TECHNICAL ARTICLE:

The application of coatings over carbon steel and alloy steel substrates has been a common practice to enhance corrosion performance and alter particular physical characteristics to make these products more suitable for specific applications. The purpose of this article is to provide a general presentation of how coatings can, and are utilized in the production of tubular products and shapes to improve product quality, widen the scope of material utilization and increase the value added during conversion of raw material to finished goods.

Traditional Uses of Coatings

The most common use of coatings has been to apply them to tubular surfaces to provide corrosion resistance to the steel substrate. Beginning with hot dipped methods of galvanizing, molten zinc had been applied to enable the use, and prolong the life of steel pipe and tubes for use outdoors or in more demanding conditions. These products have included water pipe, fence tubes, structural tubing, electrical conduits and the like. While the application of zinc imparts a significant degree of protection to the underlying metal, zinc by nature of its sacrificial characteristic provides a finite degree of protection and to a lesser extent, maintains product aesthetics. Though the zinc provides an adequate barrier for the substrate, the free zinc or outermost layer will begin to form a white powder-like surface commonly referred to as white rust. Eventually, the performance of these galvanized products was improved through the means of surface passivation utilizing a chromic acid solution to chemically alter the properties of the free zinc surface thereby making it less susceptible to the formation of white rust. Secondary coatings such as varnishes have been applied to provide further enhancements to protection and aesthetics though market demands required a more suitable alternative to these limited performance products. Currently variations of the hot dipped process include the use of zinc/aluminum alloys and aluminized steel as an alternative and enhancement to traditional galvanized products.

In recent years significant strides have been made in the development of process and coating technologies which have enabled the use of a variety of formulations for optimum performance and finished product versatility. These processes and formulations will be topic for the text of this article. Please note that the technologies presented will be limited to those have common commercial applications for finished product diameters of up to 6" or 150mm.

The Reasons for Coating Tube and Pipe Products

There are primarily three reasons for the application of coatings to tubular products. They include corrosion protection, product aesthetics and altering of the physical properties for a specific purpose.

Coatings intended to provide corrosion protection can be separated into two categories. Short term or interim protection and long term or extended protection. Generally speaking short-term protection implies that the coatings primary function is to preserve the surface of the product up to the time that it is placed into its final use. Two major considerations effecting the required level of performance for these coatings include the duration and conditions of storage prior to shipment, as well as the duration and method of transportation used in delivery of product. Products stored outdoors as well as those subject to the severe environment of ocean transport should utilize a coating which protects against damage and degrading of product quality and appearance. While surface condition may in and of itself not effect the suitability of a products use, it will most certainly adversely effect the customers perception of both quality and value of the product.

Beyond those products which require minimal storage and transportation protection are those which by the nature of their use require outstanding long term protection. Typical examples of these products would be fence tubes, greenhouse tubing and structural shapes which are subject to extreme outdoor conditions. If an adequate degree of corrosion resistance is provided to the underlying raw material it will significantly improve by its ability to withstand the effects of storage, transportation and the environment then it will have higher perceived value or greater market potential.

The application of coatings for aesthetic reasons can take two forms. The first as described above, protects the tube or pipes surface from oxidation ensuring a visually acceptable product upon delivery. Another reason for applying a coating or paint over a tubes surface include enhancement of its appearance and to incorporate secondary or post processing coatings into the tube production process. Processes and coatings are now available which, dependent upon the severity of post processing, allow the product to be pre-coated and made suitable for subsequent fabrication. This significantly reduces or eliminates the costs associated with much less efficient off-line processes.

Finally the ability to alter the physical characteristics of the substrate is yet another application for coatings. These are primarily specific use applications such as coatings applied to the interior surfaces of pipes and tubes to reduce friction to better enable the pulling of wire as in electrical conduits. Another example of this ability to alter the materials properties would include the application of either a polypropylene or polyethylene based coating for protection against hydrogen embrittlement when used for gas transmission.

Figure 1: Comparative benefits of process designs

Process Applications

Processes for the applications of coating follow two distinctly different design philosophies. This significant difference between comparative designs of coating systems relates to those processes intended for in-line or continuous coating operations and those which are conducted off-line or as a secondary operation. These off-line processes can be further grouped into two separate design characteristics, piece fed and continuous coupled. The fundamental difference between these designs are piece fed as the name implies feeds product piece by piece while continuous coupled off-line operations connect the products end to end thereby simulating the in-line, continuous type process. True in-line or continuous operations will integrate or locate the coating process into the tube or pipe making line. The equipment associated with the coating process is located post sizing or calibrating mill and prior to the flying shear or saw. In an off-line or secondary coating operation the equipment is located post shear or saw and the product is transported through the coating process in a piece fed or coupled manner. Both designs have inherent advantages and disadvantages however as shown by the accompanying table of comparison, the most efficient means of coating is an in-line operation (please see figure1). The ability to integrate the coating process into the primary tube and pipe making line minimizes or eliminates many of the issues which adversely impact production efficiencies and conversion costs. This integration of components and processes into the primary production line is one that must be undertaken with much forethought and planning. The importance of a properly designed process which addresses and optimizes all operational variables effecting the coatings performance cannot be overstressed as online experimentation and development can prove to be very costly in both revenue and quality reputation.

There are basically three fundamental aspects common to all coating processes. They are cleaning/pre-treatment, application and curing. As with any type of painting or coating operation the cleanliness of the substrate is of prime importance in optimizing performance. While some formulations are more tolerant to contaminants, all will benefit from a clean underlying surface. The level of sophistication associated with cleaning operations is directly related to the condition of the raw material being processed. If the material is a freshly in-line galvanized product then little if any cleaning is required to present a suitable surface for coating application. On the other side of the spectrum a hot band product may require a combination of both mechanical and chemical cleaning stages prior to coating application to remove mill scale and impinged rolling lubricants. In addition to removal of contaminants detrimental to coating adhesion, pre-treatments such as phosphates and surface passivates may be used in the process to further enhance coating performance and finished product quality. The method of coating application is entirely dependent upon the type of coating used as well as the specific characteristics of the product being produced. The methods most commonly used in tube and pipe applications include flood and wipe systems, spraying and lance type systems, electrostatic spray or fluidized beds, and vacuum coaters. Once again each system has distinct advantages over available alternatives for specific coating formulations and product production considerations and characteristics. Please refer to figure 2 for an evaluation of available coating application technologies. While flood and wipe systems are relatively simple and low maintenance by their design, they are limited by their inability to apply heavy film builds which limit their use for certain products. On the other hand spray systems while enabling significant coating deposition normally do not allow application of very light coating weights.

Figure 2: Evaluation of application methods

Curing equipment is mandated again by the coating formulation utilized. By definition radiation curable coatings use radiant energy in the form of light to initiate coating cure. This may take the form of light within the UV light spectrum when curing clear-coat products or may include the use of electron beam generation to provide deeper penetration of energy as is normally required when using pigmented products. While some forms of solvent-borne coatings may contain sufficient levels of solvents to enable an air-dry cure, water-borne coatings by nature of their composition require an external heat source to force off the aqueous component of the coating to promote cure. These external heat sources may take the form of induction heating devices, infra red heating devices, radiant heat sources or hot forced air curing systems.

The most important aspect of ensuring quality conformance and reliability is an integrated approach towards optimizing the intended coatings performance as reflected in the process design. Unquestionably a system which is designed around a definitive coating performance requirement will most certainly outperform a coating developed to utilize an existing process.

Coatings Technologies and Their Characteristics

Today the most prevalent coatings utilized in the production of tubular products include zinc and zinc/ aluminum coatings, waterbase emulsion coatings, water reducible/soluble coatings, solvent-borne coatings, radiation cured coatings, powder coatings and extruded or PVC based coatings. Figure 3 provides a summary comparison of coating characteristics. A more detailed description of these coatings technologies follows.

Figure 3: Comparison of coating chracteristics

Zinc and Zn/Al Coatings

Galvanizing or the coating of carbon steel through immersion into a molten zinc or zinc/aluminum alloy bath be it continuous, batch dipped or by zinc electro-deposition method is a well known practice. The results gained through this method of treatment are significant and arguably it provides the greatest degree of protection to the parent metal. For the purposes of this presentation we will consider the galvanized surface as a primary substrate which can be further enhanced by the application of pre-treatments and secondary coatings. For a more thorough discussion of the details and comparative benefits of galvanized products please see the article entitled "Galvanizing and Coating of Tubular Products and Shapes" published in the January/February 2000 issue of Tube and Pipe Technology. For a copy of the text of this presentation direct your request to the contact information located at the end of this article.

Radiation Cured Coatings

Radiation cured clear coats have been utilized in pipe and tube applications since the mid-seventies so in a sense they cannot be classified as immerging technology. However what is new is that process and pre-treatment technologies have recently been developed which enable utilization of this coatings performance potential to it's fullest extent. By nature of their rapid or near instantaneous rate of cure these formulations lend themselves well to in-line application over pipe and tubular products and enable ultra high rates of production throughput. A secondary benefit of the accelerated cure rates is that it enables the process to have a relatively small footprint and allows for immediate handling of coated products. This makes the retrofit and adaptation of this technology more useable over a broader range of installation considerations. One of the most attractive features of radiation curable such as ultra violet cured coatings is the fact that they are 100% solids by formulation and posses no VOC. This makes this technology extremely environmentally friendly and allows its use in even the strictest of regulated facilities. Surface qualities of these coatings include a high gloss finish with superb clarity and excellent abrasion and chemical resistance. An interesting attribute of the coatings base is its ability to be formulated to have a surface hardness ranging from that of glass to having an almost rubber-like characteristic. The versatility of application methods coupled with the 100% solids results in very high transfer efficiencies and enables reclamation and recycling of the coating.

Water-borne Latex Emulsion Coatings

Water-borne latex emulsion coatings are utilized for a variety of market applications in the pipe and tube industry. This coating utilizes chemical coalescing agents during the drying process in order to form a continuous film with its protective properties. This technology has its primary appeal in that it provides a relatively low cost approach to general coating requirements. Both clear and pigmented color coats are available having mid range gloss properties. The latex base promotes excellent flexibility and elongation characteristics both of which are critical to coating performance during post processing operations. Relatively low VOC content still allows for the use of this coating under current regulations and the low heat requirement for product cure results in moderate operational costs. Considerations normally associated with storage and handling of volatile formulations are minimized due to the non-flammable nature of this type of coating. Lastly, latex based formulations also enable the re-coating of previously coated surfaces with virtually all conventional types of coatings. This may be of importance when a coating is used for intermediate or longer term storage or transportation protection when production is not customer specific.

Water Reducible / Soluble Coatings

Water reducible/soluble coatings are typically utilized in the pipe marketplace where off-line coating and secondary processes are utilized. This technology is very similar to it's solvent-borne counterpart in it's curing mechanism is based upon evaporation and oxidation for the development of performance properties. Comparatively speaking this coating alternative is attractive in those applications that require lower VOC and flash point than it's solvent-borne counterpart. While the VOC component range is traditionally 2.5 to 3.0 at application it is still well below those of solvent-borne formulations. These coatings can be provided in a full array of colors possessing excellent gloss and clarity. Properly applied these products provide excellent adhesion to the substrate and the coatings surface has good abrasion and scratch resistance once adequate cure has been achieved. Satisfactory corrosion resistance can be expected upon application of a dry film thickness of between 0.5 to 1.0 mil. When utilizing off-line coating operations product transfer rates and heat input for coating cure are the significant limiting factors to production throughput. These variables should be considered when optimizing coating performance through process design. As with water-borne latex emulsion coatings these products are relatively safe to handle and do not pose the risks and concerns associated with solvent-borne or other flammable coating materials.

Solvent-borne Coatings

Solvent-borne coatings technology was one of the initial products utilized for protecting the surfaces of pipe and tubular products. Traditionally acrylic lacquer coatings have been utilized for exterior tube applications and alkyds, which dry by evaporation and oxidation, continue to be utilized by many tube and pipe producers worldwide. While these solvent based formulations have many merits, more stringent air quality regulations and corresponding VOC restrictions have had a very significant effect on their use. While post processing of effluent by means thermal oxidation can be used to overcome these environmental discharge limitations the economic considerations of this approach has restricted it's use to many producers. Recently higher solids formulations coupled with the de-listing of acetone as a VOC compound has enabled solvent-borne technology to remain a viable option in the mix of coatings available for use within the pipe and tube industry. The ease and versatility of application and corrosion resistance properties are most certainly two of the attributes that has made this technology so widely accepted. Solvent-based formulations are far more tolerant to surface contamination than their water-based counterparts and exhibit superior adhesion properties to a variety of underlying substrates. These coatings are available in a comprehensive range of colors and gloss characteristics. It should be noted that solvent-borne technology remains as the number one option for internal ID coatings applications.

Powder Coatings

For exterior applications exposed to severe environments and where corrosion performance is of utmost importance powder coatings will most likely be the coating of choice. Heat cured powder coatings differ from the preceding technologies in that the product applied is actually a powder and not a liquid. In this light powders are virtually exempt from air quality regulations other than those which address the containment of airborne pollutants in the work environment. These coatings are typically applied at high film thickness to obtain the desired performance properties. It remains generally difficult to apply at lower film thickness without incurring a rough surface which not only effects the products appearance, but also decreases the product performance properties. As powder manufacturing techniques continue to be refined thinner film builds with acceptable performance levels will most likely evolve. Powder can be applied utilizing a fluidized bed, electrostatic fluidized bed, flocking or by electrostatic gun. The transfer rate is exceptional utilizing these high efficiency methods and recycling of the powder is easily achieved provided that the powder is not contaminated by liquid or airborne particulate. Dependent upon the type of powder utilized cure can be achieved either by heating and quenching as with thermoplastic powders, heating for a specific period then quenching as with thermosetting powders or pre-heating prior to application, and curing through light sources when using ultra violet curable powders. Surface characteristics of a powder coated product include a wide variety of colors and glosses possessing excellent adhesion and corrosion protection. Unfortunately these outstanding features associated with the use of powders are offset by the economic considerations of powder processes and applied costs. While in-line powder technologies are rapidly developing and becoming commercial viable, the traditional piece fed process by which finished lengths are transported via conveyor through the process is still most prevalent.

The Value of This Technology to Tube and Pipe Producers

The value of coatings to pipe and tubular products manufacturers lies in their ability to preserve and enhance product quality, provide the opportunity to expand into new markets and increase the value added to finished products. As discussed previously, the longstanding benefit of coatings remains its ability to preserve product quality and appearance. With the exception of a few products, the marketplace for most goods is truly a global market. Producers who manufacture and deliver to local markets set the standard for product aesthetics and their perceived value. It is difficult for producers who are at a disadvantage because of their proximity to the marketplace to provide a product that maintains the same appearance of quality as locally manufactured products. Therefore coatings systems that preserve product quality and enhance aesthetics provide manufacturers a broader geographical area to effectively market their products.

In today's competitive marketplace there are two fundamental ways that operations can improve profitability. One is to improve overall operational efficiencies by reducing conversion costs and the second is to find new and innovative ways to add value to existing production capacities. Coating operations can effectively address both of these issues. Thanks to new and innovative process and coating technologies, producers can now utilize the in-line process to reduce or eliminate many of the costs inherent in off-line or piece fed processes. These new systems enhance profitability by eliminating work in process inventory and material handling costs, provide for better utilization of factory floor space and drastically reduce direct labor costs. In addition to reductions in conversion costs, major cost benefits can be derived from a reduction or elimination of claims against materials that are rejected or subject to allowances due to product degradation while in inventory or during transportation. The net result is increased profitabilty through utilizing a more efficient means of conversion.

Finally the installation of coating operations into an existing tube production facility provides the producer an immediate entrance into a new and rapidly expanding market of higher value added coated products comprising a variety of inventive new end uses.

Conclusion

In conclusion, producers must have the insight to adapt technologies that compliment and enhance existing production capabilities and versatility. The progressive producer would well be advised to consider the many benefits and opportunities brought about by innovative new coatings and coating process technologies. Their implementation will most certainly redefine market standards of product quality as well as production standards relating to conversion efficiencies.