed by allowing the surface of the product being coated to pass under a flow or level of molten zinc. It is during this immersion period that the three layers of inter-metallic alloys are formed in addition to the forth, outermost layer of free zinc. Upon exiting the zinc application section of the process, a coating weight and concentricity control apparatus is employed to control zinc deposition and coating uniformity. Once the desired coating weight and surface finish is achieved, the product is quenched to solidify the zinc and lower its temperature to a level where proper straightening and handling can be performed. After the product has been cooled, it is now ready to be passed through the sizing and or reshaping section of the mill to bring the product to its finished profile and dimensional requirements. From this point forward the remainder of the process will differ considerably dependent on the choice or combination of choices made for secondary surface treatment or coatings to further enhance finished product performance and aesthetics.

Exterior Secondary Coating Operations

While galvanized tubulars and shapes provide considerable protection against corrosion, their performance and aesthetics may be further enhanced through application of a secondary coating or hybrid coating process. In addition to galvanized substrates, un-coated black mechanical tubes may utilize these secondary in-line processes to add utility, performance and value presenting many opportunities to non-galvanized tube and pipe producers. These secondary coating operations primarily act to shield the underlying substrates by forming a distinct protective barrier which insulates the surface from potentially harmful elements. In general to be effective coatings should possess the following properties. The coating should be continuous and substantially impervious to its subject environment while remaining resistant to damage such as abrasion or scratching. It should not electrolytically accelerate the attack on its substrate. Finally the coating should exhibit good adhesion and ductility if the end use of the base product is intended for fabrication. It should be noted that coatings for application over tubular products are considered to be specific use formulations. They are meant to perform specific tasks with regard to corrosion protection, product aesthetics or in some cases to provide a materials advantage to allow the use of a more economical component. It is not uncommon for these coatings to require some degree of on-line tuning to optimize their performance and compensate for process or other production variables. Once these final modifications are completed the coatings will rarely require anything other than proper agitation to achieve their intended performance.

There are a number of coating technologies available for the many products which can be produced with the continuous galvanizing process. In general, each coating will also have process application technologies which is unique to each coating type. This section will present the four most prevalent coatings currently utilized over the outside diameter of galvanized tubulars and shapes. It is assumed that with any of the coating technologies discussed proper cleaning and pretreatment should be undertaken prior to the coating application.

Air Dry Coatings

As the name suggests, air dry coatings are formulations which require either little or no auxiliary heat source to cure the coating. Traditionally these are solvent rich lacquers which have relatively low solids contents and high solvent concentrations. These air dry formulations may be clear or contain pigments to add color to the surface of the product. This form of coating is considered to provide the least amount of corrosion protection and is generally utilized for short term storage protection. Electrical conduit is a suitable use for air dry coatings provided that inventories are only subjected to a moderately corrosive environment. Typically air dry coatings are applied by utilizing a flood and wiping apparatus. Dry film thickness is controlled by coating viscosity and wiping pressure with average films measuring approximately .0003". After application and wiping of the coating the solvents will evaporate or flash off and leave behind the thin film of solids as protection to the underlying zinc. Of all coating technologies the air dry system requires the least amount of capital equipment, requires the least amount of line length but in return provides the least amount of corrosion protection.

Heat Cured Coatings

Heat cured coatings require a heat input generally provided an induction or infrared heating unit to raise the tubing surface to a temperature where the coating becomes set or cures. These coatings are normally moderate in solids volumes and utilize a polyester resin or a waterbase acrylic latex resin base. These coatings are most often clear but may in some cases utilize a pigment to both add color as well as enhance corrosion performance. Heat cured coatings are considered to provide moderate to good corrosion protection for most tubular products and shapes. Corrosion protection can be directly related and effected by the products pretreatment method, coating dry film thickness and continuity. Most heat dry coatings are applied with good results utilizing a flood and wiping system similar to that utilized in application of air dry formulations. Dry film thickness is again controlled by coating viscosity with average depositions ranging from .0003" to as high as .0008" as dictated by the desired level of performance. Due to the flexibility and moderate cost of heat cured formulations, they are popular for use on electrical conduit, residential fence tubing and some galvanized mechanical tube applications. Cost of the equipment related to the use of heat cured coatings would be considered moderate when compared to the three other coating technologies.

Ultra Violet (UV) Cured Coatings

UV cured coatings are formulated with a photo reactive component which allows the coating to cure when an ultra violet light source is introduced. UV cured coatings are virtually 100% solids by volume which means that no solvent component is present. This full solids content coupled with the high transfer efficiencies associated with current application equipment makes UV curables the only liquid coating technology which is essentially 100% efficient. An attribute only presently matched by powder systems. UV cured coatings can be applied by any of three means, flood and wiping, spray enclosure or vacuum applicator. Coatings again may be clear or pigment. Once applied to a wet film thickness of between .0005" and .001" dependant on desired protection, the product is passed through a series of lights which excite the photo reactive nature of the coating and an almost instantaneous cure is achieved UV cured coatings are primarily used when enhanced corrosion performance will substantially improve a products useful life and value. While higher than average in applied costs, UV coatings costs are often offset by the products corrosion resistance. Products which are typically produced utilizing UV cured coatings are both residential and commercial fence tubing, some galvanized mechanical tubing applications as well as galvanized structural tubing. Capital cost of the equipment required to apply and cure UV cure coatings is higher compared to the other three coatings. However when high performance, line space and quality is an issue, UV will most often be the process of choice.

Heat Cured Powders

Heat cured powder coatings differ from the preceding technologies in that the product applied is actually a powder and not a liquid. The use of powder over tubular product is not new, for many powder has been applied off-line on conveyor type process lines. Today the technology exists to apply powder coatings in-line to produce the ultimate in corrosion performance. Powder coatings developed and grew throughout the 1970's and 1980's due their environmental desirability. Application cost and the inability to efficiently control coating weight made powder economically unfeasible for all but a limited volume market. Today powder refinement and application equipment has evolved to a point where powder is more attractive where there is an economic justification for a superior product.

Powder can be applied utilizing a fluidized bed, electrostatic fluidized bed, flocking or by electrostatic gun. Thermosetting powders when heated to a predetermined temperature undergo a chemical reaction and fuse creating an excellent adhering, highly corrosion resistant surface. As with the heat cured coating described above, powders are cured after application by means of an induction heating unit. Once the predetermined temperature is reached the product may be cooled immediately and no further processing or treatment is required. Due to the economic considerations of powder the application is at present limited to some residential and commercial grade fence tubing as well as mechanical and structural applications. Capital cost of the process associated with the application of powder is high compared to the other coating methods previously discussed, however in-line applications will prove to be far more efficient and cost effective than their off-line counterparts.

Interior Primary Coatings

As the continuous galvanizing process coats the exterior surface of welded tubular products, some form of corrosion resistance is required for internal surfaces of the product. Typically this is accomplished by spraying a specially formulated coating through a lance apparatus placed into the tube at the forming section of the tube or pipe mill. Once again the coating formulation is dependant upon the finished products end use and its associated performance requirements. Of the four product categories produced by the continuous galvanizing process, three basic coating types are used for interior finishing.

Interior Primers

These coatings are designed primarily for general corrosion resistance for use with residential or non-spec fence tubing, mechanical tube applications such as scaffolding as well as some galvanized structural products. Basically these formulations are low to mid solids by volume with either solvent or water carrier fluids. Pigments are generally used to enhance product performance and to act as an indicator for product coverage. This coating type will provide excellent corrosion resistance for most uses, with the degree of protection directly related to dry film thickness. No special considerations for application other than proper agitation and application viscosity need be given for optimal product performance.

Zinc Rich Coatings

Zinc rich coatings are intended for use when maximum corrosion resistance is desired. Zinc rich formulations represent an attempt to combine the sacrificial corrosion protection of zinc with the barrier film protection of interior primers. Finely milled metallic zinc particles are mixed and imbedded into a resin matrix. Even though the zinc particles are covered by a thin film of resin they still possess a relatively good path of conductivity to allow the zinc to retain its strong electropositive characteristics. Due to the high molecular weight of the additional zinc powder, special consideration must be given to insure that the zinc powder which settles during shipment and while in inventory is re-dispersed prior to coating application. This is accomplished by utilizing special mixing and dispersing equipment which is designed to properly agitate the coating thereby keeping the zinc element in solution and homogeneous throughout the container. These zinc rich coatings are designated for use with commercial or spec grade fence tubing, in some severe duty mechanical applications as well as galvanized structural products where the product will have to endure severe environmental conditions.

Conduit Coatings

Coatings formulated for use on conduit products must be designed to meet additional performance criteria from those intended strictly for corrosion resistance. Due to conduits use as an electrical wire way or enclosure one very important factor is its ability to allow the insertion and pulling of electrical wiring. This requirement makes the internal surface finish and a low coefficient of friction characteristics of the coating extremely critical for proper performance. In addition to the low friction characteristics, conduit coatings must also meet certain requirements set forth by various underwriting agencies such as Underwriters Laboratories. These requirements are set as standards to insure coating adhesion and flexibility as the finished product will most likely undergo bending during installation. Generally, coatings for conduit use are low to mid volume solids and may utilize either a solvent or water carrier fluid. As corrosion resistance characteristics are not as demanding as severe duty applications these conduit coatings provide acceptable results at dry film thickness of as little as .0003" to .0004".

Conclusion

Galvanizing and coating of tubular products and shapes has been utilized for many years as a means of corrosion resistance by significantly extending the useful life of carbon steel based products. However the processes which have been traditionally employed are rapidly becoming dated when compared to the efficient, cost effective and versatile process technologies now available. Todays manufacturing environment dictates that producers must establish a position of strength through efficiency and develop a competitive edge through versatility and innovation. It will be through the use of these continuous coating processes that carbon steel will continue to be the material of choice for a multitude of traditional as well as innovative new tubular products and shapes for many years to come.

For further information please contact:

Peter Chifo, Jr, President, Superior Technologies, Incorporated
- Addr: 13850 Benson Avenue, Chino, California 91710, USA
- Fax: +1 909.364.2322
- WWW: http://www.superior-tech.net
- Email.